Amanda de Oliveira Silva, Small Grains Extension Specialist
Areas of Oklahoma with dry conditions have begun to see some visible wheat injury due to brown wheat mite (Figure 1) and winter grain mite (Figure 2). Last week, Josh Bushong, OSU Northcentral Area Extension Agronomist, reported seeing fields with injury due to brown wheat mites in Alfalfa and Kingfisher counties (Figure 3). Later that week Tyler Lynch, the Senior Agriculturalist in the Small Grains Program also reported seeing wheat grain mites in our wheat plots at Alfalfa and Woods Counties.
Two common mites can injure wheat, the brown wheat mite and the winter grain mite. Producers need to remain alert so that they don’t mistake damaged wheat from small grains mites for drought or virus disease.
Figure 1. Brown wheat mite with oversummering egg
Brown wheat mite is small (about the size of this period.) with a metallic brown to black body and four pairs of yellowish legs (Figure 1). The forelegs are distinctly longer than the other three pairs. Brown wheat mites can complete a cycle in as little as 10-14 days. Oklahoma experiences multiple generations of brown wheat mites that usually peak in spring, and the last generation occurs in April. At that time, females produce a whitish egg that will over summer.
Figure 2. Winter grain mite with a kidney-shaped egg
Winter grain mite is small (about 1 mm long) with a dark blue to black body and four pairs of orange-red legs, and a small reddish spot on the top of its abdomen that can be seen under magnification (Figure 2). Winter grain mite eggs are kidney-shaped, and change from clear, to yellow to reddish-orange after several days. They are laid on leaf blades and stems or the roots near the crown. Besides wheat, many grasses serve as host plants, including barley, oats, ryegrass, and fescue. We typically experience two generations each year, a fall generation and a winter generation that cycles out in March.
Figure 3. Field infested with brown wheat mite. Photos taken by Josh Bushong at Kingfisher County on February 21, 2023.
Figure 4. Leaf stippling from brown wheat mite (left) and grain wheat mite (right) feeding. Photo on the right taken by Tyler Lynch at Alfalfa county on April 24, 2023.
Figure 5. Field infested with winter grain mite
Both mites feed by piercing plant cells in the leaf, which results in “stippling” (Figures 4 and 5). The leaves take on a characteristic brown-grayish or cast and could be mistaken for injury due to herbicide. These mites are more likely to cause injury in wheat stressed from lack of moisture or nutrients.
Figure 6. Winter grain mite hiding in residue
Threshold
Brown wheat mites are not light sensitive but are vulnerable to driving rains of more than 0.25 inches, which tend to reduce populations. Winter grain mites are more tolerant of rainfall but are very light sensitive and tend to avoid bright, sunny days and windy days, so adjust your scouting accordingly. It is best to scout for winter grain mites on still, cloudy days or early morning/late evening. On sunny or windy days, they hide under the soil surface (up to a couple of inches) or congregate under dirt clods (Figure 6). Both mites are associated with continuous wheat production. Research suggests that brown wheat mite can be economically treated when there are 25-50 mites per leaf in wheat that is 6-9 inches tall. An alternative estimation is “several hundred” per foot of a row. The best recommendation for winter grain mite is to treat when plants show visible injury, and there are still mites present.
Chemical control
Only a few insecticides include either mite species on their label. Work conducted by Dr. Gerald Wilde at Kansas State evaluated several insecticides for the control of winter grain mites. Of those registered for winter grain mites, the insecticide dimethoate (Dimethoate and other generics) is effective. Other pyrethroid insecticides, lambda-cyhalothrin (Karate, Warrior II, and its generics), gamma-cyhalothrin (Declare) and beta-cyfluthrin (Baythroid and its generics) are also effective for both mites, even if they are not specifically listed on the label.
Additional resources
Contact your County Extension office
For more information on these mites, consult fact sheet EPP-7093 Mites in Small Grains by clicking here. If you find active mite infestations in your field, consult fact sheet CR-7194 Management of Insect and Mite Pests in Small Grainsfor registered insecticides, application rates, and grazing/harvest waiting periods by clicking here.
Amanda de Oliveira Silva, Small Grains Extension Specialist and Tom Royer, Extension Entomologist
Planting date: Much of the winter wheat sown in Oklahoma is used as a dual-purpose crop. In such a system, wheat is grazed by cattle from late fall through late winter/early spring and then harvested for grain in early summer. In a grain-only system, wheat is generally planted in October, but in a dual-purpose system wheat is planted in early to mid-September to maximize forage production. Planting wheat early significantly increases the likelihood that diseases and insect pests such as mite-transmitted viruses, the aphid/barley yellow dwarf complex, root and foot rots, and Hessian fly will be more prevalent and severe. For more detailed information on planting date and seed treatment considerations on wheat, see CR-7088 (Effect of Planting Date and Seed Treatment on Diseases and Insect Pests of Wheat).
Mite-transmitted virus diseases: These virus diseases are transmitted by wheat curl mites (WCMs) (Figure 1), and include wheat streak mosaic (WSM), high plains disease (HPD), and Triticum mosaic (TrM). Of these, WSM is the most common. WCMs and these viruses survive in crops such as wheat, corn, and sorghum as well as many grassy weeds and volunteer wheat. In the fall and spring, WCMs spread to emerging seedling wheat, feed on that seedling wheat, and transmit virus to the young wheat plants. Given this disease cycle, it is easy to see several factors that determine the incidence and severity of these diseases. First, controlling volunteer wheat and other grassy weeds that serve as alternative hosts for the mite and the viruses is imperative to help limit these diseases. Often an infected field of commercial wheat is growing immediately adjacent to a field left fallow during the fall and winter (Figure 2). The fallow field contained abundant volunteer wheat and grassy weeds from which WCMs carrying Wheat streak mosaic virus (WSMV) spread into the commercial field. Wheat infected in the fall will be severely damaged the next spring. Wheat infected in the spring also is damaged, but not as severely as wheat infected in the fall. Hence, it is imperative to do yourself and your neighbors a favor by controlling volunteer wheat and grassy weeds in fields left fallow – especially, if they are adjacent to commercial wheat fields. A second factor linked to the severity of these mite-transmitted virus diseases is planting date. Early planting dates associated with grazing provides for a much longer time period in the fall for mites to spread to and infect seedling wheat. Planting later in the fall (after October 1 in northern OK and after October 15 in southern OK) and controlling volunteer wheat are the two practices that can be employed to help manage these diseases. It is extremely critical that volunteer wheat is completely dead for at least two weeks prior to planting wheat because WCMs have a life span of 7-10 days. Thus, completely killing or destroying volunteer wheat for a period of at least two weeks prior planting will greatly reduce mite numbers in the fall. The incidence and severity of these mite-transmitted virus diseases as affected by planting date can be illustrated by the number of samples that tested positive for WSMV and HPV during each of the last three years. In 2017, which was the last year mite-transmitted virus diseases were prevalent in Oklahoma, 103 wheat samples were tested by the Plant Disease and Insect Diagnostic Lab at OSU for presence of mite-transmitted viruses. Of these 103 samples, 69 (67%) tested positive for WSMV and 22 (21%) tested positive for HPV. In 2018, only 12 of 126 (10%) samples tested positive for one or both of these viruses. In 2019, only 21 samples were submitted for testing with 7 samples (33%) testing positive for WSMV (no positives for HPV). In 2020, few samples (less than 5) tested positive for any of these viruses. This lower number of positive samples in 2019 and 2020 likely was the result of an overall later planting date of wheat in the fall of 2018 due to wet conditions and in fall 2020 due to extremely dry conditions. I believethis later planting date in conjunction with more awareness and action in limiting the green bridge helped to lower the incidence and severity of the mite-transmitted viruses in Oklahoma in both 2019 and 2020. Finally, seed treatments and insecticides are NOT effective in controlling the mites or these mite-transmitted virus diseases. Regarding resistant varieties, there are several winter wheat varieties that have resistance to either WSM or the curl mites, but the adaptation of these varieties to Oklahoma is limited, and the resistance is not typically an absolute resistance to the disease. Hence, severe and continuous disease pressure especially at higher temperature (greater than about 75 F) can overcome the resistance. For more information on mite-transmitted virus diseases, see OSU Fact Sheet EPP-7328 (Wheat Streak Mosaic, High Plains Disease and Triticum Mosaic: Three Virus Diseases of Wheat in Oklahoma).
Figure 1. Wheat curl mites and symptoms of wheat streak mosaic.
Figure 2. A commercial wheat field (right) growing adjacent to a field (left) in which volunteer wheat and grassy weeds were not controlled until the spring. The commercial field begin to show WSM symptoms in late March and the disease became severe as the spring progressed.
Aphid/barley yellow dwarf (BYD) complex: Viruses that cause BYD are transmitted by many cereal-feeding aphids (Figure 3). BYD infections that occur in the fall are the most severe because virus has a longer time to damage plants as compared to infections that occur in the spring.
Several steps can be taken to help manage BYD. First, a later planting date (after October 1 in northern Oklahoma and after October 15 in southern Oklahoma) helps reduce the opportunity for fall infection. Second, some wheat varieties tolerate BYD better than other varieties; however, be aware that no wheat variety has a high level of resistance to the aphid/BYD complex.For a listing of reaction of wheat varieties to BYD, other diseases and insect pests, and agronomic traits there are several sources available including variety comparison charts from Oklahoma State University (www.wheat.okstate.edu) and Kansas State University (https://bookstore.ksre.ksu.edu/pubs/MF991.pdf), and the annual wheat variety publication titled, “Wheat Varieties for Kansas and the Great Plains by Layton Ehmke (34 Star Publishing Inc.; layton@34starpublising.com; https://thewheatfarmer.com; 1-844-643-0170). Third, control aphids that transmit the viruses that cause BYD. This can be done by applying contact insecticides to kill aphids, or by treating seed before planting with a systemic insecticide. Unfortunately, by the time contact insecticides are applied, aphids frequently have already transmitted the viruses that cause BYD. Systemic seed-treatment insecticides containing imidacloprid or thiamethoxam can control aphids during the fall after planting. This may be particularly beneficial if wheat is planted early to obtain forage. Be sure to thoroughly read the label before applying any chemical.
Figure 3. Spot in field (left) of barley yellow dwarf (BYD) as would be seen in March or April. Many types of aphids (for example, greenbug; right) transmit the viruses that cause BYD.
Hessian fly: Hessian fly (Figure 4) infestations can occur in the fall and spring. Fall infestations arise from over-summering pupae that emerge when climate conditions become favorable. In states north of Oklahoma, a “Hessian fly free” planting date often is used to help limit fall infestations by Hessian fly. However, such a planting date does not apply in Oklahoma because Hessian fly can emerge in Oklahoma as late as December (Figure 5).
Delayed planting (after October 1 in northern Oklahoma, and after October 15 in southern Oklahoma) can help reduce the threat of Hessian fly, but a specific “fly free date” does not exist for most of Oklahoma as it does in Kansas and more northern wheat-growing states. This is because smaller, supplementary broods of adult flies emerge throughout the fall and winter. A number of varieties are resistant to Hessian fly; for a listing of reaction of wheat varieties to Hessian fly, other diseases and insect pests, and agronomic traits there are several sources available including variety comparison charts from Oklahoma State University (www.wheat.okstate.edu) and Kansas State University (https://bookstore.ksre.ksu.edu/pubs/MF991.pdf), and the annual wheat variety publication titled, “Wheat Varieties for Kansas and the Great Plains by Layton Ehmke (34 Star Publishing Inc.; layton@34starpublising.com; https://thewheatfarmer.com; 1-844-643-0170). Hessian fly infestations can be reduced somewhat by destroying volunteer wheat in and around the field at least two weeks prior to emergence of seedling wheat. Seed treatments that contain imidacloprid or thiamethoxam will also help reduce fall infestations of seedling wheat, especially if combined with delayed planting and volunteer destruction. For more information on Hessian fly, see OSU Fact Sheet: EPP-7086 (Hessian fly Management in Oklahoma Winter Wheat).
Figure 4. Adult Hessian fly (left) and larvae and pupae of the Hessian fly (right).
Figure 5. Emergence of Hessian fly in Oklahoma by month from 2011-2013.
Root and foot rots: These are caused by fungi and include several diseases such as dryland (Fusarium) root rot, Rhizoctonia root rot (sharp eyespot), common root rot, take-all, and eyespot (strawbreaker). Every year samples are received in the lab that are diagnosed with root rot. Typically wheat affected by seedling/root rots are either submitted in the fall when wheat is in the seedling stage or in later May and early June as plants are maturing. Germinating seeds and seedlings have small root systems that if infected impacts seed germination and seedling emergence (Figure 6). Later in the season (late May/early June), root rots again become apparent as maturing plants are unable to obtain sufficient moisture to finish grain development especially if drought conditions are present. In mature plants, white heads often indicates the presence of root rot (Figure 7).
In 2017-2018, the incidence and severity of root rots across Oklahoma dramatically increased compared to the 2016-2017 season. This increase likely resulted from weather conditions that favored the root rots along with heat and drought in May/June of 2018 that promoted white heads to develop. Dryland (Fusarium) root rot was the most common root rot observed in 2018, and caused significant damage to wheat in southwestern, western, northwestern OK as well as the panhandle. In 2018-2019, dryland (Fusarium) root rot again became prevalent across much of Oklahoma, but was not as damaging as the previous year likely because ample moisture and cool temperatures meant that water stress on plants was much less than in 2017-2018. Root rots were only sparsely observed in 2019-2020 and only at low severity.
Controlling root and foot rots is difficult. There are no resistant varieties, and fungicide seed treatments with activity toward the root and foot rots are effective in protecting germinating seed and emerging seedlings, their activity usually involves early-season control or suppression rather than control at a consistently high level throughout the season. Often, there also are different “levels” of activity related to different treatment rates, so again, CAREFULLY read the label of any seed treatment to be sure activity against the diseases and/or insects of concern are indicated, and be certain that the seed treatment(s) is being used at the rate indicated on the label for activity against those diseases and/or insects. Later planting (after October 1 in northern Oklahoma and after October 15 in southern Oklahoma) also can help reduce the incidence and severity of root rots, but planting later will not entirely eliminate the presence or effects of root rots. If you have a field with a history of severe root rot, consider planting that field as late as possible or plan to use it in a “graze-out” fashion if that is consistent with your overall plan.
For some root rots, there are specific factors that contribute to disease incidence and severity. For example, a high soil pH (>6.5) greatly favors disease development of the root rot called take-all. OSU soil test recommendations factor in this phenomenon by reducing lime recommendations when continuous wheat is the intended crop. Another practice that can help limit take-all and some of the other root rots is the elimination of residue. However, elimination of residue by tillage or burning does not seem to affect the incidence or severity of eyespot (strawbreaker).
Figure 6. A healthy plot of wheat in the fall as a result of using a seed treatment (left); a poor stand of wheat in the fall in a non-treated plot; a healthy seedling (left) compared to two seedlings (center and right) showing symptoms of common root rot. Notice the darkened sub-crown internode on the seedlings in the center and on the right as well as the reduced top growth compared to the healthy seedling on the left.
Figure 7. White heads indicative of root rot (left); darkened roots indicative of take all root rot (center); wheat killed by dryland root rot split open to show the pinkish growth of the causal fungus, Fusarium (right).
Seed treatments: There are several excellent reasons to plant seed wheat treated with an insecticide/fungicide seed treatment. These include:
1. Control of bunts and smuts, including common bunt (also called stinking smut) and loose smut. The similarity of these names can be confusing. All affect the grain of wheat, but whereas common bunt spores carryover on seed or in the soil, loose smut carries over in the seed. Seed treatments labeled to control bunts and smuts are highly effective. If common bunt (stinking smut) was observed in a field and that field is to be planted again with wheat, then planting certified wheat seed treated with a fungicide effective against common bunt (stinking smut) is strongly recommended. If either common bunt (stinking smut) or loose smut was observed in a field, grain harvested from that field should not be used as seed the next year. However, if grain harvested from such a field must be used as seed wheat, treatment of that seed at a high rate of a systemic or a systemic + contact seed treatment effective against common bunt (stinking smut) and loose smut is strongly recommended. In 2020, loose smut in fields and common bunt in harvested grain was observed at higher incidence and severity than for several years, so I strongly recommend planting certified wheat seed that was been treated with a fungicide labeled for control of bunt and smut. For more information on common bunt (stinking smut) & loose smut, see: http://www.entoplp.okstate.edu/ddd/hosts/wheat.htm and consult the “2020 OSU Extension Agents’ Handbook of Insect, Plant Disease, and Weed Control (OCES publication E-832),” and/or contact your County Extension Educator.
2.Enhance seedling emergence, stand establishment, and forage production by suppressing root, crown and foot rots. This was discussed above under “Root and Foot Rots.”
3.Early season control of the aphid/BYDV complex. This can be achieved by using a seed treatment containing an insecticide. Be sure that the treatment includes an insecticide labeled for control of aphids.
4. Control fall foliar diseases including leaf rust and powdery mildew. Seed treatments are effective in controlling foliar diseases (especially leaf rust and powdery mildew) in the fall, which may reduce the inoculum level of these diseases in the spring. However, this control should be viewed as an added benefit and not necessarily as a sole reason to use a seed treatment.
5. Suppression of early emerged Hessian fly. Research suggests that some suppression can be achieved, but an insecticide seed treatment has little residual activity past the seedling stage and Hessian fly often infests wheat after the seedling stage.
Fall Armyworm Potential: We have seen a severe outbreak of fall armyworms infesting bermudagrass and fescues lawns this past month. The strain that is infesting these lawns is known as the “rice” strain, and it overwinters in the Florida Gulf Coast and parts of the Caribbean. We typically see the “corn” strain which typically overwinters in the Texas Gulf Coast and Mexico. The rice strain prefers rice, and grasses, and the corn strain prefers corn and sorghum and as we have seen in the past, winter wheat. This year, Oklahoma is experiencing a “double whammy” of both strains, and unfortunately, they both like wheat. So, our advice is to carefully watch your fields after they emerge and deal with any fall armyworm infestations before they take your stand.
Lanie Hale, from Wheeler Brothers sent a picture of “window paned” wheat from a field that he had scouted in 2017. He counted 3 fall armyworms per row foot from his visual count (which is treatment threshold) but when he looked closely at his photo on his computer, he saw 15 worms in an area the size of his hand (they were very tiny, and probably newly hatched). It is easy to miss some of these little worms in the field because they hide in residue and are very tiny.
Symptoms like “window pane” in the leaves indicate feeding from fall armyworm.
Look very closely for “window paned” leaves and count all sizes of larvae. Examine plants along the field margin as well as in the interior, because they sometimes move in from road ditches and weedy areas. The suggested treatment threshold is 2-3 larvae per linear foot of row in wheat with active feeding. Numerous insecticides are registered for control, but they are much more susceptible when caterpillars are small. We won’t get relief from fall armyworms until we get a killing frost, so keep vigilant!
This article was written by Dr. Tom A. Royer, Extension Entomologist
The good news is that Oklahoma has a healthy, good-looking wheat crop. Now, it must be protected from any swarming hordes of insect pests that want to eat it! Dr. Kris Giles has been surveying wheat fields in SW and Central Oklahoma. I have been collecting data from our wheat plots in Chickasha, Stillwater, and Lahoma. Dr. Giles found increasing bird cherry oat aphid (BCOA) numbers, and its main natural enemy (Lysiphlebus testaceipes) may have been set back by the record cold temperatures that we experienced in February. I have seen mixed populations of English grain aphids and bird cherry oat aphis in our wheat plots.
Sometimes, aphid infestations are overlooked. Bird cherry-oat aphid infestations do not produce visible damage until they become very numerous and English grain aphids often bury themselves in between the seeds where they blend in. So check your field for bird cherry oat aphid, English grain aphids. Bird cherry oat aphids are olive green to dark green with two rusty patches that surround their “tailpipes” (cornicles). They feed on plant juices with their piercing sucking mouthparts. They can reproduce rapidly, so fields should be scouted to make a determination as to the need to control them.
Lady beetles and most importantly, the Lysiphlebus wasp that parasitizes them, often control bird cherry oat aphids. Parasitized aphids swell up and form “mummies” that can easily be seen (below). If an aphid infestation has 10-15% mummies, the rest are probably also parasitized.
BCOA and aphid mummy
English grain aphid is larger than either greenbug or bird cherry oat aphid (0.125 inches), green with long black cornicles and legs that have alternate bands of green and black. Their appearance is sometimes characterized as “spidery”. Suggested thresholds are 5 per stem at flag leaf, and 10 per stem at head emergence through milk stage.
English Grain Aphids in a wheat head
My suggestion is to scout the field beforehand to determine if there are GROWING numbers of bird cherry oat aphids that could be or are of concern. Count bird cherry oat aphids on each of 25 randomly selected tillers across a zigzag transect of the field and note mummy activity. If 10 to 20% of bird-cherry oat aphids are mummies, and there are numerous lady beetle larvae in the wheat, consider control. If wheat heads have emerged, look for English grain aphids imbedded in the head.
Unpublished research provided by Dr. Kris Giles (OSU) and Dr. Norm Elliott (USDA-ARS) combined with studies on spring wheat from the Dakotas and Minnesota indicate that 20-40 BCOA per tiller causes 5-9% yield loss before wheat reaches the boot stage. My suggestions: if BCOA numbers average 10-20 per tiller, figure on a 5% loss, if 20-40 per tiller, figure a 7% loss, and if BCOA aphids are more than 40 per tiller, figure a 9% loss.
Estimate APHIDS PER TILLER_______ /tiller = Total # aphids ______/25 tillers
Estimate CROP VALUE $_______/acre = Expected yield ______bushels/acre X $ _____/bushel
Calculate CONTROL COSTS $______/acre = Insecticide $______/acre + Application $____/Acre
PREVENTABLE LOSS $_____/acre = Crop value $________ X______loss from aphids/tiller .
If PREVENTABLE LOSS ISGREATER THANCONTROL COSTS TREAT
IF PREVENTABLE LOSS ISLESS THANCONTROL COSTS DON’T TREAT
Here is a Table of Preventable Loss estimates for bird cherry-oat aphids for expected yields of 30 to 50 bushels per acre, expected wheat prices of $3.00, $3.50, and $4.00 per bushel, and bird cherry-oat aphid numbers of 10-20, 20 to 40, and over 40 per tiller.
This cool, rainy spring weather, while providing excellent growing conditions for wheat, is also foodstuff for “producing” armyworms. Armyworm infestations typically occur in late April through the first two weeks of May. They feed on leaves and awns, (below left) and occasionally clip the head from developing plants. The head clipping (below right) I have noticed over the years is mostly restricted to secondary tillers with very small, green heads that contribute very little to yield.
Since armyworm infestations tend to occur more frequently around waterways, areas of lush growth, or areas with lodged plants, check them first to determine the size of the infestation. Early signs of an infestation include chewed leaves with ragged margins. You may find “frass” i.e. the excrement from armyworm caterpillars, around the base of wheat stems and clipped heads. Also, look for evidence of armyworms parasitized bythe wasp Glyptapanteles militaris. This parasitoid attacks armyworms as well as several other caterpillars. When the larva emerges, it produces a cottony cocoon (below right) about the size of a Q-tip. Scout for armyworms at five or more locations looking for “curled up worms” (below left)
Armyworm caterpillars tend to feed at night, so another good strategy is to bring a flashlight, shine it on the emerged wheat heads after dusk and count armyworms that are feeding on the heads and plant stems.
The suggested treatment threshold for armyworms is 4-5 caterpillars per linear foot of row (bottom left). Generally, no control is needed if wheat is past the soft dough stage unless there is visible head clipping, and caterpillars are present and feeding.
If a producer is considering a fungicide application, this might be an opportune time to evaluate your field for bird cherry oat aphid, English grain aphid and/or armyworms. If NEEDED, combine an insecticide with a needed fungicide application to control multiple pests. Check CR-7194, “Management of Insect and Mite Pests in Small Grains” for registered insecticides, application rates, and grazing/harvest waiting periods. It can be obtained from any Oklahoma County Extension Office, or found at the OSU Extra Website at http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2601/CR-7194web2008.pdf
Our Plant Disease and Insect Diagnostic lab received samples of wheat that were damaged by brown wheat mites. Producers need to remain alert so that they don’t mistake damaged wheat from brown wheat mite for drought or virus disease.
Brown wheat mite is small (about the size of this period.) with a metallic brown to black body and 4 pair of yellowish legs. The forelegs are distinctly longer that the other three pair. Brown wheat mites can complete a cycle in as little as 10-14 days. Brown wheat mite causes problems in wheat that is stressed from lack of moisture. They feed by piercing plant cells in the leaf, which results in “stippling”. As injury continues the plants become yellow, then dry out and die. They are very susceptible to hard, driving rains which many areas have now experienced, but until then they can cause yield loss when present in large numbers.
A closeup of a brown wheat mite. Photo courtesy Franklin Peairs, CSU.
Brown what mite can severely damage wheat that is already stressed due to drought or other adverse environmental conditions.
Brown wheat mites are about the size of a period at the end of a sentence and can be difficult to see with the naked eye.
We typically experience 3 generations per year. However, in this sample, the mites had already caused considerable damage and had laid significant numbers of diapausing white eggs that tell us they have completed their last generation of the growing season and these eggs will oversummer.
Brown wheat mite eggs in soil.
Research suggests that a treatment threshold of 25-50 brown wheat mites per leaf in wheat that is 6-9 inches tall is economically warranted. An alternative estimation is “several hundred” per foot of row. If you find active brown wheat mites in your field, check CR-7194, Management of Insect and Mite Pests in Small Grains for registered insecticides, application rates, and grazing/harvest waiting periods. It can be obtained from any County Extension Office, or found at www.wheat.okstate.edu
Dr. Hunger traveled southwest Oklahoma this week, so I made a trip out Hwy. 60 yesterday to evaluate freeze injury and assess the overall condition of the wheat crop in northwestern and north central Oklahoma. Last week’s warm temperatures and wind have taken their toll on wheat in Kay, Grant, and eastern Garfield Counties. It is not too late for rain to save a partial wheat crop in these areas, but the “full yield potential” ship sailed long ago. Wheat sown behind summer crops is the hardest hit, and wheat in these fields could best be described as yellow and thin. If the weather turned and we received rain in the next week, I would predict that yield potential in these fields would still only be around the 15 bushel mark. Without rain, subtract around 15 bushels. Wheat planted behind summer fallow has held on a little longer, but is clearly showing the signs of extreme drought stress. If we receive rain in the next week (and continue to see rain) these fields could still make 20 – 30 bushels per acre. In the absence of rain in the near future, they will be 10 bushels per acre or less.
Wheat in the Lamont test plot was approximately GS 7 – 8. Flag leaves were rolled and plants were starting to abort tillers.
In addition to drought stress, we found freeze injury and greenbugs at Lamont. I was a little surprised to find freeze injury and even more surprised to find the greatest injury in the later-maturing varieties. We split several stems of early varieties such as Ruby Lee and Gallagher and did not find any injury. These varieties would have likely been at approximately GS 7 – 8 when the freeze occurred. We found significant injury in later-maturing varieties such as Endurance, but these varieties were likely only GS 6 – 7 when the freeze occurred. Conventional wisdom regarding freeze injury is that the more advanced the variety, the greater the likelihood of freeze injury. After seeing the same phenomenon last year (i.e. the greatest injury in later maturing varieties) I am changing my thinking on freeze injury and now say that all bets are off when it comes to freeze injury in drought stressed wheat.
Freeze injury was greatest in late-maturing varieties at Lamont.
Overall wheat condition started to improve around Nash and Jet, I would say that much of the wheat in this area is CURRENTLY in fair to good condition. I emphasize the currently in the previous sentence, as the only difference between wheat in the Cherokee area and wheat to the east was about one week’s worth of moisture. Some terrace ridges had already started turning blue and moisture was starting to run out. Without rain wheat in this area will rapidly deteriorate from good to poor. One consistent theme throughout the day was greenbugs. Many sites had evidence of parasitic wasp activity (i.e. aphid mummies), but the presence or absence of parasitic wasp activity varied field by field. Dr. Royer has indicated that greenbugs still need to be controlled in drought stressed wheat. If parasitic wasps are active, the best decision is to let them do the aphid killing for you. If no mummies are present, then insecticide control could be justified. The only sure way to make this determination is to use the glance-n-go sampling system.
Greenbugs were alive and well at Lamont
Parasitic wasps were keeping greenbug populations under control in this field
Active and parasitized greenbugs on the same plant
Similar to Lamont, we found freeze injury in the Cherokee and Helena areas. Many of the worst looking fields (extensive leaf burn) had only superficial injury and should recover if moisture allows. Conversely, some plants that showed no outward signs of freeze injury had injured heads within. Most fields I surveyed had less than 10% injury, but one field was a complete loss. On the surface the 10% injury field and 100% loss field looked the same, so I cannot over stress the importance of splitting stems. I have received a few additional reports of freeze injury from Kay County this morning, so it is important for producers throughout northern Oklahoma to evaluate their wheat on a field by field basis.
Plants that look healthy on the exterior could contain damaged wheat heads
A closeup of the damaged wheat head from the picture above
Although freeze injury to plant tissue in this field was severe, the wheat heads were mostly left unscathed
A closeup of a head from the freeze-injured wheat shown above. Although tissue damage is severe, the growing point and wheat head are still viable
A final note on freeze injury. Freeze injury appeared to be worst in no-till fields and in areas where residue was heaviest. Based on my observations, this was not due to winterkill or poor seed to soil contact. My best explanation is that the lack of soil cover in conventional till fields allowed stored heat to radiate from the soil surface and slightly warm the crop canopy. The insulating effect of residue in no-till fields did not allow radiant heating to occur. Given the pattern of freeze injury in fields with varying degrees of residue across the field, I feel pretty confident in this analysis of what occurred.
Please use the comment section to share pictures or descriptions of wheat in your area.
This article is provided by Dr. Tom A. Royer, OSU Extension Entomologist
Sug Farrington, Extension Educator in Cimarron County received a sample of “worms” that were collected by a producer in his wheat field. They turned out to be army cutworms.
Unlike the fall armyworm, this caterpillar overwinters in Oklahoma, tolerates cold and feeds throughout the winter months. Adult army cutworm moths migrate to Oklahoma each fall from their summer residence in the Rocky Mountains. They seek bare or sparsely vegetated fields (like a newly prepared field ready for wheat planting, or a field that was “dusted in” and had not yet emerged) and lay eggs from August through October. The eggs hatch soon after being deposited, which explains why a producer might see different sizes of larvae in a field. Army cutworms feed throughout the winter and molt seven times before they turn into pupae in the soil. Most larvae will be gone by late March and adult moths begin emerging in April and fly back to the Rocky Mountains to spend the summer.
Army cutworms. Photo courtesy Sug Farrington, Cimarron County Extension Educator.
Army cutworms can cause severe stand loss of wheat if not controlled. Cutworm damage often goes unnoticed through the winter because the caterpillars grow slowly and don’t get big enough to cause noticeable damage until temperatures warm in the spring. Unfortunately, that is also an indication of poor growing conditions due to drought (which cutworms also like), so it becomes important to check the fields for cutworms. If you notice a field at this time of year with a numbers of starlings or black birds feeding in a concentrated area of your wheat field, they are likely feasting on army cutworms!
Army cutworm injury in wheat. Photo courtesy Sug Farrington, Cimarron County Extension Educator.
Sample a field by stirring or digging the soil to a depth of two inches at 5 or more locations. Also, turn over those dried up cow patties, as they are a favorite hiding place for army cutworms. The cutworms will be “greenish grey”, and will probably curl up into a tight “C” when disturbed. A suggested treatment threshold is 2-3 caterpillars per foot of row when conditions are dry (like we are experiencing this winter) or 4-5 caterpillars per row-foot in fields with adequate moisture. Control suggestions are listed in Current Report-7194 Management of Insect and Mite Pests in Small Grains.
Army cutworms are also a potential pest of canola. Scout fields just as you would in wheat. The suggested treatment threshold for cutworms in canola is 1-2 per row-foot. Current recommendations for control of army cutworms in canola are listed in CR-7667, Management of Insect and Mite Pests in Canola.
Jeff Bedwell forwarded reports of winter grain mites in Major and Alfalfa counties over the past week. This does not appear to be a widespread problem, but growers should check wheat fields to see if winter grain mites are present. We had a lot of issues with winter grain mites and brown wheat mites in Oklahoma last year. Unfortunately, many of these fields were not diagnosed until the damage was severe and visible from a distance. In this case, a rescue treatment was still effective at controlling the pest, but some yield loss had already occurred thus reducing the return on pesticide investment.
Winter grain mites are small (about 1 mm long) with black bodies and orange-red legs. Winter grain mites complete two generations per year and the adults can live for up to 40 days. The generation we are dealing with now resulted from oversummering eggs laid last spring. The second generation peaks in March/April and results from eggs laid in January/February.
Winter grain mites on wheat near Blair, Oklahoma.
Scouting Winter grain mites are not a problem you will notice in a timely fashion while standing up. You will need to get close to the soil surface and move residue to find these pests. Winter grain mites are light sensitive and prefer calm air to windy conditions; therefore, scouting early in the morning, late in the evening, or on cloudy days generally works best. Be sure to look under residue in no-till fields and under clumps of soil in conventional-till fields.
Scouting for winter grain mites requires getting close to the soil surface and moving residue to disturb mites.
Injury
Winter grain mites feed by piercing plant cells in the leaf, which results in “stippling”. As injury continues, the leaves take on a characteristic grayish or silverish cast. Winter grain mites are more likely to cause injury in wheat if it is already stressed due to lack of moisture or nutrients. Also be advised that freeze injury can easily be confused for winter grain mite injury.
Wheat damaged by winter grain mite often has a silver or bronzed appearance from a distance
When to spray There are no established thresholds for winter grain mite. Healthy, well-fertilized wheat plants can generally outgrow injury, so it takes large numbers to justify control. If there is injury present AND large numbers of mites (~10 per plant) present in grain only wheat this time of year, you might consider control. If the wheat is to be grazed, I would simply monitor the situation in most cases and only spray if injury became severe.
What to spray Malathion is the only product labeled for wheat in Oklahoma that has winter grain mite on the label. There are many additional products, such as some of the pyrethroids and chlorpyrifos, that are effective at controlling winter grain mite, but they don’t have a specific label for them. These products can be applied under 2ee regulations; however since this pest is not specifically labeled, the user assumes all responsibility for the application of the product. It is also important to read and follow label directions regarding grazing restrictions for these and all pesticides. Consult OSU Current Report 7194 Management of insect and mite pests in small grains for a more complete listing of available pesticides.
Fall armyworms are active this fall. I checked a field of wheat this past weekend with significant damage from fall armyworms that averaged 6-7 fall armyworms per square foot. Scout for fall armyworms by examining plants in several (5 or more) locations in the field. Fall armyworms are most active in the morning or late afternoon. Look for “window paned” leaves and count all sizes of larvae.
Fall armyworm damage is characterized by window panning on wheat leaves. Injury can sometimes be greater in field margins as armyworms sometimes move in from adjacent road ditches or weedy areas.
Fall armyworms are generally most active early in the morning or late in the evening. Spray when 2-3 armyworms per linear foot of row are present.
Examine plants along the field margin as well as in the interior, because they sometimes move in from road ditches and weedy areas. The caterpillars were widely distributed in the field that I checked, suggesting that they were the result of a large egg lay from a recent adult moth flight. The suggested treatment threshold is 2-3 larvae per linear foot of row in wheat with active feeding. We won’t get relief from fall armyworms until we get a killing frost, since they do not overwinter in Oklahoma.
Add wheat streak mosaic virus to the list of possible causes of yellowing wheat in Oklahoma. Wheat streak mosaic virus is transmitted by the wheat curl mite, which oversummers on grasses such as volunteer wheat and corn. The wheat curl mite cannot survive more than two weeks without a green host, hence the recommendation to make sure that all grass plants are dead two weeks prior to planting. You can find more information on the wheat curl mite and wheat streak mosaic in OSU Fact Sheet EPP-7328 – Three virus disease of wheat in Oklahoma
The photo below is from our wheat variety trial at Kildare. As you can see there is significant yellowing in some of the plots. Our first thoughts were that either wheat soilborne mosaic virus and/or wheat spindle streak mosaic virus were causing the symptoms; however, the yellowing was present in many varieties that are resistant to both these diseases. The Disease Doctor, Bob Hunger, collected samples for analysis in the OSU Plant Diagnostic Lab. Tests showed that wheat streak mosaic was the culprit.
Wheat streak mosaic virus is responsible for yellowing at our Kildare variety trial. All varieties are affected by the disease, but as shown in this picture the severity of the reaction differs somewhat by variety.
We are facing this problem because I did not follow my own recommendations. While the plot area was mostly clean at the time of planting, there was some volunteer wheat present. We planted anyway and sprayed glyphosate right after planting. In the past wheat streak mosaic virus was primarily a northwestern Oklahoma issue and we could get by with late burndown on wheat ground in central Oklahoma. Our Kildare plots are a prime example that this is no longer the case. We have to control volunteer grasses (wheat, corn, grain sorghum, etc.) in a manner that will break the green bridge for at least two weeks prior to planting.
So, what is next for our plots at Kildare? There are some variety differences in reaction to the wheat streak mosaic virus. We will rate plots and include this information in our wheat variety comparison chart. I anticipate the plots will continue to go downhill and it is yet to be determined whether or not we will be able to harvest useable data from the location. We will certainly try again next year and apply our turndown earlier.
Over the past week, I have received a few reports of winter grain mite activity in southwest Oklahoma. Winter grain mites are small (about 1 mm long) with black bodies and orange-red legs. Winter grain mites complete two generations per year and the adults can live for up to 40 days. The generation we are dealing with now resulted from oversummering eggs laid last spring. The second generation peaks in March/April and results from eggs laid in January/February.
Scouting Winter grain mites are light sensitive and prefer calm air to windy conditions; therefore, scouting early in the morning, late in the evening, or on cloudy days generally works best. Be sure to look under residue in no-till fields and under clumps of soil in conventional-till fields.
Winter grain mites on wheat near Blair, Oklahoma.
Injury
Winter grain mites feed by piercing plant cells in the leaf, which results in “stippling”. As injury continues, the leaves take on a characteristic grayish or silverish cast. Winter grain mites are more likely to cause injury in wheat if it is already stressed due to lack of moisture or nutrients. Also be advised that freeze injury can easily be confused for winter grain mite injury.
When to spray There are no established thresholds for winter grain mite. Healthy, well-fertilized wheat plants can generally outgrow injury, so it takes large numbers to justify control. If there is injury present AND large numbers of mites (~10 per plant) present in grain only wheat this time of year, you might consider control. If the wheat is to be grazed, I would simply monitor the situation in most cases and only spray if injury became severe.
What to spray There are not a lot of pesticides with winter grain mite listed on the label, and most products have grazing restrictions. Malathion and methyl parathion have been shown to provide effective control in the past. Consult OSU Current Report 7194 Management of insect and mite pests in small grains for a more complete listing of available pesticides.
WORLD OF WHEAT 