By Tom Royer, OSU Extension Entomologist
Crop consultants have reported greenbugs in a few northwestern Oklahoma wheat fields. The hit or miss nature of greenbugs and other aphid pests in wheat fields can make scouting challenging. Fortunately, there is an online tool called the Cereal Aphid Decision Support Tool that simplifies scouting for greenbugs and also considers the effects of natural predators.
Start by going to the Cereal Aphids Decision Support Tool on your computer http://entoplp.okstate.edu/gbweb/index3.htm and selecting the Greenbug Calculator.
By answering a few simple questions, you can determine an economic threshold for controlling greenbugs. This threshold is based on the estimated cost of treating the field and the estimated price of wheat. Once a threshold is calculated, you can print a Glance ‘n Go scouting form, take it to a field and record your sampling results. The form will help you to decide if the field needs to be treatment for greenbugs. There are several things that make Glance ‘n Go a good way to make such a decision. You only have to “Glance” at a tiller to see if it has greenbugs (no counting greenbug numbers). You can make a decision to treat “on the Go” because you stop sampling once a decision is reached (no set number of samples). Finally, you can account for the activity of the greenbug’s most important natural enemy, Lysiphlebus testaceipes.
When scouting with the Glance ‘n Go system, keep a running count of tillers that have aphid mummies and a running count of tillers that are infested with one or more greenbugs. After each set of 5 stops, the Glance ‘n Go form directs you to look at your total number of infested tillers and tillers with mummies. If there is enough parasitoid (mummy) activity, you will be directed to stop sampling and DON’T TREAT, even if you have exceeded the treatment threshold for greenbugs! Why? Because research showed that at that level of parasitism, almost all of the healthy-looking greenbugs have been “sentenced to death” and will be ghosts within 3-5 days. If they have received their “sentence” you can save the cost of an unnecessary insecticide application.
Treatment thresholds will probably fall around 2-4 greenbugs per tiller, but make sure you are using the Fall (Sept.-December) form, not the spring form. If a field needs to be treated, check with Current Report CR-7194, “Management of Insect and Mite Pests in Small Grains”. If you treat for greenbugs and have a failure, please contact our Department and we will investigate further to determine if insecticide resistance might be an issue. Dr. Ed Bynum, Extension Entomologist from Amarillo, reported finding some greenbug populations in 2013 that were shown to be resistant to chlorpyrifos, the active ingredient in Lorsban 4E, and other generic products (Govern 4E, Hatchet, Nufos, Vulcan, Warhawk, Whirlwind). The bottom line: he tested some suspect greenbug populations using a diagnostic test that he developed for testing greenbugs in sorghum in the 1990’s, and found that they were resistant to chlorpyrifos at labeled rates.