Chlorpyrifos-resistant greenbugs found in Texas
This past week Monti Vandiver, IPM Extension agent for Bailey and Parmer counties, received calls from a couple of producers that each had treated a field with chlorpyrifos and did not get good control of the greenbugs. We discussed the situation and how to determine if the aphids were resistant.
click image to zoom I suggested using a diagnostic resistant kit to assay the greenbugs. I had developed the technique back in the mid-1990’s when we were having trouble with resistant greenbugs in grain sorghum (Bynum and Archer. 2000. Journal of Economic Entomology 93 (4): 1286-1292). I prepared a few kits for Monti and he tested greenbugs from one of the fields. The results from the test are shown in the following table. At the rates used to test for resistance, greenbugs would be considered susceptible to chlorpyrifos if mortality is ≥ 85%. If mortality is ≤ 40% then the greenbug population in the field can be considered resistant. The assay showed that greenbugs collected exhibit resistance to chlorpyrifos.
click image to zoom I would suspect that if a field application was made correctly at the recommended rate of chlorpyrifos, 1 pt/ac, in 3 gal to 5 gal spray by air and greater than 5 gal by a ground rig the control should be very good, unless there are resistant aphids in the field. The question then is what can be done if you have control failures.
Once a field has been treated and the control is ineffective, spraying the field again with the chlorpyrifos is unlikely to provide any better control. The list of registered products mainly belong to two insecticide classifications, organophosphate and pyrethroids. The primary organophosphate products are chlorpyrifos, dimethoate, malathion, methyl-parathion, and encapsulated methyl-parathion and the pyrethroid products are gama-cyhalothrin (Proaxis and Declare), lambda-cyhalothrin (Warrior w/ Zeon technology and Karate w/ Zeon technology), and zeta-cypermethrin (Mustang Max). And, there are newer products with a mixture of chlorpyrifos with a pyrethroid. These are Cobalt(chlorpyrifos + gama-cyhalothrin) and Stallion (chlorpyrifos + zeta-cypermethrin).
click image to zoom There have not been any recent trials with dimethoate, malathion, methyl-parathion, and encapsulated methyl-parathion in the Texas High Plains. The most recent trials have been with some of the other pyrethroids and newer formulations of Cobalt Advance and Lorsban Advance (See tables on page 3 & 4). However, these efficacy of these products in these trials were not against chlorpyrifos resistant greenbugs. Except for trials conducted from the early to mid-90s for greenbugs on grain sorghum, there is very little data on what will control field populations of chlorpyrifos resistant greenbugs. At that time laboratory studies showed that greenbugs collected from a field in Parmer County were resistant to carbofuran, chlorpyrifos, and malathion. Additional assays with mixtures of a pyrethroid (esfenvalerate) or dimethoate or malathion plus chlorpyrifos showed the addition of the pyrethroid or malathion provided slight synergistic activity (improved mortality). But, dimethoate/chlorpyrifos mixture was antagonistic (less mortality) (Archer et al. 1994. Journal Economic Entomology 87(6): 1437-1440).