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.
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.
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).
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).
Another study was conducted from 1994 - 1996 to identify best mixtures of insecticides that were available at that time and how to use insecticide mixtures for resistant greenbugs (GB) after a failure of chlorpyrifos, or as the 1st application if resistant greenbugs are expected to be in the field (Archer et al. 1999. Journal Economic Entomology 92(4): 794-803). The insecticides used in this study were chlorpyrifos, carbofuran, and malathion. Since we do not have carbofuran now, the results from the chlorpyrifos/malathion mixtures may help us in managing our resistant greenbugs. The rates for the insecticides were chlorpyrifos (4E) at 1 pt/acre and malathion (5E) at 12.8 fl. oz/acre. In summary the findings showed:
• Before any insecticide application, but GBs were a mix of resistant and susceptible GBs
• The mixture of chlorpyrifos/malathion gave good initial control (86%) but tapered off by 10 days after treatment and numbers were increasing again.
• A second application of the mixture 11 daysafter the initial application provided good control of the remaining GBs (94% to 83%) for another 10 days.
• After chlorpyrifos had already been applied
• The application of the chlorpyrifos/malathion mixture did not provide good control of the remaining GBs for 10 days after spraying
• A second application of the mixture 11 days after the mixture application did provide good control (94% to 82%) for 10 days
These data provide some evidence that chlorpyrifos/malathion mixtures may be used to improve control of fields if the greenbug infestation has some resistant greenbugs.
Unfortunately, there is no available information on how well the newer products, such as Cobalt and Stallion, will control resistant greenbugs and we do not have any good solutions when there is a control failure with chlorpyrifos. Our options are very limited, but we do know that applying multiple applications of chlorpyrifos will not control the resistant greenbugs.