Diamondback moth pest pressure has been relatively light for over a year now. Yes, we have had some short term or isolated damaging populations, but overall not as bad as “normal”. With a reduction in pest pressure and the concurrent reduction in exposure to insecticides the big question is what will resistance look like when they return (and unfortunately they always return). Our hope is that the lack of selection pressure will result in a shift toward susceptibility with multiple insecticides.
In 2024, I was not able to collect enough DBM larvae to conduct a bioassay until December. Thus, we had gone close to a year with minimal pressure. Unfortunately this population still showed poor efficacy for most of the insecticides tested.
Two populations tested in March of 2025 gave somewhat conflicting results (which is not uncommon with two field collections of this pest). Larvae collected in Colquitt County showed good efficacy of multiple products, with Radiant providing 100 percent mortality and Proclaim, Coragen and Torac showing 75-90 percent. However, the population collected in Tift County showed roughly 75 percent mortality with Proclaim and 60 percent or less for all other registered products.
The good news (hopefully) is that the most recent collection (from Worth County) showed 100 percent mortality with Radiant and Proclaim and 80 percent with Torac. Hopefully this is an indication that populations are shifting toward susceptibility to at least some products. However, it also suggests that the resistance to the Group 28 insecticides (Coragen, Exirel, Vantacor) may be more stable and require longer periods for reversion toward susceptibility (or this population was more recently exposed to these MOAs).
The other good news is that ISM-555 has shown good efficacy across all of our bioassays and field trials. The public comment period for the proposed registration has closed and hopefully we are a step closer to having a new tool for DBM control.
As always, with a new product or a product that has “started working again”, please rotate chemistries. This is especially important with older products that “work again”. The population may have shifted toward susceptibility, but the genetics have not shifted as much as the efficacy. It is very easy to reselect for resistance in these populations.