While diamondback moth has not been particularly bad this year, where populations have popped up, they have been difficult to control. I have been indirectly involved with a problem in North Carolina that has persisted for multiple weeks and has been treated extensively with insecticides that should have provided at least some control.
This past week, we were able to run a bioassay on a population from Grady County. The field had been treated multiple times but had heavy pest pressure. As always, our bioassay used leaves dipped into insecticide solutions made is if spraying the high labelled rate at 100 gpa. Mortality was checked at 72 hours after placement on the leaf disks. We ran two replications of each treatment with 10 larvae per leaf disk.
Larvae were a little large for this bioassay which resulted in high pupation rates at 72 hours; however, observations at 48 hours showed low pupation which I interpret as survival if they then pupated after the first 48 hours.
Results are shown in the graph below. Mortality was well below 50 percent for all treatments except Proclaim, Torac and ISM-555 (ISM-555 is not yet registered in the US). Torac provided about 70 percent mortality, Proclaim about 50 percent and ISM-555 90 percent.
Again, the larvae in this test were larger than preferred for a bioassay, but we had to work with what was available. I do believe the data are reflective of the control problems experienced with this population. The possible exception is Rimon which as a growth regulator works slowly and 72 hours may be inadequate for its evaluation.
While diamondback moth resistance can vary greatly from field to field, however, I believe these results do provide an indication that the general lack of heavy pest pressure with this pest this year does not necessarily mean they will be easy to control when they return.
