David Owens, Extension Entomologist, email@example.com
We found some diamondback moth pupae and some harlequin bugs in a brassica cover crop last week near Middletown, DE. Diamondback moth is notorious for developing insecticide resistance. It can cycle through a couple of generations on a single planting. Both factors mean we have to be careful with our insecticide rotations. Some of the ‘softer’ chemistries select for recessive traits and the ‘broad spectrum’ chemicals are harder to manage. The synergists PBO and DEM may help somewhat if a low level of resistance is present in a population. A recommended strategy is to break up the multiple generations into treatment windows. A treatment window lasts between 2.5 and 4 weeks. In a window, use only one or two products before switching mode of action groups. Also, diamondback moth populations tend to be greater in the fall, so use the most effective products then. If you have multiple plantings next to each other, use the same products on the plantings, using the window for all adjacent brassicas. If you can put plantings away from each other, that will help. Georgia also advises that long residual products like diamides should not be used on subsequent crops (even if a treatment window approach would allow it). On the front of an insecticide label, you should see group numbers. Products with the same number have the same mode of action. It is like selecting a breakfast food. Frosted flakes and raisin bran are both cereals and more similar than bacon. As always, follow the insecticide label.