David Owens, Extension Entomologist; owensd@udel.edu
Diamond back moth numbers have been elevated this season. Larvae are small, smooth, green, often hold their anal prolegs out in a V-shape and wiggle violently when disturbed. Imported cabbageworm on the other hand is a larger, velvety green worm that will not move when poked or prodded. Imported cabbageworms are easily controlled with insecticides, but diamond back moth are resistant to pyrethroids and resistance to other classes have been reported in various populations. Resistance management for DBM is therefore extremely important, and skipping a ‘worm product’ in favor of a pyrethroid may allow unnoticed small worms to do some extensive feeding before the next application. We have 8 effective mode of action classes for DBM. Representative insecticides include Radiant and Entrust (G5), Proclaim (G6), Dipel (G11), Rimon (G15), Confirm and Intrepid (G18), Torac (G21), Avaunt (G22), and Coragen, Verimark, Exirel, Harvanta, Durivo, Voliam Flexi (G28).
In the fall, we can expect about 3 generations of diamondback moth. A treatment window approach is the best way to manage for resistance. This works by alternating among these mode of actions (MOAs) with each application and generation, reducing selection pressure on any one product. So, in the first month of the fall plantings, use alternating MOAs, then in the second month use altogether different, alternating MOAs, and in the third month again use completely different MOAs. If one mode of action was used in transplant house, do not use it in the field. G18 materials should target small worms only.
There are numerous parasitoid wasps that attack diamondback moth and imported cabbageworm. Pyrethroids, organophosphates, and carbamates are all highly disruptive to them. These products are traditional beetle and bug products. Other effective products for flea beetles include neonicotinoids (G4), Torac (G21), Harvanta, Exirel, and Verimark (G28). The neonicotinoid class (group 4) is also effective on harlequin bugs. A few products combine both a neonic and a diamide as a premix (Durivo and Voliam Flexi) and would help preserve parasitoid activity. Bt is the most natural enemy friendly product, followed by G5 and G28 spinosyns and diamides, then G6 G22 and G21 (Proclaim, Avaunt, and Torac), then foliar G4 and G1B neonics and Ops and lastly, Lannate and pyrethroids.
Once a field has been harvested, crop residue should be tilled under as soon as possible to destroy any surviving diamond back moth larvae.