Rotating Pesticides to Delay the Development of Resistance

One of the main ways in which pesticide resistance for both insect pests and diseases is managed is through rotation among chemical families with different modes of action. Generally, if a pest population in an orchard or vineyard has become less sensitive (more resistant) to one member of a chemical family, it will also be less sensitive to other members of that chemical family. For example, if the population of brown rot fungus in an orchard becomes less sensitive to Indar, and the labeled rate of Indar no longer provides control of brown rot, then Topas, Mission and Nova will also not provide as good control of the disease. The same would apply for an insecticide group like the organophosphates (OPs).

Rotation among chemical families is an important practice for management of resistance. However, the use strategy differs depending on the biology of the target pest. Certain pests, such as oriental fruit moth and grape berry moth, have discrete generations: the adults emerge from pupae in the spring, mate, lay eggs and these eggs hatch to produce larvae that cause injury to the crop plant. All of the adults don't emerge at the same time, but all of the overwintering adults produce eggs and larvae before the next "batch" of adults develops. This trend can be followed in pheromone trap catches: the number of moths caught increases then decreases during each generation. The recommendation for this type of pest is to use an insecticide from a different chemical family for each generation. In some cases, emergence can extend beyond the period when residues of the product will be effective so a second, and possibly third, spray may be required to cover a single generation. These subsequent sprays within the same generation should be within the same chemical family (or the same product). When the next generation emerges, a product from a different chemical family should be used. For example, the general regional recommendation for OFM in Niagara is to apply Lorsban for the first generation, Delegate or Altacor for the second, Altacor or Delegate (whichever wasn't used for the second) for the third and a synthetic pyrethroid for the pre-harvest spray.

Fungal pathogens produce millions of spores (several orders of magnitude more than OFM or GBM produce eggs), but not all of them are released at the same time. Each of these spores can cause lesions that will then produce more millions of spores, but this does not happen all at the same time. There are no recognized "generations". In addition, the generation time (from when a spore infects to produce a lesion until more spores are produced on that lesion) is extremely short, as little as 5 days under optimal conditions for powdery mildew on grape. The potential for an explosive epidemic and the rapid selection for resistant isolates mean that, for resistance-prone products (anything that product that does not have an M in the group in Table 2-8 of Publication 360), the number of consecutive applications and applications per season is limited on labels. Generally no more than 2 consecutive applications from one of these families should be made. Since we have a good selection of fungicides, a more conservative approach of alternating among families for each spray and including fungicides that act on multiple sites may be even more effective at prolonging the life of products.

Insect pests such as leafhoppers, mites and aphids, like fungal pathogens, do not always have synchronous generations, particularly later in the season. On a single leaf, you can find all stages of development from egg through adult at once. Therefore, the insecticide use pattern for these is to rotate among chemical groups for each application, if monitoring indicates additional sprays are required.