In a drought year, August is the time when spider mites increase in Michigan and spray decisions have to be made. In many corn and soy fields I’ve been in, mites are present on the bottom leaves of plants, ready to explode if given the chance. There are also reports of mites starting in dry beans and sugar beets.
Hot, dry conditions obviously favor mites. Rain and irrigation can help reduce populations, but only if humidity remains high under the canopy for an extended period (48 hours) so that mite pathogens can take hold. The largest mite infestations in 2012 were in irrigated fields where the water kept plants juicy, but the foliage dried off quickly once the irrigation stopped.
In addition to dry weather, the other trigger to generate a rip-roaring infestation is to spray a pyrethroid that kills beneficial insects, but not spider mites. With the exception of bifenthrin, all other pyrethroids flare mites. If you sprayed fields recently,Michigan State University Extension suggests checking them for mites. If you plan to spray in the next few weeks, be smart about product choice to avoid mite flaring.
For more information on the best spider mite spraying practices, see the Cheat sheet for spider mite spraying in Michigan field crops.
Tips for spraying mites
- Good coverage is critical for control. Use the highest GPA practical; more water is better. Ground is usually better than air.
- Read the label to understand how the chemical acts on mites. Most insecticides kill mites but not eggs. Unless the product has longer residual, newly hatched mites recolonize quickly. Newer mite growth regulators kill eggs and nymphs, but not adults – they act slowly to reduce the population.
- Never apply the same product – or one in a similar group – twice in a field. Switch modes of action to avoid resistance.
- Selective miticides usually have low impact on beneficial insects. However, insecticides which double as miticides (like bifenthrin and dimethoate) are hard on beneficials as well as honey bees. For example, during the 2012 drought, several bee kills investigated by The Michigan Department of Agriculture and Rural Development (MDARD) focused on dimethoate use in soybeans. To avoid such a situation, check labels for specific warnings and guidelines about application to crops in bloom. Know the neighborhood, and talk to beekeepers in the area. They may be able to cover or move hives. Check the DriftWatch web site for locations of apiaries in the state. Spray in the evening versus during the day to avoid exposing foraging bees.
Note: Pre-harvest intervals range from 0 – 60 days depending on the crop x insecticide.