Concerns About Twospotted Spider Mites

August 6, 1999

· Reports of infestations of spider mites in soybeans in western and north central Illinois continue.

· Cooler temperatures have slowed down the mites' development.

· The numbers of mites have declined in some fields.

· We received one report of lack of control after miticideswere applied.

During late July and early August, people from counties in western and north-central Illinois continued to report infestations of twospotted spider mites in soybeans. In areas bypassed by recent rains, people are scouting for spider mites, and some fields are being treated. Most of the reports of injury we received recently came from Bureau, LaSalle, Livingston, and Woodford counties. Infestations in most fields have been confined to field edges.

Twospotted spider mites.

Everyone knows that infestations of twospotted spider mites are associated with hot, dry conditions. Not surprisingly, the recent cooler weather has slowed development of the mites. Twospotted spider mites can complete a generation in 4 to 14 days, depending on temperature. Fastest developmental rates occur at temperatures above 91°F. With high temperatures in the 80s during the week of August 2, increases in population density of spider mites slowed down significantly, compared with the rapid population growth that occurred in late July.

Some observers have noted that numbers of spider mites have declined in some fields. They were able to find mites in areas of the field where injury symptoms were obvious, but they found very few in the healthy, greener areas of the field, suggesting that population spread had slowed down or ceased. In fact, some people who visited the same fields more than once indicated that densities in injured areas of fields had declined from the levels they observed the week before. It is quite possible that a fungal pathogen, Neozygites floridana, has been responsible for some of the population decline. Mites infected with this disease die with 1 to 3 days. When environmental conditions are favorable, the pathogen produces spores, which spread the disease quickly. Production of spores usually requires at least 90 percent humidity (certainly exceeded during the week of July 26) and temperatures cooler than 85°F. Periods of 12 to 24 hours of such weather are necessary for extensive spread of the disease. Although temperatures during the week of July 26 were considerably higher than 85°F, the humidity might have enabled the fungus to get a foothold. Some researchers in Iowa determined that a 95 percent decline in spider density occurred within 6 days.

Most individuals who have treated soybean fields to control spider mites have been pleased with spot treating infested areas with either dimethoate or Lorsban. Although some entire fields have been treated, spot treatments have been more common. We have received one report that neither dimethoate nor Lorsban, both of which were sprayed, controlled the spider mites. During outbreaks of spider mites, we typically receive a few similar reports; the causes for poor control have not been determined easily. Because of the concern about resistance to miticides, it is easy to jump to conclusions. However, more often than not, the cause for poor control of spider mites in Illinois has been related to application. Spider mites feed on the undersides of the leaves. If the miticide does not contact the mites in the canopy, control will be poor.

If the weather becomes hot and dry again and spider mite populations have not been affected significantly by the fungal disease, expect problems with spider mites to continue. Watch fields with infestations carefully, and try to determine if the infestation is receding or escalating. We will keep you posted.--Kevin Steffey, Mike Gray

Author: Kevin Steffey Mike Gray