Field Crops News

Fungus Against Twospotted Spider Mite

Twospotted spider mites may damage soybeans, as well as field corn, and outbreaks typically occur during hot dry years. Studies were conducted in Iowa during 1987 and 1988 on the incidence of the fungal entomopathogen Neozygites sp. in twospotted spider mites on soybeans. Naturally occurring populations of this entomopathogenic fungus caused dramatic reductions in spider mite populations artificially infested on soybeans in both years. Damaging levels of spider mites built up each year, but ~30 days after infestations each year, spider mite numbers declined, coincident with the buildup of fungal infection.

These studies were conducted within a 'rainout shelter' which excluded direct exposure of the soybean plants to rainfall, demonstrating that wetting of leaves is not required for fungal pathogen activity. Also, declines in spider mite numbers were not due to the physical action of rainfall washing mites off of plants. Low levels of predatory mites or other beneficial arthropods were noted during the study.

Buildup of the fungus occurred for ~2 weeks, with infection of up to 100% of living mites sampled. In 1987 an 84% reduction in mite numbers occurred over 2 days, with an overall reduction of mite numbers up to 95% over a six day period. In 1988 the maximum rate of infection was 50%, resulting in a slower decline in mite activity. Fungal activity was very dependent upon environmental conditions, with spread of the disease occurring only during sustained cool and humid weather. Increases in fungal infection rates occurred shortly after rainfall periods which increased relative humidity and decreased maximum temperatures over several days. Previous studies have shown that Neozygites conidia are not produced when relative humidity is less than 85% or when temperatures are above 32C.

Source:

T. H. Klubertanz, L. P. Pedigo & R. E. Carlson. 1991. Impact of fungal epizootics on the biology and management of the twospotted spider mite (Acari:Tetranychidae) in soybean. Environ. Entomol. 20: 731-735.


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