Field Crops News

Seasonal Biology of Spider Mite Fungal Pathogen in Field Corn

The seasonal occurrence of the fungal pathogen, Neozygites adjarica, which often attacks twospotted and Banks grass mites in field corn was studied in southwestern Kansas in 1985. The first evidence of fungal infection was on August 5, but the major fungal infection occurred during August 14 to September 8. Fungal outbreaks occurred following several days when 8-10 hours per day had relative humidity above 80%.

The rates of fungal infection for Banks grass mites are usually higher than those for twospotted spider mites, and Banks grass mites died more quickly after infection. These differences could influence the relative numbers of the two mites species in the field. Often a shift occurs from predominately Banks grass mites early in the season to predominately twospotted spider mites later in the season. Previous research suggests that greater susceptibility of Banks grass mites to pesticides commonly used in corn is responsible for this shift. This study suggests that differential effects of the fungus could also be important. Because of the influence of environmental factors (primarily humidity) on fungal outbreaks, often it provides erratic control of spider mites on corn, and may not reduce mite numbers before economic damage occurs. Other factors that are known to influence spider mite populations include plant growth stage (lower spider mite growth and survival on senescent plants) and a variety of predatory natural enemies. Natural enemies observed in this study included the predatory spider mite, Neoseiulus fallacis, which was the most important species. Other natural enemies found in lower numbers included two predatory thrips species, the minute pirate bug, Orius insidiosus, and the spider mite destroyer, Stethorus punctum (a small lady beetle).

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