Banks Grass Mite: Oligonychus pratensis (Banks)
Description and Biology: Two species of spider mites, especially Banks grass mite and less frequently twospotted spider mite, Tetranychus urticae Koch, infest sorghum and numerous species of grasses. Spider mites are more common on sorghum grown in more arid areas. Spider mites have distinct sexual dimorphism; the female, with a body length of 0.40 to 0.45 mm, is larger than the male. After feeding, both sexes usually become deep green, with the exception of the palpi and first two pairs of legs that remain light salmon colored. Two dark spots, composed of the food contents, show through the transparent body wall. Eggs (about 50 per female) are laid in webbing on the underside of leaves. Eggs are pearly white, spherical, and one-fourth the size of the adult. Eggs hatch in three to four days. Six-legged nymphs are light green colored; older eight-legged nymphs become progressively darker green. The life cycle requires about 11 days under good conditions as many as 61 days under less favorable conditions.
Symptoms and Damage: Spider mites suck juices from the underside of sorghum leaves, beginning along the midrib of lower functional leaves. Infested areas of leaves are pale yellow initially and later become reddish on the top surface. Entire leaves may die and turn brown. When spider mites become abundant on lower leaves they spread up the plant. The undersides of infested leaves have a deposit of fine webbing spun by the spider mites. Spider mites may invade and web sorghum panicles when infestations are severe. Increase in spider mite abundance generally occurs after sorghum panicle emergence. Large numbers of spider mites reduce the ability of sorghum plants to make and fill grain until kernels reach the hard-dough stage. However, yield loss still may occur after the hard-dough stage if spider mites are abundant enough to cause lodging and related harvest losses.

Monitoring: Spider mites infesting sorghum are extremely small. The underside of lower leaves should be inspected carefully. Mites occur in colonies, first along the midrib of leaves, but later spread away from the midrib and up the plant to higher leaves. Webbing is evidence of the presence of mites. It is common for mite infestations to begin along field borders, but an infestation quickly will spread throughout a field.
Hot and dry weather usually favors rapid increase in spider mite abundance. Also, spider mites in sorghum often respond as induced (secondary) pests following insecticide applications for key insect pests such as greenbug. Rapid increase in spider mite abundance following insecticide application is thought to be due to tolerance of spider mites to some insecticides, destruction of beneficial insects and mites, and dispersal of spider mites from colonies.

Management: Control of spider mites by natural enemies is not always effective. Because spider mites increase more rapidly on moisture-stressed plants, irrigation, where available, should be timed to prevent plant stress. Also, spider mites may move from small grains, especially wheat, to sorghum. Not planting sorghum next to small grains avoids direct infestation by spider mites moving from this host crop. Varying degrees of success are achieved by chemical control. Miticide application usually is justified when one-third of leaves of most sorghum plants in a field are infested with mites. Thorough coverage with the spray mixture is required. At least 28 to 47 liters of spray mixture should be applied per hectare. In some regions during some years, spider mites are resistant to miticides. Every effort should be made to control spider mites in sorghum with the first insecticide application, because subsequent applications usually are ineffective.

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