Spider Mites
Egg - The eggs are spherical, and white to transparent when first laid. Just before hatching, they become straw colored and average 0.14 mm in diameter.
Larva - Not much bigger than eggs, the six-legged larvae are colorless except for carmine eye spots.
Nymphs - The two, eight-legged nymphal instars are difficult to distinguish. Both instars are oval, pale green to deep green, sometimes spotted, and slightly smaller than adults.
Host Plants - In view of recent taxonomic revisions which establish the carmine spider mite as a species separate from the twospotted spider mite, it is difficult to separate the host ranges of these two mites. Before the change, the twospotted spider mites (Tetranychus urticae and T. cinnabarinus) were recorded on over 180 host plants, including 100 cultivated species. Violets, chickweed, pokeweed, wild mustard, and blackberry are probably common hosts of both species from which infestations spread to nearby crops. Although these two species have been reported to infest grain sorghums since the early 1970's, forage sorghums appear to be resistant.
Damage - Feeding on corn and sorghum usually occurs on the undersides of leaves, where spider mites pierce the epidermis and extract sap. Lightly infested leaves have a stippled appearance; heavily infested leaves turn completely pale and dry up. The entire plant may die. The undersurfaces of the leaves usually have silken webs over which the mites crawl. A heavily infested plant, however, may have webs all over it, including the grain heads or ears. A rapid increase in spider mite populations is closely associated with drought conditions and symptoms are very similar from a distance. Color plate.
Life History - As a general rule, these spider mites overwinter as fertilized females resistant to low temperatures. In mild winters, they may continue to feed and lay eggs. In summer, many generations (7 or more) develop; the number of eggs laid depends largely on temperature. Within 1 to 3 weeks, eggs hatch into six-legged larvae which develop into eight-legged nymphs. There are two nymphal stages. After the larval and each nymphal stage, a resting stage occurs.
Adults mate soon after emerging from the last resting stage. In warm weather, the females soon begin laying approximately 100 eggs each. Development is rapid in hot, dry weather. A generation requires 1 to 3 weeks to mature.
Control measures, such as acaricide applications, resistant host plants, or sprinkler irrigation, are most effective in giving short-term relief of mite infestations. The need for chemical spider mite control on North Carolina corn and sorghum is rare, perhaps due to a fungal disease of the mites. For further control information, consult the current North Carolina Agricultural Chemicals Manual.
