Spider Mites

Examine leaves with a hand lens for spider mites. Frequently, infestations include a mixture of spider mite species. Adult mites are about 0.06 inch in length, have four pairs of legs, are greenish to pink or cream colored, and have various sized black spots on the body. Under warm conditions spider mites move rapidly within the colony area. Spider mites have four stages of development: (1) the oval, somewhat translucent egg; (2) a six-legged translucent immature stage; (3) an eight-legged immature stage; and (4) the eight-legged adult stage. A generation may pass in as few as 5 to 7 days in mid-summer, or in a month during cool periods. Spider mites produce webbing that is often filled with cast skins, dust, and other debris.

Mite feeding results in the destruction of chlorophyll; leaves become pale, stippled, and in later stages of infestation dry up and die. Loss of color is pronounced on the under surface of leaves before it becomes apparent on the upper side. Light infestations can be tolerated, but when heavy, can result in lowered yield and reduced quality of fruit.

Biological control is an important component of mite management. Take measures to ensure the survival of predators and parasites.

Several predators play an important role in regulating spider mite populations, including the western predatory mite (Galendromus [Metaseiulus] occidentalis), sixspotted thrips (Scolothrips sexmaculatus), western flower thrips (Frankliniella occidentalis), lady beetles (Stethorus sp.), minute pirate bug (Orius tristicolor), and lacewing larvae (Chrysoperla carnea). The western predatory mite is the same size as spider mites but lacks spots and ranges in color from cream to amber red. It is available commercially, but research has not been done on the effectiveness of releasing these predators in cucurbits. Sixspotted thrips and western flower thrips are also effective predators, but naturally occurring populations of these insects generally do not develop to high enough levels that they can provide significant control until damage has already taken place. Both species are tiny, slender insects about1 mm or less in length. Sixspotted thrips has three dark spots on each forewing; western flower thrips ranges in color from clear lemon yellow to dark brown. Monitor western predatory mites and the two species of thrips to determine if they are present in the field and their relative population density in comparison with pest mites.

Minimize dust and encourage naturally occurring predators and parasites by limiting chemical rates and the number of applications. Control field bindweed growing in or at the edges of a cucurbit field. Good water management increases plant tolerance to these pests. After runners are 14 inches long, natural enemies such as sixspotted thrips or predaceous mites usually control pest mite populations.

Biological and cultural controls and sulfur sprays (not for use on sulfur sensitive varieties) are acceptable to use in an organically certified crop.

Start monitoring for spider mites during the vegetative growth stage. No threshold is established, but when buildup is observed, either spot or completely treat the field before webbing occurs or before runners are 14 inches in length, providing no predatory thrips or predaceous mites are present. After the rows close over, ground equipment cannot get in the field, and chemical treatment must be applied by aircraft. Such treatments are less effective because it is difficult to obtain good coverage by aircraft.