Western spotted cucumber beetle

Diabrotica undecimpunctata

Description. The western spotted and the western striped cucumber beetles occur throughout California and are major pests of cucurbits; the banded cucumber beetle occurs primarily in southern California. Cucumber beetles overwinter as adults and are active by the time the earliest melons are planted in spring. Adults lay eggs at the base of plants. As soon as they hatch, larvae begin to feed on plant roots. They complete their development in the soil. There are about three generations a year.
Cucumber beetles are about 0.36 inch (9 mm) long and either have a greenish yellow background with black spots or alternating black and yellow stripes. They fly readily and migrate into cultivated areas from alfalfa and other crops, and from uncultivated lands. Cucumber beetles like moisture and dislike heat; consequently, melon fields are especially attractive in hot weather during and after an irrigation.
Western striped cucumber beetle larvae feed exclusively on cucurbit roots, whereas western spotted cucumber beetle larvae feed on a wide variety of plants including grasses, corn, legumes, and cucurbits.
Damage. Cucumber beetles are serious pests of smooth-skinned cucurbits, especially melon varieties such as honeydew, crenshaw, and casaba. While the adults prefer tender, succulent portions of plants, including the flowers and leaves, which they may destroy with their feeding, it is the damage to the surface of the melon that reduces marketable yield. When temperatures are high, adults especially feed on the undersides of young melons, scarring them. After the skin hardens, melons are much less subject to attack. Scarring in the crown of the plant is also typical of adult damage. Feeding on stems of young plants, followed by sustained winds, may result in severe stand reductions making replanting necessary. In some situations, larvae may cause serious injury by feeding on roots, and young plants can be killed. Cucumber beetles also spread squash mosaic virus.

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