Cucumber Beetles

(Adult Field Key—PDF)

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.

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.

Damaging populations of cucumber beetles are usually treated with insecticides.

Cucumber beetles are attacked by a variety of natural enemies, the most important being a parasitic tachinid fly, Celatoria diabroticae. Natural enemies are rarely effective enough, however, to reduce populations below economically damaging levels.

There are no effective cultural controls for these pests. Because spotted cucumber beetle larvae also feed on corn, avoiding planting cucurbits next to corn may help.

Cucumber beetles are difficult to control. Pesticide sprays must be directed at adult beetles. Larvae of western spotted cucumber beetle develop outside of cucurbit fields and striped cucumber beetle larvae are located on roots where they cannot be controlled.

Start monitoring for cucumber beetles after transplanting or when seedlings emerge, through the fruiting stage. Pesticide applications for adults may be necessary if there is an average of one beetle per plant during the seedling-to-4-inch-tall stage. Infestations that develop late in the season are usually not as damaging as those that begin earlier because numbers tend to be lower. Apply insecticides before beehives are introduced into the field; typically, an application is often made the day before bees are put in the field.