Cucumber Beetle

Both the spotted cucumber beetle, Diabrotica undecimpunctata howardii Barber, and the striped cucumber beetle, Acalymma vittata (Fab.), are native insects ranging from Mexico to Canada, though they tend to be most abundant and destructive in their southern range.

The adult spotted cucumber beetle is about 6mm (1/4 inch) long with a bright yellowish-green body. The head, legs and antennae are black, and 12 black spots appear on the wings. Adult striped cucumber beetles are about 5mm (../images/16 inch) long, black and yellow in color, and have three longitudinal black stripes on the wing covers. Both have beaded antennae about 2 mm long.

The eggs of both cucumber beetles are oval, orange-yellow in color, and are found in clusters of 25-50 on undersides of host leaves. The larva is about 12 mm long with a yellow-white, somewhat wrinkled body and six long, brownish-colored legs. Striped cucumber beetle larvae are more flattened on the top of the abdomen. The pupa ranges from white to yellow in color and is about 6 mm long.

Habit: Cucumber beetles overwinter as adults in sheltered places, but only the striped cucumber beetle overwinters in large numbers in Illinois. The spotted cucumber beetle migrates in from the south. In spring, they feed on nearby vegetation of fields and woodlots before suddenly appearing in large numbers on vine or bean crops. Adult beetles lay eggs in the soil at the base of the plant on which they are feeding.

Cucumbers, cantaloupes, winter squash, pumpkins, gourds, summer squash and watermelons are preferred by adult striped cucumber beetles. They also feed on beans, peas, corn and blossoms of several wild and cultivated plants. Larvae develop on these and related cucurbits. The spotted cucumber beetle has a wider host range and, in addition to cucurbits, may be found on beans, peas, potato, beet, tomato, eggplant and cabbage. The larva is the well-known southern corn rootworm which feeds on the roots of corn, peanuts, small grains and many wild grasses.

Life Cycle :Unmated adults overwinter in neighboring woodlands under leaves and trash or around the bases of plants that have not been killed by frost. Adults leave their winter sites in late March. Before cucurbits are available, the beetles subsist on the pollen and petals of many plants. As soon as cucumber, squash, or melon vines appear, beetles devour cotyledons and stems. Females of the overwintering generation lay eggs from late April through early June, each female depositing as many as 500 eggs. Depending on temperature, eggs incubate for 7 to 10 days before hatching.

Larvae feed in the soil on stems and roots for 2 to 4 weeks before pupating. First generation adults emerge from late June to early July. Over the next 6 to 9 weeks, the life cycle is repeated, second generation adults being prevalent from September to November. These later adults assemble on clover and alfalfa upon which they feed until winter. They may come out to feed during warm periods in January and February. Two generations and sometimes a partial third are produced each year.

Damage : Cucumber beetles are important pests of cucurbits. They cause four types of damage: seedling destruction, flower and foliage damage, root feeding, and transmission of bacterial wilt disease. Damage from cucumber beetles starts in the spring with feeding by adults on the seedling stage of the cucurbits. The beetles feed on newly emerged cotyledons and stems, and they have been reported to go below ground level and feed on plants as they emerge. Adults lay eggs in the soil near the seedlings and larvae soon hatch and begin feeding on roots of the cucurbits.

Larvae chew holes and tunnel into the roots. Damage by the larvae, except under dry conditions, is usually considered minor. The first generation of adults emerges in late June and early July to feed on the foliage and flowers. Feeding damage by cucumber beetles to foliage is usually very minor, but severe feeding on flowers can result in poor fruit set. The second generation emerges in September and October.

Probably the most serious damage by cucumber beetles is from transmission of bacterial wilt caused by Erwinia tracheiphila. Bacterial wilt can kill many plants in a field and seriously reduce the yield. The striped cucumber beetle and the spotted cucumber beetle have very similar life cycles and both can carry the bacteria, but both are not equally important pests on cucurbits. The spotted cucumber beetle, also known as the southern corn rootworm, is a general feeder and is a pest on other crops, peanuts and corn in particular.

The spotted cucumber beetle, however, is not considered as serious a problem as the striped cucumber beetle. The striped cucumber beetle has a more specific host range and feeds almost exclusively on cucurbits in the adult stage. The larvae are dependent on cucurbits for development; they cannot live on any other host plant. Both beetles should be monitored where cucurbits are grown.

Management : Cucumber beetles overwinter as adults in sheltered places, but only the striped cucumber beetle overwinters in large numbers in Illinois. The spotted cucumber beetle migrates in from the south. In spring, they feed on nearby vegetation of fields and woodlots before suddenly appearing in large numbers on vine or bean crops. Adult beetles lay eggs in the soil at the base of the plant on which they are feeding.

Despite the potential for bacterial wilt, we generally see wilt problems only about 1 out of 5 years. Therefore, the most common, initial concern occurs during stand establishment with direct feeding or defoliation damage on cotyledon and 1-3 true-leaf stages. The action threshold for first true-leaf plants is when SCB populations exceed 2 or more beetles/plant on 25% of the plants. Monitoring should occur weekly until initial infestations are detected. Once beetles are present, monitoring should occur more frequently to allow for a comparison of populations to the action threshold, or to detect the presence of any bacterial wilt.

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