Biology and control of cucumber beetles in cucurbits

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.

The striped cucumber beetle, Acalymma vittata, and the spotted cucumber beetle (also known as the southern corn rootworm), Diabrotica undecimpunctata howardi. They are a significant insect pest of cucurbits in the North Central Region and generally in the United States and southern Canada east of the Rocky Mountains. Both species overwinter as adult beetles under plant material near gardens and fields. In the spring, the adults become active and feed on leaves and pollen of a several different tree and shrub species. The adult beetles feed on the foliage, flowers, and fruit of cucurbits, including cucumber, watermelon, muskmelon, summer and winter squash, and pumpkins, once these become available. Feeding by adults on emerging cucurbit seedlings may kill the plants at this stage. Female beetles lay their eggs in the soil near their host plants. Larvae, the immature stages of the beetles, hatch from these eggs and feed on roots of cucurbit plants, but do not cause significant damage to the root systems. These larvae may also feed on the rinds of ripening fruits that are in contact with the soil.

The damage caused to cucurbits by these two beetles goes beyond the feeding injury they inflict. Both of these beetles also transmit the pathogenic bacterium, Erwinia tracheiphila, that causes bacterial wilt of cucurbits. When a beetle feeds on a plant already infected by this pathogen, the bacteria are picked up on the beetle’s mouthparts, and when the infective beetle feeds on a healthy cucurbit plant, the bacteria are introduced into the plant. The bacteria multiply in the xylem of the plant, eventually plugging up the xylem tissue, which prevents water and nutrients from being moved from the roots. The plant wilts, and hence the name of the disease, bacterial wilt. Eventually, infected plants die from this disease. Cucumber and cantaloupe are most severely affected by bacterial wilt.

Both the striped and spotted cucumber beetle also transmit squash mosaic virus, which causes squash mosaic, an important disease of melons and squash. This cucurbit disease causes reduction in plant growth and yield, and is a major factor in distortion and mottling of fruit, which make the product unmarketable.

Controlling cucumber beetles in cucurbit plantings will significantly reduce the occurrence of bacterial wilt and squash mosaic. Extreme caution should be used in choosing insecticides for beetle control because cucurbits are very sensitive to most of these chemicals and significant damage to the plants will occur. Malathion can cause foliar burning and should not be used. Sevin formulations that are labeled specifically for cucumber beetle control can be used on cucurbits. Sevin is very toxic to honeybees and should not be applied when bees are actively foraging. Always read the insecticide label and apply insecticides at the recommended rates. Commercial vegetable growers should consult MSU Extension Bulletin 312, Insect, Disease, and Nematode Control for Commercial Vegetables, for insecticidal control.

The use of floating row covers is very effective in keeping cucumber beetles off of the plants in small cucurbit plantings. Once flowering occurs, the row covers should be removed to allow bees to pollinate. In small plantings, the covers can be kept over the plants and plants can be hand pollinated. Rotenone can be used after the row covers have been removed to control beetles if necessary.

Melon plants
Melon plants displaying bacterial wilt
infection symptoms.

striped cucumber beetle
Striped cucumber beetle.

Spotted cucumber beetle
Spotted cucumber beetle.  

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