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How do I identify cucumber beetles?

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Content Author:
Lexi Gauger

Cucumber beetles are one of the most pervasive pests on all kinds of cucurbits. These small, colorful beetles first emerge as overwintered adults in the late spring, when they breed, feed, and lay eggs around cucurbit crops. Later in the summer a second generation of adults emerges, and in the Southern US a third generation can also occur. All types of cucurbits are at risk of damage from cucumber beetles, but Hubbard squash and zucchini are particularly attractive to the beetles, while summer squashes, butternut, acorn squash, and pumpkins are relatively less attractive.  

Spotted and striped cucumber beetles are the two most common species of cucumber beetles. Striped cucumber beetles are a pest as both larvae and adults. The larvae chew on the roots of cucurbits, causing damage that can stunt or kill young plants. Adult striped beetles feed on the leaves, stems, flowers, and young fruits, causing damage to leaves and scarring the fruits. Spotted cucumber beetles, also known as the Southern corn rootworm, only feed on cucurbits as adults. Compared to striped beetles, they are often larger, slower, and more green in color.

Image 1. Spotted (L) and stripped (R) cucumber beetles. Photos by Lexi Gauger.
cuke beetle cuke beetle stripped

One of the most damaging effects of cucumber beetles is their ability to transmit disease-causing pathogens between cucurbit plants. Cucumber beetles vector squash mosaic virus and other diseases, but the most important disease by far is bacterial wilt, caused by the Erwinia tracheiphila bacterium. Bacterial wilt causes the inside of the plants’ stems to become gummy, blocking flow of water and nutrients. This makes the plant wilt more and more until the entire plant wilts and dies off, which can lead to significant crop loss. Cucumbers and melons are the most susceptible crops, while squash and pumpkins are more resistant and watermelon is not affected. Learn more about this disease from Dr. Gleason here.

Monitoring your crops can help you avoid crop damage and loss from cucumber beetles. Check the tops and undersides of leaves, along the stems and the base of the plant, and inside the flowers for the beetles. When scouting your crops, it is important to be able to distinguish spotted and striped cucumber beetles from their many look-alikes. While the other pests can cause damage to cucurbits, they do not prefer to feed from cucurbits and do not vector diseases, so they are far less damaging.

Cucumber beetles and their look-alikes
Image 2. Cucumber beetles and their look-alikes. Infographic by Lexi Gauger.

The spotted cucumber beetle closely resembles the bean leaf beetle, which primarily affects soybeans. While the spotted cucumber beetle is usually yellow and green in color, the bean leaf beetle is usually darker and shades of yellow, green, red, orange, or brown. Spotted cucumber beetles have 12 black spots on their elytra, six on each side. The bean leaf beetle has 0-3 black spots on each side, bordered by a black stripe, and the two spots near the thorax form a triangle shape.

cuke beetles
Image 3. Spotted cucumber beetle (L) compared to bean leaf beetle (R). Left photo by Lexi Gauger. Right photo by Robert Koch.

Striped cucumber beetles have two doppelgangers, the amaranth flea beetle and the Western corn rootworm. Striped cucumber beetles are most commonly bright yellow, but sometimes can be a dark yellow or green color. They have three black stripes along their wings, with the middle stripe being split between the two wings. The underside of their body is black, and their legs are yellow at the top and turn black further from the body.

Amaranth flea beetles and their cousin pale-striped flea beetles both have a black base color, unlike the cucumber beetle’s yellow base color. Amaranth flea beetles have a yellow or red thorax and underside, two cream or white stripes down the center of their wings, and a white stripe forming a perimeter around the edge of their wings. Pale striped flea beetles have a yellow or black thorax and underside, with just one stripe in the middle of each wing.

beetles
Image 4. Striped cucumber beetle (L), amaranth flea beetle (C), and Western corn rootworm (R). Photos by Lexi Gauger.

The Western corn rootworm appears very similar to the striped cucumber beetle, with a few key differences. The stripes on the back of Western corn rootworms are usually more mottled and can give the beetle an overall dark coloring. The most key differences are that Western corn rootworms’ legs are entirely black and the underside of the corn rootworm is yellow, not black.

With practice and patience, you will be able to easily distinguish spotted and striped cucumber beetles from similar-looking pests. This is critical for recognizing the pest and disease threats that cucumber beetles present to cucurbit crops.