Identifying and Controlling Cucumber Beetles

Striped cucumber beetle
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Description

There are two types of cucumber beetles that may attack your garden: the striped cucumber beetle (Acalymma vittatum), and the spotted cucumber beetle (Diabrotica undecimpunctata). The larva of the spotted cucumber beetle is known as the corn rootworm, a severe problem for corn and other agricultural crops. 

The striped cucumber beetle has a yellow body with three black stripes along its back, while the spotted cucumber beetle is yellow, with twelve black spots.

Both are the same size, roughly a quarter of an inch long. The larvae are white grubs with brownish heads.

Life Cycle

Adult cucumber beetles overwinter in the garden, in compost piles or in trash heaps, then emerge in spring. The adults feed on weeds and other plants until their preferred food source--cucurbits (such as cucumbers, squashes, and melons) are available. Once they locate cucurbits, they will feed on the plants and the females will lay their eggs in the surrounding soil. After the eggs hatch, the larvae (now called rootworms)  feed on below-ground cucurbit roots and stems until they pupate. Then the adult insects emerge from the pupae and the cycle starts all over again.

The entire life cycle of the insect is about 8 weeks. Southern climates may see three full generations in growing season, while northern climates only see two. 

Signs of Cucumber Beetle

Cucumber beetle damage is fairly easy to spot: damage from feeding on the leaves, scarring on the fruit, and girdled stems from feeding larvae.

Most commonly, this damage is found on cucumbers, squashes, melons and pumpkins, but may also be found on tomatoes and other garden crops if cucurbits are unavailable or there are more beetles than the available cucurbits can support.

Effect on Garden Plants

Most every part of the plant can be affected: girdled stems, chewed leaves, marks on the fruit.

However, the worst damage is not from the insects themselves but from bacterial wilt. The bacteria is secreted in the beetle's stomach, and as the insects feed they spread it to the plant. The bacteria spreads to the plant's vascular system, which causes the leaves to wilt.

If not contained (via pruning off infected stems), the wilt will eventually spread and kill the entire plant. Also, plants infected with bacterial wilt attract more cucumber beetles, who will eat the infected leaves and continue spreading the bacteria throughout the garden. Cucumber beetles are also a primary carrier of cucumber mosaic virus. In other words, these are very destructive pests, wreaking both direct damage and serving as carriers for a variety of bacterial and viral diseases. 

Organic Control of Cucumber Beetles

There are a number of environmentally friendly ways to control cucumber beetles: 

  • You can protect young cucurbit seedlings from cucumber beetles by covering them right after planting with floating row covers, individual screens or cones.
  • It's also a good idea to plant cucurbits later in the season--those planted early in other nearby gardens will attract any beetles in the area, and therefore spare your plants. 
  • Also, remove and destroy (don't compost) plants infected with bacterial wilt immediately so that they don't attract more beetles to the area. Adult beetles found on plants can be hand-picked and squished.
  • Mulching the ground with straw, hay, plastic or fabric can hinder cucumber beetles from laying eggs in the ground near the plants, though it will not eliminate them entirely. 

If You Use Chemicals

Chemical pesticides are always the last resort, but because cucumber beetles are so destructive, gardeners sometimes reach for them. Applying a non-systemic insecticide such as malathion is probably the best choice, but it should be the last option after all natural remedies are tried. Use extreme care when using any chemical remedies, and always follow label directions precisely.