Cucumber beetles
Click for larger image Adult twelve-spotted cucumber beetle (Coleoptera)

Cucumber beetles feed on all cucurbits from seedling emergence to harvest. Feeding by the adults causes scarring on stems and fruit of cucurbits. However, the most important damage by the insect is the transmission of bacterial wilt, which is lethal to the vine. Adult beetles also may feed on a number of different plants, including blossoms of rose and zinnia. Cucumber beetles are about 1/3 inch long. They have a black head and their wing covers are either green with 12 black spots or alternating black and yellow stripes. They fly readily when disturbed.

Symptoms and Diagnosis

Adult beetles damage cucurbits in several ways. They may feed on young seedlings or transplants, completely consuming stems and cotyledons, and killing or stunting the young plants. The adult also damages more mature vines and fruit by chewing holes in leaves and rinds. The worst damage by the beetle is the transmission of bacterial wilt when the plants are young. The infected vines will gradually wilt and die.

Life Cycle

Adult beetles overwinter in woodlands, fence rows, under fallen leaves, under bark, and on weeds. In early summer when temperatures near 70 degrees F, the beetles become active and feed on young plants. After mating, eggs are laid in the soil near plant bases. Emerging larvae feed on plant roots for 3–6 weeks. Emerging adults appear during midsummer and feed on above ground plant parts of host plants. There are 2–3 generations a year.

Integrated Pest Management Strategies

1. Use row covers on young plants to exclude the beetles. Use floating row covers, screens, or cheesecloth to protect seedlings. Keep the covers in place until the plants begin to bloom.

2. Plant a trap crop of squash or pumpkin to attract beetles away from cucumber plants. Time the planting of the trap crop to emerge one week before the main cucurbit crop. The presence of cucumber beetles on the trap crop can help you time control practices. Trap crops can be pulled from the garden and the insects and eggs on the crop can be destroyed.

3. Plant tolerant varieties. Varieties of cucumbers tolerant to bacterial wilt are available. A couple of these are ‘County Fair ‘83’ or ‘Saladin’. Some varieties are less appealing as food for cucumber beetles. These include ‘Ashley’, ‘Chipper’, and ‘Gemini’ cucumber. ‘Heart of Gold’ muskmelon and ‘Crimson Sweet’ watermelon also resist attack from beetles.

4. Delay planting time. Planting time can influence beetle feeding damage and wilt transmission. By planting early to mid-June, some beetle feeding can be eliminated. However, this eliminates early harvest and may be too late for some varieties.

5. Insecticidal sprays may be needed if populations of cucumber beetles are high. Treat early in the season before over wintering beetles have a chance to lay eggs. Pesticides registered for use include carbaryl (Sevin), endosulfan (Thiodan), permethrin and pyrethrins.

Organic Strategies

Strategies 1, 2, 3, and 4 are strictly organic approaches. For an organic approach to Strategy 5, consult the Organic Materials Review Institute (OMRI™) for appropriate pyrethrin products.

More images:

Click for larger image
Adult twelve-spotted cucumber beetle (Coleoptera)
Click for larger image
Striped cucumber beetle (Coleoptera) on squash (Cucurbita)
Click for larger image
Wheel bug nymph (Hemiptera) eating a cucumber beetle (Coleoptera) on a potato plant (Solanum)