Cucumber Beetles

Two beetles do most of the damage to cucumbers, squash and melons in US gardens: the striped cucumber beetle (Acalymma vittatum) and the spotted cucumber beetle (Diabrotica undecimpunctata). Both are about 1/4 inch long. They chew seedlings, leaves, flowers and fruit rinds, but the bigger threat is bacterial wilt, which they carry from plant to plant and which can kill a cucumber or melon vine in days.

Identification

Striped cucumber beetle: yellow, with three black stripes running down its wing covers and a black head.

Spotted cucumber beetle: yellowish-green, with 12 black spots on its wing covers and a black head.

Eggs: tiny, oval, yellow to orange, laid in the soil at the base of host plants.

Larvae: slender, white, worm-like grubs with a brown head, about 1/3 inch long when full grown, found in the soil around the roots.

Life cycle

Striped cucumber beetles spend the winter as adults in leaf litter, hedgerows and woodland edges near last year's cucurbits. They arrive in spring about the time cucurbit seedlings come up, feed, and lay eggs in the soil at the base of the plants. The larvae feed on cucurbit roots for a few weeks, pupate in the soil, and a new generation of adults appears in midsummer. Northern gardens usually see one generation a year; southern gardens see two or more. Striped beetles depend on cucurbits; their larvae feed on nothing else.

Spotted cucumber beetles feed on many crops. Their larvae, known as the southern corn rootworm, feed on the roots of corn, peanuts, beans and grasses rather than on cucurbits. The adults overwinter in the South and fly north each year, so in northern gardens they usually show up later than striped beetles.

Damage

• Seedlings: adults feed on young leaves and stems and can kill newly emerged seedlings outright. This is the most vulnerable stage.
• Leaves and flowers: ragged holes in leaves and chewed flowers on older plants.
• Fruit: feeding scars on the rinds, especially on fruit lying on the ground.
• Roots: striped beetle larvae tunnel in the roots and stunt the plants.
• Disease: cucumber beetles spread bacterial wilt, which collapses whole vines and is worst on cucumbers and muskmelons, and squash mosaic virus. (Cucumber mosaic virus is spread by aphids, not by these beetles.) See What's going on with my cucumber plants? for wilt symptoms.

Plants attacked: cucumbers, muskmelons, squash, pumpkins, gourds and watermelons. The spotted beetle also feeds on beans, corn, peanuts and many garden flowers.

Prevention

Row covers. Floating row cover, put on at planting with the edges buried in soil, is the single most effective control. Leave it on until female flowers open, then remove it so bees can pollinate. Parthenocarpic varieties, such as many Persian and English cucumbers, set fruit without bees and can stay covered all season.

Transplants. A sturdy transplant with several true leaves tolerates feeding far better than a seedling just breaking ground.

Trap crops. Cucumber beetles strongly prefer some cucurbits over others. Blue Hubbard squash, planted around the edge of the garden a week or two before the main crop, draws beetles to it. Kill them there by hand or by treating only the trap plants, before they move to the main crop.

Cleanup and rotation. Removing vines and debris after harvest takes away some overwintering cover. Rotation helps only a little, because the adults fly in from field edges.

Control

Hand removal. Check young plants daily. Beetles are sluggish early in the morning and easy to knock into a jar of soapy water or suck up with a handheld vacuum. Look under leaves and inside flowers.

Yellow sticky traps. Cucumber beetles are attracted to yellow. Traps near the plants show you when beetles arrive and catch some of them, though they also catch beneficial insects.

Kaolin clay. A kaolin clay spray coats leaves with a fine white film that deters feeding. Reapply after rain and on new growth.

Insecticides. Pyrethrin-based products labeled for cucumber beetles kill them on contact. Spray late in the day after the flowers have closed to protect bees, and concentrate on young plants, which matter most. Insecticidal soap and neem oil do little against these hard-bodied beetles, and the Bt sold for caterpillars (Bacillus thuringiensis var. kurstaki) has no effect on them.

Natural enemies. Soldier beetles and ground beetles prey on cucumber beetles, and some parasitic flies and wasps attack them, but rarely enough to prevent damage on their own. Beneficial nematodes watered into the soil can reduce the larvae, with variable results.

Squash growers face two more cucurbit pests: squash bugs and the squash vine borer.