Cucumber beetles on Plants

Symptoms

Look first at the leaves, runners and young fruit, where adult beetles leave feeding holes and scarring. On cucumber and pumpkin fruit, the damage shows as pinhole-like scarring that lowers market value. Scarring in the crown of the plant is also typical of adult damage.

Young plants can be hurt in other ways. Stems may be chewed near the base, and wind that follows can cause severe stand loss in young plants. Small seedlings can be killed outright by feeding injury. Larvae also feed on roots, and that root feeding can kill young plants.

A third pattern is sudden wilting of individual leaves and stems. The wilting spreads until the whole plant dies, usually within 7-14 days. That window runs from the first sudden wilting to the death of the whole plant.

Scarred fruit, holed leaves and scarred crowns all come from adult feeding, while root damage comes from larvae. Pumpkin and cucumber fruit are the two crops named for pinhole-like scarring.

Causes

Adult beetles overwinter in crop debris and become active in spring as cucurbit crops are planted. Crop debris is where adults overwinter. Females lay eggs at the base of cucurbit plants. Eggs laid at the base of the plant hatch into larvae that feed on roots and crowns.

Adults feed on leaves, runners and young fruit, while larvae feed on roots. Larvae are the stage that feeds on roots, and adults are the stage found overwintering in debris. The beetles' spring activity coincides with the planting of cucurbit crops. Moist, irrigated conditions attract beetles, especially in hot weather.

Beetles also carry disease. They spread squash mosaic virus. They also disseminate the bacterial wilt pathogen to cucurbit crops. Plants are most vulnerable to bacterial wilt between emergence and bloom.

Larval root feeding can also increase the incidence of Fusarium wilt disease. Beetles harm a crop in two ways: by feeding directly and through the diseases tied to them.

Diagnosis

The facts on this page name no look-alike problems, so these checks rest on the listed symptoms and causes alone. Feeding holes and scarring on leaves, runners and young fruit are the mark of adult beetles feeding. Pinhole-like scars on cucumber or pumpkin fruit fit the same pattern.

In young plants, look for stems chewed near the base or seedlings killed outright. Chewed stems followed by sustained wind can cause severe stand loss, and larval root feeding can kill young plants too.

Timing is a further clue: adults become active in spring as cucurbit crops are planted. Moist, irrigated conditions in hot weather attract beetles, which is worth noting in your own planting.

Sudden wilting of single leaves and stems is a separate sign from feeding scars. It spreads until the whole plant dies, usually within 7-14 days. Beetles disseminate the bacterial wilt pathogen, and plants are most vulnerable between emergence and bloom. Larval root feeding can also raise the incidence of Fusarium wilt disease.

A plant with scarred leaves and fruit but no wilting shows feeding damage without the sudden collapse described above.

Treatments

Cultural measures come first. Avoid planting cucurbits next to corn, since spotted cucumber beetle larvae also feed on corn roots. Locate cucurbit fields a long distance from the previous year's cucurbit plantings to delay beetle buildup. After harvest, plow down or incorporate cucurbit crop debris to reduce overwintering sites. Choose cucurbit varieties with lower levels of cucurbitacin, which are less attractive to beetles.

Floating row covers exclude beetles, and you remove them once flowering begins to allow pollination. Yellow sticky tape or bands hung above the crop canopy trap and monitor beetles, which are attracted to the color yellow. A perimeter trap crop of Blue Hubbard squash, which is highly attractive to beetles, can surround the protected cucurbit crop. Insecticide applications are then concentrated on the trap crop instead of the main crop.

Among organic options, kaolin clay is applied as a particle-film deterrent on plants early in the growing season. Entomopathogenic nematodes can be applied to the soil around cucurbit root systems.

Chemical options follow. Carbaryl is one foliar insecticide listed for these beetles. The XLR Plus formulation is less hazardous to honey bees than other formulations when applied at night or early morning. Esfenvalerate is applied as a foliar spray to control adult beetles, repeating as necessary to maintain control. The source notes that esfenvalerate is highly toxic to honey bees.

Rotate insecticides with different modes of action between applications to manage resistance. The product label is the legal instruction, so follow it for the crop and site being treated.

Prevention

Prevention starts with where and when you plant. Cucurbit crop debris is an overwintering site, and plowing it down or incorporating it after harvest reduces those sites. Distance from last year's cucurbit plantings delays beetle buildup. Keeping cucurbits away from corn matters because spotted cucumber beetle larvae also feed on corn roots.

The period between emergence and bloom is when plants are most vulnerable to the bacterial wilt pathogen that beetles disseminate. Floating row covers exclude beetles and come off once flowering begins to allow pollination. Moist, irrigated conditions attract beetles, especially in hot weather.

Lower-cucurbitacin varieties are less attractive to beetles. Yellow sticky tape above the canopy lets you monitor beetle numbers. Blue Hubbard squash is highly attractive to beetles and is planted around the protected crop as a perimeter trap.

Plants most often affected

• Aji
• Cantaloupe
• Eggplant
• Field Pumpkin
• Garden Cucumber
• Kidney Bean
• Lingonberry
• Watermelon