Striped and spotted cucumber beetles can cause damage in vine crops, but the striped beetle is more common in Wisconsin. Feeding from larvae and adults causes direct damage to roots, leaves, flowers, and fruits. Adults can also vector fusarium wilt and the bacteria, Erwinia tracheiphila. Cucumbers and melons are particularly susceptible to bacterial wilt, and damage from this can be severe.

Appearance

The striped cucumber beetle (Acalymma vittatum) is 1/5 inch long and yellow-green in color with three black stripes running along the length of its body. It is often confused with the western corn rootworm beetle that is not a pest of vine crops but is often found feeding on the pollen of cucurbit blossoms. They can be distinguished by looking at the undersides of their abdomens. Striped cucumber beetles have black abdomens, while the abdomens of western corn rootworms are yellow-green. Spotted cucumber beetles (Diabrotica undecempunctata howardi) are yellow-green with 12 black spots on their backs and a black head.

Symptoms and Effects

Larvae feed on roots and stems and can stunt or kill seedlings or transplants. Adults feed on foliage, pollen, petals, and fruit of a variety of plants and can cause moderate to severe defoliation. Feeding on fruit can cause blemishes. Plants in the 1-3 leaf or cotyledon stages are especially vulnerable, and high adult cucumber beetle populations can completely defoliate the plants.

More importantly, cucumber beetles can transmit bacteria that cause fusarium or bacterial wilt. Adult beetles pick up the bacteria when they feed on infected weeds in early spring. The wilt is spread through the feces or contaminated mouthparts of beetles. A distinct wilting of individual lateral leaves is the first symptom of bacterial wilt followed by the entire plant wilting and dying. The disease causes plant death by plugging the water-conducting vessels. Cutting through the stem and holding the cut ends together for ten seconds can help diagnose the disease. Slowly pull the ends apart and look for white, viscous sap which is the bacteria reproducing in the xylem, or water-conducting tissue. Serious crop damage can occur if as little as 10% of the beetles are infected.

Life Cycle

Only the striped cucumber beetle overwinters in Wisconsin. They emerge in mid- to late May and lay eggs in the soil at the base of cucurbits. The beetles are attracted to the chemical cucurbitactin produced by the plants. The small white larvae feed on plant roots for 2-3 weeks before pupating in the soil. Spotted beetles migrate to northern locations in early to mid-July. This late arrival generally seldom makes them a serious problem. There is one generation of striped and spotted beetles per year.

Scouting

Plants infected with bacterial wilt will not recover. Therefore, it is important to control the beetles early in the season to prevent spread of the disease. Scout fields for adult beetles 2-3 times per week early in the season and weekly thereafter. Particular attention is needed in field edges where beetles congregate. The treatment threshold for cucumber beetles is 1 beetle per plant in melons, cucumber, Hubbard and Butternut squash, and younger pumpkins and 5 adults per plant in watermelon, other varieties of squash and older pumpkins. Beetle populations in excess of 20 per plant may transmit the bacterial wilt before insecticides have a chance to control the beetles.

Management

Host plant resistance

Cucumber varieties differ in their attractiveness to beetles. The varieties “Liberty” and “Wisconsin SMR58” are tolerant of cucumber beetle damage. In cantaloupe, “Makdimon” and “Rocky Sweet” are less attractive to beetles. Less bitter cultivars are less attractive to cucumber beetles.

Cultural control

Non-chemical control is possible in small plantings by covering the plants with floating row covers. Be sure to uncover flowering plants to allow bees to enter and pollinate the plants. Rotating crops with grain, tomatoes, or a cover crop or using perimeter trap crops can delay infestations.  If a trap crop is used, exercise care that the trap crop will not act as a reservoir for bacterial wilt. If bacterial wilt infections have already occurred, remove the diseased plants immediately to prevent the spread of the disease while insects are present.

Chemical control

Several chemical insecticides are available when beetles exceed thresholds. However, chemical control will be limited if beetle populations are already high. A systemic insecticide such as Admire^® or Belay^® can be used for control.  Contact insecticides should be applied to seedlings before transplanting and continued on a regular basis after the systemic insecticide loses effectiveness. Cucumber leaves are sensitive and can be burned by chemical sprays. Spraying in the afternoon or evening is preferable to avoid killing beneficial insects and pollinators.

Refer to the UW-Extension publication Commercial Vegetable Production in Wisconsin (A3422) for a list of registered insecticides and management recommendations.

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