Overview
In mid and late summer, you may notice little oblong-oval beetles feeding on the flowers and foliage of cucumber, squash and pumpkins, melons, and potentially on ornamental flowers. Some of these are cucumber beetles, important pests of cucurbits, while the others are corn rootworms. There are two species of cucumber beetles in Wisconsin, as well as two very similar-looking corn rootworm beetles. These native beetles in the family Chrysomelidae are about a quarter of an inch long, have beaded antennae, and often have distinctive markings.
Spotted cucumber beetle (Diabrotica undecimpunctata howardi) is bright yellow-green with 12 black spots on the elytra and a black head and legs. The larval stage is known as the southern corn rootworm because it feeds on the roots of corn and other species.
Striped cucumber beetle (Acalymma vittata) is pale yellow with three black stripes down the back and a black head.
Western corn rootworm (Diabrotica virgifera virgifera) is a little bit bigger than the others. The males are mostly dark brown or black gradating into dull yellow at the back end. Females resemble striped cucumber beetles, but their stripes are less distinct and do not extend to the end of the elytra.
Northern corn rootworm (Diabrotica barberi) is a uniform pale green.
Cucumber Beetle Life Cycles
The rootworms typically appear in cucurbits in the middle to late season mostly likely because corn (their preferred host) pollen shed has stopped. Even though they can be more abundant than cucumber beetles at that time, they usually have little impact on cucurbits – although large populations may damage pumpkin and winter squash plants and fruits.
Striped cucumber beetles overwinter as adults in protected areas (under leaves, clods of soil, and other debris) and become active in mid-spring, when temperatures begin to increase. They quickly locate host plants, and females lay groups of eggs in cracks in the soil at the base of seedling cucurbits. The slender, off-white larvae have three pairs of short legs and darker head and abdominal tip. They feed only on plant roots and generally do not cause noticeable damage to plants, so they are rarely encountered by gardeners.
These larvae pupate in the soil and later in the summer the next generation of beetles emerges.
The spotted cucumber beetle overwinters in southern states and migrates to the northern United States and Canada each year, appearing two to four weeks later than striped cucumber beetles (although there is some evidence that some may overwinter in northern areas, too). The larvae develop on the roots of corn, small grains and grasses.
Cucumber Beetle Damage
The adult striped cucumber beetle feeds almost exclusively on cucurbits, including cucumber, cantaloupe, winter squash, pumpkin, gourd, summer squash, and watermelon.
The spotted cucumber beetle has a wide host range and may feed on over 200 host plants, including beans, corn, and potatoes. Both feed mainly on the leaves, pollen and flowers, but they will feed on soft fruits, leaving scars on the fruit. Well-established cucumber, melon, and squash plants can sustain a significant cucumber beetle feeding damage to foliage without reducing yields. Severe feeding on flowers can result in poor fruit set. The beetles can kill or severely stunt young plants by feeding on the cotyledons and can girdle stems by gnawing on the tender shoots of seedlings.
and foliage (R).
These two species of beetles are also vectors of squash mosaic virus and the pathogen that causes bacterial wilt of cucurbits, Erwinia tracheiphila. The bacterium overwinters in the gut of some striped cucumber beetles and/or in the sap of alternate host plants. Once acquired, the bacterium is spread through their feces or on their mouthparts. When they feed on the leaves, they create openings for the pathogen to enter the plant.
Once inside, the bacterium multiplies in the xylem vessels, creating blockages that cause rapid wilting and death of the plant. Nothing can be done to save a bacterial wilt infected plant. Since other cucumber beetles can pick up the bacterium from infected plants and move it to healthy plants, remove and destroy any wilted plants. Bacterial wilt is most severe on cantaloupe and cucumber, less on squash and pumpkin, and rarely affects established watermelon plants.
Seedlings are more susceptible both to feeding damage and infection by bacterial wilt. Covering young plants with floating row cover, screen, or cones is an effective means of excluding cucumber beetles and preventing bacterial wilt of cucurbits in home plantings.
Remove coverings when the plants begin to flower so pollinators can access the flowers to ensure fruit is set. Some types of cucurbits are more resistant to cucumber beetles than others, but even these resistant varieties may be affected in heavy infestations, and susceptibility may vary with geographical regions. \
Foliar insecticides can be used on seedlings and older plants in larger plantings, but may not kill the beetles quickly enough to stop transmission of bacterial wilt. Always read, understand, and follow label directions when using pesticides.
Author: Susan Mahr, University of Wisconsin-Madison
Reviewers: Allen R. Pyle, Horticulture Outreach Specialist, UW-Madison Extension, Bruce Spangenberg, Horticulture Outreach Specialist, UW-Madison Extension
Revised: November 2026
