Spotted cucumber beetle
The spotted cucumber beetle is a 1/4-inch yellow-green beetle with twelve black spots and a black head. As a larva it is the southern corn rootworm. It is native across the United States but most abundant and destructive in the South, where it survives the winter. Each year it spreads north, reaching Minnesota and Wisconsin in late June or July, usually too late to do serious harm. Adults feed on cucurbit leaves, flowers and fruit and can carry bacterial wilt and mosaic viruses. The larvae feed on the roots of corn, beans and grasses as well as cucurbits. Management is the same as for the striped cucumber beetle: protect young plants and act on high numbers.
An adult Diabrotica undecimpunctata howardi on a squash flower. Photo: pending.
Identification
Adults are about 1/4 inch (6.4 mm) long and yellow-green, with a black head and twelve black spots on the wing covers. The striped cucumber beetle is similar in size but has three black stripes. In the West, the western spotted cucumber beetle (D. undecimpunctata undecimpunctata) is the same species in a different subspecies.
The eggs are yellow ovals about 0.7 mm long, laid in clusters in soil crevices at the base of the plant. The larvae are slender, white, worm-like grubs with a brown head, reaching about 3/8 to almost 1/2 inch (10–12 mm) (sources differ). They live in the soil, feed on roots and tunnel into stems, and you are unlikely to see them.
Easily confused with
- Lady beetlesBeneficial — leave itThe spots invite the mix-up. Look at the antennae: a lady beetle's are short and stubby; a cucumber beetle's are long and threadlike.
- Striped cucumber beetleAlso a pestSame size and colour, and on the same cucurbits, but with three black stripes down the back instead of twelve spots.
Damage symptoms
Adults feed on cucurbit leaves and flowers and scar the fruit, and heavy feeding on young plants and transplants causes wilting and lower yields. Like the striped species, they carry the bacterium that causes bacterial wilt of cucurbits, and they also spread mosaic viruses. Below ground, the larvae feed on roots and tunnel into stems. The larvae feed on a wide range of plants, including corn, beans and grasses, and this is the stage called the southern corn rootworm. UC IPM notes that in gardens this root feeding does not usually cause noticeable damage.
- Holes chewed in cucurbit leaves, and beetles in the flowers
- Scarred rinds on cucurbit fruit
- Young cucurbit plants that wilt after heavy feeding, or wilt and die from bacterial wilt
- Corn or bean seedlings whose roots are chewed or whose stems are tunnelled at the base, though garden damage is usually slight
- Yellow-green beetles with twelve black spots, most in midsummer in the north
Lifecycle & degree days
Unmated adults overwinter under leaves and debris near woodlands and buildings in the southern states, and move north each year. In Minnesota they arrive in late June or early July, and in Wisconsin in early to mid July. Females lay eggs in the soil at the base of the plants. One generation takes about six to nine weeks. There are generally two a year, one in Oregon and three in southern California and Alabama. In the north they do not survive the winter, and Wisconsin sees one generation.
- Generations / yr
- Generally 2; 1 in the north and Oregon, 3 in Alabama and S. California
- Egg to adult
- About 6–9 weeks
- Arrives north
- Late June to mid-July (Minnesota, Wisconsin)
- Overwinters
- Unmated adults, in the South only
Host crops
Adults feed on cucurbits and on other crops, including corn and beans. Unlike the striped cucumber beetle, whose larvae feed only on cucurbit roots, spotted cucumber beetle larvae feed on a wide range of plants, including corn, beans, small grains and grasses. UF/IFAS advises not planting cucurbits next to these larval hosts.
Monitoring & action thresholds
UF/IFAS recommends control when numbers reach five or more per mature plant, and immediately if young plants take unacceptable damage. Northern extension thresholds for cucumber beetles in general are lower on crops prone to bacterial wilt: University of Minnesota gives one-half beetle per seedling and one per older plant on cucumbers and melons, and Wisconsin gives one per plant on cucumbers, melons, Hubbard and butternut squash, and five on watermelon and other squash. In the north, spotted beetles arrive late and in numbers that rarely need treatment.
In the South, check seedlings from emergence. In the north, look for the first spotted beetles in late June or July, mixed in with striped ones on cucurbit flowers and leaves.
Biological control
UF/IFAS lists the important natural enemies of cucumber beetles as the tachinid fly Celatoria diabroticae, the fungus Beauveria and the nematode Howardula benigna, and cites a Utah list that adds ground beetles, braconid wasps and insect-killing nematodes. It notes there is no solid evidence that pathogens control adult beetles, though commercial nematode and fungal soil drenches have shown some action against the larvae. The ones listed here are background help, not a treatment.
Cotesia congregata and other Braconidae
Braconid wasps- Kind
- Wild — attract it
- Best for
- Background parasitism
Cultural controls
The same tools as for the striped cucumber beetle, with extra help from timing: early ploughing or digging, delayed planting and heavy seeding reduce the beetles' impact.
- Cover young cucurbits with row cover or screens until they flower
- Plant a trap crop along the border two weeks before the main crop
- Plough or dig the ground early, delay planting a little, and sow thickly to offset seedling losses
- Pull and destroy cucurbit plants that wilt from bacterial wilt
- Combine any spray with these methods, and rotate between insecticides that act in different ways
Chemical, last resort
- Insecticidal soapREI 12 h
- PyrethrinsREI 12 h
- EsfenvalerateREI 12 h
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
Gill, Goyal & Gillett-Kaufman, Spotted Cucumber Beetle, UF/IFAS EENY-572 (rev. 2024)
University of Minnesota Extension, Cucumber Beetles (Schuh, Hahn & Wold-Burkness, reviewed 2026)
Wisconsin Horticulture, Cucumber Beetle (Lowenstein, Groves & Liesch, 2026)
UC IPM Pest Notes, Cucumber Beetles (UC Statewide IPM Program)
UC IPM Pest Management Guidelines: Cucurbits — Cucumber Beetles and Squash Bug tables (2016)
UC IPM, Relative Toxicities of Pesticides Used in Cucurbits to Natural Enemies (2016)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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