Bean leaf beetle
A small leaf beetle, about 1/4 inch long, that chews round holes about 1/8 inch across in the leaves of snap beans and soybean. Its colour varies from yellowish-green to light brown, red or orange, usually with four black spots, but every one has a black triangle behind the head. Seedlings are the stage at risk: heavy feeding on the first leaves can kill them, while bigger plants tolerate a lot of holes. In a garden, planting after the spring flight, row cover and knocking beetles into a pail are the controls; no beneficial is sold for it.
Adult Cerotoma trifurcata on a snap bean leaf. Photo: pending.
Identification
Adults are 1/8–1/4 inch (3–6 mm) long, oval, with the head visible from above. The colour is unreliable. In Minnesota most are yellowish-green; they can also be red or orange, and Kentucky describes a range from light brown to dark red. Most have four black spots and black markings along the outer edges of the wing covers, though the spots can be missing. What they all share is a small black triangle pointing backwards at the top of the wing covers, just behind the head.
It also shares beans with the Mexican bean beetle. The Mexican bean beetle is a coppery, dome-shaped lady beetle with 16 spots whose yellow, spiny larvae feed in the open under the leaves. Bean leaf beetle larvae stay out of sight: they are white and live in the soil, feeding on the roots.
Easily confused with
- Mexican bean beetleAlso a pestA coppery, dome-shaped lady beetle with 16 spots, whose yellow, spiny larvae feed in the open under the leaves. Bean leaf beetle larvae live out of sight in the soil, on the roots.
Damage symptoms
Adults feed on the underside of the leaves and chew almost circular holes, about 1/8 inch across, scattered over the leaf. On seed leaves the feeding shows as scooped-out pits. Seedlings suffer most: a large population can strip the first true leaves and kill young plants. Later in the season the beetles also scrape the green surface of the pods. Minnesota Extension says this only affects their appearance, though Kentucky notes the beetles occasionally chew through to the bean.
- Neat, almost circular holes about 1/8 in across, scattered over the leaf
- Scooped-out pits on the seed leaves of young seedlings
- Seedlings stripped of their first true leaves, or killed
- Scarred pods where the green surface has been scraped away
- Small beetles that drop to the ground when you touch the plant
Lifecycle & degree days
Adults spend the winter under leaf litter (especially in woods), in grass clumps and in field edges. In Minnesota they emerge from mid-May to early June and move to beans to feed and mate. Females lay orange eggs in the soil at the base of the plants, and the white larvae feed on the roots for a few weeks before pupating in the soil. Minnesota usually has one generation, with a second in the south of the state appearing in late August and September. Kentucky has two and perhaps a partial third. Mild winters, or snow cover that insulates the adults, lead to more beetles the following spring.
- Egg hatch
- 1–3 weeks (Minnesota); about 11 days (Kentucky)
- Larva
- Several weeks on the roots, in the soil
- Generations / yr
- 1 in Minnesota; 2 to a partial 3rd in Kentucky
- Overwinters
- Adults, in leaf litter and grassy edges
- Best time to check
- Afternoon, 12–4 p.m.
Host crops
Snap beans and soybean are the main garden and farm hosts. The beetle also feeds on dry beans, clover, vetch, alfalfa and several related weeds. Minnesota Extension notes that it can carry plant diseases, but that snap bean varieties are not affected by them, so gardeners growing green beans can ignore that part.
Monitoring & action thresholds
Minnesota Extension's garden guide is to protect snap beans if you see 25% defoliation, or 6–10 holes per leaf, on 10% or more of the plants, especially once the first true leaves are out. The commercial version is more than 25% defoliation on over 10% of seedlings between the cotyledon and first-trifoliate stages. Kentucky's soybean figure is 30% stand loss or 30% defoliation. As the plants grow they tolerate more damage.
If you have had bean leaf beetles before, check seedlings early in the season, when they are most vulnerable. Look in the afternoon: Minnesota Extension says the beetles are easiest to find between noon and 4 p.m. Judge by the damage: the threshold is based on defoliation and holes per leaf, not on beetle counts.
Biological control
No beneficial is sold for bean leaf beetle. Minnesota Extension reports that a native tachinid fly, Medina sp., parasitises the adults at varying rates, but it cannot be bought. Its relatives are covered on the tachinid flies page. Control in a garden rests on timing and exclusion.
Cultural controls
The overwintered beetles do their damage in spring, on seedlings. Planting after that flight reduces it.
- Delay sowing: in southern Minnesota, sowing snap beans in early to mid-June minimises the damage
- Cover seedlings with floating row cover and seal the edges — the beetles are very mobile
- Hold a pail of soapy water under the plant and tap it: the beetles drop when disturbed
- Clear leaf litter and mow grass around the garden to remove overwintering sites
- Try reflective plastic mulch (white or metallic), which reduces beetle numbers on the plants
- Near alfalfa, avoid cutting it while beans are young — cutting drives the beetles into the beans
Chemical, last resort
Sources
University of Minnesota Extension, Bean leaf beetles (Schuh & Hahn)
University of Kentucky ENTFACT-131, Bean Leaf Beetle in Kentucky Soybeans (Johnson, rev. 2026)
University of Maryland Extension, Mexican Bean Beetle – Vegetables
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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