Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
Hey, what’s eating my beans? What’s eating my beans before they can grow from the seedling stage? Must be bean leaf beetles.
What are Bean Leaf Beetles?
Bean leaf beetles (Cerotoma trifurcate) are tiny ( ¼ -inch) insects that feast on the roots and seedlings of bean and soybean plants in your garden. They also like peas, cowpeas and corn. The adult beetles are characterized by four black spots on their wings and a distinctive black triangle at the top of their wing covers. The bodies can be red, orange, tan or a yellowish-green.
You can immediately spot a bean leaf beetle infestation. They chew round 1/8-inch holes in the leaves from the undersides. Later in the season, they will also attack the bean pods. Fortunately they only chew the outer layers of the pods, so the damage is mainly cosmetic. The beans are still safe to eat.
The adults hibernate in leaf litter and other garden debris during the winter. They emerge in the spring when the temperature reaches 50⁰F - 55⁰F. Bean leaf beetles at this time of year are partial to seedlings, concentrating on the cotyledons and first true leaves of your plants.
Both the males and the females spend about a week feeding on your beans before mating. The females then lay 130 – 200 orange eggs in clusters of 12 around the bases of the plants. The eggs hatch in 1 – 3 weeks depending on the temperature. The resulting white larvae feed on roots and underground stems for 30 days. They then pupate in the soil for a week. The adults emerge from the soil and begin the cycle again. In the Midwest and Northeast, there are usually 2 generations of bean leaf beetles each summer. In the Southeast with its longer growing season, there can be 3 generations.
How to Get Rid of Bean Leaf Beetles
Delay Planting Your Beans – Bean leaf beetles emerge for the soil any time from mid-May to early June depending on the temperature. If you wait to plant your beans until after the adults have emerged from the soil, there will be nothing for them to eat and they will leave in search of food. Beans only take 60 days from germination to harvest so there will be plenty of time for your plants to grow and mature if you delay planting for a few weeks.
Use Row Covers – Provided bean leaf beetles aren’t already present in your garden, you can cover your seedlings with row covers to protect them. The cloth of the row covers allows rain and sunlight to get in, but keeps out the beetles and other insect pests. You can remove the row covers once your plants are approaching maturity to allow pollinators access to the flowers. If bean leaf beetles are present in the vicinity, these larger plants will be better able to withstand their depredations.
Hand Pick the Beetles – you can also pick the beetles off of your plants by hand and drop them into a container of soapy water to kill them. Even easier than picking them off, take advantage of the fact that when they are disturbed, they drop off of the plants to the ground. Place your container of soapy water underneath your plants and gently shake them, causing the beetles to drop off into your container without having to actually touch them. I admit to being a bit squeamish and use this method.
Spray Your Plants with an Organic Pesticide – spraying your beans is also an option. Be sure to use an organic spray such as an insecticidal soap, neem oil or pyrethrin which are safe to use on food crops. Always follow the directions on the label for proper application. Be sure to spray underneath the leaves where the beetles are feeding. Also important is how close to harvest you can use the spray.
You can make your own insecticidal soap spray using a liquid (not dry) soap such as Castile soap. Mix 1 tablespoon of soap in a quart of water or 5 tablespoons of soap in a gallon of water.
Clean Up Your Garden in the Fall – In the fall, remove all dead plant material and other debris from your garden so that the beetles have nowhere to hide to hibernate through the winter. They will be forced to fly away in search of more suitable winter quarters.
© 2020 Caren White