Butter beans, botanically known as lima beans (Phaseolus lunatus), are a nutritious and popular crop. Identifying the specific creature responsible for chewing on the leaves, stems, or developing pods is the first step toward saving the harvest. Understanding the different types of pests that target various parts of the plant is necessary to implement an effective management strategy.
Insect Pests Affecting Foliage and Pods
The most commonly observed threats to the above-ground parts of the butter bean plant are insects that chew or suck fluids from the foliage and pods. The Mexican Bean Beetle (Epilachna varivestis) is a destructive pest that targets all stages of the plant. Adults resemble large, coppery-brown ladybugs marked with sixteen black spots, while the larvae are fuzzy, yellow, and covered in spines. Both life stages actively feed on the leaf tissue.
The Bean Leaf Beetle (Cerotoma trifurcata) is another foliage feeder, generally smaller than the Mexican Bean Beetle. These pests vary from yellowish-green to reddish-brown and often display black spots, sometimes including a black triangle behind the head. They emerge early to feed on seedlings but continue to damage leaves and pods throughout the summer.
Sucking pests, such as aphids, congregate on the stems and undersides of leaves, especially on new growth. These small, soft-bodied insects use piercing mouthparts to extract sap from the plant’s vascular system. Their feeding can transmit plant viruses and weaken the butter bean plant. Other generalist feeders, like the European corn borer, may occasionally bore into the main stem or developing pods.
Pests Damaging Roots, Stems, and Seeds
Some pests attack the plant from beneath the soil line or target the seeds themselves. The Seed Corn Maggot (Delia platura) is a threat to germination, as its larvae bore into and consume the soft inner parts of the planted seed. This results in poor stand establishment, causing seeds to fail to sprout or produce weak seedlings.
Wireworms, the larval form of click beetles, are thin, hard-bodied, yellowish-brown pests that tunnel through roots and stems below ground. Their feeding can stunt growth or cause sudden wilting by interrupting water and nutrient flow. Cutworms, which are moth larvae, reside beneath the soil surface and sever young seedlings at the stem base during the night.
The bean weevil (Acanthoscelides obtectus) primarily poses a problem after harvest, though infestation can begin in the field. These small, dark beetles lay eggs inside developing pods. The larvae mature inside the dried bean, emerging as adults and leaving circular holes in the stored seed.
Diagnosing the Specific Damage Patterns
Identifying the specific pest relies on interpreting the evidence left behind on the plant. If the leaves appear skeletonized—a thin, lace-like network of veins with the green tissue removed—the Mexican Bean Beetle is the likely culprit. This pattern results from larvae and adults scraping away soft material from the underside of the leaves while leaving tougher veins intact.
If the youngest leaves and stems look distorted, curled, or covered in honeydew, aphids are the cause. Honeydew is the sugary waste product excreted by these sucking insects, which can lead to the growth of sooty mold, turning plant parts black. Small, circular holes chewed through the middle of the leaves indicate feeding by the Bean Leaf Beetle.
Damage to the entire plant structure may point to larger pests. If a young seedling is missing or cut cleanly at the soil line, the cause is either a cutworm or a small mammal (vole or rabbit). Voles and rabbits also target mature plants, leaving jagged or clean-cut stems and missing pods. Small, circular exit holes drilled into a dried butter bean indicate bean weevil activity.
Implementing Effective Control Measures
Once the specific pest is identified, gardeners can implement targeted control measures. Physical control methods are the first line of defense for larger, easily spotted pests like the Mexican Bean Beetle. Hand-picking the adults, larvae, and yellow egg clusters from the undersides of leaves and dropping them into soapy water reduces pest populations.
For mammalian pests like rabbits or deer, the most reliable strategy involves exclusion through physical barriers. Fine-mesh row covers placed over seedlings can deter early-season insect pests until the plants are established. Installing a sturdy fence around the garden prevents larger mammals from accessing the plants.
Cultural practices play a large role in prevention, especially for soil-dwelling pests. Planting seeds only after soil temperatures are consistently warm reduces damage from the Seed Corn Maggot, which thrives in cool conditions. Implementing regular crop rotation, avoiding planting beans in the same spot yearly, helps break the life cycles of many soil-borne pests.
Organic chemical controls offer protection for widespread infestations. Neem oil, a botanical insecticide, works against many pests by disrupting their feeding and reproductive cycles. For chewing pests like the Mexican Bean Beetle, applying an insecticide containing spinosad, a naturally derived compound, is an effective spot treatment to reduce the population.