Bean Leaf Beetle Damage
red, yellow and black beetles chewing your bean leaves? Could be cerotoma trifurcate. What they are and what to do about damage.
What is it?
Beetles of the family, cerotoma trifurcate are a widespread attacker of bean plants. Both adults and grubs can cause massive damage to all varieties of plants.
What does it look like?
Beetles are red to yellow with a black band around their outer edge. They also have black spots on their back. Grubs grow up to 1/3 inch and are white and slender. Plants with beetle damage have holes chewed in the leaves, and may turn yellow and wilt depending on the extent of the damage.
How does it manifest?
Female beetles lay eggs at the base of plants in the soil. Eggs are reddish orange. When grubs hatch from the eggs they feed on the plant below the soil at the roots, which can kill the plants. Adult beetles feed on the undersides of the leaves and blossoms. Each year, up to 3 successive generations can cause damage if left untreated as new eggs are hatched and the feeding and breeding cycle begins anew.
What can you do about it?
Apply a carbaryl or diazinon containing insecticide when you first note damage in your bean plants. Make sure you coat the undersides of the leaves where adult feeding occurs. If damage continues, you will need to respray every 7 to 10 days until damage is no longer evident. At the end of the growing season, make sure all debris is cleared from your garden to prevent an overwintering of beetles.