Mexican Bean Beetle
Mexican bean beetles are one of the most destructive pests of many types of beans.
Both adult beetles and larvae cause major damage to bean crops by feeding on the leaves and sometimes the bean pods themselves. In severe infestations, the tissue between the leaf veins may be completely chewed away leaving a "skeleton" of just the veins.
The adult beetles are pale yellow or copper colored with 16 black spots on their wing covers. Other than the copper coloring, they sort of resemble a ladybug. In the early summer, the adults lay clusters of yellow eggs on the underside of the leaves. The eggs soon hatch into yellow larvae that are covered with black-tipped spines. The larval stage which includes four separate instars is responsible for the greatest damage to bean crops. They feed on the underside of the leaves for up to five weeks before they pupate. Adult beetles usually emerge within a week.
The greatest damage from these ravenous pests occurs during July and August. When Mexican bean beetle infestations are particularly bad all life stages can be found on the plants at any one time. Check the underside of the leaves for eggs, larvae, pupae, and adult beetles. The loss of leaf tissue due to their feeding, eventually leaves just the veins and gives a lacy appearance to the foliage.
Cultural Control Practices
- Planting crops early in the season or late in the summer and choosing early maturing varieties may help reduce damage as these beetles do the most damage in July and August.
- Select varieties that are more resistant to bean beetle attack. Pole bean varieties often tend to be more resistant than bush beans.
- Floating row covers placed over bush bean seedlings until flowering will prevent adult beetles from laying eggs on the foliage.
- Remove and destroy bean plants that are heavily damaged to prevent proliferation of new generations.
- Rake up and remove plant debris from the garden at the end of the season to reduce overwintering sites for the adult beetles.
Other Control Measures
- Hand picking the adult beetles and larvae is sometimes effective during light infestations. They can either be crushed or dropped into soapy water to destroy them. In addition, it is important to crush any egg clusters that you find attached to the underside of the leaves. When populations become higher, this method of control is not really effective.
- When populations are higher or if there is a history of bean beetle damage, other means of control may be necessary. These are most effective if applied at the first sign of beetles or their damage.
- Horticultural oil or insecticidal soap may help to control the larvae. Follow label directions.
- Bonide Eight or Captain Jack's Deadbug Brew, malathion, or pyrethrin may be used in cases of severe infestations. Always read and follow the label directions and pay close attention to the pre-harvest interval.