Mexican Bean Beetle is Becoming Active in Southeastern Pennsylvania

By Sarah Frame and John Tooker

Mexican bean beetles are an increasingly more common insect pest of soybeans in the Southern and Southeastern regions of Pennsylvania. Feeding from adult beetles and larvae skeletonize leaves, which in some cases can lead to yield loss. Often, feeding from Mexican bean beetles alone will not cause yield-limiting damage; therefore, it is important to always scout soybeans for insect damage prior to making control decisions.

Description

Mexican bean beetles undergo complete metamorphosis: egg, larva, pupa, adult. Clusters of about 40-60 bright yellow eggs are laid on the underside of host plant leaves. Within about two weeks these eggs will hatch into bright yellow, soft-bodied larvae covered in black, branched spines. The larvae will grow through four larval stages until pupation. The immobile pupae are also bright yellow and are generally attached to the underside of a leaf. Once pupation is complete, the adult beetle will emerge. Adult beetles are slightly over 1/4-inch long and terrapin-shaped. They are yellow to coppery in color with 16 small black spots. Since they are closely related, adult Mexican bean beetles and their eggs can easily be mistaken for beneficial ladybeetle species.

Life History and Crop Damage

Native to Mexico, the Mexican bean beetle now occurs in most of the United States and parts of Southern Canada. In the United States, some of the most damaging populations are found in the Mid-Atlantic states.

Adult Mexican bean beetles overwinter in Pennsylvania in residue and protected areas. As of now, this insect is typically only found in the Southern and Southeastern regions of Pennsylvania. Adults become active in late spring to early summer and search for a suitable host plant, such as green beans, lima beans, and soybeans. Soon after, they will begin to lay eggs. It is unclear whether there are one or two generations per year in Pennsylvania, but Mexican bean beetles will lay eggs throughout the entire summer, with each female laying 500-1200 eggs. Often, all four life stages can be found in the soybean canopy at the same time.

While both the adult and larval stage will feed on soybeans, the larvae cause the majority of the feeding damage from this insect pest. Adults and larvae skeletonize soybean leaves by scraping the surface of the leaf as they feed, which can desiccate the leaf in severe cases. Early in the season, this insect primarily feed on leaves. As damage from this insect intensifies by mid- to late-summer, Mexican bean beetles may also feed on the outside of soybean pods. Because damage is typically greater later in the season, double-cropped soybeans can be at increased risk of economically significant damage from this insect.

Scouting and Management

Start scouting for Mexican bean beetle adults in soybean fields in mid-May to early June. When scouting, look for damaged leaves, but also look at the underside of leaves throughout the soybean canopy for eggs, larvae, and pupae. Larger larvae are going to cause the most damage to soybeans. If you are seeing clusters of eggs and small larvae throughout the field, this is a field that will need to be scouted again within two weeks to monitor feeding pressure.
Management thresholds to spray for this pest follow general soybean defoliation thresholds. These thresholds account for defoliation from all defoliating insect pests present in the soybean field at the same time, and are as follows:

• 30% defoliation from soybean emergence until bloom
• 15% defoliation from bloom until pods begin to fill
• 30% defoliation from late pod fill until maturity

Check out this publication to learn more about soybean defoliation thresholds:  IPM Program for Soybean in Pennsylvania - A Comprehensive Approach to Controlling Invertebrate Pests Weeds and Diseases.

When making management decisions, also consider weather conditions. In years with adequate moisture, soybeans can tolerate very noticeable defoliation without influencing yield. However, in drier years, feeding from this pest can promote faster desiccation of the plant.