Mexican Bean Beetle adult (left) and larva (right). Photo by R. Hammond.
Though less common than it once was, the Mexican bean beetle still maintains a presence in Ohio, and we have been getting a few reports of economic populations in soybean this month, largely in the east-central part of the state. The Mexican bean beetle adult is a small, copper-colored beetle with numerous black spots, while the larva (immature) is yellow with black spines. The adult beetle resembles a ladybeetle because they are members of the same insect family, Coccinellidae. Mexican bean beetles are one of the few members of this family that are plant pests.
Mexican bean beetle overwinters in the adult stage in wooded areas near fields, entering soybean fields as they emerge from their overwintering sites. This insect has two generations per year in Ohio and the larvae we are seeing in August are members of the second generation. Both adults and larvae will feed on soybean leaves and other legumes. Thus, when scouting, growers need to be aware of both stages of this pest. Going into September, fields most at risk will be late-planted or double-cropped soybeans that are still green when other fields are yellowing – not only Mexican bean beetles, but also late-season bean leaf beetles, grasshoppers, and Japanese beetles will be attracted to these still-green fields. Also, Mexican bean beetles may be attracted to alfalfa as soybeans senesce, and fall-planted alfalfa near recently infested soybean fields may be at risk.
The treatment threshold for all of these defoliators in soybeans in the R1-R6 stages is 20% overall defoliation. It is important to evaluate the field as a whole, because many of these pests are worse at the field edge which may give a skewed impression of the problem. At this time of year, it is particularly important to be aware of the pre-harvest interval (PHI) of any insecticide chosen. Some PHIs are several weeks and could interfere with harvest schedules if applied too late.
For more everything you might like to know about Mexican bean beetle, visit https://doi.org/10.1093/jipm/pmv023
Observations Noted by Clifton Martin Larvae and pupae of Mexican Bean Beetle (Epilachna varivestis Mulsant)
A visit to one field revealed larvae, pupae and adults all present and defoliation was occurring throughout. Second generation larvae were abundant and the population was large. This field was planted in early April and was a likely target for overwintering adults emerging from nearby woods and treelines. Indeed, it was possible to look at an early planting directly adjacent to a later planting and see a visible decrease in the health of the plants in the early field. A little scouting revealed an abundant amount of larvae on the pods low on the plant. Of further consideration, this field would likely be ready for harvest in the second half of September. Key information about this pest includes:
- Earlier planted fields will tend to receive the majority of the overwintered adults.
- Both the larvae and adult feed on soybean leaves.
- Severe defoliation generally occurs in late summer when late instar larvae of the 2nd generation reach peak activity.
- MBB feed between the veins on a leaf creating a skeletonized appearance
- Economic thresholds for spraying are 15% defoliation at pod fill and 25% at full pod to harvest
- Pod feeding is rarely a problem in soybeans, but it can happen. In the case of pod feeding, it is even more rare for the beetle to pierce all the way to a seed, but damage to the pod can create opportunity for disease to take hold.
- Several insecticides are labeled for control.
Foliar soybean defoliation caused by Mexican Bean Beetle (Epilachna varivestis Mulsant)
Mexican Bean Beetle larvae on soybean pods
Corn, Soybean, Wheat and Alfalfa Field Guide. Bulletin 827. The Ohio State University. Page 81.
Defoliators on Soybean, OSU Factsheet ENT-39, https://ohioline.osu.edu/factsheet/ENT-39