Epilachna varivestis
While they are not a pest on every farm, some farms report significant damage from Mexican Bean Beetles (MBB) and must take action to prevent crop loss. They may be pests on snap beans, lima beans, and, more recently, soybeans. Biological control options exist which can reduce the need for insecticides.
Identification
Adults are coppery brown with black spots. They look very much like large lady beetles and in fact are closely related, but while lady beetles feed on other insects, MBB feed on foliage. Adults spend the winter in hedgerows and usually move into fields in June. Shortly after adults arrive in a bean field, they lay yellow-orange egg masses in clusters of 40 to 50 on the underside of leaves. These hatch into bright yellow, spiny, oval-shaped larvae which feed, molt several times as they grow, and pupate on the underside of leaves.
Life Cycle
Mexican bean beetles overwinter as adults, and seek out beans for feeding and reproduction. Overwintering adults colonize beans in June. There are 2-3 generations per season, usually increasing in numbers with each generation. The complete life cycle takes 30-40 days in the summer months, but closer to 60 days when temperatures are cooler.
Crop Injury
Both larvae and adults feed on beans, and feeding damage over 10-20% can cause yield loss. Feeding occurs on the underside of leaves, but causes death of leaf tissue in a lace-like pattern. Plants are most sensitive in the pod fill stage. Adults and larvae feed primarily on leaves, but when numbers are high, they will also damage pods.
Favored host crops are snap and lima beans. While soybeans are a less suitable host for development and survival, they may also be damaged by MBB.
There are 2-3 generations per season, usually increasing in numbers with each generation. Populations are usually less abundant on early plantings and may not build to damaging levels until August.
Monitoring & Thresholds
Search plants, especially the underside of leaves, for adults, eggs, larvae and feeding damage.
Cultural Controls
- Promptly destroy crop residues after harvest through mowing, disking, or flaming to reduce overwintering populations.
- Maintain wide, clean headlands and brushless wood edges.
- Avoid sequential plantings in close proximity.
- Use row covers to exclude beetles until harvest, or for as long as it is practical.
- Use reflective metallic and white plastic mulches to reduce beetle densities and feeding damage relative to black plastic or bare ground.
Management
Biological Control
If Mexican bean beetles have historically been a problem on your farm, you will very likely see them again this year. Using biological control can reduce the need for insecticides.
Pediobius foveolatus
Pediobius foveolatus is a commercially available biological control agent for MBB and has a good track record in the mid-Atlantic states and among New England growers who have tried it. Pediobius wasps are mass-reared and sold by the New Jersey Department of Agriculture. This small (1-3 mm), non-stinging, parasitic wasp lays its eggs in MBB larvae. Wasp larvae feed inside the MBB larva, kill it, and pupate inside it, forming a brownish case called a ‘mummy’. About 25 adult wasps emerge from one mummy. The parasitoids are shipped to farms as mummies or as adults. Adult wasps will emerge from mummies within 2-3 days of receipt.
Pediobius is well-suited to succession-planted snap bean crops. The first bean planting serves as a ‘nurse crop’ to establish the population of Pediobius that will be hard at work in successive plantings all summer. Control continues and in fact gets better as the season progresses and successive generations of the wasp emerge and search out new bean beetle larvae. Planning 2-3 releases at 7-10 day intervals will help ensure good timing and coverage on several plantings. After a release in the first planting, it is advisable to leave that planting standing for a while, until the new generation of wasps has emerged from their mummies.
As with any biological control, make releases as soon as the pest is present, not after it has built up to damaging numbers. The New Jersey Dept. of Agriculture Beneficial Insect Rearing Laboratory recommends two releases, two weeks in a row, coinciding with the beginning of Mexican bean beetle egg hatch. Wasps will lay their eggs in larvae of any size, but it is best to target the newly hatched young MBB larvae. This will give control before damage has been done. Thus, timing is important. Scout for egg clusters and time the shipment for when the first eggs hatch into larvae. If in doubt about the timing of the hatch, release as soon as you see the eggs—if you wait for the larvae, you may be playing catch-up. The release rate should be at least 2,000 adult wasps per field for less than an acre, or 3,000 per acre for fields of one acre or more. Mummies are frequently shipped in screen bags. Simply secure the bag to the underside of a bean plant; about 160 mummies/A, split between 2 releases for light infestations, 640 mummies/A, split between 2 releases for heavy infestations and for the home garden, a minimum of 10 - 15 mummies. Like beans, Pediobius wasps are killed by frost so annual releases are necessary.
Plan ahead by contacting the supplier to inform them of your acreage and expected release dates (based on what you’re seeing when scouting).
Contact information for New Jersey State Dept of Agriculture: Philip Alampi Beneficial Insect Rearing Lab, (609) 530-4192. You’ll also get advice on how to use the wasps from this office.
Chemical Control
Treatment with an insecticide may be warranted to prevent economic losses. A suggested treatment threshold is >20% defoliation during pre-bloom or 10% during pod formation. Several conventional and organic insecticides are labeled for use against MBB, including several products that also effectively control potato leafhopper, which is also a major pest of beans and which is active in bean fields now. Be sure to get coverage of lower leaf surfaces. Kaolin clay (Surround) may be used on seedlings and young plants to deter feeding and egg laying. For more information on chemical control options, see the New England Vegetable Management Guide.
Chemical controls for MBB and potato leafhopper could have harmful effects on Pediobius, especially on adult wasps. If releasing Pediobius, avoid sprays shortly before or after releases; apply any insecticides to a succession planting at least 5 days before release.