Using Pediobius Foveolatus as a Biological Control for Mexican Bean Beetles on Organic Vegetable Farms

EN022 (6/02)

By Kimberly A. Stoner
Department of Entomology
The Connecticut Agricultural Experiment Station
123 Huntington Street
P.O. Box 1106
New Haven, CT 06504-1106
Telephone: (203) 974-8480 Fax: (203) 974-8502
Email:  Kimberly.Stoner@ct.gov

Pediobius foveolatus has been used for biological control of Mexican bean beetles in soybeans on a county-wide scale for the last 20 years in Maryland and New Jersey. Pediobius foveolatus is a tiny wasp -- 3 mm. or about 1/9 of an inch in length -- that is a parasitoid of Mexican bean beetle larvae and squash beetle larvae. (A parasitoid is a parasite that kills its host.)

This wasp does not survive the winter in the U.S., and thus will be present only if you buy wasps from a laboratory colony and release them. An adult female wasp lays about 20 of her eggs per Mexican bean beetle larva (2nd to 4th molt). The wasp eggs hatch inside the Mexican bean beetle larva and feed on it, eventually killing it. About ten days after the eggs are laid, the body of the Mexican bean beetle larva becomes a "mummy," with the outer skin darkened, but intact, while the wasp larvae inside develop and form pupae. When the wasps emerge as adults (about seven days after formation of the mummy), they break a small hole in the skin of the mummy and climb out. They mate, and fly off to feed at flowers and find more hosts. The females gradually lose their egg-laying capacity after 3-4 weeks. They can travel several miles over one growing season.

Because Pediobius is expensive to rear and thus expensive to purchase, these instructions are for an "inoculative release." That means that the wasps are introduced early in the season in relatively low numbers, and expected to multiply over the season in the field. This strategy works best for farms with multiple, overlapping plantings of beans during the season. On an organic farm with beans growing continuously all summer, biological control with Pediobius allowed the farmer to harvest snap beans all summer until the first heavy frost for the first time in several years.

Cost and Sources (as of 2000):

• Maryland Dept. of Agriculture, Plant Protection Section, 50 Harry S. Truman Parkway, Annapolis, MD 21401. Phone: 410-841-5927. Cost $25 per 1,000 wasps plus shipping.
• New Jersey Dept. of Agriculture, Division of Plant Industry, Bureau of Biological Pest Control, P.O. Box 330, Trenton, NJ 08625-0330. Phone: 609-530-4192
• The Green Spot, Ltd. 93 Priest Rd., Nottingham, NH 03290. Phone: 608-942-8925. Cost: $29 for 25 mummies (500 wasps), $93 for 100 mummies (2,000 wasps), lower prices for large quantities plus $3.50 handling and an additional charge for shipping.
• ARBICO, P.O. Box 4247, Tucson, AZ 85638, Phone: 800-827-2847. Cost: $21.15 per unit of 120 wasps (lower prices for 6+ and 11+ units) plus $22.50 for shipping.

Sampling, Timing and Rates of Release:

Adult Mexican bean beetles start to appear in mid-June. Biological control will not work well if the average density of adults is greater than 1 adult per meter (about 1 yard) of row. To determine this, take 20 samples per week (1 yard of row each, scattered around bean plantings) in late June to early July. Count and keep records of the number of adults and egg masses you find. Whenever you find Mexican bean beetle egg masses (yellow eggs on the underside of leaves), mark the plant with a piece of rag or ribbon. Return every two days and look at the egg masses to see if the tiny yellow larvae have hatched out. As soon as you see hatching, it is time to order your first shipment of Pediobius wasps.

Rates: I would suggest 2 releases, one week apart, putting out 3 wasps per sq. m. At this rate, 1000 wasps (or 50 mummies, assuming an average of 20 wasps per mummy) would treat 3600 sq. ft. or 1200 ft. of row. Area is based on the beans that are large enough to be infested with Mexican bean beetles at the time of release. (Generally beyond the cotyledon stage). The first release should go out within a week after the first hatching is observed. If you are releasing mummies, they come in packets of screening that you tie onto bean plants. If you are releasing adult wasps, take the carton into the field, set it at the base of a bean plant, open the lid and watch the adults come out. Don’t release adults in the rain.

Because shipping costs are high (I would suggest using overnight service to avoid having the wasps overheated for long periods), it may be more economical to order enough wasps for two releases at once. The wasps will live for over 1 week in a cooler with ice packs changed regularly. (Refrigerators do not work. They keep the temperature too cold.)

If the weather is very wet during the release period, as it was in the summer of 2000, a third release may be needed. In wet weather, many of the parasitized larvae die before the new wasps can emerge.

Before turning under an old bean planting, give the wasps enough time to emerge from the mummies. An empty mummy (from which the wasps have emerged) should have one small hole, and it will be light and fragile compared to mummies with the wasps still inside. You can also clip leaves with full mummies and move them to new bean plantings.

What to Expect:

Don’t expect immediate control. The wasp population takes time to multiply in the field. The higher your initial density of Mexican bean beetles, the more damage the beetle larvae will do in the meantime while the wasp population is building up. You should see hard brown mummies on the plants from 2-4 weeks after making the first release. The biological control should reduce the numbers of bean beetle larvae making it to the pupal and adult stages in the first generation, and then should kill off the beetle larvae in the next generation before they get large and do heavy damage.

What to do if the initial Mexican bean beetle population is too high: There is no simple organic solution. Releasing Pediobius at higher rates in heavily infested fields doesn’t work rapidly enough to prevent heavy defoliation of the bean plants. Spraying once with rotenone to bring down the Mexican bean beetle density a few days before the first Pediobius release didn’t work well either. One strategy to keep the Mexican bean beetles from spreading from one planting to another is to till under the beans as soon as the larvae begin to pupate, so that no adult beetles emerge. (Often, in heavy infestations, the beans are so damaged at this point that they aren’t producing much anyway.) If the initial population is not excessively high, releasing Pediobius may help control bean beetle late in the season, and may bring down the Mexican bean beetle population in the next year.

Summary

Pediobius foveolatus has been used for biological control of Mexican bean beetles in soybeans on a county-wide scale for the last 20 years in Maryland and New Jersey. Pediobius foveolatus is a tiny wasp -- 3 mm. or about 1/9 of an inch in length -- that is a parasitoid of Mexican bean beetle larvae and squash beetle larvae. (A parasitoid is a parasite that kills its host.)