Know Your Friends

Bathyplectes, Parasites of the Alfalfa Weevil

Bathyplectes are small, non-stinging wasps that are parasitoids of the alfalfa weevil, a serious pest of alfalfa in the Midwest and elsewhere. In contrast to generalist predators that are found in many agriculture habitats and feed on numerous prey species, Bathyplectes are very specific natural enemies that occur only in and around alfalfa fields and attack only the alfalfa weevil, Hypera postica. There actually are two species of Bathyplectes in the Midwest, B. curculionis and B. anurus, often referred to simply as Bathyplectes spp. The two species are similar in appearance and habits. Adults are about 3 mm (0.12") long with black, robust bodies. Both lay their eggs in alfalfa weevil larvae, preferring to oviposit in the early instars. The wasp larva that hatches from the egg feeds internally and slowly devours the weevil larva, ultimately killing its host after the weevil has finished spinning its cocoon. The parasitoid larva then emerges from the weevil and spins a cocoon of its own. Only one parasitoid can succesfully develop in a host weevil. Bathyplectes cocoons are about 3.5 mm (0.14 in.) long, brown, and football-shaped with a white band around the middle. It is actually the cocoons of the two species that are the most visible sign of the parasitoid and also are the handiest for separating the species: in B. anurus the white band is raised, and the cocoon has the unusual habit of "jumping" when disturbed, whereas in B. curculionis the white band is not raised and the cocoons do not jump.

To understand the seasonal history of Bathyplectes spp., it is necessary first to understand the seasonal history of the alfalfa weevil host. The alfalfa weevil typically has just one generation per year, with larvae present during the spring. The alfalfa weevil adults emerge from pupae during late spring to early summer, feed for several weeks, and then spend the remainder of the summer "aestivating" in a state of arrested activity and development (diapause). Aestivation is completed by late summer or fall, and the adults become active while the weather remains favorable, then hibernate during the winter and resume feeding and laying eggs the following spring.

The adult flight activity of both Bathyplectes spp. is synchronized with the spring activity period of the alfalfa weevil larvae. The flight lasts up to several weeks, and peak parasitism levels occur 1 to 2 weeks prior to the peak in numbers of weevil larvae. B. anurus has just one generation a year, with all parasitoid pupae produced by spring parasitism undergoing diapause, and not emerging as adults until the following spring when weevil larvae are again abundant. B. curculionis, on the other hand, has a partial second generation; many of the parasitoid pupae from spring parasitism are in diapause, but some develop and emerge as adults which then must find and parasitize weevil larvae during the summer.

In comparing the two Bathyplectes, B. anurus is considered the superior biological control agent. This is due in part to B. anurus having a 50% greater reproductive potential than does B. curculionis (300 vs. 200 eggs). Also, weevil larvae are able in many cases to kill B. curculionis eggs through a process known as encapsulation. Where the two species occur together, B. anurus has tended to become dominant.

Bathyplectes are not available commercially, but they occur virtually everywhere the alfalfa weevil occurs in the Midwest. The most important consideration for enhancing the effectiveness of Bathyplectes spp. is conservation, in particular avoiding the use of insecticides when the adult parasitoids are active. Also, the cultural practice of leaving uncut "refuge" areas of alfalfa to avoid killing parasitized weevil larvae can be effective.

Finally, Bathyplectes are not native to North America but were introduced from Europe by the U.S. Department of Agriculture as part of a biological control effort against the alfalfa weevil, itself an invader. In addition to the two Bathyplectes species, three other parasitoid species (all wasps) of the alfalfa weevil that were introduced by the USDA occur widely throughout the Midwest: the mymarid wasp Anaphes luna attacks weevil eggs, the eulophid wasp Tetrastichus incertus attacks larvae, and the braconid wasp Microctonus aethiopoides attacks adults. Also, a disease caused by the fungal pathogen Zoophthora phytonomi infects and kills weevil larvae. This complex of natural enemies keeps weevil populations in check most years in most locations.

- Dave Hogg, University of Wisconsin - Madison

Drawing by Michele Schwengel

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