Bathyplectes anurus and B. curculionis
Bathyplectes spp. are small, non-stinging wasps that are parasitoids of the alfalfa weevil, a serious pest of alfalfa in the Midwest and elsewhere. They were introduced to North America in 1911 from Italy by the U.S. Department of Agriculture as part of a biological control effort against the alfalfa weevil, also a non-native.
Adults of both species are similar in appearance, about 3 mm long with black, robust bodies. Cocoons are about 3.5 mm long, brown, and football-shaped with a white band around the middle.
The cocoons of the two species 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.
The alfalfa weevil, Hypera postica, and the Egyptian alfalfa weevil, Hypera brunnipennis
The two Bathyplectes species are similar in habits. 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 successfully develop in a host weevil.
To understand the life cycle of Bathyplectes spp., it is necessary first to understand the life cycle 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, which lays approximately 300 eggs, 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. B. curculionis females lay approximately 200 eggs per year.
Together with three other parasitoid wasp species introduced by USDA, the mymarid wasp, Patasson luna, which attacks weevil eggs; the eulophid wasp, Tetrastichus incertus, which attacks weevil larvae; the braconid wasp, Microctonus aethiopoides and M. colesi, which attack adults; and the fungal pathogen, Zoophthora phytonomi; Bathyplectes spp. keep alfalfa weevil populations in check most years in most locations. The egg predator, Peridesmia discus, has recently been recovered, years after the initial releases were made. In areas where 3 or more parasites are established, alfalfa weevil is seldom of economic importance, even in areas once requiring multiple applications of insecticide.
In comparing the two Bathyplectes, B. anurus is considered the superior biological control agent, especially in hot climates. This is due in part to B. anurus having a 50% greater reproductive potential than does B. curculionis. 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.
Avoid the use of insecticides when adult parasitoids are active. Also, the cultural practice of leaving uncut "refuge" areas of alfalfa to avoid killing parasitized weevil larvae can be effective.
Not available commercially, but in the Midwest they occur virtually everywhere the alfalfa weevil occurs.