Parasitism of Alfalfa Weevil

Alfalfa weevil is a common pest that feeds as larvae and adults on the leaves of alfalfa. Both the greenish, legless, immatures with a yellow stripe on the back and the dark brown hard-bodied adults are attacked by several parasites, many of which can easily be reared from their hosts.

Collect adults and larvae in the spring from unsprayed alfalfa fields for rearing to identify any parasite species present in your location. Place the adult weevils in ice cream containers with some alfalfa stems. Cover the top of the container with a piece of cheesecloth held on with a rubberband to prevent weevil escape. Replace the stems every few days and frequently check the bottom of the containers for the white silken cocoons of the parasitoid. Place the weevil larvae in paper grocery bags, along with a handful of alfalfa stems from unsprayed fields. Fold the top of the bag over and paper clip it shut. Add additional stems every three or four days as needed. Check the condition of the weevils after 15 days. By this time there may be newly emerged light brown adult alfalfa weevils and parasitoid cocoons at the bottom of the bags.

Some common parasitoids in the Midwest include two species of Bathyplectes, a tiny ichnuemonid wasp that parasitizes the weevil larva and spins a brown, football-shaped cocoon, with a white band around the middle, outside the host pupa (the two species are easy to tell apart by their cocoons see MBCN Vol. I, No. 2); the eulophid wasp Tetrastichus incertus that attacks larvae and emerges from the mummified host body; and Microctonus aethiopoides, a braconid wasp that attacks and emerges from adult weevils. There is also a common fungal pathogen, Zoophthora phytonomi, that infects larval alfalfa weevil.

What percent of the larvae or adults you collected were parasitized? What types of parasites did you find? Did you notice whether any larvae you collected were diseased?

- Dave Hogg and AbdulAziz Mohamed, University of Wisconsin - Madison

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