No. 8 Article 6/May 19, 2006

Alfalfa Weevils and Natural Control Agents

Both Matt Montgomery, University of Illinois Extension educator in crop systems, and Duane Frederking, field sales agronomist with Pioneer Hi-Bred International, report that alfalfa weevil injury in Menard and Sangamon counties has become noticeable. Because the larvae are skeletonizing alfalfa leaves, injured fields appear gray when viewed from a distance. Because wet weather has delayed harvest of many fields, the injury could continue to worsen.


Alfalfa weevil injury to alfalfa plants (photo courtesy of Matt Montgomery, University of Illinois Extension).

Fortunately, some natural control agents are present in these alfalfa fields. Matt Montgomery found Bathyplectes anurus, a parasitic wasp, and Zoophthora phytonomi, a fungal organism, affecting alfalfa weevil populations in some fields. Although their effects on the alfalfa weevil population may be too little, too late, the presence of these natural control agents is worth noting, especially for alfalfa growers in northern counties. It is possible that both of these natural control agents could suppress alfalfa weevil populations in northern Illinois, keeping alfalfa weevil numbers below economic thresholds.

When scouting alfalfa fields for alfalfa weevils, look closely for signs of parasitism by B. anurus or B. curculionis (a closely related parasitoid) and infection by Z. phytonomi. The most apparent sign of the presence of B. anurus or B. cuculionis is the cocoons in which the parasitoid pupae are enclosed. The parasitoid cocoons often can be found inside the netlike alfalfa weevil cocoons. For more information about Bathyplectes species, check out the "Midwest Biological Control News".


Bathyplectes anurus cocoon inside an alfalfa weevil cocoon (photo courtesy of Matt Montgomery, University of Illinois Extension).


Bathyplectes cocoons (University of Illinois).


Alfalfa weevil larva killed by Zoophthora phytonomi (photo courtesy of Matt Montgomery, University of Illinois Extension).

Alfalfa weevil larvae infected with Z. phytonomi usually turn color from light green to yellow to brown as the larvae die. Alfalfa larvae killed by Z. phytonomi often look like small loaves of bread. The presence of a significant number of alfalfa weevil larvae killed by the fungus may indicate an epizootic that will wipe out the weevil population.--Kevin Steffey

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