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Alfalfa Weevil


Bug of the Month

By Dr. Boyd Mori

Adult alfalfa weevil
Adult alfalfa weevil by Shelley Barkley

The alfalfa weevil, Hypera postica (Coleoptera: Curculionidae), is one of the most important insect pests of alfalfa in North America and is especially destructive in alfalfa seed production regions in Canada.

In western Canada, insecticides are the most common control strategy used to combat alfalfa weevil in seed production fields. However, recent control failures call in to question whether insecticide resistance occurs in southern Alberta populations.

Several small wasp species present in western Canada are natural enemies of alfalfa weevil, but their ability to control alfalfa weevil populations is limited. They are small black or brown wasps, Bathyplectes curculionis (Hymenoptera: Ichneumonidae), the dominant parasitoid species, Microctonus aethiopoides (Hymenoptera: Braconidae) and Oomyzus incertus (Hymenoptera: Eulophidae).

In areas where there are established parasitoid species Alfalfa weevil seldom reaches economic thresholds, but it’s hard to determine if the right parasitoids in sufficient numbers exist in an area.

As part of an on-going project investigating the seasonal activity and occurrence of insect pests and natural enemies in seed alfalfa, graduate student, Michelle Reid, supervised by Drs. Boyd Mori (University of Alberta) and Héctor Cárcamo (AAFC-Lethbridge), is examining alfalfa weevil populations for insecticide resistance in southern Alberta.

In 2018 and 2019, she sampled adults and larvae from alfalfa seed production fields with suspected insecticide resistance (IR) and from sites never treated with insecticide. After performing internationally recognized IR vial and leaf-dip tests with deltamethrin on adults and larvae, she found the average mortality of adults at the suspected IR sites was lower compared to the never treated sites. Larvae followed a similar trend. Further testing in 2019 with deltamethrin doses up to 100X the commercial rate killed less than 50% of alfalfa weevils from sites with suspected resistance. Further studies will determine if other populations of alfalfa weevil are resistant, and the mechanisms of resistance.

*Part of this text originally appeared in the Proceedings of the Canadian Forage Seed Conference