History of the Alfalfa Weevil
in South Dakota

(click on the thumbnail for a larger image)

The alfalfa weevil (Hypera postica) was first detected in Utah in 1904.  It took
about 32 years for the weevil to reach western South Dakota via Fall River County.
An eastern strain of the weevil originating from the East Coast may have entered
Clay County in 1972.  The alfalfa weevil is currently present in all SD counties.
The county map above indicates the hypothesized entry points of the alfalfa weevil
into the state.

The eastern and western strains are identical in appearance but can be separated
by cross mating experiments and by examining their reproductive cells.  If cross
mated in the laboratory, the alfalfa weevil strains will not reproduce normally.  Although
2 alfalfa weevil strains are present in SD, their management will be identical for all
practical purposes.  However, strain determination was important during planned
releases of parasitic wasps in the 1980's.  Some parasitic wasps were efficient
only on one alfalfa weevil strain and not on the other.

Alfalfa weevils often reach economic numbers only in counties west and including
Bon Homme, Davison, and Brown counties.  For some unknown reasons,
alfalfa weevils are usually not a problem in eastern SD.  My best guess is that natural
enemies of the alfalfa weevil keep it from reaching economic numbers in the east.
Parasitic wasps and a bacterial disease are known to parasitize alfalfa weevils in SD.

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                                                                                  Prepared and posted  by Mike Catangui on May 8, 2000.