Biology of the Alfalfa Weevil
in South Dakota

        (click on the thumbnails for  larger images)

Alfalfa weevils overwinter as adults under soil clods and plant residues in shelter belts,
on the field, and along ditches and fence lines. In the spring, these adults emerge from
their overwintering sites and start laying eggs in alfalfa stems.  Adults are about 3/16
inch long, brown in color, with a dark triangular marking on the back.

The female alfalfa weevil makes a hole on the stem then inserts its eggs.  Eggs
are bright yellow in color and could be seen with the naked eye if an infested stem is
split using a pocket knife or even with bare fingers.  Each hole may contain 1-25 tiny
eggs.  Each female weevil can lay between 500 to 2,000 eggs during its life span.

In SD, alfalfa weevil larvae can typically be seen on the field from late April to early June.
Thus, alfalfa weevils are mainly a pest of first cutting alfalfa in SD.  Hatching of the eggs
is not simultaneous hence various sizes of the larvae can be seen on the field as shown
in the picture above.  Larvae are voracious consumers of leaf tissues.  They do not
consume entire leaves but rather "skeletonize" them leaving the stems and leaf veins
intact.  Infested plants take a whitish or "frosted" appearance when viewed from a
distance.

From late May to early June, larvae stop feeding then drop to the soil to transform into
cocoons.  The cocoon look like tiny white balls made of woven silk and pieces of dried
alfalfa leaves.  After about 2 weeks, adult beetles emerge from the cocoons.  Adults
generally do not cause economic damage on alfalfa although they also can make holes
on the leaves.

Adults emerging from the cocoons will over winter then come back to the alfalfa field
in the spring of the following year.  It takes about 3 months for an alfalfa weevil to
complete its development  from an egg, larva, pupa (inside a cocoon), then adult.
Adult weevils are hardy and can survive harsh SD winters to start their life cycles
anew in the spring.  

   |  SD Alfalfa Weevil Home Page  |

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