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WATCH OUT FOR THE ALFALFA WEEVIL

Dr. Burt D. Schaber, Entomologist

Last year, many alfalfa fields in southern Alberta were infested with alfalfa weevil larvae in late June, and, in some cases, regrowth of second-cut alfalfa was severely affected. In May and early June, the adult alfalfa weevils become active in alfalfa fields, and growers should become aware of how to detect their presence.

The adult alfalfa weevil that emerges in the spring is a dark gray snout beetle, about 4 to 5 mm in length, with a dark-brown stripe extending from the head to about 3/4 of the way down the back (sometimes not obvious on adults that have overwintered). Adult females lay eggs into alfalfa stems. The eggs hatch in 5 to 20 days depending on temperature. The newly hatched larvae migrate to the growing tip and feed amongst the tightly folded young leaves, but neither the damage nor the larvae are readily seen.

The greatest damage is caused by the older larvae feeding on the open leaves. Defoliation is most severe near the terminals, and, in extreme cases, the whole field has a grayish appearance. Alfalfa weevils can reduce hay yield by up to 50 per cent in severely infested fields, and the hay that is produced from these fields is of low quality.

When severe infestations are noted in first growth alfalfa, cutting early while the weevils are still in the larval stage usually reduces weevil populations sufficiently so that control is not necessary. Cutting the alfalfa and immediately removing it from the field, as in silage and deny operations, virtually eliminates the weevil population. With the usual cutting, windrowing, and baling operation, some larvae may survive by sheltering under the windrows, but usually not in sufficient numbers to damage the regrowth. However, if cool wet weather intervenes, many of the larval can survive under the windrows until regrowth begins and then damage to the second-growth often occurs.

If high populations of weevil larvae are present at the time of the first-cut alfalfa harvest, growers should watch very carefully for re-infestation of the regrowth. If the early second-cut alfalfa is infested with weevil larvae, control with a selective insecticide might be warranted.

Copyright 1993 Weekly Letter (June 2, 1993; Agriculture Canada Research Branch, Lethbridge, Alberta)