ALFALFA WEEVIL

A, Larva feeding on tip of alfalfa plant. B, Area at ground level showing a, eggs in living and dead stems; b. pupa in cocoon; c, adult weevil. (A and B, about three times natural size.) Upper left shows general feeding damage and d, larvae. Lower right shows adults, e, in lower portions of plant (all natural size.)

ALFALFA WEEVIL

The alfalfa weevil was brought to Utah from Europe about 1900 and has since spread into Arizona, California, Colorado, Idaho, Montana, Nebraska, Nevada. Oregon, South Dakota, and Wyoming. The larvae feed on the growing tips, leaves, and buds of alfalfa and may destroy most of the feed value of a hay crop or prevent the profitable production of seed. The weevil is essentially a pest of first-growth alfalfa. When the first growth is cut for hay, however, weevil larvae feed upon the basal shoots and retard the second growth for a few days to several weeks. This is especially serious in dry-land farming or second-crop seed production.

The insects winter chiefly as adults, mostly in the alfalfa fields. Soon after the snow melts, the females lay their first eggs in fragments of dead stems on the ground. After the spring growth of the alfalfa is about 6 inches high, the weevils gradually shift their egg laying to the growing plant stems. The number of eggs per female averages about 400, most of which are laid in April and May. Hatching generally begins in April, but larvae do not become numerous enough to cause economic crop damage until late May or early June, about the time the first growth of alfalfa produces buds. Meanwhile, almost all of the early larvae and many of the later ones have become parasitized by a tiny wasp, Bathyplectes curculionis, commonly called the weevil parasite. Starting about May 15, the weevil larvae complete their growth, drop to the ground, and spin lacelike cocoons, usually attaching them to fallen leaves. Parasitized larvae die after they spin their cocoons. Healthy larvae pupate inside their cocoons and change to adults in from 7 to 10 days. Weevil adults then leave their cocoons but remain sexually immature until fall or spring. Consequently there is only one generation of weevils each year.

Control: Alfalfa for hay maintain a dense, vigorously growing stand of alfalfa. Cut the first and second crops when most plants are in the bud stage. Mow the field clean and remove the hay as soon as it is cured. Do not irrigate the field for 7 to 10 days after cutting. Early spring treatment to kill the adults: Apply one-fourth pound of dieldrin or 1.5 to 2 pounds of chlordane per acre as a spray when the spring growth of alfalfa is 1 to 2 inches tall. This application will usually be made between March 15 and April 15, depending on the locality and the season. May or June treatment to kill the larvae: Dust or spray the crop as soon as plants become noticeably riddled but before many have turned gray, with 2 pounds of calcium arsenate, 1 to 2 pounds of methoxychlor, or one-fourth pound of parathion per acre. Do not cut hay treated with calcium arsenate or methoxychlor for 7 to 10 days after treatment. Leave parathion-treated hay uncut for at least 14 days.

Alfalfa for seed apply the dieldrin or chlordane treatment as for hay crops to kill adults in the early spring. When the plants reach the bud stage of development, treat with 2 pounds of DDT as a dust or 1.5 pounds as a spray per acre. This treatment is prescribed to control lygus bugs and several other pests of seed alfalfa as well as alfalfa weevil.

Do not feed alfalfa treated with DDT to dairy animals, animals being finished for slaughter, or poultry.