Up next: alfalfa weevils

Alfalfa weevil larvae now should be hatching in southern Iowa. Proper management requires timely scouting, correct identification, determining population levels, and if necessary, cultural or chemical control.

Alfalfa weevil are small; less than 5/16-inch long.

Why should fields be scouted for alfalfa weevils? Large populations of alfalfa weevil can be very destructive to first-cutting alfalfa. They remove leaf tissue, beginning with the new leaves in the top of the plant, then work down the stem to other leaves. This reduces forage quality and quantity.

When should scouting begin? Larvae begin hatching at approximately 200 degree days in fields south of I-80, and 250 degree days in fields north of this highway. The map (page 42) shows the accumulated degree days in the nine crop reporting districts and when hatching is expected to start in central and northern Iowa. Scouting can begin after the appropriate number of degree days has accumulated in your area.

Where should you start scouting? Begin on south-facing hillsides. Larvae will hatch here first because these areas warm up more quickly than north hillsides.

How do you scout for alfalfa weevil larvae? Save some time by using a sweep net to quickly and easily determine if larvae have hatched in your field. If larvae are found in the net, then collect 30 stems and look for larvae in the upper leaves. When collecting stems, do not break them too hard or you will knock off larvae still on the plant. The best way to collect all larvae is to grab the tip of the plant with one hand and break the base of the stem with the other hand, or cut it with a knife. Put stems inside a white, five gallon bucket and beat them against the side. Large larvae will dislodge and can be counted easily, but newly developing leaves must be pulled apart to accurately see small larvae hidden in the plant tip.

What do alfalfa weevil larvae look like? They have a very dark head, almost black, and are pale green with a white stripe down the back. They are about

1/16-inch long when they hatch and may be light yellow in color. After feeding for several days, they will turn green. They are 5/16-inch long when full grown.

Alfalfa weevil larvae.

Are there any other insects that look like alfalfa weevil larvae? Yes. Larvae of the clover leaf weevil look very similar, but are larger, have a light brown head, and often have the white stripe edged with pink. Clover leaf weevil larvae usually hide around the base of the plant during the day and feed mostly in lower leaves at night. They rarely cause economic yield losses and should not be counted as part of the alfalfa weevil sample.

Clover leaf weevil larva.

When should alfalfa weevils be controlled? If two or more larvae are found per stem, and 40 percent of the stems show any leaf feeding, the best option is to cut the hay within five days, if possible. This method of cultural control avoids the use of insecticides. If the crop is not mature enough to cut, then chemical control may be an option, depending on the economic thresholds.

Alfalfa weevil damage in unsprayed strips.

What are the economic thresholds for chemical control? New economic thresholds have been developed by University of Nebraska entomologists. These thresholds are for alfalfa at the early bud stage, when third- and fourth-stage larvae do 90 percent of the damage.

Economic thresholds for alfalfa weevil larvae in early bud stage alfalfa (average number of larvae per stem).

Forage value ($ per ton)
Control cost ($ per acre) 45 55 65 75 85 95 105 115
7 4.0 3.3 2.8 2.4 2.2 1.9 1.8 1.6
8 4.6 3.6 3.2 2.7 2.4 2.2 2.0 1.8
9 5.2 4.2 3.6 3.1 2.7 2.5 2.2 2.0
10 5.8 4.7 4.0 3.4 3.0 2.7 2.5 2.2
11 6.3 5.2 4.4 3.8 3.4 3.0 2.7 2.5
12 6.9 5.6 4.8 4.2 3.7 3.3 3.0 2.7
13 7.4 6.1 5.2 4.5 3.9 3.5 3.2 2.9

Source: University of Nebraska

To use the economic threshold chart, first determine the control costs in dollars per acre, then estimate the forage value in dollars per ton. Where these two values intersect in the chart is the average number of alfalfa weevil larvae per stem needed to justify chemical control. For example, if control cost is $10 per acre and forage value is $75 dollars per ton, then an average of 3.4 larvae per stem would be needed to justify chemical management.

What if the weevil count is below the economic threshold? Resample the field in 3 to 5 days. Chemical management may be needed then, or possibly the crop may have reached a stage where it can be cut.

What chemicals are labeled for alfalfa weevils? Several are shown in the chart below. Read and follow all label directions before using any insecticide.

Insecticides labeled for alfalfa weevil

Insecticide At low and high rates Harvest interval
Ambush 2E 6.4-12.8 ounces 0-14 days
Baythroid 2E 1.6-2.8 ounces 7 days
Furadan 4F 0.5-2 pints 7-28 days
Lannate LV 3 pints 0 days
Lorsban 4E 1-2 pints 14-21 days
Penncap-M 2-3 pints 15 days
Pounce 3.2EC 4-8 ounces 0-14 days
Sevin XLR+ 3 pints 7 days

This article originally appeared on pages 36-38 of the IC-478 (5) -- April 21, 1997 issue.

Updated 04/20/1997 - 1:00pm