Penn State Extension- Crop Management: Alfalfa Weevil ...

as adapted from Iowa State Extension Research Reports

Editor Del Voight – Penn State Extension

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. Typically in Pennsylvania this accumulation occurs early in the month of April.

Where should you start scouting?

Begin on south-facing hillsides. Larvae will hatch here first because these areas warm up quicker than northern 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 the most 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. Place stems inside a white, 5-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 find very small, newly hatched 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 1é16 of an inch in length when they hatch and may be light yellow. After feeding for several days, they turn green. They are 5é16 inch in length when full grown.

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

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 5 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.

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. 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 (Table 1) 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 (Table 1).

 Table 1. Economic thresholds for alfalfa weevil larvae in early bud stage alfalfa (average number of larvae per stem). Current high prices of forages will impact this chart.  Entomologists will need to adjust to this current market pricing. For now here is a relative economic chart.

	Forage value ($ per ton)
	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

Alfalfa weevil damage in unsprayed strips.

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 chemicals that can be used for alfalfa weevil control are shown in Table 2. Read and follow all label directions before using any insecticide.

Table 2. Insecticides labeled for alfalfa weevil.

Insecticide	Rate per acre at low and high rates		Cost/Acre Jan 2008 Pricing. At low and high rate.
Ambush 2E	6.4-12.8 ounces	0-14	2.94-5.87
Baythroid XL	1.6 - 2.8 ounces	7	1.68-4.71
Furadan 4F	0.5-2 pints	7-28	4-16
Lannate LV	3 pints	0	20.55
Lorsban 4E	1-2 pints	14-21	3.34-6.68
Penncap-M	2-3 pints	15	?????
Pounce 3.2EC	4-8 ounces	0-14	1.84-3.67
Sevin XLR+	3 pints	7	9.56
Warrior 1T/Proaxis	2.56-3.84 ounces	7	3.78-4.34
Mustang Max	2.24 to 4.0 ounces	3	????
Cobalt	19-38 ounce	7-14	6.67-13.35