May 12, 2006

Watch for alfalfa weevil and clover leaf weevil activity

Alfalfa weevil

Clover leaf weevil

Alfalfa weevil damage.

Based on growing degree days, alfalfa weevil larvae should be hatching from eggs statewide, and small pinholes may be visible in the new alfalfa growth near the tips of the plants. While alfalfa weevil damage has been spotty in much of Nebraska over the past few years, the potential for damage always exists. While row crop planting may be a priority for many producers at this time, if you are growing high quality alfalfa hay, take the time to monitor fields for weevils over the next few weeks.

Clover leaf weevils (CLW) are occasionally a problem; however, they are very vulnerable to fungus disease and so haven’t been pests since the late 80s to early 90s when spring rains were rare. Dry conditions the past several years may have helped them build their populations, but recent rains may have decreased populations. Clover leaf weevil larvae will be in the debris around the crowns during day. Scratching in the soil around the crowns and counting the number of larvae found per crown will help give a better idea of clover leaf weevil infestation. Their brown heads will help distinguish them from the black-headed alfalfa weevil. Table 1 compares the alfalfa weevil and the clover leaf weevil.

Both the alfalfa weevil and clover leaf weevil feed on first cutting alfalfa as larvae and regrowth of the first cutting as adults. While research conducted in northeast Nebraska has shown that clover leaf weevil larva feeding does not cause yield reduction to first cutting alfalfa, alfalfa weevil feeding can cause severe losses to yield and quality of the first cutting.

Table 1. Comparison of alfalfa weevil to clover leaf weevil. Alfalfa Weevils Clover Leaf Weevils Overwinter primarily as adults Overwinter primarily as larvae Adults brown with dark brown stripe halfway down back, 3/16 inch long Adults dark brown, pitted light brown underneath, over 1/4 inch long Larvae prefer to feed on tips Larvae feed anywhere on plant Larvae remain on plant most of the time Most larvae in soil or debris during daytime hours Larvae have black heads Larvae have brown heads Adults leave fields in June Adults may remain in fields

It is essential that fields be monitored for alfalfa weevil feeding now. Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips. The larvae are small (1/16 to 3/8 inch in length) and pale yellowish green, becoming a darker green when larger. These legless worms have black heads and a white stripe the length of the back. The alfalfa weevil larvae spend nearly all their time on the plant. They curl into a C-shape when disturbed.

Once the alfalfa is about 4-6 inches in height, take a bucket, carefully cut some stems at ground level (30 to 50 per field, from various spots in the field) and shake the stems against the side of the bucket. Count and average the number of weevil larvae per stem. Use the chart (Figure 1) to help decide whether to control alfalfa weevils. Each chart has been developed for a different alfalfa value.

To treat or re-sample depends on the average number of weevils per stem, the stem length and the value of the alfalfa. When alfalfa reaches a certain height, it may be more profitable to cut the alfalfa early rather than treat it. Insecticides registered to control alfalfa weevil larvae include Ambush, Baythroid, Cythion, Furadan, Guthion, Imidan, Lannate, Lorsban, Mustang Max, Penncap M, Proaxis, Pounce, Sevin, and Warrior.

Check the label or the UNL Entomology Web site at http://entomology.unl.edu/instabls/instabls.htm for use rates.

Keith Jarvi
Extension IPM
Northeast REC

Figure 1. Treatment thresholds for the alfalfa weevil at three alfalfa price levels.