May 1, 2009

Figure 1. Accumulated growing degree days (GDD) from Jan. 1 to April 29, using a base of 48°F. These numbers can be used to help predict alfalfa weevil activity. Egg hatch begins at 300 degree days accumulated since January 1 and maximum damage will occur from about 450 to 600 GDDs. (Map developed by Al Dutcher, State Meteorologist, High Plains Regional Climate Center).   Alfalfa weevil Clover leaf weevil   Table 1. Comparison of alfalfa weevil to clover leaf weevil. Alfalfa Weevils Clover Leaf Weevils Overwinter primarily as adults Overwinter primarily as larvae Adults are brown with a dark brown stripe halfway down the back, and 3/16 inch long Adults are dark brown, pitted light brown underneath, and over 1/4 inch long Larvae prefer to feed on tips Larvae feed anywhere on plant Larvae remain on the plant most of the time Most larvae are 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

Based on growing degree days, alfalfa weevil larvae should be present in southern Nebraska (Figure 1), and small pinholes may be visible in new alfalfa growth near the tips of the plant (Figure 2). While alfalfa weevil damage has been spotty in much of Nebraska over the past few years, the potential for damage always exists.

Despite the temptation to concentrate on row crop activities now, if you’re growing high quality alfalfa hay, take time over the next few weeks to monitor fields for weevils. Alfalfa weevil feeding can cause severe yield and quality losses to first cutting alfalfa.

Figure 2. Alfalfa weevil damage

Alfalfa Weevil Life Cycle and Potential Damage

Most alfalfa weevils overwinter as adults and then become active and lay eggs; however, some overwinter as eggs and hatch early. This has resulted in some areas in Nebraska (Panhandle and the northern tier of counties) being hit with two flushes of weevils in spring. In recent years alfalfa weevils in some areas have damaged regrowth after the first cutting due to a combination of late larval feeding and adult feeding. Be alert to this potential for damage.

 Comparing Alfalfa and Clover Leaf Weevils

Clover leaf weevils are occasionally a problem but are very vulnerable to fungus disease and so haven't been pests since the late 80s to early 90s when spring rains were rare. Clover leaf weevil larvae can be found in the debris around the crowns during day. Scratching in the soil around the crowns and counting the number of larvae per crown will help provide an estimate of the 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 and clover leaf weevils feed on first cutting alfalfa as larvae, and regrowth after the first cutting as adults (and sometimes larvae). While research 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. 

Alfalfa Weevil Scouting 

It is essential that fields be monitored now for alfalfa weevil feeding. Damage consists of small holes and interveinal feeding on the newest leaflets near the stem tips. The larvae are a small (1/16 to 3/8 inch in length), 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. When disturbed, they curl into a "C" shape.

Once the alfalfa is about 4-6 inches tall, take a net sample to establish whether weevils are present. If they are, carefully cut some stems at ground level (30 to 50 from various spots in each field) and shake the stems against the side of a 5-gallon bucket. This will dislodge the weevil larvae and make it easy to count them and calculate an average number per stem.

Scouting Tip: One way to quickly scout a field for alfalfa weevils is to use a sweep net. A 15-inch diameter net is a useful tool for establishing the presence of insect pests and is essential for some, including the potato leafhopper in alfalfa, which cannot be scouted accurately by any other method. Field sweep nets are available through many agricultural suppliers, including Great Lakes IPM, Gemplers, Forestry Suppliers, Bioquip, and others. They cost $30 - $50. Replacement nets also are available.

Treatment Thresholds for Alfalfa Weevils

Charts (Figure 3) have been developed to aid control decisions. These charts are guidelines based on available research and can fluctuate depending on growing conditions and variety. Notice that these charts use lower prices than currently available for some areas of Nebraska. With prices varying considerably depending on quality and type of bale, start by using the $105 chart and adjust the levels according to the crop health, variety, and value, as well as when you expect to use or sell it. Each chart has been developed for a different alfalfa value.

Deciding whether 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 product labels or see the Insecticide Use Tables on the UNL Department of Entomology Web site for further information on products.

Keith Jarvi
Extension Educator, Dakota, Dixon, Thurston Counties
Tom Hunt
Extension Entomologist
Haskell Ag Lab, Concord
Robert Wright
Extension Entomologist, Lincoln