April 23, 2010

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.

Alfalfa Weevil Life Cycle and Damage

The life cycle of the alfalfa weevil consists of four stages: egg, larva, pupa and adult beetle or weevil. The insect usually overwinters as an adult in protected areas outside of fields, such as windbreaks, wooded areas or other protected habitats. Some eggs are laid in the fall, but most are deposited in the spring when weevils re-enter fields.

The female weevil chews a hole into the alfalfa stem and deposits a mass of 2-25 eggs. The average female weevil is capable of laying 500-2,000 eggs during her lifetime. The eggs vary in color from yellow to brown, becoming darker as they mature, and hatch 7-14 days after they are laid. (Fall-laid eggs will not hatch until spring.) Young larvae are yellow to light green and molt or shed their skins three times, growing at each molt. When the legless larvae are mature, they are green with a white stripe down the back, have a black head, and are approximately 3/8 inch long. After feeding 14-21 days, most mature larvae move to leaves near the base of the plant or to debris on the soil surface where they form loosely woven cocoons of silk and bits of plants. The larvae then transform to the pupal stage, from which the adults emerge in about 7-14 days.

The first indications of weevil injury are small holes eaten in leaves at the growing tip during April and May. This injury becomes more apparent as weevil larvae grow. Severely damaged fields have a white or gray appearance because of the drying of skeletonized upper leaves and buds. Damage is most severe to the first crop or cutting of the season and/or to the regrowth of the second crop. Larvae generally damage the first crop, while adult weevils damage regrowth by feeding on the developing crown buds, retarding growth and preventing fields from greening up after harvest. In some years larvae surviving the first cutting also have helped prevent fields from greening up. The severity of damage is influenced by alfalfa variety, stand health, weather, growing conditions and the degree of parasitism of the weevil.

Although spring laid eggs hatch at about 300 growing degree days (GDD; see Figure 1), recent research in Nebraska has determined that larvae are present much earlier, with larvae observed after fewer GDDs in the south than in the north. This could be because fall egg laying and survival is more prevalent than previously believed, differences in weevil strain biology, or both. In any case, we suggest alfalfa producers begin scouting their alfalfa fields for larvae at about 200 GDD

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.

	(Photo courtesy Oregon State University Extension)
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

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™, Hero, 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 website 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