Insect development has been slowed this spring due to below average temperatures. As temperatures warm up, expect to see alfalfa weevil larvae in southern Nebraska and slightly later, in northern Nebraska. Although there have been no reports of alfalfa weevil in Nebraska, there have been reports of feeding in Kansas.

It is time to begin scouting alfalfa for weevils and the accompanying feeding damage. 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. This is why it's important to correctly identify the type of weevil feeding causing damage.

Clover Leaf Weevils

Clover leaf weevils are occasionally a problem, but are vulnerable to fungus disease. These pests haven't been a problem since the late 80s to early 90s when spring rains were rare. Dry conditions over the past several years in western Nebraska may have helped populations increase, although recent rains may have knocked down populations.

Scouting. To scout for clover leaf weevil, look 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. (See Table 1 for more information distinguishing the clover leaf weevil and alfalfa weevil.)

(Photo courtesy Oregon State University Extension)

Figure 1 (left). Alfalfa weevil. Figure 2. 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

Alfalfa Weevil Life Cycle

Most alfalfa weevils overwinter as adults, become active as temperatures increase, and lay eggs. Some may lay eggs in the stem during fall and, if winter is not too severe, will successfully overwinter. These eggs will hatch earlier than those laid in spring. This is most likely to occur in southern counties.

The seasonal occurrence of alfalfa weevils does not fit this general pattern in some areas in Nebraska. In the Panhandle and in the northern tier of counties there may be two flushes of weevil larvae in the spring. In the last few years some areas of the state have received damage to regrowth after the first cutting due to a combination of late larval feeding and adult feeding. This is something to be aware of after the first cutting.

While alfalfa weevil damage has been spotty in much of Nebraska over the past few years, the potential for damage always exists. Even with the pressure of planting row crops, producers growing high quality alfalfa hay should make time to monitor fields for weevils over the next few weeks.

Scouting for Alfalfa Weevils

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 long) 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 high enough to use a sweep net, take a sample to establish whether weevils are present. If they are, randomly select at least five sampling sites from across the entire field. At each site, gently pick or cut at least 10 alfalfa stems at ground level. Shake the larvae off the stems by beating the stems into a deep-sided bucket. Count the larvae and determine the average number of larvae per stem. Make sure to check for small larvae that may be enclosed in new, folded leaflets at the tips of the stems. Measure stem lengths and determine the average stem height. Use these averages in Table 2 to determine the appropriate action.

Alfalfa Weevil Economic Thresholds

Economic thresholds have been developed to aid decision making on alfalfa weevil control. Since we haven't had any reports of alfalfa weevil in Nebraska, and alfalfa first cutting is approaching, we present thresholds and recommendations for 50% bud and later in Table 2. These thresholds were derived by North Dakota State University entomologists (Beauzay et al. 2013; http://www.ag.ndsu.edu/pubs/plantsci/pests/e1676.pdf ) from a two-year study conducted at the UNL Agricultural Research and Development Center near Mead in 1990 and 1991 (Peterson et al. 1993). These guidelines can fluctuate depending on growing conditions and variety.

Deciding whether to treat or re-sample depends on the average number of weevils per stem, the stem length, treatment costs, and the value of the alfalfa. When alfalfa reaches 50% or more bud stage, it may be more profitable to cut the alfalfa early than treat it.

Insecticides

Because alfalfa weevil natural enemies have the potential to keep weevils from reaching economic injury levels, use insecticides only when necessary.

Many insecticides are registered to control alfalfa weevil larvae. See the most recent edition of the Guide for Weed Management in Nebraska with Insecticide and Fungicide Information (UNL EC130) for rates and restrictions of commonly used insecticides for alfalfa weevil larval control. They differ in their modes of action and pre-harvest intervals. Highly effective insecticides for alfalfa weevil control include those that are pyrethroids (active ingredient ends in "thrin").

Pyrethroid insecticides also can have detrimental effects on any beneficial insects present.

Resources

Integrated Pest Management of Alfalfa Weevil in North Dakota (E1676), by Patrick B. Beauzay, Janet J. Knodel, G.A.S.M. Ganehiarachchi, 2013. NSDU Extension Service, Fargo, ND.

Yield responses of alfalfa to simulated alfalfa weevil injury and development of economic injury levels by R.K.D. Peterson, S.D. Danielson and L.G. Higley, 1993. Agronomy Journal 85: 595-601.

Thomas Hunt, Extension Entomologist
Bob Wright, Extension Entomologist
Keith Jarvi, Extension Educator in Dakota, Dixon, and Thurston Counties
Michael Rethwisch, Extension Educator in Butler County

Table 2. Economic thresholds for alfalfa weevil larvae in 50% bud stage or later alfalfa prior to the first cutting (adapted from Beauzay et al. 2013)
Plant Growth Stage (Height)	Treatment Cost ($/acre)	Crop Value ($/ton)						Management Recommendation
		$50	$75	$100	$125	$150	$175
		Number of Alfalfa Weevil Larvae/Stem
50% bud or greater								Cut early
Early bud (>20 inches)	7	4.0	2.7	2.0	1.6	1.3	1.2	Cut early, or use a short PHI/PGI product
	8	4.6	3.1	2.3	1.8	1.5	1.3
	9	5.2	3.5	2.6	2.1	1.7	1.5
	10	5.8	3.8	2.8	2.3	1.9	1.6
	11	6.3	4.2	3.2	2.5	2.1	1.8
	12	6.9	4.6	3.5	2.8	2.3	2.0