Scouting Advised for Alfalfa and Clover Leaf Weevils

Scouting Advised for Alfalfa and Clover Leaf Weevils April 20, 2017

Alfalfa weevils have been reported damaging alfalfa in north central Kansas and western Nebraska. As temperatures warm up, expect to see alfalfa weevil larvae in southern Nebraska and slightly later, in northern Nebraska.

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 (Table 1).

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. Damage consisted of adults preventing regrowth after first cutting.

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 comparison chart for more information distinguishing the clover leaf weevil and alfalfa weevil.)

Alfalfa weevils
Figure 1. Below threshold levels of adult and small- to medium-sized larvae of the alfalfa weevil were found in several fields near North Platte on April 19. Low levels of pea aphids and alfalfa caterpillars (Colias eurytheme) were also observed. (Photo by Julie Peterson)

Even with the pressure of planting row crops, it is essential that producers growing high quality alfalfa hay make time to monitor fields for weevils now and over the next few weeks. In the Panhandle and in the northern tier of counties there may be two flushes of weevil larvae this spring, leading to regrowth damage after the first cutting.

Table 1. Comparison of alfalfa weevil to clover leaf weevil

Alfalfa Weevil

Clover Leaf Weevil

(Photo courtesy Oregon State University Extension)

Overwinter primarily as adults
Adults are brown with a dark brown stripe halfway down the back, and 3/16 inch long
Larvae prefer to feed on tips 
Larvae remain on the plant most of the time
Larvae have black heads 
Adults leave fields in June

Overwinter primarily as larvae
Adults are dark brown, pitted light brown underneath, and over 1/4 inch long
Larvae feed anywhere on plant
Most larvae are in soil or debris during daytime hours
Larvae have brown heads
Adults may remain in fields

 

Alfalfa Weevils

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.

In some areas of Nebraska, alfalfa weevils are not following this seasonal pattern. In the Panhandle and in the northern tier of counties there may be two flushes of weevil larvae in the spring as seems to be the case this year. 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, it is essential that producers growing high quality alfalfa hay make time to monitor fields for weevils over the next few weeks.

Scouting

Alfalfa weevil 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.

Table 1. Economic thresholds for alfalfa weevil larvae.
Plant Growth Stage
(Height)
Treatment Cost
($/acre)
Crop Value ($/ton) Management
Recommendation
$50 $75 $100 $125 $150 $175
    Number of Alfalfa Weevil Larvae/Stem  
Midvegetative
(10-15 inches)
7 3.6 2.2 1.5 1.1 0.9 0.7 Use a long-residual
product
8 4.1 2.6 1.8 1.4 1.1 0.8
9 4.7 3.0 2.1 1.6 1.2 1.0
10 5.3 3.4 2.4 1.8 1.4 1.2
11 5.9 3.7 2.7 2.1 1.6 1.3
12 6.4 4.1 3.0 2.3 1.8 1.5
Late vegetative
(16 to 20 inches)
7 3.8 2.4 1.8 1.4 1.1 0.9 Use a short to
mid-PHI/PGI product
8 4.4 2.8 2.1 1.6 1.3 1.1
9 4.9 3.2 2.4 1.8 1.5 1.2
10 5.5 3.6 2.6 2.1 1.7 1.4
11 6.1 4.0 2.9 2.3 1.0 1.6
12 6.7 4.4 3.2 2.5 2.1 1.7
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
50% bud or greater   Cut early
(Source: Integrated Pest Management of Alfalfa Weevil in North Dakota, E1676, Patrick B. Beauzay, et al, North Dakota State University 2013).

Economic Thresholds

Economic thresholds have been developed to aid decision making on alfalfa weevil control (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 (e.g., lady beetles and parasitoid wasps) 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 (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") and products containing indoxacarb (e.g., Steward). 

Pyrethroid insecticides also provide aphid control but can have detrimental effects on beneficial insects. Indoxacarb products are more selective and do not affect most beneficial insects but will not provide aphid control.

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; view abstract.