Scouting Advised for Alfalfa Weevil

Alfalfa weevil larvae
Figure 1. Adult and small- to medium-sized larvae of the alfalfa weevil. (Photo by Julie Peterson)

Scouting Advised for Alfalfa Weevil

Alfalfa weevil larvae have been reported feeding in alfalfa in southeast Nebraska. As temperatures warm up, expect to see alfalfa weevil larvae throughout southern Nebraska and slightly later, in northern Nebraska.

The larvae of alfalfa weevils feed on first cutting alfalfa as larvae, and adults (and sometimes larvae) feed on the regrowth after the first cutting.

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. Observations indicate the cause may be due to significant survival of both adult and larval weevils.

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. 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 (Figure 1). 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 1 to determine the appropriate action.

Economic Thresholds

Economic thresholds have been developed to aid decision making on alfalfa weevil control (Table 12). These thresholds were derived by North Dakota State University entomologists (Beauzay et al. 2013) from a two-year study conducted at the UNL Eastern Nebraska Research, Extension and Education 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.

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 2.1 1.6 1.2 1
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 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 2.9 2.3 1 1.6
12 6.7 4.4 3.2 2.5 2.1 1.7
Early bud (>20 inches) 7 4 2.7 2 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
50% bud or greater Cut early

*Pre-harvest interval/Pre-grazing interval
(Source: Integrated Pest Management of Alfalfa Weevil in North Dakota, E1676, Patrick B. Beauzay, et al, North Dakota State University 2013).

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.

Alfalfa root injury
Figure 2. Alfalfa root injury (highlighted by yellow arrows) caused by the clover root curculio. (Photo by Dan Keener)

Recent concerns of insecticide resistance have been noted for lambda-cyhalothrin (e.g. Warrior) as well as indoxacarb (e.g., Steward) in various field reports throughout western states. However, recent laboratory studies of field-collected alfalfa weevil populations for low desert regions of California and Arizona (Mostafa and Harrington 2020) have indicated no measurable resistance to populations in those locations. It is important to note that because of complications due to overwintering of multiple life-stages of this insect, control failure within a region could be the result of insecticide resistance or it could also be due overwinter survival of mixed life stages as discussed under “life stages” above.

Other Weevils of Note This Year

In early April, Jeff Bradshaw was contacted by a crop consultant that works in the Nebraska panhandle and eastern Wyoming. The grower was dealing with stand reduction and slow regrowth. Careful scouting revealed no alfalfa weevils or any noticeable injury from that insect species. However, once plants were dug up, they noted conspicuous, brown damage to the roots. This taproot scarring is very diagnostic to clover root weevil damage (Fig. 2) larvae. In addition, earlier instar larvae of this insect will also feed on root nodules in reduce nitrogen-fixation in alfalfa. The adults of this species feed on leaves; however, do not cause meaningful damage to the crop. For much more information and management recommendations for this insect, we suggest reading Rim et al. (2019).

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