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Alfalfa weevils are showing up in fields and can cause serious defoliation. Now is the time to get out and scout!
How to Spot
The adult alfalfa weevil is a light brown snout beetle that overwinters either down in the crowns of the alfalfa plant or other vegetation, said Matt Montgomery, agronomy education lead at Beck’s. The larvae are small, greenish caterpillars with a black head that have a white stripe on its back.
Matt Montgomery, CCA
According to Iowa State University, there are look-alikes. The alfalfa weevil is commonly confused with the clover leaf weevil, which rarely causes economic injury to alfalfa. Clover leaf weevil can be differentiated by a brown head.
Adult alfalfa weevils become active and start laying eggs when temperatures exceed 48°F. “The adults lay eggs in the stems, which begin to hatch around 250 growing degree-days (GDD) base 48 from the time they are laid,” Montgomery said. “Early feeding appears as small pin-hole damage on the upper canopy of the plant.”
Matt Montgomery, CCA
For Iowa, GDD for alfalfa weevil can be tracked by visiting the Iowa State Pest Maps and Forecasting page and comparing benchmark GDD to the map. Additionally, forecasted GDD for the upcoming two weeks can be viewed by choosing the nearest weather station.
Scouting Methods
Start scouting once alfalfa has several inches of new growth, recommended Montgomery. He suggests walking in a U-shape pattern through the field to get a representative sample. “You have to move across the field because pressure isn’t always uniform,” he said. “You will collect 30 stem samples randomly from various areas of the field in a 5-gallon bucket, and evaluate stem feeding and number of larvae per stem.”
Using the bucket, vigorously shake the stems, explained Montgomery. Shaking will knock loose the larvae. Count the larvae in the bucket and divide by the number of stems collected. You will also need an estimate of alfalfa stand height. Montgomery stresses the importance of counting larvae rather than relying on damage alone. “You’ve got to get a sense of how many are actually there, not just how much chewing you see,” he said.
Ohio State University recommends comparing the number of larvae found per stem sampled and the overall height of the alfalfa stand to make a management decision.
| Alfalfa weevil thresholds based on alfalfa height, weevil feeding, and larvae density. | |||
|---|---|---|---|
| Stand Height (inches) | Tip Feeding (%) | Larvae per Stem | Recommended Action |
| 6 | 25 | 1 | Recheck in 1 week |
| 9 | 50 | >1 | Spray |
| 12 | 75 | >2 | Spray |
| 16 | 100 | >4 | Harvest early |
Management
Decisions to treat should be based on economic thresholds, considering larval counts and plant height. “We don’t want to just spray because we see them. We want to make sure we’re actually at a point where it’s going to impact yield,” Montgomery said.
For example, if alfalfa is at least 16 inches tall and significant weevil populations are present, early cutting is preferable to chemical treatments.This saves the cost of the insecticide application and can help preserve biological control organisms. “If it’s close to cutting, that’s often your best bet. When it’s mowed, you remove their food source, expose larvae to the elements, and that takes care of a lot of the problem naturally,” he noted.
If early harvest isn’t feasible and economic thresholds are exceeded, consider an insecticide application. Remember to note the preharvest interval (PHI) of the insecticides labeled for alfalfa. These PHIs range from 0 to 28 days,depending on the product and rate per acre.
Biological Control
Don’t discount the impact of natural enemies for management. For the most part, alfalfa weevil is controlled for free by natural enemies, which include specific larval and adult parasitoids, plus general predators such as ladybugs, said Montgomery.
One of the primary natural enemies of alfalfa weevil larvae is the fungal pathogen Zoophthora phytonomi, explained Montgomery. This fungus thrives in humid conditions and can quickly reduce weevil populations. “In high humidity and moisture, the fungus will infect the larva and can reduce their bodies to a brown mass of spores. Those spores will continue to infect more weevils,” he said.
Another key biological control agent is the parasitic wasp Bathyplectes, which lays its eggs inside developing weevil larvae. “As the weevils pupate, the wasp lays an egg in them and then they convert that body into a pupa from which new adult wasps emerge,” Montgomery said.
Follow-Up Scouting is Essential
Even after insecticide application or early harvest, some weevils may survive and continue to feed on regrowth. One of the biggest mistakes growers make is assuming that a single scouting session is enough. “You need to check again after treatment and you need to check after cutting,” Montgomery stressed. “Sometimes they’ll come back on the regrowth.”
According to Michigan State University, the threshold after cutting is 6 to 8 larvae per square foot of regrowth.
By combining effective scouting, biological control awareness, and timely management decisions, alfalfa growers can mitigate the impact of alfalfa weevils while preserving forage yields.
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