Unprecedented Weevil Activity this Spring

For the second year in a row, alfalfa weevil is the most prominent spring pest for farmers. Unprecedented populations south of I-80 have been reported in 2024 and are proving to be much more severe and widespread than in previous years. We are predicting northern Iowa counties to also experience high densities in the next 7-10 days as temperatures warm up. There may be a few reasons for the alfalfa weevil surge:

1. Successful overwintering – The 2023-2024 winter was the warmest on record in Iowa, which likely led to greater than usual overwintering populations of many pests, including alfalfa weevil. If many individuals survived the winter, populations could be higher and result in more severe injury to alfalfa as fields resume growth.
2. Staggered life stages – In a normal year, alfalfa weevil larvae will feed for a few weeks in the spring, pupate, and adults will enter a summer dormancy period. Typically, those adults continue to remain dormant and overwinter, laying eggs the following spring. The main risk is to the first cutting, and subsequent cuttings are considered safe from alfalfa weevil. However, in recent years it has been noted that alfalfa weevil larvae are showing up earlier in the spring than we would expect. Entomologists suspect this is because adults will break dormancy during mild weather in the fall to lay eggs, which overwinter and hatch early in the spring. This leads to a prolonged feeding period of alfalfa weevil, which might extend past the first cutting.
3. Cool weather – Alfalfa weevil is a cool-weather insect, so recent cooler temperatures and cloud cover have been favorable for this pest. While other insects tend to slow down when it’s cold, alfalfa weevil prefers cool weather and will instead slow development when temperatures are too warm.

No matter what the reasons are for greater than usual populations of alfalfa weevil, farmers are experiencing severe defoliation from alfalfa weevil larvae that has required management in 2024 (Photo 1). This article aims to review scouting and management recommendations for alfalfa weevil.

Scouting

Perhaps the easiest way to spot alfalfa weevils before injury is apparent is by using a sweep net. Alfalfa weevil larvae and adults are readily dislodged from the plant, and a sweep net is an easy way to see what insects are present. However, a sweep net is not ideal for making management decisions. Stem counts are the most accurate way to understand whether populations warrant management. To do this, all you need is a bucket and a way to measure alfalfa. Stop in 5 areas of the field to obtain a representative sample across the entire field. At each location, break off six stems, and shake each plant into the bucket. Most of the larvae will be dislodged this way, but you may have to check the terminal leaves for smaller larvae. Count the total larvae collected from 30 stems and estimate the average plant height for the field. Use that information combined with estimated hay value and control costs for your area to determine if management is required using the threshold tables in this encyclopedia article.

Management Options

Early harvest: The most cost-effective management tool for alfalfa weevil is cutting the alfalfa. This is the preferred method once plants are at least 16 inches tall, but consider whether early harvest makes sense in terms of other goals, such as feed value. Harvesting removes the larvae’s food and shelter and exposes them to sunlight. If early harvest is chosen to manage alfalfa weevils, remember to scout alfalfa stubble for larvae or delayed regrowth.

Insecticides: If harvest is not an option, foliar insecticides are available to manage alfalfa weevil. Most options are in the pyrethroid class (Group 3A) of insecticides, but organophosphates (Group 1B), carbamates (Group 1A), and oxadiazines (Group 22) are available for use on alfalfa weevil. For organic producers, spinosyns are also labeled for alfalfa weevil (Group 5). University of Minnesota has a list of active ingredients and product names to reference for alfalfa weevil. Note that chlorpyrifos (Group 1B) is not on the list but is available to use in alfalfa for the 2024 growing season.

Some important considerations if insecticide applications will be made:

1. Pre-harvest intervals – Consult the label to determine the pre-harvest interval. This may determine which products can be applied to fit your harvest schedule. Pre-harvest intervals ensure hay is safe for consumption after harvest.
2. Insecticide resistance – Although resistance to insecticides has not yet been reported in the Midwest, alfalfa weevil has become resistant to pyrethroid insecticides in the western United States. This is certainly something we need to be aware of as resistance could develop at any time.
   • Do not assume an insecticide application was effective; visit the field 3-5 days after application to assess populations.
   • Even if alfalfa weevils are found after an insecticide application, it doesn’t necessarily mean they are resistant to the product. Timing, application rates, and coverage are all important aspects of an insecticide application, and poor timing, improper rates, and lack of coverage could impact insecticide efficacy.
   • If another insecticide application is required for alfalfa weevils or another pest during the same growing season (e.g., aphids, potato leafhoppers), use a different mode of action.
   • Consult the label to determine appropriate spray equipment and requirements such as minimum volume, and use the full labeled rate of the product.
   • If resistance is suspected, contact your local extension field agronomist or Erin and Ashley to let us know.

The development of insecticide resistance by alfalfa weevil would complicate management and underscores the need to use integrated pest management principles (i.e., scouting, prevention, cultural control, alternating modes of action, etc.) so we can preserve these tools moving forward. Insecticide resistance and regulatory action could reduce available chemical options moving forward, so stay vigilant when scouting and mix up management tools within and between growing seasons.