Alfalfa weevil can quickly eat into profits in Wyoming hay fields

Riverton — “The alfalfa weevil is the number one pest in Wyoming for alfalfa,” said University of Wyoming assistant entomologist Scott Schell during the Fremont County Farm and Ranch days, held in Riverton in early February.
Schell recommends using an integrated pest management (IPM) system to control weevil populations, adding the goal isn’t to eradicate the species but to it bring down to an economical level.
According to the Environmental Protection Agency (EPA) an IPM is a pest management approach based on a four-tiered practice that includes setting action thresholds, monitoring and identifying pests, prevention and control. It is designed to be an effective and environmentally friendly approach based on pest life cycle information and a pest’s interaction with the environment. This information is used in combination with pest control methods to manage pests in the most economical way possible with the fewest hazards to people, property and environment.
The alfalfa weevil winters as an adult, then lay eggs in alfalfa stems the following spring. Upon hatching, larvae go through metamorphosis until they reach adulthood. Each phase of its life is known as an instar and the weevil goes through four instars during life.
Each female lays between 400 and 1,000 eggs. “You can have good control one year, and low female populations can still have a huge effect on the following year,” explained Schell.
Eggs are approximately one-fiftieth of an inch long when layed and there five to 50 eggs per hole in an alfalfa stem. Schell adds that humans can’t find eggs, but there have been some insect predators introduced that can find and go after them during that stage.        
Body length is one to two millimeters when larvae hatch. From larvae they mature into a pupa, then a young adult. As young adults they feed for a while then leave the field to estivate, which is the summer equivalent of hibernation.
Development and hatching are based on the number of days of a specific temperature or a degree-day. “The weevil is 48 degrees and alfalfa is 42 degrees. It is a smart adaptation of the weevil to wait for alfalfa to be up and growing before they hatch,” commented Schell.
Degree-days vary between years but according to Schell the most important degree-day for scouting is number 425. “At this point you should scout your field to determine if you need to treat or not.”
“Lots of people use the growth of alfalfa to determine when to scout. Degree-days will tell you when eggs are hatching, but either method is effective,” added Schell.
Scouting while weevils are still immature limits crop loss and provides additional information when making management decisions. Weevils in their first and second instars will be located in the buds and their damage won’t be visible unless buds are pealed open. After weevils enter the third isntar, skeletonized leaves and other visible crop damage becomes apparent.
Schell recommends the “bucket method” over net sampling. “Everyone has a bucket and it works just as well. Net sampling is more difficult and you have to wait until they’re older.”
To utilize the bucket method producers should collect 10 stems at five random locations within a field and put them in a bucket. Schell suggests hitting fastest growing areas such as south facing slopes first.
“You’re essentially looking for something pretty small and a hand lens may be necessary. Take apart the buds you collected and look for larvae in the earlier stages. Count all the stems that show tip feeding damage and divide that number by 50 (the number of stems originally collected) to determine the percentage of tip feeding. The standard is that if you have 40 percent damage or higher you want to take some management action, be it a chemical treatment or preparation for early harvest,” explained Schell.
If a crop is within seven to 10 days of harvest the cost of treatment probably wont be worth it, but early harvest is still an option. Schell noted that producers should always figure control costs to ensure the added benefits of control are greater than the added costs.
Producers can expect around a one-ton per acre loss when six weevil larvae are present per stem. Health and vigor of the plant can be compromised and nutritional content of hay will also be affected since the weevils eat the highest quality part of the alfalfa leaf.
According to Schell an appropriate combination of management techniques can help, but working with neighbors is also important. “If you can get a really good kill you might get a multi-year benefit if you’re fairly isolated. But, if you have weevils moving in from other fields you will probably have to treat every year.”
“In Wyoming early harvest is probably the best strategy for control, and it can also be used for blister beetle control, which horse hay buyers will like. You can address it and say, ‘I know what they look like and am aware of them as a I harvest hay.’ We don’t sell as much horse hay as some places, but it is a highly lucrative market.”
Knowing what to look for and how to determine potential loss provides producers with the more information when making management decisions regarding the alfalfa weevil and other crop pests. Increased pest awareness can also reduce crop loss and increase hay marketability.
Heather Hamilton is editor of the Wyoming Livestock Roundup and can be reached at heather@wylr.net