Alfalfa weevil proving to be a worthy adversary in 2012

Phil Kaatz<kaatz@msu.edu>, Michigan State University Extension - June 07, 2012

Second cutting alfalfa regrowth is at risk of weevil attack in much of Michigan.

Reports are filtering in concerning alfalfa weevil (Hypera postica Gyllenahl) feeding on new regrowth for second cutting following first cutting harvest. Growers are encouraged to carefully examine the new tender growth for signs of alfalfa weevil feeding.

Alfalfa weevil damage on second cutting regrowth.

Alfalfa weevil cocoon.

Both adults and larvae feed on alfalfa foliage, but the larvae cause the majority of the damage.

Alfalfa weevil adults and larvae have the potential to prevent or slow regrowth by feeding on the new shoots; adults also feed on sides of the stems (bark feeding). Control may be warranted after a cutting when larvae and adults are feeding on more than 50 percent of the crowns and regrowth is prevented for three to six days.

Michigan State University’s Enviro-weather website has an alfalfa weevil development model to provide producers a tool for predicting possible insect damage. This model predicts when damage from alfalfa weevil is expected based on degree-day accumulations calculated from data recorded at Enviro-weather stations.

Growing degree-day (GDD) accumulations (base 48 degrees Fahrenheit) begin January 1. Egg hatch is expected beginning at 300 GDD and small larvae (first and second instars) are expected to feed between 301 and 438 GDD, producing light leaf feeding.

Major leaf feeding occurs as larvae grow and is expected when third and fourth instars feed (between 439 and 595 GDD). Feeding stops when larvae pupate at 596 to 810 GDD. The table below highlights the current GDD for some areas of the state that are below the 810 GDD level.

Alfalfa weevil development model predictions

Location	GDD May 5	Predicted date for 810 GDD	Predicted GDD May 11
Ithaca	805	Wed., June 6
Sandusky	739	Sun., June 10
Pigeon	699	Mon., June 11
Munger	759	Sat., June 9
Hawks	575	---	690
Benzonia	678	---	783
Chatham	459	---	561

Scouting is recommended for all fields that have been cut to check for regrowth feeding activity. If small and large alfalfa weevil larvae are found, consider spraying for larvae control. If you are past the GDD for your area, continue to closely monitor growth due to adult weevil feeding.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).