NDSU Extension - Mercer County

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Alfalfa Weevils Emerging

Submitted by Craig Askim, Extension Agent, Agriculture and Natural Resources  

Based on the May 2016 degree day accumulations for alfalfa weevils in North Dakota, adult weevils are emerging. Adult alfalfa weevils have been observed at low densities in the Minot area by T.J. Prochaska (NCREC, NDSU). Adults are about ¼ inch long and brown-golden with a blunt snout and dark brown longitudinal stripe in the center of the back. Antennae are elbowed and clubbed. They overwinter in debris and alfalfa stubble. Eggs are laid inside the stems of alfalfa, and are small and cream colored. Mature larvae are about ⅜ inch long with a black head capsule and a green-wrinkled body. A white stripe, running lengthwise, can be observed across the top.

The degree days for alfalfa weevils use a base development temperature of 48°F and show that adults are active mainly in the southeastern area with 200 accumulated degree days (DD). Egg hatch begins at 300 accumulated DD. To assess the DD model, go to the NDSU’s NDAWN website and Applications – Insect DD. Then, click on the Map tab and select 48°F for your base temperature and Degree Days (DD) for your map type. Although adult weevils will feed on the foliage causing defoliation, most of the economic defoliation is caused by larval feeding. Peak feeding occurs about mid-June (or 504 - 595 accumulated DD) when larvae are in the 3rd to 4th instar. For alfalfa grown for hay, the most cost-efficient method is to cut the alfalfa early before feeding damage occurs. A treatment threshold of 40% tip feeding is recommended.

Weather plays a huge factor in how fast or slow the outbreaks will occur. Producers are encouraged to scout fields on a regular basis because alfalfa weevil outbreaks can occur in a very short period of time. Along with plant development, many chemicals are registered to be used to control alfalfa weevils. Alfalfa weevils must be active during the chemical application to get complete control and in many cases a follow-up application may be required.

Source: Janet Knodel NDSU Extension Entomologist

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