Alfalfa Weevil Serious Pest

Alfalfa is the second most important crop in Montana after small grains, and alfalfa weevil is the most serious pest of alfalfa in the High Plains region. The USDA Agricultural Research Service in Sidney has been monitoring weevil populations since 2009, and last year (2012), represents the first year in which economically damaging levels of weevils were observed in irrigated fields in the Yellowstone and Missouri river valleys, according to Tatyana Rand, a Research Entomologist with the USDA-ARS-Northern Plains Agricultural Research Laboratory in Sidney.

Many growers noticed this pest last Spring when their fields started to turn grey, and there was much speculation as to what it actually is, Rand said. In fact, the alfalfa weevil is a snout beetle in the family Curculionidae. The adults are brown with a very beetle like appearance, easily recognized by most. However, it is the larvae (grubs) which are actually damaging in alfalfa fields, Rand said. Larvae are about 1/20-3/8 inches long, depending on age, and later stages are bright green with a black head and a white stripe down the back.

“During severe infestations, like we saw in 2012, weevil larvae can substantially defoliate plants resulting in severe first-cutting losses if not controlled,” Rand noted. “They can also retard post cutting re-growth and decrease stand density over the longer term.”

MSU Extension has excellent information on how to monitor and manage this pest, according to Rand, which can be found on the Internet at:http://www.ipm.montana.edu/Training/PMT/2006/AlfalfaWeevil.pdf. Another just-released Extension resource from North Dakota State University will also be available on the Internet soon, she said. In the meantime, interested persons can contact Rand (406-433-9439 or [email protected]) for a copy of that publication entitled “Integrated Pest Management of Alfalfa Weevil in North Dakota.” While there are several insecticides available for managing high numbers of the pest, generally the most effective method is early cutting, Rand noted. Avoiding pesticide use is encouraged where possible as they also kill desirable insect species such as honeybees in addition to the alfalfa weevil.

Due to its widespread economic impact across the US, alfalfa weevil was the target of a nation-wide biological control program carried out by the USDA in the 1980’s. Five parasitoid wasp species that attack and kill weevils were released in the MonDak region, but almost nothing was known about whether they actually became established here and how important they are in keeping weevil numbers down, Rand noted.

“We are currently determining which natural enemies (predators and parasitoid wasps) are dominant in Eastern Montana alfalfa fields and what management approaches might be useful in conserving and promoting these beneficial species to maximize biological control of weevils,” Rand said. Initial results indicate that at least two parasitoid wasps are present in the region and rates of parasitism are generally high (averages in 2009 and 2010 were 58% and 29% respectively), suggesting that parasitoids play an important role in keeping weevil numbers down in most years.

However, the dominant parasitoid, Bathyplectes curculionis, has a hard time keeping up (it kills relatively fewer weevil larvae) when weevil numbers are high (Figure 2a). A second parasitoid, Oomyzus incertus, does better under these conditions, but is much rarer, so does not fully compensate for the loss of activity by B. curculionis in years of high weevil density (Figure 2b). Future work will focus on how to augment numbers of these parasitoids, particularly the rarer wasp species, as well as scouting for, and potentially re-introducing, other parasitoid wasps that are highly effective in controlling weevils in other parts of the US.