Check fields for alfalfa weevils

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Editor's Note: The following article was written by Phillip Glogoza, University of Minnesota Extension.

A mild winter and early spring have sparked speculation on what may happen with different insects, such as the alfalfa weevil. This early-season pest can reduce hay yields.

Alfalfa weevils normally affect alfalfa around the timing of the first cutting of hay, although the actual timing varies from year to year as influenced by temperatures.

Daily temperatures are recognized as playing a significant role in how fast insects develop. Scientists have developed models for insects capable of causing large-scale economic losses. The models are referred to as degree day (DD) models. They use the daily average temperature and the experimentally determined lower developmental threshold temperature.

This knowledge is used to estimate the rate of physiological development and to forecast egg hatch and other biological events. They tend to be more accurate in predicting biological events than calendar dates, though not always.

An alfalfa weevil predictive model has been available for many years. University of Wisconsin Extension summarizes the model at www.soils.wisc.edu/uwex_agwx/thermal_models/alfalfa.

Alfalfa farmers who become familiar with the model will gain insight about when weevils, particularly the larvae, are most likely to be feeding. The goal of these models is to alert the crop manager to the need for timely field checks and to avoid missing an infestation.

Field observations from southern Minnesota indicate that the overwintering adult weevils began moving into alfalfa fields the week of April 15. The adults are there to feed and females to lay eggs in stems. The models would suggest that early larval hatch is underway and checking fields to determine population levels would be wise.

These early checks can provide an indication as to whether infestations at the field level are significant enough to warrant early cutting or insecticide treatments. Early cutting to avoid foliage loss is preferable when timing permits so beneficial insects that naturally suppress weevils are conserved.

As crop managers monitor alfalfa and weevil development, remember that warmer temperatures will accelerate development; cooler temperatures will slow it down. Moisture stress may also interact in the plant-insect relationship. If plants are not actively growing due to stress, the feeding injury can be significantly increased.

Using as many of the available tools as possible will help crop managers better understand the timing of biological events in the fields and lead to better management decisions.

More educational information on forage crops can be found at www.extension.umn.edu/forages.



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