Why You Should Consider not Spraying for Alfalfa Weevil in Western Virginia

Crop and Soil Environmental News, December 1998

Tom P. Kuhar, Rod R. Youngman and Curt A. Laub
Department of Entomology, Virginia Polytechnic Institute & State University

The alfalfa weevil has been a major insect pest of alfalfa in Virginia since the 1950's. For decades, it was virtually impossible to grow alfalfa profitably without spraying insecticides to control the weevil. As a result, the practice of applying a springtime insecticide to alfalfa has become routine for many Virginia growers. Surveys have shown that 80-90% of alfalfa growers still apply an insecticide to control alfalfa weevil in Virginia. However, our research has revealed a number of reasons for not spraying to control this pest, particularly in the ridge and valley regions of western Virginia, where much of the alfalfa is grown in the state.

A few reasons to not spray:

1. Density makes the pest. It is important to realize that the mere presence of alfalfa weevils in alfalfa is not an indication that economic yield loss will occur. In fact, alfalfa weevil infestation levels often do not exceed the economic injury level in western Virginia. A survey of over 100 commercial alfalfa fields that we conducted in Virginia from 1996-98 revealed that only 26% of the fields located in the Shenandoah Valley and 15% of the fields located in the southwestern region of the state contained alfalfa weevil populations high enough to justify an insecticide spray. In fact, alfalfa weevil larvae were barely detectable in many of the surveyed fields.

2. Timing of the pest infestation should be considered. Colder winter climates in Virginia's ridge and valley areas typically result in reduced alfalfa weevil egg laying and slower egg development compared with warmer climates, such as in the Piedmont. Consequently, most of the infestations of alfalfa weevil larvae peak late in the spring in western Virginia (Table 1). This can affect pest management decision-making in two ways. First, because weevil larvae attack alfalfa later in the season, the crop has had more time to grow and produce more leaves, and thus is better able to withstand greater densities of alfalfa weevil without suffering yield loss. Secondly, because the crop has had sufficient time to grow, early harvest is often a better option than applying an insecticide.

3. Natural enemies can successfully control the alfalfa weevil. The USDA initiated a large-scale biological control effort in the late 1950's to help manage the alfalfa weevil. Numerous parasitic wasps (Figures 1 through 3), specific to the alfalfa weevil, were released into the northeastern United States including Virginia. After 20 years of releasing and redistributing the parasitoids throughout the country, populations of the alfalfa weevil were brought in check in many states. Today, only about 15% of alfalfa fields in the northeast are at risk to economic damage from this pest. In addition, there has been about a 75% drop in insecticide use for alfalfa weevil in these states. This natural control is usually stable from year to year, and its continuation often depends on the avoidance of insecticide use. Our research has shown that all of the key parasitoids of the alfalfa weevil are established in Virginia and that parasitization is high, particularly in fields not sprayed with insecticides. In addition to the parasitoids, a naturally occurring fungus, Zoophthora phytonomi (Figure 4), also can cause high mortality of alfalfa weevil larvae, particularly during periods of ample rainfall in the spring. This fungus was present in 83% of the alfalfa fields sampled in our survey.

4. Not spraying conserves other important natural enemies. Not all insects are pests. In fact, alfalfa is one of the best early-season reservoirs of beneficial insects, such as lady beetles, minute pirate bugs, big-eyed bugs, damsel bugs, ground beetles, spiders, and lacewings. These insects feed on other insects and keep the populations of many potential pests in check. Insecticide use should be avoided whenever possible to enhance the survival of these beneficial insects.

5. Avoiding unnecessary sprays saves you money. By spending just a few minutes scouting for alfalfa weevil, you will be able to quickly identify which fields are at risk to weevils and need to be treated, and which fields are not. Such timely information can only help save you money and increase your profits. The bucket-shake method, as described on page 123 of the 1998 Pest Management Guide for Field Crops (VCE Publication 456-016), is a proper method for scouting alfalfa weevil.

¹Data collected from 3 fields in Orange County in 1997 and 4 fields from Campbell County in 1998.
²Data collected from 3 fields in Augusta County in 1997 and 4 fields from Rockbridge County in 1998.
³Data collected from 3 fields in Montgomery County in 1997 and 1998.

Figure 1	Figure 2
Figure 3	Figure 4

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