Alfalfa Weevil Is Starting To Hatch

By John Tooker

Alfalfa weevil is just becoming active in the southern tier of counties. On the map below, which is from the PA-PIPE* system, the earliest portion of the population is becoming active; the first instar larvae should be just starting to feed in southwestern and south central counties, while weevils in the rest of the state should still be in the egg stage (Fig. 1). Young alfalfa weevil larvae cause pin hole-sized damage to leaves near the tips of plants often on unfolded leaves. Older larvae consume leaves that are more open, typically leaving jagged edges. Much of this early season feeding does not result in economic loss, but it is good to recognize the damage and be aware of which fields have greater activity.

Those fields with larger populations will of course be where economically significant populations are more likely to develop. Economic thresholds for alfalfa weevil are determined from the size of plants, the value of the hay, the cost of insecticidal treatment, and the number of larvae per 30 stems of alfalfa. To sample weevil larvae, systematically select 30 stems from across a field and shake them into a bucket. If the number of larvae exceeds the threshold, a treatment might be warranted. See our fact sheet for thresholds and more details on alfalfa weevil biology.Many compounds are available for controlling alfalfa weevil. See the Agronomy Guide for details.

[*PA-PIPE stands for “Pennsylvania Pest Information Platform for Extension and Education,” which is a web-based set of maps that tracking temperatures and crop and pest development across the state. For more of thesemaps, click here, follow the link at the top of the page, choose “Public Map”, select the date (sometimes yesterday seems to work better than today), and then the mapping options are on the right side of the page. Below the temperature options, you will some weed options; these show expected germination of a variety of weed species.]

Figure 1. PA-PIPE map from 14 April 2015 showing expected activity for the earliest 5% of the alfalfa weevil population in Pennsylvania. According to the legend below the figure, the yellowish color (that covers most of the state) shows where we expect weevils to still be in the egg stage. Tan colors in the southern part of the state shows where we expect the first instar larvae to have begun hatching from eggs and started feeding.

Source:psu.edu