• Alfalfa weevils have been active and feeding in southern Illinois and insecticide sprays were made in some areas where pressure warranted.
• In southern areas, alfalfa weevil eggs are laid in both fall and spring, and weevil activity starts early enough to seriously damage the first cutting of alfalfa.
• In the northern areas, the first crop harvest often takes place in time to reduce or eliminate the weevil threat.
• Delayed cutting due to wet conditions this spring can increase the potential for feeding damage by alfalfa weevil.

As of May 1st, insect degree days suggest that some weevil larvae may still be small in some areas while in others, larvae are in the late instar stages with the potential to cause major feeding damage (Table 1 below). Recent reports from southern Illinois have indicated that alfalfa weevils have been active and feeding on alfalfa stands. Although adult weevils will seek shelter late in the fall and overwinter, in southern Illinois and other areas of this same latitude, alfalfa weevils overwinter primarily in the egg stage.  Eggs are laid in stems of alfalfa in the fall and hatch in the spring when about 200 heat units (base 48°F) are accumulated.  This normally coincides with about 8 inches of new spring growth of alfalfa.  Areas farther north (north half of Illinois and most of Iowa & Wisconsin) rarely have to deal with alfalfa weevil damage to first cutting alfalfa because most of the weevil eggs are laid in the spring.  Feeding by weevil larvae normally doesn’t start in these northern areas until about 300 heat units (base 48°F) are accumulated. The first crop harvest in northern areas often takes place in time to reduce or eliminate the weevil threat.

Table 1. Accumulated degree days for alfalfa weevil in Illinois as of 5-1-19. (Source: Kelly Estes, IL Pest Survey Coordinator. Pest degree day calculator)

In southern Illinois, insecticide applications have been made in fields where pressure warranted. As a general threshold, insecticide treatments are advised when between 30 and 40% of the stems show obvious feeding and there is an average of 3 or more larvae per stem, if at least 7-10 days remain before the crop can be harvested.  Keep in mind that this is a “static threshold” that does not account for the value of the crop or the cost of treatment. Dynamic thresholds will consider all these factors and help guarantee that the insecticide application “pays off”.  See Table 2 below for an example of dynamic thresholds.

Table 2. Dynamic action thresholds for alfalfa weevil that considers the price of hay, plant height and cost of treatment. (Source: John Tooker, Department of Entomology, PSU)

Several insecticide options are available to manage alfalfa weevils. Pyrethroids (various), OP’s (Chlorpyrifos products including certain pre-mixes), Indoxacarb and a premix of a pyrethroid + diamide (Besiege) are labeled against alfalfa weevils. Be sure to consider the preharvest interval (PHI) when making insecticide selections as they can vary from 0-21 days depending on product and rate. Some reports from the 2019 season have indicated the need to re-spray. If cool and wet weather was prevalent during the time the application was made, that could have resulted in slower weevil activity and less “exposure” to the insecticide. Additionally, keep in mind that a portion of the eggs are laid in the spring, and these will hatch later than those laid in the fall. Sometimes seeing live larvae feeding in fields previously treated doesn’t necessarily mean that the insecticide did not work but that another flush of weevil larva has become active. Normally this later flush doesn’t threaten alfalfa, as the first cutting usually happens before the larvae start feeding. However, if the first cutting is delayed, the risk of feeding damage by these late flushes increases.

The use of degree day systems and scouting are key to properly time insecticide applications. For IL, the online pest degree day calculator can be found at: https://www.isws.illinois.edu/warm/pestdata/ For more information on management options or scouting in general contact your local FS crop specialist.