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Alfalfa Weevils and Biological Control Agents

May 11, 2001
By now we have reported the status of alfalfa weevils throughout the state for several weeks in the Bulletin. The weevils have done their damage to the first crop in southern Illinois, have caused significant damage to the first crop in some fields in central and western Illinois, and currently are present and causing injury in northern counties. Matt Montgomery, Extension unit educator in crop systems, Springfield, and Mike Roegge, Extension unit educator in crop systems, Quincy, have reported that damage in some fields in their areas was extensive and that insecticide applications have been ongoing for at least a couple of weeks. Russ Higgins, IPM educator, Matteson Extension Center, Will County, found as many as six larvae per stem and "major defoliation" in a field in Grundy County on May 6. All of these folks indicated that many growers were caught by surprise, as if the alfalfa weevil larvae appeared overnight. Obviously this underlines the importance of early and frequent scouting.

Figure 2 shows the accumulated degree-days (base 48°F) from January 1 through May 7, 2001. Figure 3 shows projected degree-day accumulations (base 48°F) from January 1 through May 21. Based on Figure 1 and field observations, alfalfa weevils are well into their business throughout northern Illinois, so we will cease printing the degree-day maps. As always, our hats are off to Bob Scott with the Illinois State Water Survey, who diligently produces these maps (and others) for us throughout the spring. Thanks, Bob.

As alfalfa producers in central and northern Illinois continue or begin to contend with alfalfa weevil larvae in the first crop, producers who have made the first cutting need to watch the regrowth carefully. Both fully grown larvae and adults feed on the buds of regrowing alfalfa and delay "greenup." Damage to regrowing alfalfa can significantly reduce dry-matter yield by stunting growth. To determine whether the regrowth will be affected economically, count alfalfa weevil larvae and adults from five randomly selected square-foot samples in a field. If the alfalfa stubble has been completely defoliated for 3 to 5 days, or you observe 50% defoliation on 30 to 50 stems samples, or you find four to eight larvae and/or adults per square foot, an insecticide to prevent further damage is warranted. Use the lower thresholds if alfalfa is drought stressed and control costs are low; use the higher thresholds if rainfall is abundant and control costs are high. Don't treat if 50% of the larvae are dying from disease. Insecticides suggested for control of alfalfa weevils and preharvest intervals for insecticides were presented in Bulletin issues no. 3 (April 13, 2001) and no. 5 (April 27, 2001), respectively.

Biological control agents have been active throughout southern and central Illinois, but unfortunately, they usually have made their presence known after the alfalfa weevils had caused significant damage. Nevertheless, watching for evidence of the fungal disease organism Zoophthora phytonomi and parasitic wasps, Bathyplectes species, is important if you are scouting for weevils. Matt Montgomery provided some pretty good photographs of an alfalfa weevil larva killed by Z. phytonomi and of Bathyplectes cocoons. Note that the dead alfalfa weevil larva is dark brown and dried up; the white line along the back of the larva is barely visible. A Bathyplectes cocoon typically is oval and brown with a distinct light-colored equatorial band, encasing the pupal stage of the parasitoid as it changes from a larva to an adult. The parasitoid larva finishes feeding within the alfalfa weevil larvae and emerges from the dead host to construct its cocoon inside the weevil's cocoon.

Alfalfa weevil larva killed by the fungal organism Zoophthora phytonomi.

Cocoons of the alfalfa weevil parasitoid, Bathyplectes species.

During his forays in alfalfa fields, Matt Montgomery also found some pea aphid mummies--dead aphids from which parasitoids had emerged. The globular, somewhat copper-colored aphid mummies are additional evidence that biological control agents are at work. And in the case of pea aphids, parasitoids, predators, and pathogens almost always combine to keep the numbers of aphids below economic levels. So natural biological control can and does work for some of our potential pests in Illinois.

Pea aphid mummy, evidence of parasitism.

Alfalfa weevils will be present in some stage of development in most of Illinois throughout May, so keep your eye on them. However, as temperatures increase, the adults eventually leave alfalfa fields seeking shelter from the heat. The adults won't return to the alfalfa fields until the fall to begin the cycle all over again.--Kevin Steffey

Author: Kevin Steffey

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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