Alfalfa Weevils Are Being Noticed

April 23, 1999
Highlights:

· Some insecticide applications have occurred in southwestern counties.

· Development of alfalfa weevils is on track in central counties.

· Determine the differences between alfalfa weevil and clover leaf weevil larvae.

· Watch for the effects of natural enemies on alfalfa weevils.

· New alfalfa insect management publication is available.

Although the recent cooler temperatures slowed things down a bit, alfalfa weevil activity has continued to increase slowly but surely throughout the southern half of Illinois. I received at least one report of some spraying activity in some fields in southwestern Illinois, although the need for insecticide application was not confirmed. However, Ron Krause, Southern Illinois University research station at Belleville in St. Clair County, reported that signs of alfalfa weevil damage were quite evident at their site.

Mike Roegge, Extension Unit Educator/Crop Systems at the Adams/Brown Extension Unit, received a sample of weevil larvae from southern Pike County during the latter part of the week of April 12. He reported that the larvae ranged in size from 1/8 to 3/8 inch long, and wondered whether some of the larvae might be clover leaf weevil larvae (more on that later). However, upon further examination, Mike determined that all of the insects were alfalfa weevils. Some of the larger larvae may have developed from fall-deposited eggs. Matt Montgomery, Extension Unit Assistant/Crop Systems at the Sangamon/Menard Extension Unit, found an average of one first-stage larva per plant and some plants with pinhole-feeding injury during a survey of fields from southern Sangamon County to northern Menard County on April 19 and 20. According to heat-unit accumulations (Figure 1), alfalfa weevil development in central Illinois is right on track.

Figure 2 shows the projected heat-unit accumulations (base 48°F) from January 1 through May 3, 1999. Remember that these projections derive from actual heat-unit accumulations to a given date (in this case, April 19) plus projected heat-unit accumulations, based on historical weather data, from that date to a future date (in this case, May 3). Deviations from average historical weather data will slow down or accelerate accumulation of heat units. The projections in Figure 2 suggest that alfalfa weevil activity will be in full swing throughout most of Illinois by May 3. Forecasts for warm temperatures for the next several days probably ensure this. A peak of third-stage larvae from spring-deposited eggs could occur in southwestern counties around May 3 (accumulation of 575 heat units).

Now, about the clover leaf weevil: Although this insect often is found in alfalfa and clover fields, it rarely causes economic damage in Illinois. However, it is similar in appearance to the alfalfa weevil, so misidentifica-tions occasionally occur. A comparison of these two species might help avoid confusion. A fully grown alfalfa weevil larva is 3/8 inch long and pale to bright green with a dark-brown head and a white stripe along the center of the back. A fully grown clover leaf weevil larva is longer (1/2 inch) and "fatter" than an alfalfa weevil larva. The head of a clover leaf weevil larva is light brown, and the white stripe along the back is bordered with
light-red or pink accents. During the day, alfalfa weevil larvae usually are found on the plants, and clover leaf weevil larvae are found on the ground among the debris. When you find weevil larvae in an alfalfa field, make sure you identify the species correctly.

As people begin scouting alfalfa fields in earnest, looking primarily for alfalfa weevils and symptoms of the injury they cause, everyone should be alert to the activity of natural enemies. During some years, two parasitic wasps, Bathyplectes curculionis and B. anurus suppress populations of alfalfa weevils, as does a fungal disease caused by Zoophthora phytonomi. The effects of these natural enemies often occur too late to prevent alfalfa weevils from causing economic damage, especially in southern Illinois. However, under certain conditions, the impact of natural enemies could be significant. The recent cool, wet weather we have experienced is ideal for development of this fungus. Zoophthora phytonomi is capable of causing epizootics in alfalfa weevil populations that cause weevil numbers to "crash" in a matter of 3 to 4 days. As you scout for alfalfa weevils, keep your senses tuned to the presence of diseased alfalfa weevil larvae, which usually first appear off color (slightly yellow) and then turn brown as they die. Dead weevil larvae often are observed curled around leaflets on the plants. If you observe these characteristic signs of disease infection, think twice before you make a decision to treat the field.

The Bathyplectes wasps also are important natural control agents in alfalfa cropping systems. Both parasitic wasps are probably too small (3 mm) to observe directly. However, the adults are present in the fields when alfalfa weevil larvae are active, and their peak densities probably occur 1 to 2 weeks before peak densities of alfalfa weevil larvae. A female wasp deposits an egg in a weevil larva; early instars of the weevil are preferred. The egg hatches and the parasitoid larva feeds on the weevil larva, killing it after the weevil completes its silken cocoon. The parasitoid then spins a hard, brown, football-shaped cocoon (about 3.5 mm long) that has a raised, white, equatorial band (see Figure 3). You may find the cocoons of the parasitoids inside weevil cocoons. The B. anurus larva has the unusual ability of causing its cocoon to "jump" several centimeters upward. This trait probably enables the parasitoid to avoid predation and hyperparsitism.

One final note about management of alfalfa weevils (and other alfalfa insects, for that matter): Several entomologists and agronomists from Oklahoma State University, Iowa State University, the University of Wisconsin, and the University of Minnesota cooperated to produce a very nice publication entitled Integrated Pest Management of Alfalfa Insects in the Upper Midwest. The publication is full of excellent photographs and good information, including a set of economic thresholds for alfalfa weevil based on numbers of larvae per stem, plant height, and value of the alfalfa (Table 1). The publication is available from the Leopold Center for Sustainable Agriculture, Iowa State University, 209 Curtiss Hall, Ames, Iowa 50011-1050, telephone (515)294-3711, e-mail leocenter@iastate.edu. Anybody serious about protecting alfalfa from insect injury should have a copy.--Kevin Steffey

Author: Kevin Steffey