One never knows what a new year will bring in terms of pest problems in field and forage crops. We have discussed the differences between alfalfa weevils and clover leaf weevils frequently in issues of the Bulletin but usually only in the context of being able to distinguish the two species. We have stated that clover leaf weevils rarely cause damage to alfalfa because at least two fungal organisms usually keep their numbers below economic levels in Illinois. However, we have always been concerned that producers might mistake clover leaf weevils for alfalfa weevils and apply insecticides unnecessarily.|
Well, the drought conditions we have experienced apparently have changed things. Matt Montgomery, Extension unit assistant in Sangamon County, visited a couple of alfalfa fields on April 18 that were "pretty much destroyed by clover leaf weevil." He reported finding dozens of larvae in 6-square-inch areas; only stems remained in some of the infested areas. The alfalfa was only 6 to 8 inches tall, the growth having been retarded, probably due to dry weather.
Alfalfa field in Sangamon County damaged by clover leaf weevil larvae. (Digital photograph submitted by Matt Montgomery, Springfield, Illinois.)
So what can we make of this? It seems logical that the fungal organisms have not suppressed the population of clover leaf weevils during the prevailing dry conditions, at least not in the two fields Matt visited. Most fungal organisms thrive in moist environments. I recall reviewing a poster prepared by some entomologists in Nebraska a few years ago. They indicated that densities of clover leaf weevils in some areas of their state were quite high and caused severe damage to several alfalfa fields. The weather conditions that year were unusually dry.
As you continue to scout alfalfa fields, you should be able to distinguish an alfalfa weevil larva from a clover leaf weevil larva. A full-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 full-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.
Alfalfa weevil larva.
Clover leaf weevil larvae. (Digital photograph submitted by Matt Montgomery, Springfield, Illinois.)
Clover leaf weevil larvae on the soil surface in an alfalfa field. (Digital photograph submitted by Matt Montgomery, Springfield, Illinois.)
Matt also observed some cocoons on the soil surface and some sickly- looking (yellow in color) larvae. The former suggests that clover leaf weevil larvae are completing their development, at least in south-central Illinois. The latter suggests that a fungal disease is beginning to exert its influence. As you scout alfalfa fields, be aware of all possible situations with regard to both clover leaf weevils and alfalfa weevils.
Should clover leaf weevils be controlled with insecticides? We always hope that natural enemies reduce densities of clover leaf weevils below economic levels. However, when the damage appears significant, it is logical to wonder whether insecticide application is necessary. The economic threshold presented in Pest Management of Alfalfa Insects in the Upper Midwest, published in 1999 by the Leopold Center at Iowa State University in Ames, is five or more healthy larvae per crown. If you determine that an insecticide is necessary, check the labels of insecticides registered for control of alfalfa weevils; many of them also are registered for control of clover leaf weevils. Although we no longer list clover leaf weevils in our annual recommendations, a need to control them may arise during certain years, and 2000 may be one of those years.--Kevin Steffey