Alfalfa Weevil Larvae Infected with Fungal Pathogen

Published: May 5, 2010

The threat from alfalfa weevil larval infestations has substantially drop in central and northern Missouri during the past few days. Most fields statewide are approaching or have reached the growth stages where harvest of "first cutting" is possible. In addition, Ben Puttler (retired entomologist associated with the MU Division of Plant Sciences) has reported the presence of the fungal pathogen Zoophthora spp., in most central Missouri alfalfa fields. This fungus is present in fields each year, but is dependent on the occurrence of wet, warm conditions to be expressed and become an effective biological control agent of alfalfa weevil larvae. Once found in an alfalfa weevil population, a majority of larvae in a field will die within just a few days. When scouting for alfalfa weevil larvae in the field, the first indication that the fungus is present in the weevil population is a change in color of the larvae from their normal lime green color to more of a yellow color. Once yellow larvae are observed, the alfalfa weevil larval population will collapse within a few to about 7 days later depending on weather conditions. In the past, several researchers have attempted to develop commercial formulations of the fungus for use on alfalfa weevil larval infestations, but with little success. Because the fungus is almost always present in alfalfa fields, the application of additional fungal spores in alfalfa fields is of little value. Weather is the controlling factor which determines whether this fungus is expressed in and kills alfalfa weevil larvae annually. Recent wet and recent warm conditions have resulted in effective control of the moderate numbers of weevils found in northern and central Missouri fields this spring. However, in most years the fungus appears too late for early control of alfalfa weevil populations in southern counties of Missouri.