Alfalfa Weevil Larvae Infected with Fungal Pathogen

Published: May 19, 2009

A fungal pathogen has appeared in most alfalfa weevil populations in central and northern Missouri. The pathogen was found about a week ago in larvae in central Missouri and can now be found in most fields statewide. The fungus, Zoophthora spp., is present in fields each year, but is dependent on the occurrence of wet, warm conditions to be an effective biological control agent of alfalfa weevil larvae. Ben Puttler, retired entomologist from the Biological Control Laboratory in Columbia, monitors for the presence of this fungus in alfalfa weevil larvae annually by collecting larvae and determining the presence of the fungus under laboratory conditions. He did find the fungus in field collected larvae about 10-14 days ago at which time he predicted the collapse of alfalfa weevil populations statewide. 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 days to about 10 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 each year. Our 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.