INFECTION OF THE RED IMPORTED FIRE ANT BY BEAUVERIA-BASSIANA THROUGH VARIOUS ROUTES OF EXPOSURE

The entomophagous fungus, Beauveria bassiana, is a potential biological control agent of the red imported fire ant, Solenopsis invicta, but the best method of exposing the colony to the pathogen is unknown. Different application methods revealed that immersion of adult ants in a conidial suspension was more effective and practical than spraying. Although B. bassiana is capable of penetrating insect cuticle, there was little evidence that infection occurred by the penetration of the cuticle of most areas of the ant's body, with the exception of the tarsi and by oral routes. Scanning electron microscopy revealed the presence of conidia, many of which were germinating on the tarsi suggesting this as a possible site of penetration. Another route of infection was found through the use of conidia formulated in baits. Liquid baits containing conidia were more effective in causing mortality among workers, whereas solid baits containing conidia were more effective in causing mortality among brood. Through dissections, microscopic examinations, and the use of a culture method to reveal the presence of viable fungus, adult workers were found capable of filtering the conidia from liquid. While germinating conidia were found in the crops of large major workers that had ingested conidia-containing liquid baits, most workers filter out the conidia and place them in their buccal cavity, which appears to a source of fungal invasion. The contamination of soil with conidia did not cause the ants to abandon the soil as a nest media nor was increased ant mortality observed. Infected ants were removed by nest mates from the colony and placed on the refuse pile prior to possible fungal sporulation and where dry conditions further reduced sporulation. The removal of infected colony members prior to fungal sporulation and subsequent reduced sporulation may reduce colony reinfection. © 1992.