Soil Antagonism Affecting the Dose—Response of Workers of the Red Imported Fire Ant, Solenopsis invicta, to Beauveria bassiana Conidia

Laboratory experiments conducted under four sets of conditions evaluated the effectiveness of Beauveria bassiana to infect and kill Solenopsis invicta workers. A LC50 of 1 × 106 conidia/ml was estimated for ants directly sprayed with conidial suspensions and subsequently transferred to cups with or without sterile or nonsterile nest soil. A LC50 of 1 × 103 conidia/mm2 was estimated for ants exposed to a plastic surface covered with fungal conidia. When conidia were mixed into nest soil, the LC50 was 1 × 102 conidia/g of sterile soil compared with 2 × 109 conidia/g of nonsterile soil. This suggests that antagonistic factors detrimental to fungal infection were present in the nonsterile soil. LT50s varied between 5.7 and 7.0 days for ants sprayed with 107 and 108 conidia/ml suspensions, respectively.