EFFECT OF LONG-TERM CRYOGENIC STORAGE AND CONIDIAL SUSPENDING AGENTS ON THE VIRULENCE OF BEAUVERIA-BASSIANA TOWARD SOLENOPSIS-INVICTA

Storage in liquid nitrogen has become a widespread tool in collections of microbial germplasm. However, little information on the effects of cryogenic storage on the virulence of entomopathogenic fungal strains is available. In this work we compared the virulence of two subcultur es of a Beauveria bassiana strain, one of which was maintained for 4 y ears in liquid nitrogen and the other which was maintained under stand ard laboratory conditions, passed through susceptible hosts (harvester ants, Pogonomyrmex barbatus) and artificial media. Concurrently, we e valuated the effects on virulence of three methods used to obtain B. b assiana conidia in aqueous suspensions (centrifugation, sonication, or the detergent Tween 80). Adult workers of the red imported fire ant, Solenopsis invicta Buren (EIS menoptera: Formicidae), were used for bi oassays. We compared treatments consisting of combinations of maintena nce conditions and conidial suspension methods. Each treatment was tes ted at two conidial concentrations (5 x 10(7) and 5 x 10(7) conidia/ml ). Ants were shaken in conidial suspensions and incubated in moist cha mbers. Mortality among ants at Day 6 postinoculation was used for data analysis. Strain maintenance conditions (storage in liquid nitrogen o r maintenance under standard laboratory conditions) did not have an ef fect on mortality levels of S. invicta workers nor did conidial suspen sion treatments have a significant effect on mortality. Percentage mor tality was mainly dependent upon the conidial concentration used and r anged from 70.7 to 94.2% (mean values) for the high concentration and from 24.0 to 72.5% for the low concentration. Storage in liquid nitrog en (-196 degrees C) did not significantly modify the virulence of this B. bassiana strain, compared to that of the same strain maintained me tabolically active in a thermal window of 4-25 degrees C on live hosts and artificial media. (C) 1995 Academic Press, Inc.