Virulence of Seven Isolates ofBeauveria bassianaandMetarhizium anisopliaetoCoptotermes formosanus(Isoptera: Rhinotermitidae)

Abstract

Three strains of Metarhi;:;ium anisopliae (Metschnikoff) Sorokin and 4 strains of Bearweria bassiana (Balsamo) Vuillemin were evaluated for potential use as remedial control ageuts against Coptotermes fonnosmws Shiraki. The 7 isolates were screened for relative pathogenicity, and the median lethal time eliciting 50% mortality (LT50) was calculated. [n general, the M. allisopliae strains were more virulent, with lower LTso values, than were the B. hassiana strains. However, B. hassilllla strain 787 also showed a high level of virulence. The LTso values ranged from 0.13 to 4.5 d. Determination of the median lethal concentration required to achieve 50% mOltality (LCso) with the remaining 6 isolates showed that B. bassiana strain 787 had the lowest LCso valne but was closely followed by all 3 M. allisopliae strains. The LCso values for 4 of the isolates ranged from 33 to 40 conidia per termite. The isolates were tested for transmissibility and survivorship under simulated nest conditions. Termite workers dusted \vith dry conidia (6 X 105 to 2.4 X 10 7 conidia per termite) were capable of transmitting the pathogen to other colony members. The isolates differed in their inoculum potential, rate of mortality, and response by other workers to fungus-killed cadavers. However, all isolates showed the ability to grow, sporulate, and produce mycosis under these artificial conditions, indicating the potential for causing an epizootic in treated nests. Based on these remIts, B. hassimw strain 787 was thought to possess the highest potential as a remedial control agent for C. fomwsanus because of the moderately low LTso value of 2.9 d and the low LC50 value of ,'33 conidia per termite in combination with its transmissibility and performance in l1C'stingmaterial.