Virulence of Seven Isolates of Beauveria bassiana and Metarhizium anisopliae to Coptotermes formosanus (Isoptera: Rhinotermitidae)

Abstract

Three strains of Metarhizium anisopliae (Metschnikoff) Sorokin and 4 strains of Bearweria bassiana (Balsamo) Vuillemin were evaluated for potential use as remedial control agents against Coptotermes formosanus Shiraki. The 7 isolates were screened for relative pathogenicity, and the median lethal time eliciting 50% mortality (LT50) was calculated. In general, the M. anisopliae strains were more virulent, with lower LT50 values, than were the B. bassiana strains. However, B. bassiana strain 787 also showed a high level of virulence. The LT50 values ranged from 0.13 to 4.5 d. Determination of the median lethal concentration required to achieve 50% mortality (LC50) with the remaining 6 isolates showed that B. bassiana strain 787 had the lowest LC50 value but was closely followed by all 3 M. anisopliae strains. The LC50 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 with dry conidia (6 × 105 to 2.4 × 107 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 results, B. bassiana strain 787 was thought to possess the highest potential as a remedial control agent for C. formosanus because of the moderately low LT50 value of 2.9 d and the low LC50 value of 33 conidia per termite in combination with its transmissibility and performance in nesting material.