Screening of tropical isolates of Metarhizium anisopliae (Hypocreales: Clavicipitaceae) for virulence to the sweet potato weevil, Cylas formicarius (Coleoptera: Brentidae)

The sweet potato weevil (SPW), Cylas formicarius, is a serious pest of sweet potato in Australia and Papua New Guinea. Ten strains of Metarhizium sp. isolated from Australian soil samples were evaluated for their growth characteristics and screened for virulence to adult SPW under laboratory conditions. All isolates except QD62 (48.6%) had moderate to high germination (66–97%), and all took 2 to 4 days to sporulate at 25 °C. The optimal temperature for radial growth for the majority of isolates was 30 °C, and there was a significant interaction between isolate and temperature (P < 0.05). Isolate QS155 showed the fastest radial growth at 30°C. The internal transcribed spacer sequences showed slight variations among the isolates; however, all isolates were shown to be Metarhizium anisopliae. Isolates varied greatly in their virulence. At 10 days after inoculation (DAI) by immersion in a suspension of 1 × 107 conidia/ml, 9 of the 10 isolates were virulent, causing 80-100% mortality of adult SPW. Only two isolates (QS001-6 and QS155) caused more than 50% mortality at 5 DAI. In dose-mortality bioassays, isolate QS155 had the lowest 20-day LC50 and LC90 values; however, there were no statistically significant differences in mortality among the three most promising isolates tested (QD66, QS001-6 and QS155). These results show that M. anisopliae isolate QS155 has potential as a microbial control agent for SPW, and that further evaluation under glasshouse and field conditions is warranted.