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 × 107conidia/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.

Despite considerable efforts in developing sweetpotato cultivare resistant to two species of sweetpotato weevils, Cylas formicarius and C. puncticollis, no cultivar has yet been bred, utilizing previously identified sources of resistance, and cultivated in any appreciable area for weevil control. Escape, nonpreference, tolerance, and antibiosis, and the possible mechanisms involved do not seem to exist in the sweetpotato plant or cannot be utilized because of the peculiarities of ecology and host plant relationship of the weevils. Since weevil infestation of roots does not adversely affect the growth and reproduction of sweetpotato plants, it is possible that weevil resistance to root infestation does not exist in this crop.