Different susceptibility in the two sympatric sweet potato weevils, Cylas formicarius and Euscepes postfasciatus, to the entomopathogenic fungus Metarhizium anisopliae

Authors: Reddy, Gadi V.P.; SHIMIZU, SUSUMU - West Japan Junior College; ICHINOSE, KATSUYA - National Agricultural Research Center For Kyushu Okinawa Region

Submitted to: Agricultural Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/11/2023
Publication Date: 9/14/2023
Citation: Reddy, G.V., Shimizu, S., Ichinose, K. 2023. Different susceptibility in the two sympatric sweet potato weevils, Cylas formicarius and Euscepes postfasciatus, to the entomopathogenic fungus Metarhizium anisopliae. Agricultural Sciences. 14/12277-1291. https://doi.org/10.4236/as.2023.149086.
DOI: https://doi.org/10.4236/as.2023.149086

Interpretive Summary: Sweet potato weevils seriously damage sweet potatoes in tropical and subtropical regions. These weevils are controlled effectively by chemical insecticides, often with incorporation with cultural managements such as field sanitation, the use of earlier harvest, and tillage between lines and/or watering to avoid soil cracking. In this regard, biological control agents may be considered for possible agents to control these weevils, if they move actively for the search of hosts and maintain or even increase their population by self-multiplication. The isolate of SMZ2000, in both suspension and granular formulation was tested for its lethal effects on two sympatric sweet potato species in laboratory. The most efficient treatments were the conventional chemical management and two applications of the isolate over the ground surface, both of which showed similar efficacies to reduce both the population and the reduction of damaged tubers by these weevils in both years. Any treatments were not evaluated sufficient for their efficacy on the weevils due to its relatively low occurrences. Based on these results, the isolate SMZ2000 can be considered to possess the potential as a biological control agent for the management of sweetpotato weevils. However, more field studies are needed to confirm its potential to control the weevil.

Technical Abstract: Laboratory and field experiments were performed to evaluate the pathogenicity of an isolate of the entomopathogenic fungus Metarhizium anisopliae to the two sympatrically occurring weevil species, Cylas formicarius and Euscepes postfasciatus. In the laboratory bioassays, suspension of conidia, = 106 CFU/ml, caused mortalities >80% on adults of both weevils in seven days after inoculation. It took longer time 20 days grain formulation of the isolate adhered on rice grains of = 5 g/m2 (107 CFU/g) to attain similar mortalities of E. postfasciatus, but no evident mortality was obtained in C. formicarius. The grain formulation was thus less effective on C. formicarius than the suspension. Field trials were carried out over two years from 2013 to 2014, in which adults of E. postfasciatus were released two times during the field experiments for enhancement of damage on plants by this weevil, whereas the other weevil species was left to naturally invade the experimental plots by flying. The results of the experiments revealed in both years that two applications of the isolate in grain formulation, equivalent to 50 kg/hectare, sprayed manually over the ground surface reduced the infestation of plants and tuber damage by weevils of both species as much as the conventional chemical insecticide applications. The occurrences of weevils at harvest were not significantly different among treatments. The potential and possible uses of the fungus are discussed for the management of these two weevil species.