An indigenous Metarhizium sp. strain amended in compost enhances sweetpotato growing system and management of Cylas formicarius (Coleoptera: Curculionidae)

Authors: WONG, LANDON - University Of Hawaii; WANG, KOON-HUI - University Of Hawaii; Myers, Roxana; SIPES, BRENT - University Of Hawaii

Submitted to: BioControl
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2025
Publication Date: 8/6/2025
Citation: Wong, L.G.K., Wang, K., Myers, R., Sipes, B.S. 2025. An indigenous Metarhizium sp. strain amended in compost enhances sweetpotato growing system and management of Cylas formicarius (Coleoptera: Curculionidae). BioControl. https://doi.org/10.1007/s10526-025-10340-2.
DOI: https://doi.org/10.1007/s10526-025-10340-2

Interpretive Summary: Beauveria and Metarhizium are entomopathogenic fungi (EPF) that parasitize insects and have been commercialized into biological control agents. They have wide host ranges, including the sweet potato weevil whose larvae damage sweet potato by creating galleries and causing a bitter flavor. The objective of this research was to evaluate the efficacy of local and commercial EPF against sweet potato weevil in laboratory and field settings. Beauveria had high efficacy in laboratory studies and the number of unmarketable sweet potatoes was reduced in Beauveria treated field plots compared to untreated plots. The local isolate Metarhizium KO-002 as a compost amendment reduced sweet potato weevil populations and damage caused to sweet potato roots while improving early plant growth and vigor. In addition to providing insect control, Metarhizium may establish an endophytic relationship in plants, improving their development and overall health. Both EPF species show promise as tools in the management of sweet potato weevil and incorporation of Metarhizium into compost is a feasible method for delivery of the fungus providing enhanced sweet potato growth.

Technical Abstract: Two commercial products, Botanigard® (active agent Beauveria bassiana Vuillemin [Hypocreales: Cordycipitaceae]) and Met Master® (active agent Metarhizium anisopliae [Metchnikof] Sorokin [Hypocreales: Clavicipitaceae]), and a Hawaiian isolate of Metarhizium anisopliae Ko-002 were evaluated for managing sweetpotato weevil, Cylas formicarius Fabricus (Coleoptera: Brentidae). Choice arena tests indicated M. anisopliae Ko-002 neither attracted nor repelled C. formicarius but did suppress the insect’s feeding. In the greenhouse, inoculation with M. anisopliae Ko-002 enhanced sweetpotato growth. Virulence of commercial products against C. formicarius larvae were tested in the laboratory. MetMaster® did not result in any C. formicarius larvae mortality while Botanigard® resulted in 30% mortality of the insects. Field plots treated with Botanigard® resulted in lower sweetpotato yield, no change in C. formicarius damage, and did not lower sweetpotato weevil population density compared to the untreated control. The Botanigard® may have had unintended effects on natural predators of C. formicarius. Met Master® had no effect on sweetpotato yield, C. formicarius damage, nor sweetpotato population density as compared to the untreated control. Metarhizium anisopliae Ko-002, applied as a compost amendment at planting, did not affect sweetpotato yield, but reduced C. formicarius damage, and lowered sweetpotato weevil population density. Field plots amended with Metarhizium anisopliae Ko-002 had higher plant vigor ratings and were taller than plants in unamended plots. Entomopathogenic fungi, especially the locally isolated M. anisopliae, can improve management of the C. formicarius and enhance plant growth when incorporated as a compost amendment.