Phylogenetic variation of the green muscadine fungus, Metarhizium anisopliae (Metchnikoff) Sorokin, and its virulence to larvae of the sugarcane longhorn stem borer, Dorysthenes buqueti Guerin (Coleoptera: Cerambycidae)

Abstract

The sugarcane longhorn stem borer (SLSB), Dorysthenes buqueti Guerin (Coleoptera: Cerambycidae) has recently become a serious insect pest of sugarcane in Thailand and effective biological control agent must be evaluated. The green muscadine fungus (GMF), Metarhizium anisopliae (Metchnikoff) Sorokin is a species complex of entomopathogenic fungi, which includes many cryptic subspecies and species. It has been reported that GMF infects and kills the sugarcane longhorn stem borer (SLSB), D. buqueti Guerin, so that GMF is a possible biological control agent of SLSB. Molecular analyses were conducted to gain a better understanding of the taxonomic position of GMF Thai strains. Virulence bioassays were carried out on four isolates of GMF to 5th-9th instars of SLSB. This study revealed that an isolate from Khon Kaen (KK) showed the highest virulence to 5th-9th instars of SLSB. In biological control, an aqueous suspension containing 1 x 10⁸ conidia/mL of KK isolate was best from the viewpoint of a tradeoff between the economic cost/benefit of the mass production cost and the consequent mortality after application. Comparing suspensions containing 1 x 10⁸ conidia/mL with those containing 1 x 10¹³ conidia/mL, 100,000 times as much quantity of suspension can be obtained from the same quantity of conidia, though the difference in the D. buqueti mortality was relatively small. Six isolates of GMF from SLSB in Thailand were likely a cryptic species, although further molecular analysis using factor 1-alpha sequences is needed.