Metarhizium majus and Metarhizium robertsii show enhanced activity against the coleopteran pests Holotricha serrata and Oryctes rhinoceros

Abstract

Studies were conducted to systematically isolate Metarhizium isolates from the insect cadavers and soils of South India. Morphological and PCR amplified sequences of 5.8S ITS regions and RNA polymerase II largest subunit (RPB1) gene regions were used to identify the isolates at species level. Eight Metarhizium isolates were isolated and initially identified by morphological and microscopic studies. Further identification was confirmed through 5.8SrRNA ITS and RPB1 analysis. They were identified as three isolates of M. robertsii J.F. Bisch., Rehner & Humber sp. nov. (ArMz3R, ArMz3S and ArMz6W), one isolate of M. majus (J.R. Johnst.) J.F. Bisch., Rehner & Humber (VjMz1W) and four isolates of M. anisopliae (WnMz1S, NlMz2S, BgMz2S and DhMz4R). Topical conidial suspensions (TCS) and powder based formulations (PBF) of the eight indigenous isolates of Metarhizium spp. that were isolated from insect cadavers and soils of South India were tested against coleopteran pests Holotricha serrata L. and Oryctes rhinoceros L. that cause serious damage to sugarcane and palm trees respectively. Against H. serrata TCS of M. robertsii (ArMz6W) was the most effective with an LC₅₀ of 6.893í—10⁵ cfu/ml and caused 100% mortality against the 3^rd instar larvae in 5 days; PBF elicited an LC₅₀ of 7.502í—10⁵ cfu/ml with 96% mortality in 10 days. Against O. rhinoceros TCS (LC₅₀ of 9.75í—10⁵ cfu/ml) of M. majus (VjMz1W) caused 90% mortality in 7 days and the PBF (LC₅₀ of 9.57í—10⁵ cfu/ml) caused 86% mortality in 14 days. The results establish that M. robertsii is highly effective against H. serrata and against O. rhinoceros, M. majus was the most effective. The TCS formulations of these two strains can be readily deployed for field applications.