Genetic Structure Of The Masson’s Pine Caterpillar Isolates Of Beauveria Bassiana In China And The Irpathogenicity To Silkworms

As the concern to environmental protection and biological safety increases, biologicalroles are being raised. Beauveria bassiana insecticides have been used more and morewidely in southern China. However, the release of the insecticides is sometimes thought tobe the causal agent of the white muscardine of silkworms, causing conflicts betweensericulture and biological pest control. As the sericulture moves westernwards from easternChina, the conflict is becoming more and more serious where pine plantations andmulberry orchards are overlapped. The issue has been in serious debate on if the release ofBeauveria insecticides will cause or enhance silkworm muscardine, which affects theapplication of the fungal insecticides. Therefore, it is important that a study by mordernmeasures and methods on the issue is needed to provide evidence and answer on the toughissue which has been puzzling research and production.To trace if the resource of silkworm muscardine in south part of China is relatated tothe indigenous strains of B. bassiana causing natural muscardine of the Masson’s pinecaterpillar, Dendrolimus punctatus, and the production isolates of the insecticides released,ISSR techniques was used to cluster327pine caterpillar isolates of B.bassiana from8provinces of southern China. The custering result suggested that their phylogenetic lineageis related to their geographic origins.64representatives from them in8provinces and12silkworm isolates from8provices were made for clustering and PCA analyses. The resultsrevealed that the2group isolates were obviously isolated and independent, with rather fargenetic distance, suggesting that the muscardine originated not from exotic isolates, butfrom domestic indigenous silkworm strains.To scientifically rate the causal risk of the pine caterpillar isolates to silkworms and toensure the representativeness and accuracy,3pathogenicity tests were made with the64pine caterpillar isolates from8provinces and12silkworm isolates from4provinces to the3rdinstars of the silkworm.The result showed that either the caterpillar isolates except oneor the silkworm isolates were pathogenic to a certain degree, however, with muchsubstantial difference. Under the concentration of3×10⁷conidia mL^-1, the mean infectionrate of30caterpillar isolates was10.2%, significantly lowed than that of the8silkwormisolates （89.6%）; under the concentrationof1.5×10⁸conidia mL^-1, the mean infection rateof34caterpillar isolates was36.4%, extremely lower than that of6silkworm isolates（90.6%）; infection of the pine caterpillar isolates under1.5×10⁸conidia mL^-1was extremely lower that that under3×10⁷conidia mL^-1, while those of the silkworm isolatesunder the2concentrations were not significantly different. Under the concentration of1.5×10⁸conidia mL^-1, LT₅₀’s of the4caterpillar isolates comparatively highly virulent tothe silkworm averaged7.83d （7.45⁸.20d）,4.21d shorter than3.62d, mean of LT₅₀’s of6silkworm isolates （3.43³.72d）. Based on the comparison of lethal concentations of B1and B5, virulent silkworm isolates, to those of D26and D22, the least and the mostvirulent caterpillar isolates, the virulence of silkworm isolates was132¹5733times ashigh as that of the caterpillar isolates; based on the comparison of their lethal dosegs, hevirulence of silkworm isolates was43²201times as high as that of the caterpillar isolates.The comparisons strongly indicates that all pine caterpillar isolates are obviously hostspecific.An integrated analysis based on the molecular tracing and pathogenicity test revealedthat the white muscardine of silkworms, common and occuring throughout China is notassociated with the white muscardine of the Masson’s pine caterpillars, naturally occuringor induced by release of B. bassiana insecticides, and the risk of epizootic prevalence insilkworms artificially induced by application of the fungal insecticides was very low.