RESUMEN
In a laboratory assay, it was shown that B. bassiana BCC48145, BCC2660, and P. lilacinum TBRC10638 were the three strains that exhibited the highest insecticidal activity against chili thrips, causing 92.5% and 91.86% and 92.3% corrected mortality, respectively. The fungi B. bassiana BCC48145 and P. lilacinum TBRC10638 were selected for greenhouse spraying. Cytotoxicity test of the extracts from both fungi evaluated against 4 animal cell lines: KB; human oral cavity carcinoma, MCF7; human breast adenocarcinoma, NCI-H187; human small cell lung carcinoma and GFP-expressing Vero cells, showed none-cytotoxic to all cell lines. An efficacy validation in the greenhouse showed that P. lilacinum TBRC 10638 was more effective than B. bassiana BCC48145 and could control the thrips up to 80% when using the fungus at 108 spores/mL. The LC50 values of P. lilacinum TBRC 10638 against chili thrips based on total thrips count from two experiments were 1.42 × 108 and 1.12 × 107 spores/mL when the fungal spores were sprayed once a week. The optimal concentration of P. lilacinum TBRC 10638 spores for effective control of chili thrips was determined at 1.41 × 109 spores/mL. The average efficacy of P. lilacinum TBRC 10638 for thrips control from 3 field trials was 30.08%, 14.39%, and 29.92%. This result was not significantly different from that of the chemical insecticide treatment group, which showed efficacy at 19.27%, 14.92%, and 19.97%. Furthermore, there was no difference in productivity among the different treatment groups. Our results demonstrated that P. lilacinum TBRC 10638 is a promising biocontrol agent that could be used as an alternative to chemical insecticide for controlling chili thrips.
RESUMEN
The reducing clade III polyketide synthase genes, including pks15, are highly conserved among entomopathogenic fungi. To examine the function of pks15, we used targeted disruption to investigate the impact of Beauveria bassiana pks15 on insect pathogenesis. Southern analysis verified that the Δpks15 mutant was disrupted by a single integration of the transformation cassette at the pks15 locus. The Δpks15 mutant had a slight reduction in radial growth, and it produced fewer spores. Our insect bioassays indicated the Δpks15 mutant to be significantly reduced in virulence against beet armyworms compared to wild type (WT), which could be partially accounted for by its markedly decreased ability to survive phagocytosis. Total haemocyte count decreased sharply by 50-fold from days 1-3 post-inoculation in insects infected with WT, compared to a 5-fold decrease in the Δpks15 mutant. The mutant also produced fewer hemolymph hyphal bodies than WT by 3-fold. In co-culture studies with amoebae that have phagocytic ability similar to that of insect haemocytes, at 48 h the mortality rate of amoebae engulfing Δpks15 decreased by 72 %, and Δpks15 CFU decreased by 83 % compared to co-culture with WT. Thus, the Δpks15 mutant had a reduced ability to cope with phagocytosis and highly reduced virulence in an insect host. These data elucidate a mechanism of insect pathogenesis associated with polyketide biosynthesis.