Toxicity of the Bacterial Formulation of Flavobacter runzhou to Spodoptera exigua Larvae

The non-spore-forming bacterium,Flavobacter runzhou,is a recently found insect parthogen that infects numerous lepidoterous larvae.The virulence of the parthogen against larvae of the beet armyworm,Spodoptera exigua Hubner(Noctuidae,Lepidoptera) was evaluated by the leaf-dip method.The pathogen caused higher mortalities to young larvae than to old larvae,the cumulative mortality(5.50×107 conidia/ml) at three days after the initial infection was 96.15% for the first instar,80.00% for the second,67.86% for the third,and 67.86% for the last instar larvae,respectively;and at four days after the initial infection,mortality of the first instar larvae was 100%,that of the 2nd to forth instar larvae was ca.90%.The LD50 was 3.93×106 conidia/ml for the first instar,6.34×106 conidia/ml for the second,8.59×106 conidia/ml for the third,and 1.02×107 conidia/ml for the last instar larvae.The LT50 to the same instar larvae was shortened with increase in concentration of the pathogen formulation,while it was extended with increase of larval instars at the same concentration of the pathogen formulation.