Toxicity Of Bacterium Flavobacter Runzhou To Spodoptera Exigua Larva And Its Interaction With Parasitism By Meteorus Pulchricornis

The beet armyworm Spodoptera exigua(H(u|¨)bner) is a polyphagous pest on numerous crops over the world.Biological control can be an effective approach to the persistent management of this noxius pest.Parasitoids and pathogens are two important groups of natual enemies that provide potential for the biocontrol.To develop an effective management design for control of the beet armyworm,we conducted a series of experiments about the toxicity of the bacterium,Flavobacter runzhou gen.nov.,sp.nov.,GXW15-4,the host selection of the parasitoid Meteorus pulchricornis Wesmeal between healthy and pathogen-infected larvae.The main results were summarized as following.1.Toxicity of the bacterial formulation of Flavobacter runzhou to Spodoptera exigua larvaeThe bacterium,Flavobacter runzhou,is a recently found insect pathogen that infects numerous lepidoterous larvae.The virulence of the pathogen against larvae of the beet armyworm,Spodoptera exigua H(u|¨)bner(Noctuidae,Lepidoptera) was evaluated by the leaf-dip method.The pathogen caused higher mortalities to young larvae than to old larvae, with the cumulative mortality 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;at four days after the initial infection mortality of the first instar larvae was 100%,that of the 2^nd to forth instar larvae was ca.90%.The LD₅₀ was 3.93×10⁶ cfu/ml for the first instar,6.34×10⁶ cfu/ml for the second,8.59×10⁶ cfu/ml for the third,and 1.02×10⁷ cfu/ml for the last instar larvae.The LT₅₀ to same instar larvae was reduced with the concentration of the pathogen formulation,while it was extended with increase of larval instars at the same concentration of the pathogen formulation.2.Host preference of Meteorus pulchricornis between healthy and infected larvae by two bacteriaThe paired-choice was used to observe the selection behavior of the parasitoid Meteorus pulchricornis between healthy and infected 2nd to 4th instar larvae by Bt and F.runzhou. The number of first antenna-tapping and first ovipositor-drilling did not differ significantly between healthy and pathogen-infected host larvae.But the number of antenna-tapping and of ovipositor-drilling during three-hour observation period was different,to some extent, between healthy and pathogen-infected host larvae.3.The pathogen-transmission of Meteorus pulchricornisThe 3^rd instar larva of the beet armyworm,after infected by bacterium F.runzhou,was exposed to parasitism.The wasp that exercised ovipositor-drilling was then exposed to a healthy host larva,which was observed for survival.The larvae exhibited low mortality, whether the infection time of previous host larvae was short or long,which suggested little transmission of the parasitoid through oviposition.4.Survival of parasitized host larvae when infected by bacterium F.runzhouThe 3^rd instar larva,one and three days after being parasitized by Meteorus pulchricornis,was infected by the bacterium F.runzhou,and was observed in survival.The results showed that the mortality of treated larvae was significantly higher than that of the control after four days of infection by 70%,and after five days of infection by 86%.