| Xenorhabdus nematophila HB310 (Xn HB310) with high virulence, screened in our laboratory, is an entomopathogenic bacteria which symbiotically associated with entomopathogenic nematode Steinernema carpocapsae. This strain can secrete the insecticidal toxins with broad-spectrum insecticidal activity. However, while the virulence of Xn HB310 on different insects vary a lot. In order to clarify the main factors that affect the insecticidal activity and its insecticidal mechanism, the midgut environments of different sensitive insects were compared. In addition, the formulation of suspension concentrate for Xn HB310 was developed, and the synergism of Xn HB310 mixtured with seven different kinds of insecticides were determined respectively. The results were as the followings.1. By analyzing and comparing the midgut environment of different sensitive insects, the results showed that there was no correlation between the insecticidal activity of Xn HB310 and pH value of insect midgut, because Xn HB310 all could remain high insecticidal activity in the gut environment of 4 ~ 10 pH value. The midgut enzymes from different sensitive insects had no influence on the insecticidal activity of the toxin from Xn HB310. There was certain correlation between the PM and the insecticidal activity of Xn HB310. The thinner was the PM, the more sensitive was the insect to Xn HB310. Furthermore, Fluorescent whitening agent FB-28, which could damage the PM, showed the synergism with Xn HB310 against third instar larvae of Helicoverpa armigera.2. No Xn HB310 bacteria was isolated from its hemolymph of Locusta migratoria manilensis after feeding Xn HB310. The result indicated that it is the toxin protein but not living cell which plays a crucial role in the whole insecticidal process.3. We obtain the optimum formula--40% Xn HB310 suspension concentratea (SC) By screening of various additives. The formula was as follows: Xn HB310 cell 40%, OP-10 4%, Emulsifier 600# 2%, Xanthan gum 0.1%, Potassium sorbate 0.1%, Methyl paraben 0.03%, Sodium Dehydroacetate 0.2%, Glycol 5%, Organic silicon 0.6%, whole milk powder 2% and water 45.97%. The experiment on pot plant showed that the 40% Xn SC have a good control efficiency on 3rd-instar larvae of P. xylostella larvae. The control efficiencies of 40% Xn HB310 SC of 50 times, 100 times and 200 times dilution were 97.56%, 90.24% and 80.49% respectively, and obvious better than the 50 times cell suspension dilution.4. We respectively determined and evaluated the synergistic effect of Xn HB310 mixtured with seven different kinds of insecticides on 3rd-instar larvae of P. xylostella by the leaf dipping method in laboratory. The results showed that the mixture of Xn HB310 with cypermethrin had remarkable synergistic effect which the cooperative virulence index was 30.9. When the mix-proportion (w/w) of Xn HB310 with cypermethrin ranged from 1006.4:2.6 to 816:4, the mixtures showed synergistic effect and the mix-proportion of 952:3 had the highest common-toxicity coefficient (CTC, 335). When Xn HB310 was mixed with dursban, Imidacloprid or emamectin benzoate respectively, the mixtures all showed additive effect. The cooperative virulence indexes of the three mixtures were 9.6, 6.9 and -18.6, respectively. On the contrary, when Xn HB310 were mixed with spinosad, chlorbenzuron or abamectin, the mixtures all showed antagonistic action and the values of each cooperative virulence indexes were -48.4, -24.9, -57.1 respectively. |
PDF Full Text Request