Study On The Action Mode And Insecticidal Mechanism Of Xenorhabdus Nematophila HB310 To Plutella Xylostella L.

Xenorhabdus nematophila HB310 strain, an entomopathogenic bacterium symbiotically associated with entomopathogenic nematode Steinernenza carpocapsae, appeared high oral toxic activity against several species of insect. In order to explore the feasibility of the X. nematophila HB310 strain as bioinsecticidal agent, the action mode and mechanism of the culture broth and insecticidal protein from HB310 strain to Plutella xylostella (diamondback moth (DBM)) were studied systematically. Helicoverpa armigera (cotton bollworm) larvae was researched at the same time. The study will provides the basis for the development and utilization of X. nematophila HB310 in the future, The main results are as follows:1. Thought testing action mode of X. nematophila HB310 to the DBM, It was found that the culture broth and insecticidal protein from X. nematophila HB310 showed high stomachic action and antifeedant function. The bacteria had no contact toxicity. They showed no evident oviposition deterrence to the adults of P. xylostella yet. After ingesting the culture broth(5.997×10⁸cell/mL), the rectified mortality of the 2nd, 3rd, 4th instar larvae of DBM were 100%, 54.39% and 50% respectively at 72 hours. LC₅₀ of the crude insecticidal protein extract to the 2nd instar larvae of DBM was 0.0416mg/mL. After ingesting toxinâ…¡(51.9μg/mL), which was purified by native-PAGE from the crude insecticidal protein extract, mortality of the 2nd instar larvae of DBM was 73.33% at 72 hours.Sublethal effects of X. nematophila HB310 on the feeding, development and fecundity of DBM was also studied in the laboratory. The results showed that treatment of sublethal concentration of the culture broth (9.9955×10⁵cell/mL) and the crude insecticidal protein extract (0.0284mg/mL) could significantly reduce the subsequent weight of pupa, pupation percentage, emergence rate, longevity and fecundity of live DBM.2. The effects of X. nematophila HB310 to the DBM eggs were tested. The results showed the culture broth not only could distinctly declined the hatching rate of the eggs, but also reduced the livability of DBM neonates. The hatching rate of the eggs treated with the culture broth was 58%, it was less 37% than that of the control, the livability of the neonates treated by the culture broth was 55%, it was less 45.83% than that of the control, whereas the crude insecticidal protein extract can distinctly declined the livability only, the livability of the neonates treated by the crude insecticidal protein extract was 33.76%, it was less 61.98% than that of the control3. After investigating the midgut main protease activity of DBM and cotton bollworm larvae, we found that the toxinâ…¡could certainly inhibit the the activities of weak alkaline trypsin-like enzyme, active alkaline trypsin-like enzyme and chymotrypsin-like enzyme of DBM larvae. Then, the time of feeding and dosages of toxinâ…¡were tow factors that can affect the inhibition to the activity of the midgut main protease, the more both the time and dosages,the more inhibitable to the main protease activity of DBM larvae. toxinâ…¡ appeared the similar effect on the main protease midgut of H. armigera larvae.4. Toxinâ…¡could enhance the activities of carboxylesterase, esterase, acetylcholinesterase and polyphenoloxidase of H. armigera larvae.