Effect Of Gut Bacterial Communities On Growth And Development Of The Diamondback Moth, Plutella Xylostella(L.)(Lepidoptera:Plutellidae)

Diamondback moth (DBM), Plutella xylostella L.(Lepidoptera:Plutellidae) is one of the most destructive pests of cruciferous vegetables worldwide. To know the effect of gut micro flora on the devolopment and reproduction of DBM, we altered gut microflora by oral antibiotics. Firstly, the most suitable antibiotic for altering microbes of DBM’s gut was selected through the antibiotic susceptibility test, the minimum inhibitory concentration (MIC) of the antibiotic was determined by the broth macro-dilution. Secondly, DBM was fed on Chinese cabbage leaf soaked in antibiotic solution with MIC, and4th instars larvae were dissected to detect the variation of intestinal microflora. Meantime, the devolopment and reproduction were studied. The important results are as follows:1. Antibiotics screeningA total of24kinds of antibiotic were selected for the susceptibility test, and bacteriostatic ring size was compared, the ciprofloxacin was detected as the most potent antibiotic due to it’s bigger bacteriostatic circle diameter (39.4±0.5mm) than the sensitive threshold (21mm). The MIC of ciprofloxacin was found as1μg/ml for intestinal bacteria of DBM by the broth method.2. Variation of DBM intestinal bacteria by antibioticsThe larvae were fed on Chinese cabbage leaf soaked in1μg/ml ciprofloxacin solution till4th instars before dissection, while the larvae fed on Chinese cabbage leaf soaked in0.1%triton were used as control. Traditional culture and PCR-DGGE methods were combined to detect the variation of the intestinal bacteria of DBM. Enterobacter and Enterococcus were detected from the control group by traditional culture method. The number of them were108cfu/moth, suggesting that they were the dominant bacterial community. The number of Enterococcus was105cfu/moth in antibiotics group which was less than that of control group. This indicates that antibiotic can affect the number of culturable bacteria of DBM. The dominant bacterial communities of control detected by PCR-DGGE were Firmicutes (Enterococcus) and Proteobactera (Citrobacter), including Klebsiella, Chroococcidiopsis, Pantoea. The dominant bacterial communities of antibiotic group were Cyanobacteria (Chroococcidiopsis) and a new kind of bacteria B, and few (Enterococcus) were also detected. There is significant variation in gut microflora of DBM between control and treated groups. Enterobacteriaceaes were eliminated due to treatment.3. The effect of variation in intestinal bacteria on the devolopment and reproduction of DBMAfter feeding antibiotic, the variation of intestinal bacteria affects the devolopment and population parameters of DBM. The result showed:the developmental duration of larvae and pupa in antibiotic group were longer than that of control group, the weight of pupa was higher too. There was no significant difference between two groups for oviposition, oviposition period and adult longevity. Net reproductive rates (R0), intrinsic rate of increase (r) and finite rate of increase (γ) were not significantly different between two groups. The mean generation time (T) of antibiotic group was significantly longer.On the basis of above facts, we can conclude that the Proteobacteria (Enterobacteriaceae) and Firmicutes (Lactobacillaceae) bacterias in the intestinal micro flora of DBM were significantly changed due to treatment. The variation of intestinal microflora affect the devolopment of DBM, but no effect on it’s population growth.