بررسی اثرات غیرکشندگی باکتری Bacillus thuringiensis روی حلزون Achatina fulica

The snail Achatina fulica (Gastropoda, Achatinidae) is one of the most significant agricultural pests due to its wide distribution and broad host range among various plants. The most common method of controlling this species is the use of the chemical molluscicide metaldehyde, which, besides its incomplete efficacy, has definitive negative impacts on the environment. Therefore, the search for alternative control methods, such as biological control, has become essential. The bacterium Bacillus thuringiensis (Bt) is one of the most important biological agents for controlling insect pests, but its use as a biological control agent against agricultural mollusks has not been comprehensively studied. In this study, The direct and indirect (endophytic) effects of Bt , prepared as a commercial product within the country, were investigated on the snail A. fulica. Lettuce (Lactuca sativa), a preferred host of the snail, was cultivated in a greenhouse and used for snail feeding trials. Nutrient Agar (NA) was used to culture the bacterium in its vegetative phase. The effects of Bt were evaluated under several treatments: dipping lettuce leaves into a suspension of bacterial spore-crystals (at concentrations of 5×108 and 109 CFU/ml ), a suspension of the bacterium in its vegetative phase without crystals (5×108 cell/ml), a suspension of Xanthomonas translucens (control 1), and sterile distilled water (control 2) on both adult and non-adult A. fulica individuals. The results showed that none of the treatments had observable effects on adult snails. However, non-adult snails treated with the 109 CFU/ml concentration stopped feeding on lettuce leaves from the second day of treatment onward for a duration of 21 days. To confirm the presence of the bacterium in the gastrointestinal tract of the non-adult individuals that had ceased feeding, their digestive tracts were aseptically removed at 14 and 22 days post-treatment. The suspension resulting from homogenization in a porcelain mortar with sterile distilled water was cultured on Nutrient Agar plates. Five days post-incubation, the presence of Bt spores and crystals was confirmed. Subsequently, the potential of Bt to become endophytic was studied using the method of spraying leaves and irrigating 6-week-old lettuce plants with 10 ml of bacterial suspension at a concentration of 8/7‎×‎108 CFU/ml. Data obtained 10 days post-inoculation concerning the endophytic presence of the bacterium in the leaves indicated that this process was only successful via the spraying method. Leaves of plants treated with the bacterial suspension via the roots lacked endophytic bacteria. The effect of the bacterium in its endophytic state on both adult and non-adult snails is under investigation.