Antagonistic effects of Rhamnolipid biosurfactant on larval gut microbes and impact on the growth and development of the silkworm

Abstract

The demand for bio-based insecticides has significantly increased during the last decade due to the contamination of the environment following massive pesticide use. The present study aims at evaluating the insecticidal activity of rhamnolipid (RL) biosurfactant (from
 0.01 to 1 %) orally administered to fifth instar larvae of the silkworm, Bombyx mori, a model among Lepidoptera. The bioassay showed that above 0.5% RL caused significant mortality in the larvae and pupae of B.mori. To evaluate the mode of action of RL, we carried out
 antibacterial activity assays against larval gut microbes of B.mori. Interestingly, a fall of the microbial population and a susceptibility of host beneficial bacterial species, like Bacillus megaterium, Bacterium IMBL3, Enterococcus mundtii and Bacillus cerus, towards RL were
 observed by using plate sensitive assay and agar well diffusion assay, respectively. The results of the present study confirm that RL cause
 the breakdown of beneficial interactions between the larval gut and microbes, leading to a high rate of larval mortality due to the digestive upset in the B. mori gut. This work provides a platform of knowledge to set up promising strategies for the biological control of insect pests.
 Keywords: Rhamnolipid, antibacterial activity, Bombyx mori, insecticidal activity, gut microbes