Submitted to: Biopesticides International
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/25/2014
Publication Date: 6/1/2014
Citation: Gelman, D., Martin, P.A., Blackburn, M.B., Rojas, M.G., Hu, J.S. 2014. Insecticidal Activity of Chromobacterium subtsugae on the Sweet Potato Whitefly, Bemisia tabaci, Biotype B. Biopesticides International. 10(1):11-22.
Interpretive Summary: White flies are a serious insect pest on greenhouse-raised plants. White flies often develop resistance to insecticides after repeated chemicals sprays, thus additional means for their control are required. Insecticidal bacteria are an alternative means for control of this insect, but few bacteria have been identified that affect white flies. In this study, using a feeding assay, a cell-free extract of a characterized purple bacterium known as Chromobacterium subtsugae was shown to reduce the lifespan of white flies. At least part of this toxicity may have been due to the presence of a whitefly-toxic sugar, ribose, produced within whiteflies that had ingested the bacterial extract. This information will be of interest to scientists interested in alternative means for biological control of insects, and to industrial scientists interested in commercialization or scale up of insecticidal bacteria that act to kill insect pests by novel mechanisms.
Technical Abstract: Chromobacterium subtsugae crude extracts contain compounds that are toxic to nymphal and adult Bemisia tabaci. When fed on artificial diet containing 10% of the supernatant of an aqueous cell-free extract of C subtsugae, the number of 2nd and 4th instar nymphs and of emerged adults was significantly lower in experimental as compared to control groups. Feeding an artificial diet containing 5% and 10% of an aqueous cell-free extract of C. subtsugae to adult B. tabaci reduced mean day survival from 5.9 days to only 2.8 and 1.8 days, respectively. The molecular weight ranges of the toxic C. subtsugae compounds were found to be 3 – 10 kDa and = 300 kDa. Both the higher and lower molecular weight toxins were resistant to extraction with methanol and drying. The higher molecular weight toxin(s) was also resistant to boiling while the toxicity of the lower molecular weight toxin(s) was slightly, but significantly, reduced by boiling. Upon overnight refrigeration of the crude C. subtsugae extract, some material precipitated out and upon centrifugation, activity of the high molecular, but not the low molecular weight components in the supernatant, was reduced. It is likely that the high molecular weight toxin had bound to the precipitate, since activity in the supernatant was not restored after vortexing, but was restored upon sonication. Incubation with a general protease, amylase or lipase did not destroy the activity of the supernatant prepared from a cell-free extract of C. subtsugae, and incubation with trypsin or chymotrypsin did not destroy the activity of the lower or higher molecular weight toxins. After reverse phase high pressure liquid chromatography (RP-HPLC) of a supernatant prepared from crude C. subtsugae extract, toxic activity was only detected in Fractions 47 through 49. Ribose (a sugar that is toxic to B. tabaci) was detected in B. tabaci fed on diets containing 10% C. subtsugae supernatant. Although levels of this monosaccharide were 900 times less in the supernatant than levels present in diets containing 2.5% ribose, levels were only 13 times less in B. tabaci fed on diet containing C. subtsugae vs. diet containing 2.5% ribose. Thus, it is possible that the increased levels of ribose in B. tabaci fed on diets containing C. subtsugae supernatant may contribute to the reduction in B. tabaci mean day survival as compared to controls.