A study from the Ohio State University reported for the first time, the mechanisms how nematodes and their symbiotic bacteria differ in virulence against white grub species. The results clarified a nature picture that the low susceptibility of Europe chafer to Heterorhabditis bacteriophora is not due to the symbiotic bacteria, but is due to the nematode’s poor ability to penetrate through either the gut wall or the cuticle to vector the bacteria into the blood. Therefore, future research was suggested to be focused on how to improve the penetration ability of the nematode.
While nematodes are effective in killing white grubs, the use of nematodes against these pests is still limited, and one of main reasons is differences in the virulence of nematode species and strains to white grub species. Thus, one of studies conducted by OSU researchers was to determine the mechanisms of differences in the virulence of different nematode species to the white grub species. As bacteria play a key role in the virulence of nematodes, this study compared the virulence of Photorhabdus temperata and Xenorhabdus koppenhoeferi, the symbiotic bacteria associated with the nematodes Heterorhabditis bacteriophora and Steinernema scarabaei, respectively, to three white grub species Japanese beetle, Europe chafer, and masked chafer. Both bacteria were pathogenic to all three grub species even at as low as 2 cells per grub. However, the median lethal dose at 48 h post injection and median lethal time at 20 cells per grub showed that Photorhabdus temperata was more virulent than Xenorhabdus koppenhoeferi to masked chafer. There were no differences in virulence of two bacteria against Japanese beetle and Europe chafer. Although Heterorhabditis bacteriophora carrying Photorhabdus temperata is less pathogenic than Steinernema scarabaei carrying Xenorhabdus koppenhoeferi to Europe chafer, Photorhabdus temperata grew faster than Xenorhabdus koppenhoeferi both in vitro and inside insect larvae. The researchers then tested the pathogenicity of oral- and hemolymph-introduced Heterorhabditis bacteriophora to Europe chafer to determine whether nematodes are able to successfully vector the bacteria into the hemolymph. Hemolymph injected Heterorhabditis bacteriophora were pathogenic to Europe chafer indicating successful bacterial release, but orally introduced Heterorhabditis bacteriophora were not. Dissection of grubs confirmed that the orally introduced Heterorhabditis bacteriophora were unable to penetrate into the hemolymph through the gut wall. Therefore, they conclude that the low susceptibility of Europe chafer to Heterorhabditis bacteriophora is not due to the symbiotic bacteria, but is due to the nematode’s poor ability to penetrate through either the gut wall or the cuticle to vector the bacteria into the hemolymph. Author: Ruisheng An and Parwinder Grewal. Credits/Source: Biological Control 2007, 43:310-316
