Abstract
We determined virulence of seven Heterorhabditis bacteriophora strain GPS11 inbred lines possessing superior infective juvenile longevity, and heat and ultra violet radiation tolerance against white grubs Popillia japonica and Cyclocephala borealis. At 1 and 2 weeks after treatment, inbred line A2 was significantly more virulent towards P. japonica compared to the parent strain GPS11 and inbred lines A7, A8, A12 and A21; and line A2 caused significantly higher C. borealis mortality than lines A6 and A12. Penetration, encapsulation and survival of two inbred lines, A2 and A12, that showed the highest and lowest virulence against both grub species were then assessed. There were no differences between the two lines for the total number of nematodes penetrated in either P. japonica or C. borealis within the first 24 h, but a significantly higher percentage of penetrated nematodes were alive in line A2 compared to the line A12 in both grub species. P. japonica immune response over time to hemocoel-injected nematodes of A2, A12 and the parent strain was further investigated. While all injected nematodes were encapsulated at 6 h post injection, non-encapsulated living nematodes were detected at 12 and 24 h post injection, showing the breakage out of encapsulation. A higher percentage of non-encapsulated living nematodes and a lower percentage of dead nematodes were found in line A2 as compared to the line A12 after 12 h post injection. These data suggest that virulence differences in the studied H. bacteriophora inbred lines are not due to differences in nematode penetration or recognition by the grub immune system, but are related to the ability of the infective juveniles to break out of encapsulation.
Highlights
Entomopathogenic nematodes (EPNs) Steinernema and Heterorhabditis are lethal insect parasites.The nematode infective juveniles (IJs) penetrate the insect host generally through natural body openings and release mutualistic bacteria (Xenorhabdus spp for Steinernema and Photorhabdus spp for Heterorhabditis) in the hemocoel
As combating the host immune response plays an important role in successful infectivity of EPNs, we further investigated the fate of the hemocoel-injected nematodes in P. japonica, overtime
Separate bioassays were conducted for P. japonica and C. borealis to compare the virulence of inbred lines and the parental strain GPS11 of H. bacteriophora
Summary
Entomopathogenic nematodes (EPNs) Steinernema and Heterorhabditis are lethal insect parasites. The nematode infective juveniles (IJs) penetrate the insect host generally through natural body openings and release mutualistic bacteria (Xenorhabdus spp for Steinernema and Photorhabdus spp for Heterorhabditis) in the hemocoel. As white grubs are an important pest complex requiring effective control [4], interest in the commercial development of more potent nematode strains is high. Previous studies [5,6] show that, from among the many strains of Heterorhabditis bacteriophora, the GPS11 strain possesses superior traits, including higher virulence against several white grub species, longer longevity of IJs, and higher stress tolerance compared to the commercial strains. We tested seven inbred lines possessing these fixed superior traits for differences in virulence against two white grub species, Popillia japonica and Cyclocephala borealis, to identify a “super” inbred line with multiple desirable traits fixed against potential genetic deterioration. We hypothesized that the differences in the virulence of these inbred lines is related to the ability of IJs to defend against the host immune response, but not to the rate of their penetration into the host
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