Antagonistic interactions between the biocontrol agents Beauveria bassiana and Heterorhabditis indica

Abstract

The effect of the pathogenic fungus, Beauveria bassiana, on the invasion and proliferation of the entomopathogenic nematode, Heterorhabditis indica, in larvae of the waxmoth, Galleria mellonella, was investigated. Simultaneous application of both pathogens did not cause any significant decrease in nematode penetration (6.0%) in comparison to infection with nematodes only (7.4%). However, when the insect larvae were preinoculated with B. bassiana for 24, 48, 72, or 96 h before nematode application, the mean percentage penetration was significantly reduced to 1.7, 1.4, 1.7, and 0.0, respectively. When the larvae were infected with nematodes only (control), the number of nematodes collected, after 2 weeks after the death of the insect, was 106,000 nematodes per larva (i.e., 14,380 nematodes per one injected juvenile (InjJ)). However, when the G. mellonella larvae were simultaneously inoculated with both agents, the total number of nematodes produced was restricted to 35,000 nematodes per larva (5,866 nematodes per InjJ). A more dramatic effect was observed when larvae were preinoculated with B. bassiana before nematode application. The 24 h preinoculation period resulted in the decreased harvest of 2,160 nematodes per larva (1,256 nematode per InjJ), while the other preinoculation regimes caused a total inhibition of nematode production. Increasing the number of nematode juveniles inside preinoculated by fungus larvae by injection reduced the inhibitory effect of the fungus on the development of hermaphrodites. The observed results indicate the occurrence of antagonistic interactions between the two pathogens inside the infected insect.