Abstract
Pathogenicity and immunity effects of both the entomopathogenic nematode, (Heterorhabditis zealandica) and the entomopathogenic fungus (Beauveria bassiana) on the last larval instar of the greater wax moth, Galleria mellonella L. (Lepidoptera: Pyralidae), were investigated. Both pathogens were either applied separately or in a combination. The fungus was inoculated first, followed by the nematode on time manner 0, 2, 4, and 6 days. LC50 values for both pathogens were calculated and then used for determination of the changes in immune response-mediated phenoloxidase (PO) and detoxifying enzymes glutathione S-transferase (GST) and non-specific esterase (EST) activities. The results indicated that a positive correlation was found between pathogen concentration and host mortality percentage. LC50 values were 6.49 IJs/larva for H. zealandica and 3.1 × 102 conidia/ml for B. bassiana. Synergistic interactions were found in all combined applications. The degree of synergism increased (reaching 100% mortality), when the nematode was applied 2 days post-fungal infection. PO activity increased significantly (p < 0.05) in a time-dependent manner post-B. bassiana infection. In contrary, the combination of H. zealandica + B. bassiana or H. zealandica alone produced a significant suppressive effect on PO activity over time. GST activity increased significantly (p < 0.05) in 36 h, then decreased at 48 h post-combined application, while the fungal infection enhanced significantly GST activity in time-dependent manner than the control and other treatments. EST activity increased significantly (p < 0.05) in both combined application and the single nematode infection than the single fungal infection, which increased during the initial period only. The increased mortality rates and suppression of phenoloxidase and glutathione S-transferase enzymes, following the combined application suggests a strong synergistic effect between both pathogens. It could be concluded that the tested combined pathogens are compatible element for integrated pest management.
Highlights
The greater wax moth, Galleria mellonella Linnaeus (Lepidoptera: Pyralidae) is one of the most serious pests of honeybee products due to the destructive feeding habit of its larvae
Entomopathogenic nematodes (EPNs) from the genus Heterorhabditidae are characterized by a symbiotic association with the bacteria of the genus Photorhabdus, in the intestine of the free-living infective juveniles (IJs) of these nematodes (Ansari et al 2003)
Nematode–fungus interactions To study the interactions between the two tested biocontrol agents, G. mellonella larvae were exposed to B. bassiana and H. zealandica at concentration of 3.1 × 102 conidia/ml (7 IJs/larvae) respectively, each alone or in various fungus-nematode combinations
Summary
The greater wax moth, Galleria mellonella Linnaeus (Lepidoptera: Pyralidae) is one of the most serious pests of honeybee products due to the destructive feeding habit of its larvae. Conidia of EPFs impart mortality in the target pest by overcoming the complicated host defense-related multiple mechanisms (Hussain and Ming-Yi, 2013). This process requires a prolonged period to impart significant mortality in target pest. Previous studies clearly described that host defense-related enzymes play a crucial role by safeguarding the pest from toxins and pathogens. No data are available on the activities of defense-related enzymes in G. mellonella in response to invading fungal pathogen with nematodes. The interaction effects between Heterorhabditis zealandica and B. bassiana in the lepidopteran, G. mellonella larvae, individually and/or combined were investigated The effect of both pathogens on insect enzymatic activity was studied
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