Abstract
BackgroundMetarhizium anisopliae, a soil-borne entomopathogen found worldwide, is an interesting fungus for biological control. However, its efficacy in the fields is significantly affected by environmental conditions, particularly moisture. To overcome the weakness of Metarhizium and determine its isolates with antistress capacity, the efficacies of four M. anisopliae isolates, which were collected from arid regions of Yunnan Province in China during the dry season, were determined at different moisture levels, and the efficacy of the isolate MAX-2 from Shangri-la under desiccation stress was evaluated at low moisture level.ResultsM. anisopliae isolates MAX-2, MAC-6, MAL-1, and MAQ-28 showed gradient descent efficacies against sterile Tenebrio molitor larvae, and gradient descent capacities against desiccation with the decrease in moisture levels. The efficacy of MAX-2 showed no significant differences at 35% moisture level than those of the other isolates. However, significant differences were found at 8% to 30% moisture levels. The efficacies of all isolates decreased with the decrease in moisture levels. MAX-2 was relatively less affected by desiccation stress. Its efficacy was almost unaffected by the decrease at moisture levels > 25%, but slowly decreased at moisture levels < 25%. By contrast, the efficacies of other isolates rapidly decreased with the decrease in moisture levels. MAX-2 caused different infection characteristics on T. molitor larvae under desiccation stress and in wet microhabitat. Local black patches were found on the cuticles of the insects, and the cadavers dried without fungal growth under desiccation stress. However, dark black internodes and fungal growth were found after death of the insects in the wet microhabitat.ConclusionsMAX-2 showed significantly higher efficacy and superior antistress capacity than the other isolates under desiccation stress. The infection of sterile T. molitor larvae at low moisture level constituted a valid laboratory bioassay system in evaluating M. anisopliae efficacy under desiccation stress.
Highlights
Metarhizium anisopliae, a soil-borne entomopathogen found worldwide, is an interesting fungus for biological control
This study aimed to evaluate the capacity of M. anisopliae isolate MAX-2 for infection under desiccation stress, and develop a valid laboratory bioassay system in testing the efficacy of M. anisopliae under desiccation stress with sterile Tenebrio molitor L. larvae in a substrate with low moisture content
Sterile culture of host insects T. molitor larvae were successfully reared in sterile wheat bran substrates with 15% moisture content at 25°C under natural day light, and cultured for more than five generations before use for the tests (Figure 1e)
Summary
Metarhizium anisopliae, a soil-borne entomopathogen found worldwide, is an interesting fungus for biological control. Metarhizium anisopliae (Metschnikoff) Sorokin is a fungus that is often found in soil, and can infect more than 200 species of insects [2]. This fungus is one of the first fungi used in biological control experiments. M. anisopliae is less virulent in the field than in the laboratory [3,4] because the environmental conditions in the soil may diminish its pathogenicity. The conidia of M. anisopliae attach to the cuticle of the host via germ tubes. Neutral trehalase has an important function in environmental stress response in many organisms, including Metarhizium spp. [6]
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