Nutrient-fortified metarhizium anisopliae granules for the management of soil-borne insect pests of sweetpotato

Nutrient-fortified metarhizium anisopliae granules for the management of soil-borne insect pests of sweetpotato

Sweetpotato (Ipomoea batatas) is a small but important vegetable crop in Australia, with a total annual production of about 100,000 tonnes, estimated to be worth AU$100 million. Sweetpotato is susceptible to damage by many root herbivores like wireworms, nematodes, and weevils. Wireworm damage is sporadic but significant in sweetpotatoes, despite the deployment of insecticides. Inferior performance of soil insecticides and the discouraged use of hazardous insecticides has encouraged research into biological control agents such as entomopathogenic fungi (EPF). Entomopathogenic fungi like Metarhizium anisopliae are frequently used in integrated pest management programs and may be a suitable candidate for wireworm control. The capacity of an EPF to grow in soil may determine the outcome of wireworm control in sweetpotato fields. This study assessed the resporulation potential of nutrient-fortified fungal granules on different soil types with various levels of reduced soil microbes. The infectivity of the resulting resporulated fungal granules in soil was also assessed using larval mealworms (Tenebrio molitor) as a model insect under the laboratory and glasshouse conditions.

When the resporulation of fungal granules was examined in response to nutritive additives, the combination of 20% w/v compressed baker's yeast (Saccharomyces cerevisiae) and 20% w/v corn starch (Zea mays) induced the greatest fungalresporulation and this formulation was used for subsequent experimentation. Soil type had no effect on fungal resporulation from granules; however, the role of soil treatment (sterilised, pasteurised, and non-sterile soils) was significant. Fungal resporulation was significantly pronounced in sterilised soil possibly due to the reduction in the background, resource-competing soil microbes (bacteria and fungi).

Despite the variability of fungal resporulation among sterilised, pasteurised, and non-sterile soil, the overall mealworm mortalities caused by the resporulated fungal granules were not different. However, the fungal infectivity against larval mealworms was significantly higher on sterilised soil than on non-sterile soil under glasshouse conditions. The inability of fungal granules to resporulate on non-sterile soil in the glasshouse may have resulted in low mealworm mortalities.

Although soil fumigation only causes short-term suppression in the antagonistic microorganisms, this was sufficient to maximise resporulation of fungal granules. The fungal ranules inoculated on fumigated soil resporulated profusely and the resulting granules killed at 100% larval mealworms after 10 days. Additionally, the fungal conidia from resporulated granules had excellent germination, which was significantly greater than those found germinating on non-sterile soil. The information and knowledge gained during this research have important implications for the biological control-based management of insect pests, especially in high-value horticulture like sweetpotato.

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