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Insects such as locusts and grasshoppers can reduce the effectiveness of pathogens and parasites by adopting different defense strategies. We investigated the behavioral thermopreference of Locusta migratoria manilensis (Meyen) (Orthoptera: Acrididae) induced by the fungus Beauveria bassiana, and the impact this behavior had on the fungal mycosis under laboratory conditions. By basking in higher temperature locations, infected nymphs elevated their thoracic temperature to 30–32.6 °C, which is higher than the optimum temperature (25°C) for B. bassiana conidial germination and hyphal development. A minimum thermoregulation period of 3 h/day increased survival of infected locusts by 43.34%. The therapeutic effect decreased when thermoregulation was delayed after initial infection. The fungus grew and overcame the locusts as soon as the thermoregulation was interrupted, indicating that thermoregulation helped the insects to cope with infection but did not completely rid them of the fungus. A significant enhancement in the number of haemocytes was observed in infected thermoregulating locusts, reaching levels that were even higher than those observed in the controls. In contrast, haemocyte concentration was severely reduced in infected insects that did not thermoregulate. In infected non-thermoregulating locusts, the reduction in haemocyte number was accompanied by an increase in fungal blastospore concentration that was obvious in the haemolymph by day four. In contrast, no circulating blastospores were found in the haemolymph of infected thermoregulating locusts three days post-inoculation. We also examined the phagocytic activity of infected insects in vivo by using fluorescein isothiocyanate (FITC)-labelled silica beads. The proportion of beads that was engulfed by haemocytes in infected, thermoregulating insects was similar to that in the controls throughout the experiment, whereas the rate of phagocytosis in infected, non-thermoregulating insects progressively decreased after infection. These findings demonstrated that behavioural thermoregulation can adversely affect B. bassiana mycosis in infected L. migratoria manilensis, thereby limiting the development of lethal entomopathogenic fungi in locusts. This is apparently accomplished through an increase in the levels of haemocytes, leading to greater phagocytic activity under certain environmental conditions.
Background
Beauveria bassiana (Bals.) (Hypocreales: Cordycipitaceae) is an entomopathogenic fungus that has potential as a biological control agent against many insect pests. This study was conducted to optimize the conditions for large-scale production on rice of B. bassiana and evaluate its virulence against the bean flower thrips Megalurothrips usitatus (Bagnall) (Thysanoptera: Thripidae).
Results
The optimal substrate quantity for conidia production was 200 g of rice per container (2.1 l), with substrate having a 52% moisture content, being supplemented with 2% cooking oil. In stage production of a batch, 150 ml of conidia suspension (at 24.33 × 107 spores/ml) was incubated for 10 days in SDA liquid medium amended with 4% glucose and 1.5% yeast extract. The SDA medium was then used to inoculate one container of the rice substrate. The optimum ratio of conidial suspension to substrate (v/v) was 20–25%. Virulence of the spore powder harvested from the rice substrate was same as that of produced on the SDA solid media against thrips. The level of control from an application of spore powder of the thrips M. usitatus was similar to that from chemical the pesticide spinetoram when applied in a cowpea (Vigna sinensis Endl) field.
Conclusions
The optimized rice substrate system can be used for a large-scale production of B. bassiana spores, which can be used for field control of thrips.
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