The biology of the slug-killing Tetanocera elata (Diptera: Sciomyzidae) and its potential as a biological control agent for pestiferous slugs
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Laboratory experiments were carried out to determine the effects of controlled temperatures (14oC, 17oC, 20oC, 23oC, and 26oC) and ambient outdoor and laboratory temperatures on the development and predation capacity of Tetanocera elata (Fabricius) (Diptera: Sciomyzidae), potential biocontrol agent of pestiferous slugs. In addition, predatory behaviour of the third-instar larvae was observed and categorised using infrared digital recordings. Results suggest that of the temperatures tested, the optimum for egg hatching is 14oC with a median duration of 13 days and a hatch rate of 52% (the highest hatch rate of all temperature conditions examined). The low hatch rate (5%) for eggs stored at 2-3oC indicates that storing eggs at low temperatures does not seem to be a viable option for T. elata. The effects of temperature on the larval stages of T. elata were found to be similar to those for the egg stage. In general, as temperature increased, the duration decreased. However, percentage larval survival at constant high temperatures (26oC and 23oC) was poor (0% and 4% respectively). Nevertheless, larval survival rate at ambient outdoor temperatures (which rose to 30oC) was 52% suggesting that larvae can tolerate higher temperatures but not for prolonged periods. The recommended optimum temperature of those tested for rearing T. elata larvae is 20oC which had the greatest percentage larval survival (62%) with a median duration of 44 days. In addition, the sex of adult flies can be predicted prior to emergence using puparial weights which could prove useful in mass culture and biocontrol release programmes. No significant difference in the median number of slugs killed per larva at each temperature was detected with each larva killing 6 - 8 slugs. An analysis of digital recordings of the behaviour of third instar T. elata larvae indicate three different strategies for prey finding: 1) searching and attacking; 2) searching and waiting; 3) waiting. Previous contact with a slug was not always required for a larva to actively search for a slug. Slugs were immobilised by a larva with the head being the preferred site where feeding took place although the head was not always the site at which the initial attack took place. The results of this study are discussed in the context of developing optimum conditions for the rearing of T. elata with the view of using it in biological control trials. Recommendations for future work are also discussed.
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