T he original aim of my semester-long Minor Project was to simultaneously test the effect of an increase in temperature and pathogen load on pathogen transmission to a native stingless bee via flowers. However, due to the nature of biological research and working with pathogens, this aim became challenging to investigate in a single semester. This led me to pursue an equally fascinating interaction, similarly overlooked in the literature, that was easier to control – temperature and insecticide. Though many insects are able to regulate their body temperature via behavioural and physiological mechanisms, they will ultimately lose muscle control under extreme hot and cold conditions, referred to as critical thermal limits. Such limits are relevant to tropical ecosystems where many species tolerate a narrower temperature range and often live closer to their upper thermal limit i.e. their critical thermal maxima (CTmax). I used a laboratory experiment to test whether acute exposure to insecticide reduced the thermal tolerance of the stingless bee, Tetragonula hockingsi.
I provided acclimated bees with four sublethal doses of fipronil in a sucrose solution, with a sucroseonly control, and measured their CTmax after an exposure period using a dynamic temperature ramp. I maintained additional bees at a constant temperature for the duration of a temperature ramp to assess whether responses were due to insecticide treatment only. I chose to use the insecticide fipronil as it is systemic, has broad-scale applications (from crop protection to ant baiting programs) and has been widely implicated in bee declines.
My results revealed mean CTmax to be significantly lower in bees which received higher doses of fipronil than those given sucrose only. However, bee deaths during my non-ramped control precluded me from making strong conclusions about an interaction, thus I plan to run more experiments in the coming weeks to test additional insecticide doses. I found T. hockingsi to be highly sensitive to fipronil, and future research should investigate exposure pathways and seek to quantify insecticide residue levels in pollen and nectar. A Workplace Integrated Learning student who assisted on my project was inspired to pursue this avenue for their Honours project this year. Overall, understanding the physiological tolerances of bees, such as CTmax, will be useful in predicting biotic responses to a changing climate and heat events.