Year

2015

Degree Name

Master of Science

Department

School of Biological Sciences

Abstract

The scarcity of information on the effects of pesticides on Australian native vertebrates constrains the development of biologically relevant risk assessments in Australia for the registration of insecticides. This thesis investigates the sensitivity of two dasyurid marsupials, Sminthopsis crassicaudata (Gould 1844) and S. macroura (Gould 1845) to an organophosphorous (OP) insecticide, fenitrothion (O,O-dimethyl-O- (3-methyl-4-nitrophenol)-phosphorothioate), a common locust control agent in Australia. The effects of fenitrothion on a suite of physiological measures that affect the ability of animals to survive in free-living conditions (namely, locomotory and thermogenic functions, metabolic performance, body mass, haematocrit and haemoglobin levels as well as plasma and brain cholinesterase activity) are investigated in relation to time since exposure in S. macroura.

The concern that endemically old and unique Australian vertebrate fauna might display high sensitivity to pesticides used for locust control provoked examination of the acute oral toxicity of fenitrothion for the fat-tailed dunnart S. crassicaudata and the stripe-faced dunnart, S. macroura in Chapter 2 of this thesis. Using the Up-and-Down method for determining acute oral toxicity, S. crassicaudata and S. macroura were found to have estimated median lethal doses (LD50) of 129 mg kg-1 (95% confidence interval (CI) = 74.2 - 159.0) and 97 mg kg-1 (95% CI = 88.3 - 120.0) respectively. These values are 10 - 14 times lower than the reported LD50 values for a similar-sized eutherian mammal, Mus musculus (L. 1758) (LD50 = 1100 - 1400 mg kg-1) and lower than all other reported mammalian LD50 values. Such wide inter-specific variation in sensitivity to fenitrothion may be a consequence of underlying differences in the metabolic pathway for fenitrothion detoxification in mammals and a possible explanation for the increased toxicity of fenitrothion to dunnarts, as compared with other mammals, is proposed. The unexpectedly high sensitivity of these Australian marsupials to fenitrothion emphasises the importance of adequately evaluating the risks of pesticides to endemic Australian fauna.

Agricultural pesticides applied for locust control have the potential to exert structural and functional effects on Australian arid zone ecosystems by impacting endemic and evolutionarily unique species occupying the same habitat as the target insect. In Chapter 3 I examined the impact of fenitrothion on a suite of physiological measures that affect the ability of animals to survive in free-living conditions; namely, locomotory and thermogenic functions, metabolic performance, body mass, haematocrit and haemoglobin levels. Plasma and brain cholinesterase activity in relation to time since exposure to pesticide were also determined. An orally applied dose of 90 mg kg-1 fenitrothion reduced running endurance in S. macroura by 80% the day after exposure concomitantly with a reduction of approximately 50% in plasma and 45% in brain acetylcholinesterase activity. These adverse effects disappeared by 10 days postexposure. Maximal metabolic rates reached during running were unaffected by pesticide as were body mass, haemoglobin and haematocrit levels. Maximal coldinduced metabolic rate (measured as the peak 2 min metabolic rate attained during cold exposure), the time taken to reach peak metabolic rate upon cold exposure, the cumulative total oxygen consumed during the shivering thermogenesis bout and body temperature before and after cold exposure were unaffected by fenitrothion. Dunnart rectal temperature showed a reduction of up to 5 °C after exposure to fenitrothion, but returned to pre-exposure levels by 10 d post dose. Such physiological compromises in otherwise asymptomatic animals demonstrate the importance of considering performance-based measures in pesticide risk assessments.

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