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A survey of pesticide accumulation in a specialist feeder, the koala (Phascolarctos cinereus)

Marschner, C, Higgins, DP & Krockenberger, MB 2017, Bulletin of Environmental Contamination and Toxicology, vol. 99, no. 1, p. 303-307.

This study is the first to examine exposure to and effects of pesticides in koalas. In all tissue samples from koalas that inhabited areas impacted by horticultural, agricultural or urban development, pesticide levels were consistently below the limit of detection. This finding suggests that, for these koalas, chronic ongoing or acute exposure to pesticides was unlikely.

  During necropsies of 57 koalas that had died of unrelated causes, samples of liver tissue were taken. These animals came from habitats in Queensland and New South Wales affected by or adjacent to horticultural, agricultural and urban activity. Typically, fatty tissues are sampled for pesticide screening as many pesticides are lipophilic, tending to combine with lipids or fats. As koalas lack fatty tissues, however, the liver was sampled as the next most suitable tissue. Previous studies have reported levels of pesticides in mammals, birds, reptiles and fish to range between from 40 µg/kg tissue to around 3 mg/kg tissue. In this investigation, pesticide levels were consistently below the detection limit of 50 µg/kg tissue.

  In the past 50 years, use of agricultural and horticultural pesticides in Australia has increased by 836%. Given this remarkable increase in pesticide usage in combination with the encroachment of the agricultural and horticultural industries into koala habitat across the species’ range, it is somewhat surprising that pesticides were not detected in the livers of koalas. Some of the pesticides tested for in this study have relatively low lipophilicity, reducing the likelihood of the chemicals bioaccumulating in tissue. Additionally, it is possible that koalas can rapidly detoxify and thus prevent the accumulation of pesticides in tissue. Chemical biomagnification is more likely with an animal’s increasing trophic level. Although the koala is a folivore, its unique diet may make it vulnerable to the effects of pesticides. Eucalypt leaves contain plant secondary metabolites which are metabolised with the aid of cytochrome P450 enzymes. In other species, pesticides have been found to inhibit these enzymes. If this also occurs in the koala, exposure to pesticides may reduce the animal’s ability to metabolise potentially toxic plant secondary metabolites and consequently disrupt food intake.

  Despite a dramatic increase in pesticide usage in recent decades, records of use are not readily accessible, and thus its impacts on wildlife are difficult to monitor. Although no evidence of pesticide accumulation in koalas was found in this study, other negative effects cannot be excluded. It will be important to continue to monitor the potential effects of pesticides in koalas as increasing habitat fragmentation will create more opportunities for exposure to agricultural, horticultural and urban contaminants.


Summarised by Joanna Horsfall


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