The impact of avermectin usage on the ecology of dung insect communities and the potential implications for foraging birds

Webb, Lisa (2004) The impact of avermectin usage on the ecology of dung insect communities and the potential implications for foraging birds. PhD thesis, University of Glasgow.

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There is concern that the use of avermectin worming products in livestock and the subsequent presence of avermectin residues in dung could negatively affect the survival and development of dung-breeding insects in pastures. Such a reduction in natural populations of dung insects has potential implications for the vertebrate predators that forage in pastures for invertebrate prey. This study compared the abundance, diversity and assemblage structure of adult dung insects (Aphodius, Cercyon and Sphaeridium beetles and yellow dung flies, Scatophaga stercoraria) between pastures grazed either by avermectin-treated or untreated cattle. Sampling was conducted using dung-baited pitfall traps in cattle-grazed pastures in Ayrshire, South West Scotland from April to July of 2002 and 2003. Twelve fields grazed by cattle that were not anthelmintically treated and fourteen grazed by cattle receiving either a doramectin or an ivermectin product were sampled. Six 'untreated' and six 'treated' fields were sampled in both years of the study while all other fields were sampled in only one of the years. Generalized Linear Models were used to investigate the significance of potentially influential factors for the abundance, diversity and assemblage structure of the dung insects under study. Those factors included avermectin treatment, seasonality, pasture management intensity, weather and various habitat variables. A multivariate ordination technique was used to explore differences in the species compositions of dung insect communities in study pastures. In general, the factors found to be consistently significant for patterns of variation in dung insect abundance and diversity were year, seasonality and weather. There was no significant difference in the abundance of adult Cercyon beetles or yellow dung flies between pasture grazed by treated or untreated cattle. Sphaeridium beetles were trapped in mimbers that were not sufficiently high to be modelled. Significantly more Aphodius dung beetles were trapped in fields grazed by treated cattle and evidence from additional fields trials suggested that this phenomenon could have been due to an avoidance of dung from avermectin-treated cattle. Wing length asymmetry was higher in yellow dung fly populations in pastures grazed by avermectin-treated cattle, suggesting that individual flies may have undergone developmental stress in dung from treated animals. However, higher asymmetry in treated fields could not be solely attributed to avemiectin exposure and other potential reasons for the difference in asymmetry are considered. Furthermore, there was no evidence that this possible sublethal effect impaired the overall density of yellow dung fly populations in pastures. Variation in the size stmcture of Aphodius dung beetle assemblages was mainly due to seasonality and there was no effect of avermectin treatment. Therefore, the profitability of prey items for foraging birds is more likely to be a function of seasonal occurrence than due to any difference associated with avermectin treatment. Furthermore, basic observations of the foraging activity of birds in pastures did not show any major differences in the foraging activity of birds between pastures grazed by treated and untreated cattle. Hence, the availability of dung insects for foraging predators is more likely to fluctuate according to variation in season, weather and year than it is with avermectin treatment. However, the unsuitability of dung from avermectin-treated cattle for Aphodius dung beetles could potentially reduce beetle abundance in pastures when an alternative 'untreated' dung resource is not available. A survey of the use of anthelmintics in livestock farms in South West Scotland was conducted to help guide the selection of study sites and to allow any results to be set in a wider context. This study was conducted in a region where dairy fanning is predominant. The questionnaire survey indicated that aspects of livestock management and anthelmintic treatment on dairy farms increase the availability of avermectin-free dung in the landscape. Therefore, it cannot be disregarded that the observed minimal effects of avennectins on dung insects may have been mitigated by the presence of 'untreated' dung for insects to colonise. It is proposed that in areas where avemiectin-free dung is limited, either because of farming type or geographical area, negative effects associated with unsuitability of dung from treated animals on populations could occur. Such effects could be mitigated by management practice. For example, only young animals should be treated and unnecessary treatment of immune adults should be avoided. Where possible, avemiectin-treated cattle should be grazed in pasture adjacent to pasture that is grazed by untreated livestock. On grazed grassland being managed to benefit insectivorous species e.g. waders, a non-avemiectin wormer or one of the less toxic avermectins could be used to worm livestock.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Davy
Keywords: Ecology, entomology.
Colleges/Schools: College of Medical Veterinary and Life Sciences
Supervisor's Name: McCracken, Prof. Davy and Nager, Dr. Ruedi
Date of Award: 2004
Depositing User: Enlighten Team
Unique ID: glathesis:2004-71339
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 10 May 2019 10:49
Last Modified: 28 Jul 2021 15:08

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