The cascading trophic accumulation of aldicarb in a carrion ecosystem: the forensic implications
Motolo, Tshepiso Christinah
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Entomotoxicology is a relatively new discipline in forensic entomology which deals with the study of drugs in insects of forensic importance. The toxicant under investigation is the active ingredient found in an insecticide commonly known as “Two steps”. This was the first study to investigate the forensic implications of the toxicant, aldicarb, in a carrion ecosystem. Insects that feed on a deceased person who had toxicants in his or her system will also ingest the toxicants. In cases where the decomposed tissues of the deceased have degraded and can no longer be used for traditional toxicological analysis, insect specimens can be utilised as an alternative toxicology matrix. A high performance liquid chromatograph-Ultra Violet detector (HPLC-UV) was used to detect the toxicant in entomological specimens. Varying concentrations of aldicarb were mixed with chicken livers and presented to Chrysomya chloropyga fly larvae and Thanatophilus micans adult beetles. There was a correlation between the aldicarb concentrations found in entomological specimens and the concentration present in the chicken livers. An experiment was also set up to test for the accumulation of aldicarb in a secondary trophic level of a carrion food chain. To this end, C. chloropyga larvae that were exposed to the toxicant at varying concentrations were presented as a prey item to predatory Chrysomya albiceps fly larvae and adults of the predaceous Saprinus splendens beetle. Although the toxicant was recovered from these predators, there was no correlation between the toxicant concentrations in them and that that the C. chloropyga larvae were exposed to. Another iteration of this experiment was to test which of the post-feeding life stages of flies can also be used as an alternative toxicological matrix. It was postulated that the toxicant would be eliminated during the post-feeding stages of the fly and that some of the toxicant might be deposited in the cuticle of larvae and consequently also in the pupal casings. It was found that since the emergent adult flies of C. albiceps and S. cruentata still contained trace amounts of the toxicant, the pupae of these two species would still be suitable alternative toxicology sources. The toxicant was not be picked up in the emergent adult flies of C. chloropyga and it is unclear up to what point the toxicant remains present in its pupae. The toxicant was not picked up in the pupal casings of any of the flies. However since the extraction method used might have been inadequate to release the toxicant from the chitinised matrix of the pupal casing, a verdict cannot be made regarding excluding the pupal casings as alternative toxicology source. When calculating a post mortem interval (PMI) based on the Developmental Model, it is imperative to know whether or not the deceased was exposed to toxicants since toxicants can potentially influence the growth rate of forensic indicator species. To test the effect of aldicarb on the development of forensic flies, C. chloropyga, C. albiceps and Sarcophaga cruentata larvae were exposed to a lethal dose of aldicarb. Larvae were measured and weighed at 24 hour intervals, pupal development was tracked by noting morphological landmarks every 24 hours and adult fitness was assessed based on the ability to reproduce. Aldicarb slowed down the total development rate of C. chloropyga and accelerated that of C. albiceps but had no effect on S. cruentata. The necessary PMI adjustments should be made for the calliphorid life stages that were exposed to the toxicant as larvae. It was furthermore noted that the toxicant did not affect the reproductive fitness of all species examined.