The effects of organophosphate exposure on non target terrestrial and aquatic organisms following different exposure regimes : linking biomarker responses and life-cycle effects

Jordaan, Martine Saskia (2010-12)

Thesis

ENGLISH ABSTRACT: The use of organophosphate pesticides is still an integral part of commercial farming activities and these substances have been implicated as a major source of environmental contamination in South Africa. Evidence exists that many non target animals in and around agricultural areas are at risk of being affected due to the mobile nature of pesticides and the intermittent nature of pesticide application. The extent to which non-target animals are affected by exposure to two organophosphates (azinphos-methyl and chlorpyrifos) was investigated through monitoring selected biomarker responses and life cycle effects under laboratory conditions in two selected test species. A representative species from both the aquatic and terrestrial environment was used as these two compartments of the environment are inevitably linked due to the mobility of pesticides from the area of application to surrounding areas. The earthworm Eisenia fetida was used as test organism in the terrestrial environment while the fish Oreochromis mossambicus served as representative of the aquatic environment. Juvenile life stages of both species were subjected to standard acute toxicity tests which showed that for both species, juvenile life stages were more sensitive to both pesticides than adults. It was also illustrated that azinphos-methyl is more toxic than chlorpyrifos to both species. Both test species were also subjected to an intermittent exposure regime in order to assess the effects of repeated pesticide application on biomarker, life-cycle and behaviour responses. The results indicated that for similar exposure regimes, azinphos-methyl was more toxic to E. fetida than chlorpyrifos and detrimentally affected all endpoints investigated. The present study suggests that exposure concentration may have a more pronounced effect in inducing a toxic response than exposure interval, irrespective of the pesticide used. In addition to this, E. fetida was unable to avoid the presence of these pesticides in soil, even at concentrations as high as 50% of the LC50 value, indicating that the presence of pesticides in the soil pose a realistic threat to earthworms and other soil dwelling organisms. Biomarker responses, morphological effects and feeding behaviour was assessed for O. mossambicus and similar to the terrestrial toxicity experiments, there was evidence to suggest that in the case of an intermittent exposure scenario, azinphos-methyl was more hazardous than chlorpyrifos to this species. For the majority of endpoints that were investigated, it appeared that exposure interval played a more important role in inducing an effect than exposure concentration. At a shorter exposure interval, the majority of endpoints showed no difference between higher and lower exposure concentrations, while at a longer exposure interval the effects of exposure concentration became evident. In addition, feeding behaviour was affected by pesticide exposure in a dose-dependent manner. The present study yielded important results that improve the understanding of biological impacts of pesticide pollution on the environment. This can aid in optimising farming practices such as pesticide application not only in terms of eradicating the pest organisms, but also in terms of mitigating the environmental effects associated with large-scale pesticide use, thereby ensuring sustained biodiversity in these areas.

AFRIKAANSE OPSOMMING: Die gebruik van organofosfaat plaagdoders is ‘n integrale deel van kommersiële landbou aktiwiteite maar hierdie middels is ook ‘n prominente bron van omgewingsbesoedeling in Suid-Afrika. Daar is bewys dat verskeie nie-teiken diere in en om landbouareas geaffekteer word weens die nie-statiese aard van plaagdoders in die omgewing, sowel as die herhalende aard van plaagdodertoediening. Die graad waartoe nie-teiken diere geaffekteer word deur die plaagdoders azinphos-metiel en chlorpyrifos is ondersoek deur die monitering van verskeie biomerkerresponse en lewenssiklus-effekte in geselekteerde toetsspesies binne ‘n beheerde laboratoriumomgewing. ‘n Verteenwoordigende spesie van beide die akwatiese en die terrestriële omgewing is gebruik aangesien hierdie twee dele van die omgewing onlosmaaklik verbind is weens die beweging van plaagdoders vanaf die area van toediening na omringende areas. Die erdwurm Eisenia fetida is gekies as toetsorganisme vir die terrestriële omgewing en die varswatervis Oreochromis mossambicus het gedien as verteenwoordigende spesie vir die akwatiese omgewing. Onvolwasse diere van beide spesies is onderwerp aan standaard akute toksisiteitstoetse en daar is gevind dat, vir beide spesies, onvolwasse diere meer sensitief vir die betrokke plaagdoders is as volwasse diere. Dit is ook gevind dat azinphosmetiel giftiger is as chlorpyrifos vir beide spesies. Beide toetsspesies is ook onderwerp aan ‘n chroniese blootstellingsregime om die effek van herhaalde plaagdodertoediening op biomerker-, lewenssiklus- en gedragsresponse te ondersoek. Die resultate van die herhaalde blootstelling het aangedui dat vir soortgelyke blootstellingsregimes, azinphos-metiel giftiger is as chlorpyrifos vir E. fetida en dat beide middels alle eindpunte wat ondersoek is, nadelig affekteer. Die huidige studie toon ook bewyse dat blootstellingskonsentrasie ‘n meer prominente effek as blootstellingsinterval kan hê in die teweegbringing van ‘n toksiese respons. Verder was E. fetida nie in staat om die teenwoordigheid van die plaagdoders in grond te vermy nie, self nie by konsentrasies so hoog as 50% van die LC50 waarde nie. Laasgenoemde resultaat dui dus aan dat die aanwesigheid van plaagdoders in die grondomgewing ‘n realisitese bedreiging inhou vir erdwurms en ander grondorganismes. Soortgelyk aan die terrestriële toksisiteitseksperimente, was daar getuienis vir die verhoogde toksisiteit van azinphos-metiel relatief tot chlorpyrifos vir O. mossambicus. Dit blyk dat blootstellingsinterval ‘n meer prominente rol as blootstellingskonsentrasie speel in die teweegbringing van effekte vir die meerderheid van die eindpunte wat ondersoek is. In die geval van ‘n korter blootstellingsinterval het die meerderheid van eindpunte wat ondersoek is geen verskille getoon tussen ‘n hoër en ‘n laer konsentrasie nie, terwyl met ‘n langer blootstellingsinterval daar ‘n aanduiding was dat blootstellingskonsentrasie ‘n meer prominente rol gespeel het. Verder is gevind dat voedingsgedrag in O. mossambicus geaffekteer is op ‘n konsentrasie verwante manier. Die huidige studie toon resultate wat ‘n belangrike bydrae kan lewer tot die begrip van die biologiese impakte van organofosfaat plaagdoders op die omgewing. Die resultate kan gebruik word vir die optimisering van boerderypraktyke soos plaagdodertoediening, sodat laasgenoemde effektief is vir die beheer van pes-organismes, maar ook die impakte van grootskaalse plaagdodertoediening kan minimaliseer en sodoende die biodiversiteit binne hierdie areas sal beskerm.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/5312
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