Sublethal effects of organophosphates on wolf spider enzymes: A dissertation submitted in partial fulfilment for a Bachelor of Science with Honours at Lincoln University

Abstract

Wolf spiders (Lycosa hilaris), an abundant arthropod in pasture throughout Canterbury, have potential as bioindicators of environmental contamination. Organophosphates are a class of insecticides widely used in New Zealand to control grass grub and porina caterpillars. The activity of cholinesterase (ChE) and glutathione S-transferase (GST) enzymes were assessed in wolf spiders as biomarkers of organophosphate contamination in agroecosystems. Spiders were exposed to field-simulated rates of chlorpyrifos (CP) and diazinon (DZ) under laboratory conditions. In terms of survival, CP and DZ were more toxic to male than female wolf spiders, but gender-specific differences in ChE and GST enzyme activities were not evident. Gluthathione S-transferase enzyme activity was not affected by either pesticide. Cholinesterase enzyme activity in male spiders was inhibited to 74% (P<0.00) by DZ and 51 % (P<0.00) by CP of activity in controls at field-rate soil concentrations under laboratory conditions.

The ChE enzyme response in wolf spiders to DZ was further investigated under field exposure conditions. Three hours after spraying the pasture with DZ, male wolf spiders in mesocosms were placed in replicate plots. Mortality was recorded at 24 h and survivors collected for subsequent ChE and GST enzyme analysis. This process was repeated every 24 or 48 h for 8 days. Mortality was 20% at 24 h and at this time, a significant inhibition in ChE enzyme activity (87%, P<0.001) was observed in the surviving spiders. Cholinesterase enzyme activity gradually returned to normal levels 6 days following pesticide application. The low mortality that occurs at recommended field rates, coupled with a clear and sustained inhibition of ChE to a single application of DZ, indicate the potential use of this enzyme in wolf spiders as a biomarker for field evaluations of exposure to organophosphates.