Toxicity and repellent action of Coffea arabica against Tribolium castaneum (Herbst) adults under laboratory conditions

Abstract

Coffea arabica parchment extracts and caffeine isolated from the plant were evaluated randomly against 1-week-old adults of red flour beetle, Tribolium castaneum for fumigant toxicity and repellent action under laboratory conditions. The effects on detoxification enzymes and neuroenzyme was also determined. Among the various extracts prepared sequentially using hexane, dichloromethane, ethyl acetate and methanol as solvents, the dichloromethane extract did repel insects on contact (EC50 = 4380.65 ppm). The repellence was more prominent when an olfactory response was measured (EC50 = 2571 ppm). The active ingredient isolated from dichloromethane extract was identified as caffeine which showed very strong repellency as compared to the extract. In terms of toxicity of this extract, a significant mortality was recorded in fumigation assay (LC50 = 5555 and 791 ppm, 24 and 48 h post-treatment, respectively). However, caffeine did not induce similar toxicity as the dichloromethane extract. The studies on the impact on detoxification enzymes of T. castaneum showed that dichloromethane extract inhibited carboxyl esterase activity, which possibly led to high toxicity. However, caffeine inhibited glutathione-s-transferase and induced carboxylesterase enzymes. It was, therefore, obvious that C. arabica parchment crude extracts have dual effects against T. castaneum adults, i,e, fumigant toxicity and repellent effects. However, the active compounds responsible for the two activities are surely different as caffeine could only induce repellent action against the beetles and the toxic compound needs to be identified, which is presently being investigated.