Insecticide cross-resistance spectra and underlying resistance mechanisms of insect pests of vegetables.

Abstract

Insecticide resistance spectra were investigated in seven species of insect pests ie. five aphid species (Aphis gossypii, A. craccivora, Myzus persicae, Lipaphis erysimi and Toxoptera citricidusi, a diamond-back moth species Plutella xylostel/a and a leaf minor species Liriomyza huidobrensis. Insects, except L. huidobrensis, were collected from the vegetable fields of Gannoruwa Agricultural Research Station and colonised in the laboratory. L. huidobrensis was from Nuwara Eliya vegetable farms.

Adult insects were exposed to three different insecticides; malathion (an organophosphate), propoxur (a carbamate) and pennethrin (a pyrethroid) using insecticide impregnated papers of different dosages. LDso and LD90 values were obtained by establishing log-probit mortality curves. Activity levels of insecticide metabolising enzymes ie. carboxylesterases, glutathione-S-transferases and oxidases, were investigated using biochemical. Quantitative changes of carboxylesterases were studied by native polyacrylamide gel electrophoresis. Qualitative changes were studied by detecting the rates of malathion metabolism. Resistance of the insect target site, acety1cholinesterases, to insecticide inhibition was tested biochemically.

The highest resistance to all three insecticides was shown by L. huidobrensis and M persicae. LDso values (ug/cnr') of L. huidobrensis population for malathion, propoxur and pennethrin were 1680.0, 13640.0 and 400 respectively. For M persicae, these values were 408.0, 128.0 and 24.0. A. craccivora was the most susceptible to malathion (LDso= 4.8) while T. citricidus (LDso= 0.2) and A. gossypii (LDso= 0.4) had the lowest resistance for propoxur and pennethrin respectively. Results of biochemical assays showed that acety1cholinesterases of L. huidobrensis are completely altered giving high resistance to organophosphates and carbamates. High activity levels of insecticide metabolising enzymes were found in M persicae, L. erysimi, A. gossypii and P. xylostella. Quantitative changes of carboxylesterases were found in all the pest species except T. citricidus. Qualitatively different (fast metabolising) carboxylesterases were found only in P.xylostella.