DETECTION OF INSECTICIDES RESISTANCE IN SPODOPTERA EXIGUA (LEPIDOPTERA: NOCTUIDAE) DEPENDS UPON INSECT COLLECTION METHODS

Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) is a polyphagous pest with high migratory potential, which could contribute significantly to the population outbreaks. Due to indiscriminate use of insecticides the pest has developed resistance to several groups of insecticides. We were interested to establish if the fourth instar larvae of S. exigua collected from potato and cabbage would be more resistant than the adults collected using light traps. The results of bioassays suggest that the toxicity of pyrethroids to S. exigua was significantly lower for field populations (P 0.01) compared with laboratory susceptible population (Lab- Pak). Levels of resistance to pyrethroids in S. exigua collected from potato field were in excess of 20–fold for 8 populations out of 10 populations. The population was significantly more resistant to organophosphates tested {average LC = 306. 20 (237.39-395.18)} than 50 pyrethroids {average LC = 146.25 (114.44-186.98)}. The observed resistance in the 50 population collected from cabbage was significantly (P 0.01) higher for spinosad, indoxacarb and chlorfenopyr with a resistance ratio of 702-fold, 1219- and 436-fold respectively compared with Lab-Pak. A possible explanation for variation in resistance to insecticides on different host plants is that different secondary metabolites in cabbage or potato plants have elicited different metabolic changes in S. exigua, such as activation or inhibition of metabolic ability, detoxification enzymes or a general increase in esterase activities. The results are discussed in relation to role of different host plants in insecticides resistance management.