The Influence of Food Type on the Response of Sitophilus zeamais (Motschulsky) (Coleoptera: Curculionidae) to Cypermethrin 
Oyeniyi, E. A., Gbaye O. A. and Boboye D. J.

NJE Vol. 34: 133-143, 2018


Abstract: The effect of food type on the response of maize weevil, Sitophilus zeamais (Motschulsky) to cypermethrin was investigated. Weevils were reared on maize, wheat and sorghum at ambient temperature (28±2ºC) and relative humidity (75±6%). Twenty grams of each food type was doused with one millilitre of different concentrations (0.00001-0.1%) of cypermethrin. Twenty emerged weevils from each food were then exposed to each pre-treated food type. Two control experiments were also set up (one with neither cypermethrin nor methanol and the other with only methanol). The mortality was assessed after 6, 12, 18 and 24 hours post-treatment. Weevil mortality varied with host food, cypermethrin concentration and exposure time. All weevils exposed to cypermethrin treated maize and sorghum grains, respectively exhibited complete (100%) mortality at 0.1% after 24 hours’ exposure while there was incomplete mortality (96.88%) in weevils exposed to 0.1% cypermethrin after 24 hours’ exposure. The lethal concentration values also showed that susceptibility of S. zeamais to cypermethrin was least in wheat-reared weevils (LC75: 0.052 g ml-1and LC95: 275.890 g ml-1) while highest susceptibility was observed in maize-reared weevils (LC75: 0.009g ml-1; LC95: 0.813 g ml-1). The sorghum-reared weevils however showed the median susceptibility (LC75: 0.018g ml-1; LC95: 106.530 g ml-1) to this pyrethroid. Also, irrespective of the host food, there was significant reduction and inhibition (p<0.05) in the adult emergence and developmental period of S. zeamais in treated grains when compared to that of untreated and solvent-treated grains. Similarly, weight loss in the treated maize grains was also significantly lower (p<0.05) than that of untreated and solvent-treated grains. This study reveals that the host food on which weevil developed influenced considerably the degree of tolerance to cypermethrin. It could therefore be recommended that crop merchants, peasant farmers and pest control managers should be mindful of host food on which S. zeamais is found when controlling them using cypermethrin insecticide. This will help to prevent the problem of pest resistance that is currently associated with other synthetic insecticides.

Keywords: Cypermethrin, Sitophilus zeamais, Weight loss, Adult emergence, Lethal concentration

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