SYNERGISTIC QUALITIES OF SELECTED PLANT EXTRACTS ON POTENCY OF PYRETHRINS AGAINST THE MAIZE WEEVIL, SITOPHILUS ZEA-MAIS (MOTSCH.) (COLEOPTERA: CURCULIONIDAE)

Abstract

Post-harvest losses of grains are a major hindrance to attainment of food security in Kenya due to infestation of coleopteran grain pests such as weevils. Although synthetic chemical insecticides are being used in their control, overuse and misuse of these insecticides have resulted in problems of pesticide resistance, environmental contamination, pest resurgence and even consumer poisoning. Several studies have been done on utilizing plant essential extracts and oils in control of storage pests but their limitations, such as inconsistencies in efficacy, lack of persistence and residual effect have hindered their use as stand-alone products for pest management. Piperonyl butoxide (PBO) is often the synergist used to enhance efficacy of insecticides like pyrethroids and pyrethrum formulations. Synergists enable the use of an active ingredient in very small quantities by preventing detoxification within the insect thus un-synergised formulations are rarely used. This study aimed at evaluating synergistic qualities of selected plant extracts on potency of pyrethins formulations against the maize weevil, Sitophilus zea-mais (Motsch.) (Coleoptera: Curculionidae). Plants extracts (Black pepper seeds, Nutmeg seeds, Coriander leaves and roots) “synergists” used were selected on the basis of possessing a methylenedioxyphenyl (MDP) ring structure similar to that of PBO. Full dose response, dose-mortality, synergism experiments were carried out on S. zea-mais at four concentrations of synergists, each at four ratios (synergist: pyrethrins) while infestation was carried out on maize treated with plant ectracts-pyrethrins formulations. Topical application of pyrethrins/synergist/formulation on S. zea-mais was done in triplicate in a Completely Randomised Design. Experiments were conducted under controlled laboratory conditions of 27 ± 2OC and 60 ± 5% RH with normal day light hours. Probit analysis was used to determine the lethal concentration (LC) of pyrethrins to the S. zea-mais. Analysis of Variance was used to obtain the mean mortality differences of S. zea-maisat P ≤ 0.05 while Duncan’s Multiple Range Test was used to rank significant concentration means within a synergist. Results obtained found LC20 value for pyrethrins to be 2,200 ppm. To obtain LC50, 14227 ppm, 13780 ppm and 8868 ppm of pyrethrins wwere required over a 24 h, 48 h and 72 h exposure time respectively. Black pepper seed hexane extract (BPSHE) and PBO after 48 h exposure were statistically significant (P ≤ 0.05) with the average percentage mortalities of 10% and 20% at 10,000 ppm and 20,000 ppm respectively, while PBO and Coriander leaves hexane extract (CLHE) were significant P ≤ 0.05 after 72 h. PBO was the most toxic synergist (36.67 ± 3.33%) followed by CLHE (26.67 ± 3.43%) at 20,000 ppm. In the formulations of synergist: pyrethrins, CLHE, Nutmeg seed hexane extract (NMHE) and Black pepper seed methanol extract (BPSME) at ratio 1:1 were statistically significant (P ≤ 0.05) 24 h after exposure. PBO registered higher percentage (83.33 ± 12.02%) mortality followed by CLHE (46.67 ± 3.33%), BPSME (43.33 ± 6.67%) and NMHE (26.67 ± 3.33%) at ratio of 3:1 while BPSHE was most effective synergist at ratio 2:1. BPSME and NMHE co-toxicity values were below 20 and -20 respectively while in PBO, BPSHE and CLHE values were above 20. Low concentrations of pyrethrins were required to achieve higher percentage mortality of S. zea-mais when exposure time was extended to 72 h. The toxicity of plant extracts tested was low hence qualified as potential synergists to replace the standard, PBO in pyrethrins formulations. BPSHE was a better synergist than PBO at a concentration of 5,000 ppm followed CLHE while BPSME was an additive to the pyrethrins while NMHE was antagonistic to pyrethrins at 1,000 ppm. It is recommended that time of exposure of a synergist and an insecticide play a critical role in high mortality rates of S. zea-mais regardless of the ratio of synergist: pyrethrins and should be considered when formulating insecticides.