Synergistic Evaluation of Selected Plant Extracts on Potency of Pyrethrins against the Maize Weevil, Sitophilus zea-mais (MOTSCH.)

Abstract

The interest in utilization of botanical insecticides, particularly essential oils have become increasingly relevant in the control of insect pests. Many plant products and their bioactive compounds with repellent, antifeeding or insecticidal activity against stored product insect pests have been reported however, their levels of control are still wanting. The major issue is that the oil yields obtained are low, unstable and high costs involved in production to use on a commercial scale. Use of these oils in combinations or in formulations with botanical actives as synergists or stabilizers, may make them economically viable insecticides. The present study focussed on evaluating plant extracts (Black pepper seeds, Nutmeg seeds, coriander leaves and roots) as potential synergists in formulations with pyrethrins for possible use in protection of stored grains against Sitophilus zea-mais. Full dose response and synergism experiments were carried out on S. zea-mais at four concentrations of synergists at ratio 1:1 (synergist: pyrethrins) Topical application of synergist/ formulation on S. zea-mais was done in triplicate using Factorial Design in a completely Randomized Design layout. 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 and ANOVA was used to obtain the mean mortality differences of S. zea-mais at P ≤ 0.05. LC20 value for pyrethrins was found to be 2,200 ppm. In formulations of synergist: pyrethrins, coriander leaves extract (CLHE), Nutmeg seed extract (NMHE) and Black pepper seed methanol extract (BPSME) 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%). BPSME and NMHE co-toxicity values were below 20 and -20 respectively while in PBO, BPSHE and CLHE values were above 20. Plant extracts BPSHE and CLHE and PBO synergized the toxicity of pyrethrins in all concentrations over 72 h exposure duration with co-toxicity factors above 100. BPSME was shown to be an additive in the formulation and only a synergist at 20,000ppm while NMHE was antagonistic to pyrethrins. Increasing the concentration of a plant extract did not correspond to increased efficacy of the formulation. Efficacy of the synergist- pyrethrins formulations at lower concentrations can be economically viable since average percentage mortalities will not be different from that of higher concentrations when time of interaction is prolonged. It is recommended that BPSHE and CLHE can be synergists to pyrethrins when formulations are exposed to insects for 72 h duration