The Potential of Bioactive Protectants of Maize Grains against Sitophilus Zeamais in Western Kenya
In African tropical zones, cereals are kept as grains or flour in storage structures and constitute important source of income for farmers. Protecting stored product in this case is an emergency to ensure food security for people living in these areas of deficiency where agriculture is not highly productive. Some of the pursued measures include physical, traditional, and biological methods; and application of synthetic pesticides. The above techniques have some problems including consumer sensitivity, affordability by resource poor farmers, not eco-friendly, development of insect resistance and instability of the synthetic pesticides and, hence need for continual search for new ones. This study sought to reduce post-harvest losses by use of traditional protectants whose knowledge appear to be disappearing ; and establishing scientific basis for the bioactivity of products derived from plant materials and ashes against S. zeamais as a stored product insect pest. The protectant materials evaluated included ground plant powders, solvent extracts, essential oils and their blends, and ashes. Minimal activity was recorded from plant powders with highest mortality observed from L. javanica at 20% treatement with mean mortality of 44%. One hundred percent (100 %) mortality of weevils was recorded for all the ashes bioassayed. Ash derived from bagasse at 700oC for 12 hours evoked the highest mortality. Essential oils from L. javanica and M. pyrifolia and their blend showed higher mortality than actelic super 2% concentration at dose of 50 g/ 90 kg of grains. Probit analysis of the essential oils for LD50 values gave 0.147µl /100g of grains for L. javanica, 1.029 µl /100g of grains for M. pyrifolia and toxicant LD50 value of 0.199 µl /100g of grains for the blend of the 2 oils. Co-toxicity coefficient of the blend of the essential oils was 178.94 implying the oils were synergistic. Chemical analysis of the essential oils of the two plants using GC, GC-MS and GC-co injection with digitized standards identified 18 constituent compounds. The major components in L. javanica were, 1-imidazol-1-yl-3-methyl-but-2-en-1-one (38.356%), 4-tertbutylphenol (10.597%) and 2-tert-butylphenol (23.453%) and from M. pyrifolia, (1Z,7Z)-3-isopropyl-2-methylene-cyclodeca-1,7-diene(α-humulene)( 29.576%), 5-isopropyl-2-methyl-bicyclo[3.1.0]hex-3-ene (7.088%) and 1,1,7,7a-tetramethyl-2,3,5,6,7,7b-hexahydro-1aH-cyclopropa[a]naphthalene (6.472%). Chemical analysis of the four different ashes done using XRFA, AAS and gravimetric analysis confirmed eleven different oxides (SiO2, Al2O3, CaO, MgO, TiO2, MnO, Fe2O3, CuO, ZnO, K2O and Na2O). SiO2 was generally the most abundant in all the ash spercimens and active against S. zeamais. LD50 values for bagasse ash was 6.07mg/20g of grains while the toxicant LD50 of the mixture of bagasse-L. javanica essential oil was 3.328 mg/20g of grains. Co-toxicity coefficient of bagasse-L. javanica essential oil blend was 232.93, showing a synergistic effect. Mortality tests of the solvent extracts of the plants showed best results generally from methanol extracts. Repellency tests of L. javanica and M. pyrifolia essential oil on S. zeamais were carried out on filter papers and with a Choice Bioassay System. The two oils were found to be repellent to the weevil. None of the plant’s essential oils and ashes bio-assayed had significant effect on the germination of maize seeds. The protective ability of essential oil-ash mixtures, plain ash from bagasse and essential oil from L. javanica on long-term storage was carried out for 12 months. Plain ash from bagasse and L. javanica-bagasse blend reduced grain damage to 1.65 and 1.63 % respectively as compared to actellic super, L. javanica essential oil and untreated control at 8.17, 8.23 and 10.09% respectively. Using such combinations is advantageous as the essential oil and bagasse ash have different modes of action and chemical composition and this will slow down the development of resistance. These combinations can serve as affordable, available and biodegradable alternatives or supplements to the costly and less environmentally friendly synthetic post harvest protectants.