ASSESSMENT OF EFFICACY OF UNSTABILIZED PYRETHRINS AND DIATOMACEOUS EARTH ADMIXTURE ON SITOPHILUS ZEAMAIS IN MAIZE GRAINS

Abstract

Maize crop provides source of livelihood to all groups of people that depend on it. Its adaptability to different agro-ecological zones has led to its increased production and consumption worldwide. In Kenya maize is regarded as the country‟s staple food crop. A large proportion of the estimated 30% post-harvest losses are attributed to storage insect pests including Sitophilus zeamais (maize weevil) which thrive in tropical climates. Chemical insecticides used have raised concerns on undesirable environmental and human health effects as well as effects on other non-target organisms. In this study, a laboratory investigation was carried out to explore the possibility of controlling S. zeamais with a admixture of unstabilized pyrethrins and diatomaceous earth. Adult S. zeamais were seeded into glass jars containing maize grains with different ratios of the admixture. Maize grains in separate jars were treated with pyrethrins alone and diatomaceous earth alone and also seeded with S. zeamais. Untreated maize grains in other jars were seeded with adult insects as the control. Actellic Super Dust – a commercial grain protectant containing admixture of organophosphorous (1.6% pirimiphos-methyl) and synthetic pyrethroid (0.3% permethrin) was included for comparison purposes. Mortality was monitored in all the treatments at intervals of 7th, 14th, 21st and 28th day respectively. Deposition of pyrethrins, pirimiphos-methyl and permethrin in grain were determined on treatment day (24hrs). Residues of these compounds were determined at 90th and 180th day after treatment to establish degradation levels. Comparison of insect pest mortalities was done using one-way analysis of variance (ANOVA) and significant differences separated using Tukey‟s HSD test. Deposition quantities were analyzed using chi square (χ2). Results showed that admixtures containing increased quantity of pyrethrins resulted in higher mortality of S. zeamais on 7th, 14th and 21st days respectively. However, by end of 28th day, all the different ratios had showed 100% mortality. Initial deposited pyrethrins onto the maize grains showed no significant difference with the calculated pyrethrins content before application. Residue analysis showed that pyrethrins degraded in large proportion over the 180th day period. Milled grains also showed low residue levels indicating high degradation resulting from milling. Based on this study it is recommended that combinations of unstabilized pyrethrins and diatomaceous earth be utilized as grain storage protectant.