Effects of diatomaceous earth on cowpea field pests in Uganda
Nakato, Gloria Valentine
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Optimal cowpea production in Uganda is severely limited by an array of cowpea insect pests, which infest and damage the crop both in the field and in storage. In absence of host plant resistance, insecticide application is a suitable alternative, as currently there is no single variety with resistance to all insect pests. Insecticides provide rapid and effective control against important insect pests and the broad spectrum of action of many of them makes it possible to use one or a combination of insecticides to control a complex of insect pests attacking the cowpea crop. Currently, cowpea production cannot be sustained without the use of insecticides and the main direction in the control of a complex of insect pests on cowpea has been the chemical approach. Previous studies have shown that continuous use of chemicals in the control of field insect pests is environmentally unfriendly and detrimental to our health. This therefore necessitated the need for better alternatives that are affordable to farmers as well as environmentally friendly with no health implications. Diatomaceous earth (DE) provided the best and available alternative. Diatomaceous earth is an inert substance that abrades the chitinous layer of insects, resulting into dehydration and death. Field experiments were conducted during the growing seasons of 2005B, 2006A and 2006B to assess the effectiveness of DE together with selected synthetic insecticides towards the control of cowpea field insect pests. The study aimed at improving cowpea productivity through reduction of field insect infestations by application of DE, and if found effective could be included in the integrated insect pest management of cowpea. More specifically the study sought to a)evaluate the effectiveness of different dosage rates of DE against cowpea field insect pest complex, b) compare effectiveness of DE and selected insecticides against cowpea field insect pest complex, and c) determine the effect of field treatment on carry-over populations of storage insect pests. Trials were established at National Agricultural Research Laboratories Institute (NARL), Kawanda, as Randomised complete block design (RCBD) with five replications for the field experiments, and as complete randomized design (CRD) for the laboratory experiments. Cowpea variety ‘ebelat’ was used exclusively and was planted at a spacing of 60 x 20 cm. The effectiveness of DE against cowpea field insect pests was studied using five different dosage rates of 0, 10, 20, 30, and 40g/m2 at budding, flowering, 50% pod formation, 50% pod filling, and 50% physiological maturity. Insect pest populations at each growth stage as well as insect damage on pods and seeds were recorded. The study focused on the following pests: aphids (Aphis craccivora Koch), thrips (Megalurothrips sjostedti (Trybom)), Maruca vitrata, and the pod sucking bugs. Aphid assessments were done from budding to flowering stage, thrips and M. vitrata were assessed during the flowering stage while pod sucking bugs were assessed during pod formation, pod filling and physiological maturity stages. The following were the insect pest control strategies used: DE (as a dust and wettable powder), Fenitrothion and tobacco leaf extract. When DE was applied as a wettable powder, it was observed that at dosage rate of 20 g/m2 aphid and thrip populations were significantly (P<0.05) reduced, and at 40 g/m2 pod bug populations were reduced. As a dust, DE of 40 g/m2 was very effective in reducing aphid and pod bug populations. The effectiveness of DE dust and wettable powder at 40 and 20 g/m2 respectively was compared against tobacco extract and fenitrothion. Fenitrothion was very effective in reducing insect pest populations in all seasons. In 2006A, tobacco extract was as effective as fenitrothion in reducing all insect pest populations, while in 2006B, DE as a wettable powder was equally effective in reducing aphid populations. Carryover effects of storage insect pests were examined in storage by monitoring on a daily basis and any insects that emerged were collected, identified and discarded. The only emergent insect specie in storage was Callosobruchus maculatus (Fabricius). The least emergences were recorded in tobacco extract except for 2006A, where DE (dust) had the least emergencies. In summary, our results indicate that DE has the potential to reduce field insect pest populations but has to be used in combination with other chemical, cultural and physical methods to increase its effectiveness. These findings suggest that for effective control of cowpea field insect pests, integrated pest management is the ultimate solution.