Effects of plant extracts on the larger grain borer, Prostephanus Truncatus Horn (Coleoptera: Bostrichidae)

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Mukanga, Mweshi
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The study conducted to access the insecticidal activity of selected plant extracts:Eucalyptus Eucalyptus globules., Guava, Psidium guajava l., Neem, Azadirachta indica A. Juss., Tephrosia, Tephrosia vogelii F (Hook) and water Hyacinth Erchhornia crassipes (Martius) Solms-Laubach, against the Larger grain borer, Prostephanus truncates (Horn). The effects of plant extracts were determined in two ways: dry powder trials and petroleum ether leaf extracts experiments. Split plot design with three replications was used for all the experiments. The bioassays were conducted at a temperature of 27oC and 70 + 5% relative humidity. The dry leaf powders were applied in six doses: 0.1,0.5,0.25,0.5,1.0,2.5 and 5 grams per 100g dried cassava chips or flour to determine the reproduction inhibition, repellence, food preference and anti- feeding deterrence of the extracts on P. Truncates. Quantification of the reproduction inhibition was made at 14, 28, 42, 56 and 70 days after infestation. The results of reproduction inhibition study showed that the leaf powders partially suppressed reproduction of parent adult population. The numbers of larvae, pupae and adult in leaf powder treated cassava were significantly (P ≤ 0.01) fewer compared to the untreated cassava. Significantly low numbers were produced in Neem treatment, followed by Tephrosia and Water Hyacinth whereas Guava had a comparably higher numbers of P. Truncatus. Dry leaf powder application at 2.5 and 5.0g per 100g dried cassava were superior to the lower doses, 0.1, 0.25 and 0.5g/100g. No larvae or pupae was produced in Actellic super (1.6% Pirimphos methyl and 0.3% Permethrin) treated cassava (applied at 50g/90kg dried cassava) as the adults were killed soon after introduction. The mean percent weight loss due to P. Truncates feeding on leaf powder treated cassava was between 13-18% compared to 39% in untreated cassava, after 70 days of storage. Actellic super treated cassava incurred 0.23% weight loss during the same period. Repellency effect and food preferences were assessed for five days consecutively, while the anti-feeding effect was assessed 7 days after introducing the insects in the treatment vials. Tephrosia, Neem and Water hyacinth extracts had strong repellent and anti-feeding effects on P.truncatus while Eucalyptus and guava were moderate. Negative orientation in response to treated cassava was observed. Prostephanus truncates preferred the untreated cassava more than leaf powder treated cassava. Mortality due to the petroleum ether leaf extracts was compared in the vapour, residual surface film exposure, topical application and P. Truncates larval dipping methods. The petroleum ether leaf extracts were applied at the rate of 0.1, 0.25, 0.5 and 1% concentration. Mortality was assessed after 24 hours in the vapour method, 72 hrs in the residual film surface exposure method and 120 hrs in the topical application method while the larval dipping experiment was conducted over 20 days. Toxicity of petroleum ether leaf extracts was plant specific and dose-dependent. In the vapour test, Neem extracts were highly toxic to P.truncatus, followed by Tephrosia and Water hyacinth. When the insects were exposed to the residual surface film, it was found that Eucalyptus was more effective against the insects. Insect mortality increased with increased exposure time. In the topical application, Tephrosia and Water hyacinth extracts were highly toxic. When P. truncatus larvae were dipped in the petroleum ether leaf extract, mortality was highest in Tephrosia, followed by Neem and Water hyacinth. It is likely that these materials could be used as storage protectants.
Dinoderus truncatus , Cassava , Cassava--Diseases and pests