Impact of doubling the recommended dose of Magicforce (Lambda-Cyhalothrin + Dimethoate) on major associated arthropods and performance of watermelon

Abstract

Some crop growers hold the view that application of pesticides at higher than the manufacturer's recommended doses results in better pest control and crop productivity. The veracity of this perception was evaluated in field experiments at the experimental field of Federal University Wukari. A recommended insecticide and acaricide; ® Magicforce (Lambda-cyhalothrin 15g/L + Dimethoate 300 g/L) was evaluated against insect pests of watermelon (Citrullus lanatus Thunb.) and other associated beneficial arthropods. The  experiments were laid in a Randomized Complete Block Design with five treatments (control inclusive). Data collected includes  arthropod (pest and beneficial) densities, crop growth, and number of fruits at the early-fruiting stage which were analyzed using  variance analysis after appropriate transformations. Student's t-test was used to compare early- and lateseason's variables while  correlation and regression analyses were used to determine relationships between key variables. Results showed that plots treated with double the highest manufacturer's recommended dose of ® Magicforce (cost implication of ₦ 144,000) had lower pest (leaf beetle species and Bactrocera cucurbitae) and beneficial arthropod (predatory ants, spiders and Apis mellifera) densities than those treated with the lowest (cost – ₦ 48,000) and highest (cost – ₦ 72,000) recommended doses. Their plant growth (vine length and number of leaves) and fruit production were however comparatively lower though largely statistically comparable. The ® relationship between the quantity of Magicforce applied and number of fruits produced were though positive, 2 2 only moderate and insignificant in both early- (r = 0.665, R = 44.3%, p = 0.220) and late- (r = 0.659, R = 43.4%, p = 0.227) crops. The results revealed that the application of double the manufacturer's recommended dose of ® Magicforce (with its comparatively higher cost implication of 100 – 200%) suppressed growth  of watermelon by 1.96 – 6.20%, and impeded fruit production by 9.14 – 13.30%. While there is need to verify the mechanism of this key finding, the need to source for genuine pesticides and follow manufacturer's recommended doses a re highlighted. Keywords: Arthropods, Insect pests, Organophosphate, Pesticide over-dose, Pyrethroid, Watermelon