Tri-Trophic Insecticidal Effects of African Plants against Cabbage Pests

Blankson W Amoabeng,Geoff M Gurr,Louis Munyakazi,Helen I Nicol,Catherine W Gitau,Phil C Stevenson,Nicolas Desneux

doi:10.1371/journal.pone.0078651

Abstract

Botanical insecticides are increasingly attracting research attention as they offer novel modes of action that may provide effective control of pests that have already developed resistance to conventional insecticides. They potentially offer cost-effective pest control to smallholder farmers in developing countries if highly active extracts can be prepared simply from readily available plants. Field cage and open field experiments were conducted to evaluate the insecticidal potential of nine common Ghanaian plants: goat weed, Ageratum conyzoides (Asteraceae), Siam weed, Chromolaena odorata (Asteraceae), Cinderella weed, Synedrella nodiflora (Asteraceae), chili pepper, Capsicum frutescens (Solanaceae), tobacco, Nicotiana tabacum (Solanaceae) cassia, Cassia sophera (Leguminosae), physic nut, Jatropha curcas (Euphorbiaceae), castor oil plant, Ricinus communis (Euphorbiaceae) and basil, Ocimum gratissimum (Lamiaceae). In field cage experiments, simple detergent and water extracts of all botanical treatments gave control of cabbage aphid, Brevicoryne brassicae and diamondback moth, Plutella xylostella, equivalent to the synthetic insecticide Attack® (emamectin benzoate) and superior to water or detergent solution. In open field experiments in the major and minor rainy seasons using a sub-set of plant extracts (A. conyzoides, C. odorata, S. nodiflora, N. tabacum and R. communis), all controlled B. brassicae and P. xylostella more effectively than water control and comparably with or better than Attack®. Botanical and water control treatments were more benign to third trophic level predators than Attack®. Effects cascaded to the first trophic level with all botanical treatments giving cabbage head weights, comparable to Attack® in the minor season. In the major season, R. communis and A conyzoides treatment gave lower head yields than Attack® but the remaining botanicals were equivalent or superior to this synthetic insecticide. Simply-prepared extracts from readily-available Ghanaian plants give beneficial, tri-trophic benefits and merit further research as an inexpensive plant protection strategy for smallholder farmers in West Africa.

Highlights

Cabbage, Brassica oleracea var. capitata L. (Cruciferae) is an important temperate vegetable crop that grows well in other climatic regions throughout the world [1]
First trophic level: plant yield and damage In the major rainy season open field experiment S. nodiflora, C. odorata and N. tabacum as well as Attack® had significantly higher head weights than the water control whilst the last two mentioned botanical treatments were superior to the conventional insecticide (Table 1)
A similar observation was made for Bossmate® which failed to control B. brassicae in a field experiment in Ghana resulting in a reduced yield of plots sprayed with Bossmate® compared to plots sprayed with garlic, chili pepper and Attack® [23] The lack of control by this conventional insecticide was attributed to resistance in the aphid population and this is likely to have been the case in the present study

Summary

Introduction

Brassica oleracea var. capitata L. (Cruciferae) is an important temperate vegetable crop that grows well in other climatic regions throughout the world [1]. (Cruciferae) is an important temperate vegetable crop that grows well in other climatic regions throughout the world [1]. Despite the importance of cabbage, there are a number of biotic constraints, including insect pests, which hamper its production and consumption [2]. Diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae) is of European origin and one of the most destructive pest of crucifers worldwide [3,4]. Yield loss attributed to DBM can be as high as 100% [2,5]. Global cost of control and yield loss attributed to DBM has recently been estimated between US$ 4 and 5 billion per annum [6,7]. In Ghana, DBM is considered the most important pest of cabbage [8]

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Conclusion