Shade trees decrease pest abundances on brassica crops in Kenya

Solène Guenat,Mattias Jonsson,Riikka Kaartinen

doi:10.1007/s10457-017-0159-5

Solène Guenat, Mattias Jonsson + Show 1 more

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https://doi.org/10.1007/s10457-017-0159-5

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Abstract

Agroforestry practices may mitigate the current loss of biodiversity and ecosystem services due to deforestation and agricultural intensification. To examine the effects of agroforestry on the ecosystem service of pest regulation, we assessed pest abundances and biological control potential in shaded and open kale (Brassica oleracea L. acephala) fields in Western Kenya. Specifically, we compared the abundance of pest aphids and caterpillars, ground-dwelling ants, spiders and predatory beetles, and examined aphid parasitism rates, predation rates on diamondback moth eggs, attack rates on surrogate caterpillars and bird predation on aphids. Shade trees effectively reduced abundances of aphids, caterpillars and increased abundances of spiders and predatory beetles, but neither affected ant abundances, or predation and parasitism rates. Our results suggest that presence of shade trees can decrease pest abundances, but that this is not only due to improved biological control by natural enemies but also due to microclimatic conditions affecting pest performance and bottom-up processes such as changes in plant quality and soil conditions. We encourage studies exploring simultaneously how top-down and bottom-up processes affect pest regulation in agroforestry settings.

Highlights

Increased cover of agricultural land-use, occurring mainly at the expense of natural habitats, and agricultural intensification are primary causes for the current biodiversity loss and degradation of ecosystem services (Foley et al 2005)
Before testing the effect of shade trees, we examined whether bird predation had an effect on aphid abundance, aphid parasitism rates and plant damage with a Mann–Whitney nonparametric test
We found no direct evidence that biological control was affected by shade, as neither predationnor parasitism rates were affected by the presence of shade trees

Summary

Introduction

Increased cover of agricultural land-use, occurring mainly at the expense of natural habitats, and agricultural intensification are primary causes for the current biodiversity loss and degradation of ecosystem services (Foley et al 2005). Agroforestry is a land management practice whereby woody perennials are planted together with crops and/or livestock (Lundgren 1982) It can enhance several ecosystem services, including pest regulation (Schroth et al 2000; Tscharntke et al 2011; Pumarino et al 2015; Kuyah et al 2016). Many studies have shown that landscape complexity and presence of trees in agricultural habitats can improve regulation of specific pests (Bianchi et al 2006; Tscharntke et al 2011; Pumarino et al 2015) in both temperate and tropical climates, with crops as diverse as cabbage and coffee, and with natural enemies ranging from vertebrates (birds and bats) to invertebrates (parasitic wasps, ants, hoverflies and spiders) Other mechanisms such as modification of micro-climate or changes in crop nutritional value can explain differences in pest regulation between agroforestry and conventional practices (Schroth et al 2000)

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