Fallow fields and hedgerows mediate enhanced arthropod predation and reduced herbivory on small scale intercropped maize farms – δ13C and δ15N stable isotope evidence

Abstract

Fallow fields and hedgerows enhance agricultural landscape heterogeneity and boost predatory arthropod diversity, though there is little direct quantitative evidence of their value in mediating pest suppression. Here, stable isotope analyses were used to quantify the influence of fallow field presence, fallow size proportion of farms, hedgerow volume and hedgerow type, on arthropod pest consumption by six predatory taxa. Levels of herbivory by four arthropod taxa on three vegetation groups: maize, beans and non-crops – were also considered. This was achieved through mixing model analysis with δ13C and δ15N stable isotope to identify and estimate food-source proportions incorporated into tissues of arthropod consumers collected from ten maize farms intercropped with beans in western Kenya. Fallow fields’ presence supported predation on Lepidopteran herbivores by attracting predatory bugs and wasps, and mediated lower herbivory levels. Fallow size proportion corresponded to higher taxa-level predation by ants and spiders on phytophagous beetles, and overall increase in bean herbivory, but predicted no difference in maize or non-crop herbivory. Higher hedgerow volumes did not affect overall predation pressure, but supported increased bean herbivory by Lepidoptera, and lower maize consumption by Orthoptera. Exotic hedgerows supported highest wasp predation on Lepidoptera while spiders consumed significantly more Orthoptera and phytophagous beetle pests on indigenous-hedgerow farms. Similarly, exotic hedgerows corresponded to enhanced Lepidoptera bean herbivory while trophic linkage of maize and non-crops to pests overall, were stronger with indigenous hedgerows. Thus, in addition to thicker hedgerows, predatory spillover impacts of wasps, bugs and beetles can be enhanced with fallow presence and exotic hedgerows, and that of spiders and ants, with larger fallows. Overall, the choice, combination and application of farm habitat structural features to maximize predatory arthropods’ contributions to pest biocontrol requires knowledge of focal crops, dominant pest groups and their most important predators. The findings demonstrate that fallow fields and hedgerows are significant in supporting the biological component of integrated crop pest management.