At Which Spatial Scale Does Crop Diversity Enhance Natural Enemy Populations and Pest Control? An Experiment in a Mosaic Cropping System

Abstract

The importance of plant richness to enhance the presence, biodiversity and efficiency of natural enemies in agricultural systems has largely been studied and demonstrated these last decades. Planting and preserving non-crop plants or manipulating crop richness in fields are practices that have proven their efficiency. However, the impact of crop-richness continuity in space and time on pests and natural enemies at a landscape scale remains poorly studied. In a two-year study, we assessed the effect of crop richness (single crop vs. multiple crops) on pest and natural enemy abundance and spillover in a field experiment in north-east China. Overall, we found crop diversity had a limited impact on pest and natural enemy abundance at the spatial scale tested (0.025 vs. 0.2 ha). The total pest and natural enemy abundances were not different between single-crop and multi-crop plots in either year, and the community composition at the functional group level was mostly determined by the crop but not crop diversity. However, we found that crop diversity influenced the numeric response of ladybirds to aphids in wheat; their negative response (higher abundance where aphid abundance was lower, suggesting predation) was attenuated in multi-crop plots (no correlation of aphid and ladybird abundance, suggesting the use of alternative resources). This pattern was not found in maize. Finally, crop succession enhanced the spillover of ladybirds from wheat and maize to cotton plots but with limited benefits for aphid control. Because of these limited impacts, we hypothesized that crop diversity may benefit natural enemy populations and enhance pest control at larger spatial scales; while we found similar abundances of ladybirds between our small (0.025–0.2 ha) plots and in large (2 ha) close-by cotton fields, aphid abundances were more than ten times higher in large cotton fields. Our study highlights the need to accurately estimate the spatial scale at which crop biodiversity may benefit pest control, in relation to the ecology of the target pest and natural enemies.

Functional Traits Mediate the Natural Enemy Response to Land Use at the Local Scale

“…However, several other studies have shown the complexity of such a relationship, limiting our capacity for generalization [9,10]. Some possible causes of such a complex response to landscape heterogeneity (composition and configuration [11]) are the scale of analysis, the effect of within-farm management practices, and natural enemies' life histories [6,[12][13][14]. In this sense, a mechanistic understanding of the biophysical controls on the natural enemy community is essential for decision making at the local and landscape levels.…”

“…There is significant evidence of the role of landscape heterogeneity in providing natural contributions to human beings, but this information is often contradictory. On the one hand, several authors have found a strong positive relation between non-arable areas and natural enemies' abundance and diversity [3,12,14,26], emphasizing the importance of heterogeneous landscapes in determining biological pest control [5,14]. On the other hand, numerous studies have shown that natural enemies' responses to landscape heterogeneity may also be neutral or negative [4,9,27], featuring a context-dependent result (sometimes including management practices or other farm-level drivers).…”