Increasing crop field size does not consistently exacerbate insect pest problems

Abstract

Increasing diversity on farms can enhance many key ecosystem services to and from agriculture, and natural control of arthropod pests is often presumed to be among them. The expectation that increasing the size of monocultural crop plantings exacerbates the impact of pests is common throughout the agroecological literature. However, the theoretical basis for this expectation is uncertain; mechanistic mathematical models suggest instead that increasing field size can have positive, negative, neutral, or even nonlinear effects on arthropod pest densities. Here, we report a broad survey of crop field-size effects: across 14 pest species, 5 crops, and 20,000 field years of observations, we quantify the impact of field size on pest densities, pesticide applications, and crop yield. We find no evidence that larger fields cause consistently worse pest impacts. The most common outcome (9 of 14 species) was for pest severity to be independent of field size; larger fields resulted in less severe pest problems for four species, and only one species exhibited the expected trend of larger fields worsening pest severity. Importantly, pest responses to field size strongly correlated with their responses to the fraction of the surrounding landscape planted to the focal crop, suggesting that shared ecological processes produce parallel responses to crop simplification across spatial scales. We conclude that the idea that larger field sizes consistently disrupt natural pest control services is without foundation in either the theoretical or empirical record.

Pest species respond differently to farm field size

“…Rosenheim et al. ( 8 ), in PNAS, tested this concept using a large, multicrop dataset collected from different agricultural systems to investigate whether the size of fields planted to a single crop or the amount of a crop in the surrounding landscape is positively related to pest abundance. Their key finding is the absence of a consistent relationship between pest abundance and the area of host crop at either the field or the landscape scale.…”

“…The latter suggests reduced biological control of herbivores in large pure stands because these monocultures do not provide suitable habitat and resources for natural enemies. However, the generalization of these concepts to agricultural systems across broad spatial scales and the idea that large-scale monocultures intensify pest problems lacks rigorous theoretical or empirical support ( 8 ). Field studies in agricultural crops, primarily involving small plots, as well as studies in natural systems and modeling studies have shown that the effects of increasing field size on pest population density can be inconsistent ( 12 – 17 ).…”

“…Rosenheim et al. ( 8 ) took advantage of existing data compiled by independent crop advisors or farm staff during regular sampling of pest populations or crop injury to inform decisions regarding pesticide applications. The dataset comprised >20,000 field-year observations spanning five crops and 14 pest species that included a wide range of field sizes within and across cropping systems in three countries.…”

“…To extend these results, the authors ( 8 ) estimated the prevalence of host crops in the nearby landscape to examine the relationship between local crop production patterns and pest infestation intensity. For each focal crop, the amount of the same crop in the local landscape was measured and related to pest pressure.…”

“…Applying approaches similar to those of Rosenheim et al. ( 8 ) will be useful to identify taxa and cropping systems in which pests have predictable responses to crop concentration, particularly those systems with a negative relationship. Leveraging this knowledge will be challenging, but annual crops provide an opportunity to test proactive structuring of crop fields and landscapes.…”