The effects of crop diversification on herbivorous insects: a meta‐analysis approach

Abstract

Abstract. We used a meta‐analysis to evaluate the hypothesis that diversified crops are subject to lower density of herbivorous insects. This method consists of integrating findings of independent studies by calculating the magnitude of treatment effects (effect size). A literature review covering a period of over 10 years yielded twenty‐one studies that contained sufficient information for the meta‐analysis. The data were analysed according to three criteria: yearly replications of the same study were considered independent data so that research repeated over time would have greater weight; a second meta‐analysis was conducted with results of only 1 year because of possible lack of independence between results of the same study; the third approach was to remove studies involving Brassica spp. from the analysis because they are redundant from the standpoint of testing a general hypothesis of crop diversification. The three analyses resulted in effect sizes of 0.50, 0.35 and 0.27, respectively, and all were statistically significant. These results correspond to 69%, 63% and 60% differences between average insect densities for treatment (diversified crops) and control groups. We considered the last criterion for data analysis as the most appropriate because of lowest bias from non‐independence between samples. Therefore we concluded that crop diversity caused moderate reduction of herbivorous insect populations. The implications and limitations of meta‐analyses are discussed.

Ecosystem Services in Biologically Diversified versus Conventional Farming Systems: Benefits, Externalities, and Trade-Offs

“…Meta-analyses suggest that diversification schemes generally achieve significant positive outcomes including natural enemy enhancement, reduction of herbivore abundance, and reduction of crop damage, from a combination of bottom-up and top-down effects (Letourneau et al 2011). Specifically, in a meta-analysis of 21 studies comparing pest suppression in polyculture versus monoculture, Tonhasca and Byrne (1994) found that polycultures significantly reduced pest densities by 64%. In a later meta-analysis with a nonoverlapping set of studies (45 articles comprising 552 total comparisons), Letourneau et al (2011) found a 44% increase in abundance of natural enemies (148 comparisons), a 54% increase in herbivore mortality (221 comparisons), and a 23% reduction in crop damage (99 comparisons) on farms with species-rich vegetational diversification systems (including within or around the field) than on farms with species-poor systems.…”

Agricultural Intensification and Ecosystem Properties

“…The reduction in plant species richness that accompanies agricultural intensification leads to changes in the community composition of the pest complex-herbivorous insects, their natural enemies (predators and para-sites), and the microbial community attacking crops (9). The low planned diversity of monocultural agricultural systems typically results in greater crop losses from an insect pest complex that is less diverse but more abundant (10,11). The trend for higher insect pest densities in monocultures is especially strong for specialist herbivores with a narrow host range.…”

Agroecology: Principles for the Conversion and Redesign of Farming Systems

“…Tonhasca and Byrne [36] analyzing 21 studies comparing pest suppression in polyculture versus monoculture, found that polycultures significantly reduced pest densities by 64%. In a later meta-analysis involving 148 comparisons Letourneau et al [37] found that farms with species-rich vegetational schemes exhibited a 44% increase in abundance of natural enemies, a 54% increase in pest mortality, and consequently a 23% reduction in crop damage when compared to monoculture farms.…”