Non-bee insects are important contributors to global crop pollination

Abstract

Wild and managed bees are well documented as effective pollinators of global crops of economic importance. However, the contributions by pollinators other than bees have been little explored despite their potential to contribute to crop production and stability in the face of environmental change. Non-bee pollinators include flies, beetles, moths, butterflies, wasps, ants, birds, and bats, among others. Here we focus on non-bee insects and synthesize 39 field studies from five continents that directly measured the crop pollination services provided by non-bees, honey bees, and other bees to compare the relative contributions of these taxa. Non-bees performed 25-50% of the total number of flower visits. Although non-bees were less effective pollinators than bees per flower visit, they made more visits; thus these two factors compensated for each other, resulting in pollination services rendered by non-bees that were similar to those provided by bees. In the subset of studies that measured fruit set, fruit set increased with non-bee insect visits independently of bee visitation rates, indicating that non-bee insects provide a unique benefit that is not provided by bees. We also show that non-bee insects are not as reliant as bees on the presence of remnant natural or seminatural habitat in the surrounding landscape. These results strongly suggest that non-bee insect pollinators play a significant role in global crop production and respond differently than bees to landscape structure, probably making their crop pollination services more robust to changes in land use. Non-bee insects provide a valuable service and provide potential insurance against bee population declines.

Keywords: bee; beetle; fly; insect pollinator; unmanaged pollinator.

Fig. 1.

The contribution by honey bees, other bees, and non-bees to crop pollination. Data from individual crop studies were standardized by z-scores before analysis. (A) Pollination considered as a function of visits*single-visit effectiveness among guilds for the nine studies with effectiveness and visitation data. Note that per capita effectiveness in each guild is measured only in a subset of dominant species in each study. (B) The contributions of different insect groups to visitation (i.e., percentage of visits). (C) The relative effectiveness of honey bees, other bees, and non-bees as measured by pollen deposition or fruit set per visit, combined across the 11 crop studies for which data were available. Letters depict post hoc test differences (at P < 0.05) among pollinator groups.

Fig. 2.

The contribution of different insect groups to flower visitation across the 37 crop studies for which visitation data were available. Crops are ordered, left to right, from mostly bee-dominated to mostly non-bee–dominated.

Fig. 3.

Regression coefficients (i.e., slopes ßi ± 95% CI) representing honey bee, other bees, and non-bee contributions to overall fruit set and distance from natural/seminatural habitat. (A) Overall fruit set measured by seed set across 19 crop studies, estimated from the relationship between visitation and fruit set variation. Visitation by other bees increased fruit set (i.e., the average slope is positive, and CIs for regression coefficients did not include zero). The average regression coefficients across crops for non-bees increased fruit set (i.e., positive mean), but CIs minimally overlapped zero. (B) Distance from natural/seminatural habitat was measured across 23 studies. Visitation by other bees was negatively related to distance from natural/seminatural habitat (i.e., the average slope is negative, CIs for regression coefficients did not include zero). Visitation by honey bees and non-bees was not related to distance from natural/seminatural habitat (i.e., the average slope is negative, but confidence intervals overlapped zero for both taxa). Data from individual crop studies were standardized by z-scores before analysis to permit direct comparison of slopes.