Effects of tillage on a ground-nesting, crop-pollinating bee
Agriculture is a dominant form of land use globally and a leading driver of biodiversity loss. The documented negative impacts of agriculture on biodiversity are likely linked to increased disturbance. Tillage practices are a major form of soil disturbance that negatively impact different macroinvertebrates, including beetles and earthworms. Tillage is also thought to negatively impact ground-nesting bees, which represent roughly 70% of described bee species. Detailed observational studies on ground-nesting bees in agricultural fields, however, provided conflicting results and some species persist in such highly disturbed landscapes. We conducted a two-year, large-scale tillage experiment on the squash bee, Peponapis pruinosa, and asked how tillage impacts offspring survival and timing of emergence. In summer 2012, we established P. pruinosa nesting aggregations in twenty 10 foot by 10 foot plots. In fall 2012, we randomly assigned the tillage treatment to half of those plots. The following summer we trapped emerging offspring and used an over-dispersed Bayesian Poisson model to analyze the data.
Tillage negatively impacted offspring survival; the mean change in emergence between tilled and control cages was large (-53%). However, the results were highly variable and there is a 9% posterior probability that tilling has a neutral or positive impact on offspring survival. This suggests that some individuals nested below the tillage zone (e.g. deeper than 40cm). Bees in tilled plots emerged later than those in control plots. This delay may be due to changes in soil moisture and temperature, which bees are thought to use as emergence cues. Although tilling has a negative impact on P. pruinosa in the short term, some individuals survive this disturbance allowing the population to persist. Future research will use computational modeling to determine how the frequency and intensity of tillage may impact the long-term persistence of this species and those with similar life histories.