Hypothesis
Adding managed Apis mellifera or Bombus impatiens hives to pumpkin fields would raise bee visits to flowers and fruit yield beyond what wild bees provide.
What failed
In a two-year comparison of fields supplemented with honey bee hives, bumble bee hives, or nothing, neither kind of supplementation increased visitation by those bees or pumpkin yield.
Broken assumption
Placing managed hives next to a crop increases pollination of that crop above the wild-bee baseline.
Why
Yield did rise with visits by honey bees and bumble bees across all fields, so the bees matter as pollinators; adding hives simply did not raise their visit rate, and wild bees such as Peponapis pruinosa were also prominent visitors.
From the published paper: Petersen et al. PLoS ONE, 2013. ↗CC BY