Over the past decade, honey bees in the United States have experienced unprecedented declines.  Scientists believe that these bee losses are caused by a variety of factors including pesticide exposure, loss of floral resources in the landscape due to intensification of agriculture, and infestation of hives by parasitic mites.  For the past ten years, bee keepers have reported average losses of ~30% each winter with some bee keepers reporting upwards of 90% colony losses (Bee Informed Partnership).  Although bee keepers can “replenish” their colonies to some extent by splitting the remaining healthy colonies into multiple hives, these losses are not sustainable in the long run for beekeepers or modern agriculture.

In Wisconsin, a number of crops depend on insect pollination in order to produce a crop, including apples, cranberries, pumpkins, cucumbers, cherries, and raspberries.  Many growers of these crops bring in managed honey bees each year during bloom to ensure good pollination.  With the decline in honey bees, however, growers are finding that managed hives are becoming more scarce and expensive.  The good news is that wild, native bees are also good pollinators.  In fact, more than half of all bee visits to apple blossoms are by wild bees!  By using bee-friendly management practices, growers can encourage wild bee populations on their farm and rely less on honey bees.

In our current research project, we are working with Wisconsin apple growers to understand how one management practice influences wild bee behavior.  Specifically, we are studying how understory mowing during bloom influences the rate at which wild bees visit apple blossoms and, as a result, effects fruit set.  By mowing the understory, growers remove non-crop flowers such as dandelions.  Dandelions in the understory could draw bees away from the apple blossoms leading to reduced fruit set.  Alternatively, leaving the understory unmowed could make the orchard more attractive to wild bees, drawing them in from the surrounding landscape.  So should apple growers mow the understory or not?  Does mowing the flowers cause wild bees to visit apple blossoms more often, increasing fruit set?  Or should growers leave the flowers in the understory in an attempt to draw more bees into the orchard from the surrounding landscape?

Grower collaborators

In the first year of our study, we observed bees and measured fruit set at 19 apple orchards across southern Wisconsin.  These orchards represented a wide diversity of operation scale, management practices, and production goals (e.g., fresh fruit, cider).  The orchards ranged in size from 0.5 to 160 acres and included small scale, backyard orchards as well as huge production operations.  Management practices included conventional, organic, and no-spray.  Fifteen orchards had honey bees present during apple bloom while four did not.  We also chose these orchards to span a range of different landscape types as previous research has shown that wild bees respond to habitat in the landscape.  We visited each orchard four or five times over the course of the spring to meet growers, set up our study equipment, and collect data.

Preliminary results

We found fewer bees on apple blossoms when dandelions were present in the understory (i.e. not mowed), although this trend may not be statistically significant (fig. 1).  This pattern is likely due to bees being drawn away by the highly attractive dandelion flowers.  The same trend was found in our fruit set data with slightly lower fruit set in areas where dandelions were present (fig. 2).  Regardless of whether the difference in fruit set is statistically significant, the level of fruit set in both mowed and unmowed areas was greater than what apple growers aim to get for optimal fruit production (~10%).  That means that, despite any differences in wild bee behavior when dandelions are present or absent, fruit set was more than sufficient for a good crop.

Future direction

This fall and winter we will continue to analyze our data to determine whether the patterns we found are influenced by the presence/absence of honey bees or differences in habitat in the surrounding landscape.  Next spring we will continue our research following similar methods to 2016.  We look forward to being out in the orchards again.

Further resources

Wild bee conservation: The Xerces Society for Invertebrate Conservation, www.xerces.org

Honey bee health: The Bee Informed Partnership, www.beeinformed.org

(research update originally published in Fresh Magazine)

This article was posted in Agroecosystems, Lab News, Pollinators.