Understanding the Relationship between Pollination Services and Yield in Blueberry
Lisa W. DeVetter
Washington State University, NWREC
Pollination is the transfer of pollen to the plant stigma, which is a necessary prerequisite for seed and berry development in many species of flowering plants. Many species of insects perform pollination, but single-species pollination by the European honeybee (Apis mellifera), or “honeybee”, is the most common in agriculture. Concerns about pollination services have been increasing, particularly with reported declines in honeybee health due to Colony Collapse Disorder and other underlying factors threatening honeybee health. This concern extends to blueberries, which have been reported to have poor pollination and fruit set in Washington. The objectives of this presentation were to provide an update on current research addressing honeybee activity in Washington blueberry and to provide recommendations on ways to promote pollination.
Pollination may be limited by several factors. Weather can be a significant challenge in western Washington because springs are often cool and wet, which are unfavorable conditions for honeybee activity. Floral morphology can also limit pollination, with cultivars having narrow flower openings and longer flower length being more challenging for pollinators to physically access pollen. Bumblebees (Bombus spp.) can overcome this challenge through sonication, which entails vibrational motion that causes pollen to be shed and more easily accessed by bumblebees. Honeybees do not have this ability and are therefore less effective pollinators relative to bumblebees. Landscape features and field management can also impact the populations of pollinators, both wild and domestic (see “Promoting Pollination” for more details).
Knowledge of honeybee health, pollination services, and pollination limitations in Washington blueberry is currently lacking. A pollination study was initiated in 2014 and the objectives of this project were to: 1) Survey and evaluate honeybee activity and its role in yield within Washington highbush blueberry; and 2) Determine the primary limitations of pollination. Long term, the goal of this project is to assist in the development of guidelines and provide information on management practices to promote pollination for the specific conditions found in Washington blueberry production.
Information on year one of a two-year project was presented at the Washington Small Fruit Conference (data presented from Canada are not included in this proceedings report). Data were collected from 16 sites in Washington [10 sites in western Washington (Skagit and Whatcom counties) and 6 sites in eastern Washington (Benton County)]. Four of the eastern sites were certified organic. Honeybee visitation data were collected from established ‘Duke’ plants at 30-100% bloom, with 30 plants measured per site. Data collected include: honeybee visitations, honeybee colony strength/health, pollinator diversity, estimated yield, berry size, and seed number. Visitation was assessed at least three separate days per site and were only collected on days >55°F from 10 AM to 4 PM, which is standard protocol for pollination measurements.
Honeybee visitation rates were significantly greater at organic sites relative to conventional sites across all locations (Figure 1). Visitation was also slightly greater for conventional sites in eastern Washington relative to western Washington. However, conventional fields at both locations were below the recommendation of having 4 to 8 bees/bush/minute (shaded green in Figure 1). Hive numbers across all sites ranged from 1.5 to 6 hives/acre and increasing hive density was not related to increasing visitation rates nor yield. Estimated yield was also not strongly related to honeybee visitation, although there was a slight relationship between increasing visitation and larger berry size. The health of honeybee hives was evaluated by counting the number of bees entering a hive per minute, repeated throughout the course of the bloom period. A healthy hive should have 100 or more bees entering the hive per minute. None of the hives met this recommendation (Figure 2), despite measurements being taken during the time of the day when bees are most active. Pollinator diversity and seed number data are still being analyzed. We hope to repeat this study in 2015 and include more organic sites in western Washington in order to clarify the relationship between increasing honeybee visitations and organic management.
Figure 1. Average number of honeybee visitations per minute in eastern and western Washington blueberry (2014).
Figure 2. Honeybee hive strength in eastern and western Washington blueberry, assessed as the average number of honeybees entering a hive in one minute (2014).
Several tactics can be utilized to increase pollination services in blueberry fields. First, it is advised to discuss changes in management with beekeepers so that an agreement is reached before any action is pursued. One approach is to reduce the number of pesticides, including herbicides. Herbicides can decrease floral diversity and lead to declines in pollinator services. Recent research is beginning to clarify the role of pesticides on honeybee health. The additive and synergistic effects of fungicides, adjuvants, and other products has been shown to decrease honeybee brood health. Decreases in brood health leads to smaller broods, less foraging, and decreases in pollination by honeybees. Insecticides are particularly dangerous and should be avoided during bloom. How to Reduce Bee Poisonings from Pesticides (PNW 591) by Hooven et al. (2013) is a publication that outlines strategies to reduce honeybee poisoning from pesticides.
Some recommendations include:
Other was to enhance pollination include planting hedgerows or native flower strips along the border of fields. Research in Michigan has shown this strategy can lead to increases in blueberry yield and profits because hedgerows/flower strips appear to supplement domestic honeybees with wild pollinators. Lastly are honeybee attractants, which are designed to mimic bee pheromones and enhances pollination. Research done by the DeVetter Small Fruit Horticulture program has tested some of these products and found slight increases in honeybee visitation, but no increases in yield when attractants were applied at low or high rates.
Factors causing declines in honeybee health and ways to promote pollinator services is a very active area of research. Please contact Lisa DeVetter (email@example.com) if you have any questions or would like to access some of the information mentioned in this report.
*Acknowledgements: Ramesh Sagili (OSU), Elizabeth Elle, Tim Lawrence (WSU), Karina Sakalauskas (BC Blueberry), Sean Watkinson, Carrie Scott, Ben Guadagnoli, China Moss, Rachel Rudolph, Doug Walsh, Tora rooks, Yajun Li, and Marina.
Special thanks to grower cooperators and Leighton Overson for project assistance.
This project was funded by the Washington Blueberry Commission.