Phenology and Its Value to Beekeepers

It’s important to know what blooms when and where

by Denise Ellsworth

Spring can be slow to arrive to Ohio. Here in the Buckeye state, the days of March are usually full of gray skies and partially frozen soils, with only hints here and there of warmer days to come. Silver maple and speckled alder are two of the first trees to bloom, both with subtle flowers eagerly visited by honey bees ready to start the season. Gradually Spring color splinters the gray hues, and Spring arrives.


The yellow flowers of the forsythia shrub are a sure sign of spring in Ohio. Forsythia is a fairly modern addition to our landscapes; most commonly-grown species were introduced from Asia in the mid-1900s. Several plants native to North America also serve as harbingers of Spring, including the wetland-loving skunk cabbage, spring beauty wildflowers and the lovely white-blooming serviceberry tree – all visited by honey bees in need of pollen and nectar to build the Spring colony.

Serviceberry’s name tells of this plant’s importance as an indicator of Spring. Sometimes called sarvis-berry or simply ‘sarvis,’ the serviceberry (Amelanchier spp.) earned this common name because its time of bloom coincides with the thawing of cold Winter soils. In times past, funeral services could be held for those who died over the Winter once serviceberry bloomed; soils would be thawed enough to allow for burial.

Serviceberry’s other common names, Juneberry and shadblow, also speak to the tree’s role as a phenological indicator. Phenology is the study of recurring biological events, such as bloom time, animal migration and insect emergence. These events follow the same sequence from year to year, regardless of the speed at which the events unfold. The common name shadblow comes from the coincidence of bloom time with the swimming of shad upstream to spawn in spring; the name Juneberry describes the ripening of the fruit in early Summer.

The seasonal appearance of flowers and insects are examples of phenological events that have been recorded for centuries. Because the development of both plants and insects is temperature dependent, plants can accurately track the environmental factors that determine when insects are active. For this reason, plant phenology can be used to predict insect activity. Plant phenology can also be used to track long-term climactic changes, such as global climate change.



Using the principles of phenology, Dr. Dan Herms, professor of entomology at The Ohio State University, created an extensive biological calendar based on years of observation of plant and insect activity at Ohio Agricultural Research and Development Center’s Secrest Arboretum in Wooster, Ohio (find calendar here). Several times a week between 1997 and 2004, Dan walked through the arboretum, monitoring the phenology of 91 ornamental plant species and/or cultivars, and 43 key insect and mite pest species.

This intensive, long-term research project has demonstrated that phenological events occur in virtually the same order each year. By entering an Ohio zip code, visitors to the Ohio State Phenology Calendar website can acurately pinpoint development events in their region in Ohio. The biological calendar is used extensively by extension personnel, pest managers and horticulture professionals to monitor plant development events, predict pest activity and schedule pest management appointments.

To expand on the phenology calendar, Dan and I developed the Ohio State Phenology Garden Network in 2004. At 36 gardens located throughout Ohio, project volunteers planted identical plant material consisting of 17 common trees and shrubs and 15 perennials, the blooming sequence of which spans the growing season. The Network is a collaborative research and education effort that involves numerous Ohio State Master Gardener programs, local parks, schools, arboreta, and other partners, including the grounds of Bee Culture magazine’s headquarters in Medina, Ohio. At each site, volunteer citizen scientists engage in the age-old practice of tracking plant bloom and linking these phenological events with insect pest activities, as well as using the gardens as demonstration and teaching tools for gardeners and professional horticulturists.

In 2012, I changed positions with Ohio State University from a county extension educator to the program director of the honey bee and native pollinator education program, housed within the OSU Department of Entomology. To reflect this change in focus, we added 11 pollinator-attractive native perennials to 28 of our phenology network gardens. Beginning this Spring, our volunteers will track the span of bloom and pollinator visitation on all network garden plants. These data will paint a picture of which plants are visited by which pollinators, and how bloom times overlap to provide forage across the growing season.

While many plant lists exist to help beekeepers, gardeners and farmers to select plants important to honey bees and other pollinators, most of these lists include information about the timing of bloom using calendar dates (April – May, for example), if at all. As any Midwesterner who travels south in Spring knows, April in central Michigan is a world away from April in northern Kentucky or southern Virginia, making the calendar-based information almost useless. By linking time of bloom to Growing Degree Day units (or GDD, a measure of accumulated heat), the bloom timing and sequence can be interpreted across a wider region. Growers hoping to span the Spring and Summer with a sequence of bee-friendly forage plants will be better able to select suitable plants.

From a practical perspective, Ohio beekeepers have been excited to use Ohio State’s phenology calendar to help them prepare for specific bloom events. Because the calendar can show which plants are flowering in a specific region of the state and which are coming into bloom soon, beekeepers can be ready to add honey supers before black locust comes into bloom, for example. This is also helpful for beekeepers who manage bees at multiple locations across the state – areas that may have significant variation in bloom progression. One clever Ohio beekeeper has tracked plant phenology and correlated this to his hive management practices, such as swarm week (correlated with bloom of daisy fleabane), time to raise queens (dogwood bloom), and time to create nucs (chicory in bloom).

Another practical application of the calendar pertains to the prediction of plant pest activity. Horticultural and pest management professionals in Ohio have used the phenology calendar for over a decade to help refine the timing of pest management strategies, such as pest monitoring and pesticide application. Pesticide and other pest management recommendations from OSU Extension previously relied on calendar dates in order to time applications. Because phenological phases in the state can vary by three weeks or more from Cincinnati in the south to Cleveland in the north, recommending a pesticide application in “early May” could be costly and useless, since the pest may be past its most susceptible stage or not yet susceptible in the far reaches of the state. Using plant and pest phenology to time management strategies refines the timing and improves pest management results.

Eastern tent cat webbing

Eastern tent cat webbing

Since many beekeepers are also farmers, gardeners or land managers, the phenology calendar is useful to give a heads-up to specific pest activity. When border forsythia first comes into bloom, for example, Eastern tent caterpillar eggs begin to hatch. These caterpillars can damage fruit trees by feeding on early foliage. Once alerted to the timing of egg hatch, apple growers can monitor trees for signs of the pest’s webbing in the crotches of tree branches. These early webs can be easily removed and destroyed with a gloved hand before leaf damage occurs, eliminating the need for pesticide applications. Other plant/pest indicators include full bloom of lilac and hatch of pine needle scale, full bloom of arrowwood viburnum and bagworm egg hatch, and first bloom of ‘June Bride’ littleleaf linden and Japanese beetle adult emergence.

Residents of states that border Ohio can make use of the Ohio State Phenology Calendar by entering an Ohio zip code from a geographically similar area. Additionally, other states are conducting similar research to track plant and insect phenology, although we have yet to locate a resource as comprehensive as Ohio’s calendar. Still, curious beekeepers and naturalists can make use of the principles of phenology to create a biological calendar by tracking bloom events and insect activity in their own locality throughout the growing season. Maybe the serviceberry’s bloom this spring will provide a phenological reminder to grab a notebook and pen and begin to take notes on the sequence of biological events near you.

Learn more about phenology and Ohio State’s phenology research.

Denise Ellsworth is the Program Director for honey bee and native pollinator education, The Ohio State University Department of Entomology in Wooster, Ohio.