Tracking the Complex Relationships and Timing of Plants and Pollinators

Posted: May 23, 2019

With increasing attention to pollinators and native plants that sustain those native pollinators, one may wonder if warmer-than-average or cooler-than-average temperatures have an effect on the emergence of pollinating insects.

Many people are discovering and joining native plant groups and other informal learning groups on social media, capturing pictures of plants in flower, wildlife, insects, and other aspects of nature through various seasons. One unexpected benefit of using social media to document and share photos is that they can act as a record of phenology—the study of recurring biological phenomena and their relationship to weather. As I recently discovered when a “memory” from last year on the same day popped up on my timeline, my peach trees bloomed a full two weeks earlier last year than this year.

Some big variations in March temperatures may have caused some plants, like these peach trees, to flower later. According to, the second week of March this year saw temperatures about 10 degrees (F) below than the historical average. During the following week, there were a few days that were 20 (F) degrees above the historical average. Many plants respond to photoperiod, meaning that either shorter or longer days prompt changes in plants’ lifecycles. Temperature also has a strong effect on development rate.

Together, day length and temperature determine the actual timing of flowering. Many plants require a passage of a period of cold temperatures before development can proceed. For some species, flowering dates can be predicted by the number of days above a certain temperature threshold. In the years when temperatures are at or above the historical average temperature, earlier flowering may occur.

Though there are other ways for pollination to occur (wind and birds, for example), many plants rely solely on insects for successful reproduction. In fact, numerous pollinating insects and flowering plants co-evolved, changing over very long periods of time to the benefit of both insect and plant. Like many plants, most insects found in Eastern U.S. forests break diapause (a period when development is delayed) if they’ve experienced a sufficiently long chilling period. If ambient temperatures are still cool and days are overcast, though, they may remain inactive, sheltering in leaf litter or another protective spot.

In some cases, pollinators and flowering plants response to the same environmental cues, such as number of days at a particular temperature, while other insect-plant pairs may respond to different cues naturally or as a result of weather variability. For example, trees—with their roots insulated by soil—may flower even during cool temperatures. Research on big sagebrush populations in Utah and Idaho by B.A. Richardson has shown that these plants may accommodate different flowering dates of up to two weeks.

Bees, among our most important pollinators, are more particular. Bee colonies can be affected by a single cold night in March, when they are trying to maintain colony temperatures at 93 degrees to protect new eggs. If some eggs become unviable because of late freezing temperatures, the colony will struggle to rebuild the colony and be able to stockpile enough food to sustain them.

With increasing attention to pollinators and native plants that sustain those native pollinators, one may wonder if warmer-than-average or cooler-than-average temperatures have an effect on the emergence of pollinating insects. We do know that the relationships between flowering plants and pollinators are intricate and complex. Studies have shown the relationships are so intricate that, among other strategies to accommodate the needed pollinators, some plant species regulate their temperature (by following the sun, for example) to attract insects. Bumblebees prefer warmer flowers when outside temperatures are cool, but when the ambient temperature exceeds the 86 degrees (F), they choose cooler flowers. The bees modify their flower choices in order to maintain their body temperature at the 86 degrees needed for them to fly.

The reality is that although we know of these complex relationships between many pollinators and flowering plants, there are thousands upon thousands of these interdependent relationships that have not yet been studied. This makes it difficult to understand, in detail, how lasting changes or extreme fluctuations in weather patterns may affect numerous species of flowering plants and pollinators. However, many scientists agree that such changes are likely to influence the geographical distribution and local abundance of both plant and pollinator. There may also be disruptions to the geographic overlap of flowering plants and pollinators, as well as when they flower or emerge.

So, what can one do? Commonly, landowners think of creating gardens with diverse flowers to attract and sustain pollinators. Having a diversity of tree species in our forests is just as essential. Entomologist Doug Tallamy of University of Delaware has found that a whopping 534 butterfly and moth species are supported by oaks. Black cherry trees support 456 species of butterflies and moths. Poplars, blueberry bushes, maples, and elms each support more than 200 species of pollinators. Woodland owners can play a significant role in supporting pollinators by maintaining a diversity of tree species. As Tallamy teaches, “You don’t have to save biodiversity for a living, but you can save it where you live. And you should.”

A few resources for learning more are via the Pollinator Conservation Association and the USDA Forest Service. If you would like to order a copy of “A Woodland Nature Journal” to track some of your own seasonal and yearly observations, contact The Center for Private Forests at Penn State at 1-800-235-9473.