Honey Bees and Climate Change: A Hypothesis
Throughout my career my focus has been on field and laboratory research. My mentors of years past were quite negative about offering opinion to a public audience in the absence of data. Subsequently, I have previously avoided doing so. I usually write reports that are backed by data, and which, compile data to analyze a technical question or perspective. However, being invited to provide this opinion editorial presented an interesting opportunity to reflect on the following casual and experiential observations I’ve made during years of fact-seeking research.Much has been published and debated in recent years on the effects of pesticides, parasites, diseases and habitat loss on pollinators, while I have noticed only limited attention being paid to the potential influences of climate change on honey bees and other pollinators. Admittedly, this is a tough topic of study not entirely suited for short-term laboratory or field research. My professional involvement in honey bee field research has afforded me, and researchers I’ve worked with, the opportunity to closely monitor many honey bee colonies through their annual cycle, including overwintering and early spring foraging behavior. When honey bees stir from their hives at the advent of spring weather, when daytime air temperatures exceed 50° Fahrenheit, the colonies are usually at their lowest number of adult foragers. It is essential that the foragers take full advantage of the early spring nectar flow and accompanying availability of pollen.
This early foraging is critical to the colony in its effort to rapidly build food resources in the hive to support high productivity of offspring that will mature and add to the population of foragers (and worker bees) which, in turn, increase incoming food stores and perpetuation of the intense pace of reproduction. The storage of nectar, as honey, and pollen, as bee bread, must be of sufficient quantity to feed the burgeoning larvae and adult population in the hive and build stores adequate to get the colony through the usual late-summer (mid-July through September) dearth of nectar and pollen and through the following winter. If adequate food resources are available, and harvested, during the period from spring thaw to mid-summer, the necessary rapid growth of the colony in both adult strength and food storage, will be accomplished. If, for any reason, the foragers from a colony are unable to take full advantage of the early nectar flow, colony development will be impeded and survival through the ensuing winter will be in jeopardy.
Based on observations of overwintered honey bee colonies and bee behavior during early spring, primarily in North Carolina and central Oregon, I have developed a hypothesis about two current, and possibly transient, weather conditions that are challenging honey bee populations. At this point in the climate change chronology, we are experiencing gradually warming winter temperatures, characterized by occasional, increasingly common, but still sporadic mid-winter temperature spikes when air temperature rises above 50° Fahrenheit and, at times, into the mid- and upper 60s. Intuitively, one would think warmer mid-winter temperatures would enhance overwinter survival, and this may be the case, if these intermittent temperature spikes did not stimulate unrewarded foraging efforts, with large numbers of bees taking flight in search of food that doesn’t exist. The unrewarded expenditure of energy leads to greater consumption of hive food stores with little, or no, additional food gathered.
When the daytime air temperature then returns to winter conditions in a day or two, and sometimes suddenly, foragers can be caught away from the hives and unable to return as temperatures plummet to lethal levels, further reducing the forager population available to harvest the early spring nectar flow when it finally occurs. While this phenomenon has occurred historically from time to time, I don’t believe it has occurred at the frequency observed during recent winters.
Additionally, our slowly warming temperatures are gradually changing what used to be late winter months into early spring months. What’s different about the “new” early spring is that diurnal temperature swings are greater. The deleterious influence of this to honey bees is related to the fact that honey bees generally do not forage in temperatures below 50° Fahrenheit. The daytime temperatures during many of our current early spring days don’t reach or exceed 50° until late morning to mid-day and then fall below 50° again in late afternoon. This affords honey bees only 4 to 5 foraging hours per day. Because many plants, particularly trees and various wildflowers, are not similarly impeded by the fluctuating diurnal temperatures (provided night-time temperatures remain above freezing) they proceed through their budding-out and flowering chronology, producing the early nectar flow under conditions that do not allow honey bees to take full advantage of it. This timing misalignment has serious impact on the ability of honey bee colonies to build adequate food stores to facilitate the colony growth necessary to survive the subsequent winter.
Not all pollinators are as sensitive to cooler temperatures as the honey bee. I often see certain varieties of bumble bees in central Oregon foraging on early-blooming plants in 40-degree temperature and long after honey bees have retired to their hives for the day. Therefore, I cannot, with certainty, extend this hypothesis to pollinating insects, in general. I am concerned, however, that native pollinators are experiencing population stress related to fluctuating climate conditions, especially when I consider the weather issue in synergy with the loss of quality habitat across the continent. But, for honey bees, the influence of fluctuating weather conditions may be a major cause of over-winter colony failures in North America.