Time to Chill

Mason bees make their cocoon and go into hibernation, Friday, Nov. 6, 2015. by Randee Daddona

Mason bees make a cocoon and then go into hibernation.

While marveling at the puffy white and pink flowers that make fruit trees insanely gorgeous this time of year, cheer the chill-hour requirements that make this wonder possible. It takes a specific number of accumulated chill hours—longer days and warmer temperatures—for many deciduous trees (those that lose their leaves in winter) to begin to grow normally again in the spring. In fact, each species requires a set amount of cumulative hours where temperatures are between 32 and 45 degrees (i.e., chill hours) in order to produce flowers and set fruit. Chill hours can vary year to year depending on the number of cold fronts each winter. In some cases, warm temperatures (e.g., greater than 59 degrees) can even negate previously accumulated chill hours. As a general rule, most pears have a high chill-hour requirement, most apples are in the middle, and most peaches and other stone fruits have low chill-hour requirements.

Mason bees make their cocoon and go into hibernation, Friday, Nov. 6, 2015. by Randee Daddona

Tree fruits and especially apples are a significant part of New York’s agricultural economy. In fact, New York was ranked second in apple production nationally in 2012 with a production value of about $250 million. Although there were no reports commercial orchards were affected by this past winter’s warm spells, growers always take chill hours into account. Knowing the magic chill-hour range for a region helps orchardists select cultivars genetically inclined to grow well. The fruit trees will flower too early and be damaged by late frosts if one selects a low chill variety for a cold area. Little or no fruit will be produced if a high-chill variety is selected for a warm area. Chill hours for apples range from 400 to 1,800 hours, with Gala and Red Delicious at the low end of the scale and Macintosh and Empire at the high end of the scale.

The forecasted effects of climate change on early spring flowering crops are interesting. By the 2050s in New York State, expected warmer winters may mean that popular cool-season apple varieties like Macintosh and Empire will no longer receive sufficient winter chill hours for commercial viability. On the flip side, New York’s projected longer and warmer growing seasons may provide better opportunities for longer-season varieties like Fuji and Granny Smith. Since orchards often remain in production for decades, discussions on whether to replant whole sections or develop new cultivars are taking place from California to New York.


OK, so how does all of this information relate to bees? I am a beekeeper after all. The native mason bee (Osmia spp.) is increasingly being used to pollinate early spring crops, including apples, apricots and cherries. Adult mason bees overwinter in individually spun cocoons, and researchers have found that winter survival and their vitality after emerging is heavily dependent on winter temperatures. Bees overwintered at 39 degrees or less had the highest survival rates and vigor. Bee cocoons overwintered at 45 degrees were the most pathetic with increased mortality, decreased longevity and low body fat. Luckily I keep my mason bee cocoons in my refrigerator to control their overwintering environment. But, wild mason bees do not have such cushy existences and, as we saw this past winter, can experience major swings in temperature. Mason bees are just one example of a bee that enters a dormant state and depends on cold days to overwinter: leafcutter bees, bumblebees and digger bees do, too.

I really appreciate a red, juicy, crunchy, sweet apple, and I sure as hell love my bees and want the best for them. My memories of winter used to melt away with the first signs of spring. Now I see the beauty of many cold winter days in each spring blossom and each bee that flies by.