While the calendar was turned to May last month, it felt more like March in Ottawa. The miserable, cool weather limited pollinator activity in the garden, slowed plants emerging from the ground, delayed blooms, and prevented seed germination.
Temperature and pollinator activity
Throughout the month, local gardeners shared my concerns about the dearth of pollinators. One gardener pollinated his apple trees with cotton swabs — just in case. Another wondered if there will be fewer berries for birds on native shrubs this year. And another questioned whether the milkweed would pop up before Monarchs arrived to lay eggs on it?
Last year, I learned that excessive heat harms bees. When the temperature hovered around 40°C last summer, I noticed only a few honey bees in the garden. How urban heat affects bee populations summarizes how high temperatures affect native bees.
On the flip side, what is the impact of cooler than normal temperatures? How warm does it needs to be for native bees to become active? The University of Maine Extension article, Understanding Native Bees, the Great Pollinators: Enhancing Their Habitat in Maine, states:
“The most likely place to find bees is in the flowers of native plants, when the day is sunny, relatively calm, and the temperature is above 70°F. To be active, fly and feed, bees need to be warm. A few species are active below 60°, but most prefer temperatures above 72°. Wind makes flying more difficult because it requires more energy.”
In Celsius, that means it needs to be at least 15.5°C for the more cold-hardy bees, and 22°C for the rest. According to the Weather Network records for May 2019, there were 16 days above 15.5, and only 2 above 22. Not very good weather for native bees. It doesn’t seem that the weather was favourable for honey bees either because I saw only one in my garden this month.
In Bumblebee Economics (at Ottawa Public Library), Bernd Heinrich explains that bumblebees are more tolerant of cooler weather because they can shiver to warm up. I did frequently see queen bumblebees foraging and looking for nest sites in my yard. It will take several weeks, after these queens choose nest locations and lay eggs, before the first brood of workers will be out and about.
Luckily, bees aren’t the only pollinators. Flower flies are also more tolerant of cooler temperatures. I did spot some in the garden, so hopefully they picked up some of the slack.
Most popular flowers
In May, the popular flowers for queen bumblebees were on my ‘PJM’ rhododendron, long-blooming lungwort, and crabapples. The bumblebees liked the flowers so much that they even visited them after they had dropped onto the ground. While I was watching, to my surprise, the queen bumblebees ignored my native perennials, shrubs, and trees.
For smaller native bees and other pollinators, the winners were serviceberries, crabapples, and wild strawberries. It looked like some native bees were nesting under the plentiful strawberry plants that I use as ‘living mulch’ in my hedgerow. Perhaps they preferred foraging close to their nests instead of flying up to the shrubs and trees.
May 2019 trees and shrubs in bloom
May 2019 flowers
A few notes about my monthly tables
As I made May’s tables, I thought of a few points worth mentioning here.
For many plants, I have multiple clumps in the front and back yards, and in differing degrees of sun. The plants in more sun bloom earlier that ones in more shade, which prolongs bloom times in the tables.
If plants have no pollinator value, should I bother including them? I’m going to include them in the tables anyway because I have been surprised to see pollinators visiting ‘worthless’ flowers. A few weeks ago, my son pointed out a queen bumblebee drinking nectar from periwinkle flowers. It wasn’t visiting the Merrybells, Dutchman’s breeches, or Hellebores in bloom. Maybe my patches of those native plants are still too small for them to notice.