The Case of the Missing Bumblebees

Bumblebees are my favourite kind of bee. They’re big, gentle, furry constant companions in the garden – or should be. In late June and early July, I had no bumblebees in my gardens at all. The yard was a lonely place without them. After a lot of fretting, I decided to investigate why there were no bumblebees, so I could do something about it.

Suspect 1: Wool Carder bee thugs

I first blamed male European Wool Carder bees for chasing all the bumblebees away. I really do dislike them and their bullying behaviour. Male Wool Carder bees choose an area with abundant flowers and patrol it, so females they mate with will have a secure supply of food. This year I had multiple males claiming territory in my front and back gardens. They rammed and chased each other, as well as other bees. Here’s a Youtube video, Wool carder bee – defending territory, that shows a male bee driving away another bee. They are easy to spot because they have a distinctive hovering flight, and a stocky, hairless, striped body.

The local Wild Pollinator Partners has an informative blog post about Wool carder bees. Although there are alarmist stories on the Internet about Wool Carder bees, it turns out that I don’t really need to worry about them. I may not like Wool Carder bees, but they are not guilty of driving all the bumblebees from my garden.

Suspect 2: Bumblebee-mimic robber fly

One day I noticed a large bumblebee, maybe a queen, carrying another bumblebee. In the book A Sting in the Tale (at the Ottawa Public Library), by Dave Goulson, I had read about internal bumblebee nest conflicts, but never about a queen carrying a worker away. So strange.

It turns out that the larger ‘bee’ is actually a bumblebee-mimic robber fly with quite an amazing disguise. It is eating the smaller worker bumblebee! Visit the Wild Pollinator Partners blog posts Big-eyed bumblebee and Bumble bee imposter to read about robber flies. The robber fly in my photo is Laphria thoracica. Its large eyes, short antennae, and 2 wings (as opposed to 4) indicate that it is a fly instead of a bee.

Did this robber fly kill all my bumblebees? Probably not. It must have fled the murder scene because I never noticed it again. Also, robber flies eat other kinds of insects too, not just bumblebees. If it stuck around it could have eaten some of the Japanese beetles in my yard.

Suspect 3: Crab spider

I’ve noticed more insect diversity in my garden this year. Many of them predators and parasitoids that feed on other insects. Their presence indicates that I have created a healthy ecosystem in my yard.

One morning, this Goldenrod Crab Spider was in my back garden waiting to ambush a foraging bee when it landed on a Knautia macedonica flower. Crab spiders grab insects with their long front legs, inject them with paralyzing venom, and suck out the contents of their body.

I got tired of watching this spider. Perhaps it also got tired of waiting here. I never saw it again. He was quite poorly camouflaged on the red flower, so maybe the bees noticed it just like I did. Besides, Bicoloured Striped Sweat Bees were visiting these flowers, not bumblebees.

Suspect 4: Mother Nature

Mother Nature delivered a strange mix of weather extremes this spring and summer. Bumblebees had a difficult time with the cold, late spring and a very hot, humid July. Was the weather to blame for the lack of bumblebees?

The cold spring weather caused queen bumblebees to emerge late, and start their nests late. Once a queen bumblebee lays eggs, it takes about a month for them to develop into the first batch of workers. By early June, I saw the first workers. I also still saw queens foraging, which meant that the workers weren’t able to return enough food to the nest, so the queens had to join in to help. Bumblebee colonies are already under severe time constraints to gather resources and grow as quickly as possible, so the late start jeopardized their success.

By the time I started to see more worker bumblebees in the garden, the extremely hot and humid weather hit. According to Bernd Heinrich, in Bumblebee Economics (at Ottawa Public Library), Bumblebees can only forage when their body temperature is between 30°C and 44°C. They have adapted to function in cool temperatures; their furry bodies, and ability to shiver to warm themselves, allows them to forage and heat their nests in cooler temperatures. If it’s too hot, they can’t forage, and they don’t store large surpluses of pollen and nectar to use in the meantime.

Wild fluctuations in temperature, such as our cold spring and very hot early summer, are a consequence of climate change, not Mother Nature’s whims. The normal dips and wobbles in the Polar Jet Stream (Wikipedia) become more persistent and are amplified, causing odd weather extremes, according to the Discover Magazine article What’s up with all this wild, weird weather. For information on studies of how climate change is threatening bumblebees, visit Bumble bees being crushed by climate change, Climate change is killing off bumblebees, and Bumblebees affected by 2018 extreme UK weather. Sadly, bumblebees seem to be the climate-change canaries in the garden.

Missing flowers?

Were the bumblebees out there visiting other gardens, instead of mine? The strange weather caused some plants to bloom late. My anise hyssop plants, which have been a bumblebee favourite for the past two years, bloomed weeks later than before and I lost several of my largest plants. When it finally did start to bloom, there were no bumblebees around.

I searched around the neighbourhood to see if bumblebees were in other gardens with different plants. I did find a few on non-native rugosa roses and false spirea. I asked other gardeners if there were bumblebees in their gardens and what plants they were visiting. Based on their replies, it seems that bumblebee numbers were down elsewhere too. Others were seeing bumblebees on the following native plants: spiderwort, spirea shrubs such as meadowsweet and steeplebush, and St. John’s wort. I’ll be adding these to my garden to ensure that I have back-ups and a consistent succession of blooms.

This mystery turned out to be more complicated than I expected, and there’s no specific villain to blame. In the end, since mid-July, there have been so many bumblebees visiting my garden — I am so relieved! While they had a late start, they are now abundant. I appreciate their company more than ever.