There’s a new bug in town …
Firebugs have chemical beacon called ‘aggregation pheromone’ that causes them to congregate
I was sitting on a rock at the Leslie Street Spit one evening last week, soaking up the gifts of late summer as dusk dropped an indigo curtain over the park. A sliver of crescent moon was setting in the west, just visible over the city’s twinkling skyscrapers. Behind me, over Lake Ontario, Jupiter was a glowing opalescent orb in the eastern sky. The fresh green air, perfumed with the fragrance of white clover and goldenrod, was filled with the chorus of crickets.
In the midst of my reveries, the phone in my pocket buzzed. It was my niece Annie, sharing photos of some fire-engine-red bugs she’d seen while walking her dog Ruby earlier in the day. They were bugs she’d never come across before, so she used an app on her phone to identify them. According to the app Seek, by iNaturalist, they were European firebugs (Pyrrhocoris apterus).
When Annie first noticed the bugs, they were crawling one by one, helter skelter, through the grass and leaf litter under a large tree. About 30 minutes later, the bugs had ceased their disorderly scramblings and had congregated into a loose mass of several hundred insects. They seemed particularly sensitive to vibration. When she walked on the ground near them, they scattered as quickly as they had amassed.
“It’s as if one of them sent out a signal, calling the others — like they got an email telling them to attend a Zoom meeting,” Annie said when I contacted her to learn more about the novel crea
tures and where she had seen them. She texted me a map of the bugs’ location on a residential street in south Scarborough, and the next morning I went to see if I could find them.
Following Annie’s detailed map, it was easy to locate the bugs. There was a mass of the striking red-and-black insects basking on a chunk of granite in the warm morning sun. A massive, healthy-looking linden tree towered overhead. Looking very carefully, I was able to find dozens more of the bugs scurrying through the leaf litter on the ground. I observed them for a while, took some photos, then went home to learn more.
There’s a good reason neither of us had ever seen these insects before — they’re the new bugs in town. Hailing originally from Europe (hence their common name), they were first discovered in North America in Utah in 2008. They didn’t arrive in Canada until 2017 — likely in plants imported from Europe — but their range is expanding rapidly. In the Greater Toronto Area, firebugs are mostly found under linden trees (Tilia species) where they feed on the trees’ fallen seeds with specialized mouthparts that pierce the seeds and suck out their contents.
That was all good, but I still wanted to know what caused the firebugs to congregate.
It turns out Annie was right about the signal. Firebugs have a chemical beacon called an “aggregation pheromone” that calls them to come together. They have other chemical messengers, too — “alert pheromones” — that trigger an alarm response that can quickly dissolve aggregations.
In the winter, European firebugs enter what biologists call “diapause,” a period similar to hibernation in mammals that allows the insects to survive cold weather. In the spring, as temperatures rise and day lengths increase, the insects gradually become more active, eventually forming large aggregations that facilitate mating.
But with mating season long over, I wondered what was causing the bugs to amass now, in mid-September.
Sandy Smith, a professor of forest health at the University of Toronto, told me by email that a combination of shorter day length and cooler temperatures induces the firebugs to seek sheltered overwintering sites, and this leads to congregations.
“They will continue to gather in clusters to increase their body temperature,” Smith said. “In order for them to remain active and feed, they need to be warm either by gathering together or exposing themselves to sunlight…. Ultimately, they are all moving to overwintering locations, either in built structures or in naturally sheltered areas such as tree hollows and leaf litter.”
If the bugs overwinter in the crevices of a tree, there’s no problem. But if they end up choosing “built structures” in which to spend the cold months, they can become what the city of Toronto’s website calls “a nuisance to homeowners.” Note the word nuisance, rather than danger. The city’s official page on the insects says that despite their “warning colouration and threatening name, European firebugs are harmless to humans as well as their (tree) hosts.” If you see them around your home this fall, the city warns against squishing them, as they “can emit a foul odour when crushed.” Instead, the web page recommends using “high-pressure water from a garden hose sprayed directly on the insect congregations.”
Smith says that although European firebugs are “clearly invasive in North America, they are an interesting species that do not seem to be harmful.”
No need to worry, then — at least for now.