They're cute, fuzzy, smaller than most people's pinkie fingers -- and if you touch them, you may regret it.

The black and white tussock moth caterpillar lives in elm, ash, oak and hickory trees, and earlier this week Transport Canada, which has offices on Sparks Street, issued a warning to its employees that the caterpillar had been spotted in downtown Ottawa.

Many caterpillars can be handled without any concerns, but not the tussock moth caterpillar, said Bob Anderson, a research scientist with the Canadian Museum of Nature.

Covered in poisonous spines

"The caterpillars have little spines all over the body. And those little spines contain venom," Anderson told Ottawa Morning's Robyn Bresnahan on Friday. "And if you brush against the caterpillar, the venom will be injected into your skin."

Most people will get a red welt that stings for "about 15, 20 minutes" and fades into a mild itch, says Anderson.

The tussock moth caterpillar might look cute but it packs a venomous punch, says Bob Anderson, research scientist at the Canadian Museum of Nature. (Stu Mills/CBC)

But in very rare cases, the caterpillar's venom can cause an anaphylactic reaction, he added. In those cases, symptoms can include headaches and nausea, and people should seek medical treatment.

The caterpillar also incorporates some of those tiny spines into its cocoon, said Anderson, rendering it poisonous as well.

Anderson said there's no sign that the population of the tussock moth caterpillar -- which is native to a wide swath of North America that includes Ottawa -- is any greater this summer than in years' past.

"I think it's probably somebody (with Transport Canada) just noticed them. They're very, very obvious when you see them on the trees," he said.