A southern Ontario health unit and naturalists alike are warning parents to keep kids away from a venomous caterpillar prevalent this time of year.

All summer, the head naturalist at Ojibway Nature Reserve in Windsor, Ont., thought nothing of the small, white insects crawling along the tree leaves in the area.

"For a long time I just thought it was a cute little fuzzy caterpillar," said Paul Pratt. "Recently I found out they're actually venomous."

Pratt is talking about hickory tussock moth caterpillars. They're approximately 5 cm long, white, fuzzy, with a few big tufts of black hair near its front and back. These hairs are connected to poison glands.

If you touch these tufts, the glands will excrete venom, according to Pratt.

"You can get a reaction much like getting in a patch of stinging nettles," he said. "For most people it will sting and itch for 20 minutes or so before it goes away, so it's best not to handle them."

'It's best not to handle them.'- Paul Pratt, Ojibway Nature Centre

Symptoms can range from a slight skin reddening to a burning sensation with swelling and pain, or even nausea.

"You have to be careful, especially letting small children pick up cute, white fuzzy caterpillars," he said.

Dhara Bhavsar is a Grade 4 student who visited Ojibway Nature Reserve with her class. They learned about the hickory tussock's venomous properties during the field trip.

She said, "I think it's really neat, that it's different colours."

"It can give you an allergic reaction and I don't really like those," Bhavsar added.

Her classmate Harmeet Singh agreed.

"Our leader said sometimes they can be itchy. I never knew that caterpillars can be poisonous," he said.

Their colours may attract children, but it's really to ward off potential prey.

"Just like monarch butterflies are bright and orange because if a blue jay eats one, it'll throw up. So, they're advertising, 'I'm distasteful.' The caterpillars are basically doing the same thing," Pratt said. "Basically, it's a warning for animals, 'don't try to eat me or pick on me because you'll pay for it.'"

Pratt said there seems to be more of the caterpillars this year than in those previous.

The Middlesex-London Health Unit is advising parents and caregiver to ensure that children in their care avoid touching hickory tussock moth caterpillars.

Dan Flaherty, communications manager for the Middlesex-London Health Unit, told CBC News he had to re-issue a news advisory originally sent out a couple years ago because the number of calls about the caterpillar has increased this fall.

The health unit recommends those experiencing a reaction to wash the affected area with soap and water as soon as possible. In the case of itching or swelling, apply calamine lotion and ice packs.

Flaherty said the original concern was raised in 2011 when a young boy dropped a caterpillar down the back of a shirt of a classmate's shirt.

Flaherty said the the itching and burning is "instant" upon touching the caterpillar.

Common in later summer

Pratt said the hickory tussock moth caterpillar is most common from July to September. They then grow into an orange-coloured moth in the summer. But, beware the cocoons as well, because the caterpillar hairs are used in the shell to discourage predators.

These white insects are related to the wooly bear variety, which are rusty brown and black, but aren't poisonous.

It feeds on the leaves of hickory, walnut, ash, elm and oak trees in preparation for over-wintering in its cocoon.