The McKellar Island Bird Observatory in Thunder Bay, Ont., has planted more than 150 milkweed plants to help revive the monarch butterfly population.

The effort was in honour of Canada 150, though it wasn't funded by a government program, said John Woodcock, a retired biologist who bands birds at the observatory.

He just wanted to do something to help the insects, he said, adding "their numbers are dwindling."

The use of pesticides and the changing temperatures in the butterflies' winter homes in Mexico have contributed to their decline, Woodcock said.

Milkweed is the only plant the monarch caterpillar eats, John Woodcock told CBC. (John Woodcock/supplied ) Though mature monarch butterflies eat a variety of plants, monarch caterpillars only eat milkweed, he explained.

Woodcock said he doesn't expect to see the impact of the new plants until next year but he's already seeing results from a pilot project he started at his home two years ago.

"This year we collected 20 caterpillars off these plants," Woodcock said. "The reason we collect them is we bring them indoors, and we raise them from caterpillar to chrysalis to butterfly, which we release, and thereby we avoid predation."

Around 90 per cent of wild caterpillars are eaten before turning into butterflies, Woodcock said, so raising them indoors speeds up the population growth.

Now here we are turning into a butterfly factory - John Woodcock

Woodcock has been hatching the butterflies in a mesh laundry hamper in his yard.

He won't be raising caterpillars from all 150 observatory plants at home, he said, but added, "We'll do a sample for sure just to get more people interested."

Woodcock credits Dan Fulton of Urban Greenscapes for igniting his own interest in saving the monarchs.

"In talking to Dan, we got interested in it, and now here we are turning into a butterfly factory," he said, laughing.