MILTON, Wis. — Last April, Dave Bendlin spotted a monarch in his garden about five weeks ahead of the usual time the butterfly first appears in spring.

Bendlin of Milton did not know it then, but the early arrival was a harbinger of good news.

This summer, struggling monarch numbers in the Upper Midwest have shown the strongest comeback in years.

"The number of monarchs has been phenomenal," Bendlin told the Janesville Gazette . "All summer long we have seen large numbers."

Monarch Watch, a citizen-science program that monitors monarchs, confirms what Bendlin witnessed — a perfect storm for the striking orange, black and white butterflies.

"These are the most extraordinary conditions for monarchs that we have had since 2001," said Chip Taylor, founder and director of the Kansas-based Monarch Watch. "This is going to be the largest population we have seen in a decade."

The professor of ecology at the University of Kansas projects an overwintering population in Mexico of 5 hectares, which is twice as big as the one last winter. A hectare is 2.47 acres.

Scientists cannot count the millions of butterflies that spend winters in fir trees in the mountains west of Mexico City. So they count in hectares the amount of space the trees occupy.

The highest recorded number was 18.9 hectares in 1996-97.

Since the early 2000s, monarchs have been in decline with a record low of less than a hectare in the winter of 2013-14.

The decline has been due to habitat loss both in Mexico and the United States, less predictable weather and the advent of herbicide-resistant crops, which has led to a decline in the amount of milkweed available in the butterfly's range.

Monarchs feed on the nectar in flowers, but they lay their eggs only on milkweed, which the caterpillars exclusively eat.

Taylor explained how conditions favored monarchs this spring, when they migrated north from Mexico.

"They made it successfully to Texas, where there was ample milkweed, and temperatures were higher than normal," Taylor said. "They produced a large first generation."

As temperatures rose, butterflies continued moving north, mating and dying along the way. Eventually, after multiple generations, they reached the northern United States and Canada.

"How well these generations move north in May and early June depends on temperatures," Taylor explained. "Temperatures were favorable for large-scale movements to the north and northeast. Monarchs reached the northern breeding areas successfully at the right time and in good numbers. That allowed the population to develop rapidly."

But the current uptick does not ensure another good year in 2019.

"What we saw this year may never happen again because the climate is changing," Taylor said. "It's getting very warm in Texas in March, which may push the monarchs north too soon. If it is too hot or too cold as they move north, monarch reproduction goes down."

He also said there is another big unknown.

"Will fall conditions favor survival during the migration?" he asked. "We don't really know how to answer this question."

To help dwindling numbers of monarchs and other pollinators, people such as Bendlin and his wife have planted many varieties of native plants in their yards.

"Plants with open, flat flowers are what the pollinators like best," Bendlin explained. "The simpler the flower, the more nectar it produces. A lot of hybrid varieties have more petals but less nectar."

Bendlin, of the Rock County Conservationists, said the group has worked with 11 Rock County schools to plant pollinator gardens. Pollinator gardens are rich in nectar-producing, mostly native plants.

The group also works to educate people about how to create a healthy environment for butterflies and other pollinators in their yards.

Bendlin offered basic tips: Dig up some lawn and plant nectar-rich flowers, such as native prairie plants; do not use pesticides; and do not mow native plants, such as milkweed, that grow in ditches.

Pollinator gardens are full of rewards, including a steady stream of butterflies and other insects from spring to fall.

"There's also a sense of satisfaction in knowing we made a difference," Bendlin said.

Conservationist Emily Scheunemann said a person does not need a big yard to make a difference.

"If everyone does a little bit, it all adds up," she said. "Citizens can do their part by planting native plants and milkweed, even if they live on postage-stamp property."

She and her husband, Larry Scheunemann, created an oasis for pollinating insects and birds at their rural Whitewater farm.

She called this year's migration extraordinary.

"Citizens have observed huge masses of monarchs," Emily Scheunemann said, "and they report big roosts at night."

Earlier this month, the Scheunemanns tagged migrating monarchs just as they have done for at least 15 years.

Monarchs born in Canada and in the northeastern United States are now on their 2,500-mile journey to Mexico — a place where they have never been. Some can still be seen bouncing between blossoms or across roads in southern Wisconsin.

Larry Scheunemann called gardening for pollinators "really gratifying."

"It's a labor of love," he said. "When you take time to look at the different plants and all the different pollinators, it is really amazing."

An AP Member Exchange shared by the Janesville Gazette.