PLAINS (AP) - Mary Malinowski's garden in Plains Township blooms with clusters of purple flowers of the common milkweed, planted to attract monarch butterflies.
But the last two years, the milkweed's broad, flat leaves have been free of monarchs, their caterpillars or their eggs.
"This year, so far nothing," she said. "But the years before, they were always here before the first of June."
Butterfly observers and scientists are warning that the monarch, North America's most famous butterfly, is in trouble. Overwintering populations counted in Mexico are at their lowest in 20 years, according to data collected by Mexican biologists and compiled by MonarchWatch.org, a website run by University of Kansas professor Dr. Orley "Chip" Taylor.
Scientists are tracing their decline to a loss of milkweed, a crucial food and habitat source in the butterfly's complex life cycle.
Each fall, monarch populations escape the cold of the Midwest, and northeast U.S. and southern Canada by flying southwest to high-elevation forests of central Mexico, where they cluster on oyamel fir trees.
Their journey takes about two months, with monarch fluttering 25 to 30 miles a day, stopping to fuel up on amino acid-rich nectar from plant species such as pentas and lantanas, according to MonarchWatch.org.
It takes several generations of monarchs to make the journey each year. Eggs hatching in late summer and early fall become the adults that travel to and from Mexico, living eight or nine months.
Those born in spring and summer transform from egg to caterpillar, chrysalis, then adult in about a month. They live another two to five weeks, the website states.
The butterflies rely on common milkweed, which grows along roads, in farm fields and other landscapes with disturbed soils. Monarchs lay eggs and their caterpillars feed exclusively on 30 different species of milkweed.
But milkweed has declined drastically over the past 20 years as farmers have adopted herbicide-tolerant crops in tandem with greater use of milkweed-killing herbicides, according to Taylor's blog. Housing and commercial developments and the federal ethanol mandate also has led to milkweed removal.
Taylor estimates 167 million acres of monarch breeding habitat have been lost since 1996, making sanctuaries such as Malinowski's garden increasingly rare.
She hopes her the butterflies return. After her husband bought her a butterfly bush in 2006, Malinowski became fascinated with the colorful insects.
"The thing grew to like 10 feet tall - it was huge!" she said "I had every butterfly in the world in my yard."
She also planted several species of milkweed, encouraging others to do the same.
"It doesn't have to be common milkweed either," she said. "There are so many pretty milkweeds you can put in your garden."
This year's unusually cold spring killed that butterfly bush, she said. Cold conditions this year also likely drove a decline of many species of butterflies, according to butterfly expert with Penn State University.
Dr. Robert "Butterfly Bob" Snetsinger, a professor emeritus of entomology, said he had seen no monarchs in Centre County as of late June. Butterflies that overwinter as larvae could have frozen in the soil, he said.
"The whole butterfly population is down this year," he said.
In the Snetsinger Butterfly Garden in State College, named for Snetsinger because of his work converting the 3 acres from farm field to butterfly oasis, even common species such as clouded sulphur and orange sulphur have been spotted in smaller numbers this year.
He confirmed that herbicide tolerant crops and increased herbicide use are causes of decline in monarchs and other species. Farms in Amish country in southcentral Pennsylvania tend to have higher and more diverse butterfly populations, he said, because of more traditional farming practices.
In his area, spraying for the invasive gypsy moth set butterfly populations back a few years ago, he said. In 2007, he noted 32 species of butterfly in the Snetsinger garden. By 2010, that number had declined to 17 but have recently bounced back to 26, he said.
He doesn't think monarchs will go completely extinct, but emphasized the importance of planting milkweed.
"There's a real major effort this year to plant milkweed all across the country," he said. "This is a big year for the monarch butterfly."
MonarchWatch.org is giving away free milkweed to schools and nonprofit organization. Visit the website and click the "Free Milkweeds" link to learn more.