Every year thousands of monarch butterflies dance through the air over North Florida, traveling between their winter refuge in Mexico and their regular homes along the U.S. Atlantic coastline. The colorful pageant attracts flocks of tourists who are eager to bear witness to this sprightly migration.

Every year, though, there have been fewer and fewer of the princely insects to see.

A new University of Florida study — at nearly four decades, the longest of its kind — has found that the number of caterpillars and butterflies in North Florida has been declining since 1985.

Since 2005, the numbers have dropped by 80 percent.

"It's alarming," said associate professor Jaret Daniels, a co-author of the study.

He pointed out that if a beloved and widely known species such as the monarch can be pushed so easily toward extinction, imagine how much more imperiled are other, less well-known ones.

"It exemplifies all the issues we're having to deal with in conservation," he said.

The scientists involved in the study say the causes of the decline are not entirely clear, but they believe there are two major factors at work.

One is the destruction — by development or agriculture — of areas that had been planted with native milkweed, the favorite food of young monarchs. The other is the widespread use of a herbicide called glyphosate, often applied to farmers' fields to kill weeds. One of the weeds it kills is milkweed.

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The study was launched 37 years ago by an internationally known monarch expert named Lincoln Brower. He died this summer at age 86, just before the study was published in the Journal of Natural History.

Brower began studying monarchs in the 1950s. He was instrumental in locating the fir trees in Mexico where they spend the winter before heading back for Florida. In their winter home, about 80 to 100 miles west of Mexico City, the trees are thick with the orange insects and when they flap their wings, the sound is like thousands of leaves rustling.

"It has the most complicated migration of any insect known," Brower told the Chicago Tribune in 1998. "Somehow they know how to get to the same trees every year. It's a highly specific behavior that is unique to the monarch butterfly."

During their return trip to the U.S., a new generation of monarchs is born en route. They grow from larvae to caterpillars, then turn into butterflies to continue the rest of their flight.

That's why checking on their population every time they landed in North Florida allowed scientists to determine what was going on with their population numbers.

"Florida is kind of a staging ground for the recolonization of much of the East Coast," Daniels said. "If these populations are low, then the northern populations are going to be at a similar abundance level."

The most famous spot in Florida to watch monarchs is St. Marks National Wildlife Refuge, which sponsors an annual Monarch Butterfly Festival every fall. That catches the butterflies on their way south.

Under Brower's direction, a team of scientists closely monitored how many monarchs showed up each spring heading north. Their selected site was a single herbicide-free cattle pasture in Cross Creek, onetime home of acclaimed Florida author Marjorie Kinnan Rawlings, who wrote The Yearling.

"Lincoln made the connection with that family 37 years ago and they've been incredibly gracious to continue to allow us access," Daniels said.

At the study site 20 miles south of Gainesville, the team gathered every year to examine the pasture's milkweed plants for any caterpillars. They also captured adult monarch butterflies to check the growth or decline of the population.

The 37 years they spent doing this is roughly the equivalent of 140 generations of monarchs.

One finding of the study: Monarchs time their departure from Mexico in the spring so it coincides with the optimal growth of milkweed in Florida and other Southern states. While adult monarchs can eat a variety of plants, the young ones' diet consists of nothing but milkweed. The plants contain toxins that they store up to ward off predators.

Florida is home to 21 native species of milkweed. Three types are best for the monarchs, Daniels said: swamp milkweed, pinewoods milkweed and, of course, butterfly milkweed.

Although Brower has died and the study has been published in a scientific journal, the work will go on, Daniels said. The hope is that the monarch migration will continue too.

Contact Craig Pittman at craig@tampabay.com. Follow @craigtimes.