WEST LAFAYETTE, Ind. (WLFI) - More than one million monarch butterflies are making their way south for the winter.
Many have been visible through the region over the past few weeks. But the migration is not as big as it used to be.
According to Entomology Professor Jonathan Neal, the monarchs know when to migrate based on the amount of daylight.
"When the days are getting shorter, they're triggered to migrate south. Now in about February when the days are starting to get longer, that's reversed and they'll start to migrate north,” said Neal.
According to the US Forest Service, monarchs are the only butterfly known to make a two-way migration.
Other species can survive winter as larvae or adults. But monarchs cannot survive in cold temperatures.
Neal said the monarch population has been on a decline.
"Compared to 20 years ago, populations are much, much lower," said Neal.
The United States Fish and Wildlife Service have considered monarchs a threatened species since 2014.
One of the factors leading to the decrease is less milkweed.
It is the primary food source for monarch caterpillars.
"Farmers are better at controlling milkweed in their fields than they used to be, and so there's less milkweed now than there was in the past," said Neal.
But Neal says groups like INDOT have created methods to help increase the amount of milkweed.
"They're changing the way they mow the ride away, so they let the milkweed and other plans grow up," said Neal.
Neal said a greater food source could help increase the monarch population.