The Monarch butterfly’s bright colors begin to show up in Pennsylvania in late July, but there is data that suggests that those sightings are becoming less common.

One of the believed causes of the decline has been the loss of a plant which gardeners can grow in their backyards.

The migrating monarch population in North America, by far the largest in the world, has been decreasing over the past two decades.

“A lot of things have changed over the years that have impacted their population,” said Chad Schwartz, director of science and education for the Lehigh Gap Nature Center.

While monarchs have been considered for the Federal Endangered species list, they are not currently listed. However a European organization recently classified them as endangered, putting a spotlight on the population decline. The International Union for the Conservation of Nature, which put the insect on its “red list,” estimated that the population has decreased by 22-72 percent over the last decade.

Population estimates vary widely because they are made by scientists who look at trees in mountainous areas of Mexico, where the insects spend the winter months. Butterflies are also tagged with small stickers and tracked. Monarchs from the Lehigh Gap have been recovered in Mexico.

A lot of things have to go right for small winged insects to fly thousands of miles twice a year. Multiple factors have affected their ability to complete the journey, including an increase in severe storms and deforestation in Mexico.

“There’s nothing random about nature, it’s on purpose. It’s all perfectly balanced,” said Mari Gruber, a butterfly farmer who owns Bear Mountain Butterfly Sanctuary in Penn Forest Township.

One factor can be directly affected by backyard gardeners. Milkweed is a family of plants which monarchs feed on. The caterpillars that become monarch butterflies feed exclusively on milkweed, and caterpillars go through multiple generations during each migration.

Increased use of herbicide has reduced the amount of milkweed along the migratory pathways used by the birds.

Scientists have been encouraging gardeners to grow varieties of milkweed and pollinator gardens to promote the existing monarchs. Schwartz suggests butterfly milkweed, which has colorful flowers and doesn’t grow as fast as common milkweed.

“They’re pretty plants to have in your garden,” Schwartz said.

Many people already grow butterfly bush, but milkweed has the added benefit of providing food for caterpillars. Butterfly bush is also an invasive species.

The nonprofit Live Monarch will mail out free milkweed seeds to anyone who sends a self-addressed stamped envelope to “Live Monarch - 2022 seed campaign, P.O. Box 1339, Blairsville, Ga., 30514.”

Monarchs will migrate thousands of miles in a matter of weeks in order to reach their winter homes. But typically they don’t travel far to feed. So gardeners who plant milkweed not only make a contribution to the overall population, they’ll also have more monarchs on a regular basis.

“They won’t go long distances to get nectar, so if you’re building more pollinator gardens, you’re going to see more butterflies,” Gruber said.