San Francisco (Associated Press)-A conservation organization is planting more than 30,000 milkweed plants in California, hoping to provide a new breeding ground for Western monarch butterflies.

San Francisco Chronicle Reported On Monday, the River Partners team worked with others and the California Department of Fish and Wildlife to plant along the Sacramento, Feather and Kern Rivers.

These plants are considered vital because orange and black butterflies lay eggs on them. Their caterpillars also eat them.

Every winter, butterflies fly south from the Pacific Northwest to California. Earlier this year, researchers stated that there are fewer than 2,000 butterflies each winter—a sharp drop.

Francis Ulep, a restoration biologist at River Partners, said: “Now is the critical time to do this.”

Scientists said that due to the expansion of housing and the increase in the use of pesticides and herbicides, the milkweed habitat of butterflies along their migration routes has been destroyed, so the number of butterflies in western states is extremely low.