A conservation group is planting more than 30,000 milkweed plants in California in the hope of giving Western monarch butterflies new places to breed.

The San Francisco Chronicle reported Monday that the River Partners group has joined with others and the California Department of Fish and Wildlife to plant along the Sacramento, Feather and Kern rivers.

The plants are critical because the orange-and-black butterflies lay their eggs on them. Their caterpillars also eat them. Their caterpillars also eat them.

The butterflies head south from the Pacific Northwest to California each winter. This year, researchers said an annual winter count recorded fewer than 2,000 of the butterflies — a massive decline.

“There couldn’t be a more critical time to be doing this,” said restoration biologist Francis Ulep of River Partners.

Monarch butterflies that live east of the Rocky Mountains migrate to mountains in Mexico during the winter from October to late March. The eastern monarch butterflies account for the majority of the population in North America.

Scientists have said the butterflies are at critically low levels in Western states because of destruction of their milkweed habitat along their migratory route as housing expands and use of pesticides and herbicides increases.

You can learn more about monarch butterflies and their migration patterns at journeynorth.org/monarchs.