A conservation group is planting more than 30,000 milkweed plants in California in the hope of providing new breeding grounds for western monarch butterflies.

The San Francisco Chronicle reported Monday that the River Partners group has partnered with others and the California Department of Fish and Wildlife on the plantations along the Sacramento Feather and Kern Rivers.

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

The butterflies migrate to California from the Pacific Northwest each winter. Earlier this year, researchers said an annual winter census recorded fewer than 2,000 of the butterflies – a massive decrease.

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

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Scientists have said that due to the destruction of their milkweed habitat along their migration route, butterflies in the western states are at critically low levels as habitation expands and pesticide and herbicide use increases.