Christina O'Haver, The Union Democrat

Fewer monarch butterflies have been spotted in California over the past decade, possibly due to habitat destruction and climate warming, entomologists say.

The Western Monarch Thanksgiving Count showed 1 million fewer butterflies at counting sites across the western United States in 2011 than 1997.

"Lots of people have their own opinion, but its an insect, and insect populations fluctuate a lot," said David Marriott, founder and director of the Monarch Program in San Diego.

Marriott believes the insects are losing their migration instincts, possibly the result of warmer winter temperatures.

Typically, the butterflies seen in the Mother Lode and throughout the western U.S. overwinter in eucalyptus groves along the California coast, Sacramento-based entomologist Baldo Villegas said.

Marriott also suspects the monarch population has become more dispersed as a result of new eucalyptus groves. He said people plant the trees for decoration or as windbreaks for agricultural fields.

Steve Heydon, senior museum scientist at UC Davis' Bohart Museum of Entomology, said the drop could alsoreflect the destruction of milkweed plants during land development. Milkweed serves as a source of food and breeding ground for the insects.

He also said more humans living in the butterflies' migratory paths could disrupt migration.

The number of eastern monarch butterflies - which overwinter in a fir forest in central Mexico - has also plunged, according to the annual census of the insects that was released last week by WWF-Mexico.

In the past year alone, the population has dropped 59 percent, the study showed.

The colonies that overwinter in Mexico spend summers in Canada and the midwest and eastern U.S.

Scientists attribute the decline in Mexico to pesticides destroying milkweed.

Illegal logging has historically been cited as a factor in the species' downfall, destroying overwintering habitats south of the border. However, a 2012 aerial survey showed almost no detectable logging, according to The Associated Press.

Like other butterflies, monarchs are pollinators and, especially at the caterpillar stage, an important food source for predators.

"If they disappear, it won't make any difference in the ecosystem at large, but what it tells us is that other species that we don't notice so much are possibly suffering the same problems," Heydon said. "It's just an indication of ill-health in the ecosystem."