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On the day after Thanksgiving my Dad and I hiked up the Nelson Trail that follows the Tule River just east of Camp Nelson.  We were surprised to find oodles of ladybugs in several locations along the trail.  I had heard about this phenomenon of hibernating ladybugs but hadn't seen it myself until then.  It seems that for nine months or so out of the year there are huge clusters of hibernating ladybugs gathered on fallen trees and in shrubs in the coast ranges and the foothills of the Sierra Neveda.  They tend to be in open areas where the sun shines through the trees.  Laboratory experiments suggest that the clustering is stimulated by reduced temperatures and less light. In the Sierra ladybugs have been found as high as 6,000 feet clustering and surviving under snow drifts! The ladybugs are basically hibernating and do not eat but live off their accumulated aphid fat.

Why the huge clusters? One suggestion is that predators (birds) are discouraged from eating ladybugs because of the obnoxious odors emitted by the large group. But another major motivating factor may be for mating...
Hibernating ladybugs along the Nelson Trail
Giant Sequoia National Monument