Why only here? Tim Smith answered the question in the Missouri Conservationist Ask the Ombudsman column several days later. To quote him:
"Each fall, during a warm-up following the first cold weather, the insects gather on the sunny sides of houses and other structures as they look for cracks and crevices where they can find shelter from the coming winter. Many will survive the winter and appear again in the spring as temperatures warm and they try to exit the house."
Tim provided more information on the MDC Fresh Afield blog last year. According to Wikipedia, they were brought to the United States in 1916 to control insect pests of plants, but were not successful. In 1988 they were observed in numbers in New Orleans, and since then they have spread. By 1995 they were occasionally found in the Midwest and became common in 2000.
Subsequently, they have also contributed to the decline in native ladybugs, presumably by out competing them. They also have reached pest status to the higher biped mammals, both because of the swarming numbers, their little bites and unpleasant odors and the tendency to move into our buildings. For information on these pests including control recommendations, check out http://ohioline.osu.edu/hse-fact/1030.html