Although some of the Porcupine Caribou occasionally remain on the North Slope for the winter, the Porcupine Caribou usually travel south and east to Canada. When they do stay on the North Slope, the Porcupine Caribou usually move westward from the Coastal Plain area and mingle with caribou from the Central Arctic Herd. As the mosquitoes emerge in late June and early July, the caribou gather into enormous post-calving aggregations, sometimes numbering in the tens of thousands. For example, in 1987, over 93,000 caribou assembled in one group south of Camden Bay. The caribou seek areas where breezes and cooler temperatures reduce the harassment by mosquitoes, and when there is no wind, the caribou move continually.
Cold winds offer relief from the mosquitoes and permit the caribou to rest and feed freely.
The tundra provides a perfect environment for mosquitoes and other insects who emerge in late June and July continually harass the caribou. The warble fly, which looks like a small yellow and black bumblebee, lays its eggs in the fur and the legs or abdomen of the caribou. The larvae soon hatch, burrow under the skin, and travel to the back. Caribou commonly carry over one hundred larvae. It is not until May and June of the following year that the larvae cut exit holes, crawl out and drop to the ground to develop into mature flies.