Aphids that must have come from the irrigated valleys were trapped in isolated areas of the Colorado and Mojave deserts. Potted alfalfa plants set out in the same area as the traps on the Colorado desert caught an average of two aphids each in a week. Enough aphids were caught in every location to have infested fields planted there. The number caught indicated that aphids disperse across desert barriers.
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Ross Dickson (1959) studied this question.