Monarch Butterfly Voltinism: Effects of Temperature Constraints at Different Latitudes

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Abstract
Monarch butterflies have previously been though to produce three to five generations in the warm southern latitudes of North America and one to two generations in cooler northern latitudes. This life history pattern is based solely on lower temperature constraints on reproduction and is unsupported by data. In contrast, we show that monarch breeding is distributed in space and time in relation to both lower and upper temperature constraints. Thus in the hot south (Florida [USA]) monarchs produce two generations between the arrival of overwintered migrants and the onset of lethally high early summer temperatures. This is followed by a further three generations at 16.degree. latitude further north in Wisconsin. Our field-based observations are consistent with five years of predictions for Florida and Wisconsin using a simple model based on the day degrees available for reproduction and larval development, between experimentally determined lower and upper lethal temperatures. Such a pattern of fewer generations in warmer than cooler climates suggests that the offspring of spring migrants are also migratory to escape lethally high southern temperatures.