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A year like no other…
There has been no year in the 117 years of climate records for the United States that matches what we have all experienced in the last 12 months. We have been tracking how monarchs are affected by climate for a number of years and there have been obvious patterns, like cooler March temperatures in Texas are associated with larger overwintering populations in Mexico. Similarly, warmer May temperatures are more favorable since they favor re-colonization of the northern breeding areas. Also, warmer summers in the northern breeding range are better for monarch production.
This year, however, is off the charts. March was the warmest recorded in Texas – not a good start based on past records. May was also warm, allowing monarchs to move into the northern breeding areas earlier by two to three weeks and in numbers that were unprecedented. Early arrival in the north in the past has been associated with population declines but in those years the average summer temperatures were in the normal range. This summer is starting out to be different from other summers with June temperatures exceeding the norm over most of the northern breeding area. What will happen this year with early arrivals followed by a warm summer, possibly one of the hottest summers ever recorded?
Not only has it been warm, it’s been hot and dry – perhaps too hot and dry for good monarch reproduction. And, then there are the milkweeds and nectar plants to consider. Plants grew rapidly this spring with many species blooming 10-30 days earlier than normal. Plants that typically flower in the fall began blooming in June and reports continue of water stressed plants blooming early. Milkweeds were no exception with flowering being earlier almost everywhere, raising the question as to what their condition they will be in the last week of July and the first week of August when most of the eggs are laid that produce the migratory generation. If the milkweeds are past their prime, and are senescing, will this diminish the size of the last generation?
There are also questions as to whether large numbers of monarchs overshot (that is, flew beyond) the limits of milkweed. Unprecedented numbers of monarchs have been reported from the Prairie Provinces of Canada where milkweeds are scarce, from the Maritime Provinces where monarchs are usually few, and even from Newfoundland, an area with virtually no milkweed. As you can see, there are many questions but little basis for making a prediction as to the size of the fall migration. If there is sufficient rainfall and normal to above normal temperatures (+/-80F) in the northern breeding area through August, the migratory population may be the largest since 2003 – perhaps 6-7 hectares. On the other hand, a continuation of the extreme heat and drought conditions, a track that seems equally likely, could result in another overwintering population in the 2-3 hectare range – well below the long-term average of 7 hectares.