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Last year was a disaster for monarchs. The spring, summer, and fall droughts in different portions of the country limited reproduction in the spring and summer and survival of the migrating population in the fall. The result was the lowest overwintering population in Mexico recorded to date – only 28.3 million monarchs. Late in the winter (early March) severe storms added significantly to the usual mortality of the overwintering butterflies. Worried about the coming year I ran a few calculations and estimated that as few as 4.9 million females survived to lay eggs in the southern states in March and April. I had a hard time envisioning how the population could recover; however, they did and the overwintering population this year could be 80-100 million butterflies.
How did they recover? The explanation appears to be weather extremes and fire ants. Severe droughts not only affect monarchs but all other insects that feed on soft bodied insects – such as predatory and parasitic wasps, and fire ants. In most instances, populations of predators and parasites decline even further than those of their prey – and recover more slowly. The droughts in Texas were followed by heavy fall rains, rain throughout the winter, and even good spring rains. The result was a lush spring in Texas and one with relatively few predators and parasites. Monarchs and numerous other butterflies that migrate out of Texas in the spring produced enormous numbers of offspring which migrated into the midwest in May and June. The numbers of monarchs were sufficient to recolonize nearly all of the breeding area even up to 50 degrees N (Winnipeg) and reproduction was normal to above normal in most of the breeding areas throughout the summer.
My speculation about the role of fire ants received a boost recently with the arrival of a newsletter from the fire ant research group at Texas A & M University. For months they had been fielding “Where are the fire ants”? questions from people all over Texas and Oklahoma. It appears that fire ants are down over large areas of Texas and seem to have disappeared from much of the Red River Valley in Oklahoma. They attributed the decline to the drought of 2000 and flooding that followed in the fall and winter.
Let’s hope the fire ants stay down for awhile!