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The Great FTC MigrationJune 5, 2001

Forest tent caterpillar

Most of the tiny FTC emerged on April 29th and 30th whether they were in Forest Lake or International Falls. For the next three weeks, the weather was ideal and lush, green foliage was abundant. Life was good. Then the weather turned nasty. It was rainy, windy and cold (even freezing). The caterpillars bunched together in tight masses to tough it out, on tree stems, on buildings and even on stop lights. Since they don't feed when the temperatures are below 59F, very little foliage was consumed for almost a week. These critters are hardy. Inclement weather may delay development and allow parasites and predators to make a few inroads but, in the big picture, this won't put a dent in the population. Or the defoliation.

forest tent caterpillar defoliation

Where caterpillars are abundant, their food has run out. When this happens, the caterpillars move out....in large numbers. And speaking of mass migrations, we are on the verge of another great FTC migration. Wildebeests are certainly much larger than the average forest tent caterpillar, but the wildebeest migration on the Serengeti pales in comparison to the great FTC migration of Minnesota in terms of the total number of animals involved. There can be as many as four million caterpillars per acre and as much as four million acres infested this year. Mass migrations, which have already occurred to some extent, will continue to be obvious until mid-June when the FTC begin to pupate.

FTC is being reported in areas further south than expected. Reports of isolated infestations have come in from Minneapolis, St. Paul, Golden Valley, Oak Park Heights and Forest Lake and rural Wright County. While defoliation is not likely to be wide spread, the distribution of the reports indicates pockets of defoliation will likely show up in additional areas.

many forest tent caterpillars on a trail sign

Caterpillars averaged between 1 and 1 1/4 inches long in Grand Rapids on May 29th. Statewide, FTC are expected to finish feeding in mid- to late-June. The sarcophagid flies (friendly flies) will likely be abundant in areas that experienced defoliation in the past year or two. The flies are native parasites of the pupal (or cocoon) stage and eventually help end the outbreak.

Defoliation is expected to end in mid-June with the moths appearing around July 4th.