FTC or Gypsy Moths? You be the judge.

People are talking about both insects and getting the stories mixed up, adding to the general hysteria about them. Here are a few tips about how to tell these defoliators apart.

Unlike forest tent caterpillars, gypsy moths are not natives

Currently there are no permanent populations of gypsy moth, Lymantria dispar, within the state, although there are a few isolated populations being treated and/or monitored by the MN Dept. of Agriculture. Three areas within Minnesota will be treated this year for gypsy moth populations; two sites in the Metro area and one in Houston County (extreme southeastern county). Reports in the Metro area indicate that the infestation in Minneapolis is quite heavy with a possibility of defoliation if not treated (over 250 egg masses per acre are needed for defoliation). To date, gypsy moths have not caused any defoliation. All the defoliation seen in the news over the last year or two has been caused by forest tent caterpillar, not the gypsy moth. The advancing "front" of gypsy moth infestation currently runs through the middle of Wisconsin extending south and east to the Carolinas. The gypsy moths got their name because of how efficiently they hitch rides as humans travel or move goods around the globe. Projections indicate the gypsy moth will become a permanent Minnesota resident within 10-15 years, with severe defoliation likely to occur three to five years after that. So there's no need to get excited just yet, unless we bring the moth home with us the next time we visit out east.

Will the real "army worm" please stand up

First, the term "army worm" is a misnomer. There is a caterpillar whose common name is armyworm, but it's an agricultural pest and not a critter that chews up aspen leaves. The insect most Minnesotans are referring to when they use the term, is the forest tent caterpillar, Malacosoma disstria, a native forest pest that tends to roam "en masse" during large outbreaks, creating the image of matching armies (hence the nickname). Last year, forest tent caterpillar defoliated 7.7 million acres of forestland, striping trees, covering roads and buildings, and frustrating anyone outdoors. Forest tent caterpillars are expected to defoliate 3 to 4 million acres of forest-land this year. While forest tent caterpillars prefer aspen, they also feed on oak, basswood, birch, ash, maple and elm. Aspen forests in northern Minnesota were among those hit the hardest during last year's defoliation event. But they weren't the only forests chewed up. There were even small pockets of defoliation as far south as Anoka County in the Metro area. Some of the areas that were hardest hit last year will likely see a slight decline in the population this year. Then hopefully the "friendly fly," forest tent caterpillars' most effective natural enemy, will catch up with the outbreak and help force population numbers down.

Forest tent caterpillar & gypsy moths have similar hosts and life cycles

Forest tent caterpillars overwinter in glossy cylindrical egg masses found wrapped around host twigs. The caterpillars, which hatch in late April, feed through June (see photo). Caterpillars are blue with black and have white-colored foot-print shapes along their backs. The buff-colored adult moths, with 1 or 2 darker bands on their wings, emerge in early July to mate and lay more eggs.

Gypsy moths overwinter in the egg stage, in mustard-colored, fuzzy egg masses about the size of a quarter. But the flat egg masses are not found just on the branches. They can be in any protected spot outdoors. The eggs hatch in late April and the caterpillars feed through June. Gypsy moth caterpillars are dark colored and quite hairy. Pairs of red and blue raised dots can be found along their backs (see photo). The darker brown male moths and creamy white females, both with brown scalloped lines on their wings, emerge in July and August to mate and lay eggs. So once the gypsy moth becomes established in Minnesota, gypsy moth caterpillars will be out feeding at the same time as forest tent caterpillars. That means they may actually compete with one another for food.

forest tent caterpillar gypsy moth larva
Comparison of forest tent caterpillar and gypsy moth biology and potential impacts
 Forest Tent CaterpillarGypsy Moth
Caterpillar stageLate April-JuneLate April-June
Caterpillar markingsBlack and blue stripes & white "foot prints"Black with pairs of red and blue dots
Moth stageEarly JulyJuly-August
Moth markingsBrown bands on buff-colored wingsDark brown scallops on white/cream (female) or brown (male) wings
Eggmass stageOverwinter on twigsOverwinter on anything outdoors
Eggmass featuresGlossy, brown cylinderFlat, fuzzy, yellow-brown
HostsAspen, oak, basswood, birch, ash, maple, elmOak, aspen, basswood, birch, willow, tamarack
Native?YesNo
MN resident?YesNo
Expected tree mortalityAlmost none, unless associated with droughtAverages 25-35% during the first outbreak in an area
Potential impactsForest resources, recreation & outdoor aestheticsForest resources, timber industry, wildlife habitat, recreation & outdoor aesthetics

Like their life cycles, their host range is similar. Both prefer aspen, oaks and basswood, among other species. In other states where they co-occur, gypsy moths have not been a serious problem in areas with forest tent caterpillars outbreaks, so how the two insects will interact to impact their host species is unknown. It's not known how often gypsy moth outbreaks will occur in Minnesota or what conditions favor outbreaks. We won't know until it gets here whether or not we can expect gypsy moth outbreaks to happen concurrently, consecutively or entirely independently of forest tent caterpillars outbreaks. But we do know they will happen in many of the same forest stands because of their host preference, which is bad news for those living, working or recreating in areas of our state dominated by oak and aspen.

Prepare now to mitigate gypsy moth impact

There are things we can do now to guard against the gypsy moth and the future damage it will cause. Be aware of the gypsy moth when traveling to eastern states, or when receiving shipments of goods from eastern states. Inspect your belongings and notify an authority if you find something you suspect are gypsy moth life stages. If you own forestland, contact a local forester about ideas to improve stand health to protect your forest investment. Forest management can lessen the likelihood of future defoliation in some stands and in others it can lessen the rate of associated tree mortality. Talk to your resource professional and take action soon to allow your forests the time to respond to these management efforts.