Getting Ready for Spring


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April 27, 2001

FTC: What's on the menu?

Editor's note: Keep this next to the phone as a ready reference for homeowners with questions.

Forest tent caterpillar is expected to be widespread in northeastern Minnesota this year and has the potential to occur throughout the northern half of the state. Slightly over two million acres were defoliated in 2000. The acreage of defoliated trees is likely to be much more this year.

The forest tent caterpillar (FTC), Malacosoma disstria, is a native defoliator of hardwoods, especially, aspen and birch trees in northern counties and basswood and oaks in central and southern counties. Forest tent caterpillars are often mistakenly called "armyworms". Widespread outbreaks of FTC occur at intervals of ten to twenty years and are two to three years in duration. Outbreaks peaked in 1922, 1937, 1952, 1967, 1978, and 1990. Populations collapse due to starvation, predation and parasitism. Populations of the "friendly flies", native parasites, build up as the FTC populations peak and become nuisances themselves.

The table below gives a projected schedule of FTC life stages. The caterpillars will emerge from their eggs about the time the aspen first begin to get their leaves this spring. Defoliation will begin to be noticeable by the end of May but the heaviest defoliation and biggest nuisance will occur in June.

Projected schedule* for FTC life stages and host tree foliage 

Host tree buds break Apr 18 - May 10
FTC hatch Apr 18 - May 15
FTC major feeding June 5 - June 25
Host trees refoliate Starts as soon as FTC are done feeding, finishes in mid-July
FTC moths fly

and lay eggs

July 1 - July 15

* = The timing of biological events varies from year to year because timing depends on weather and location. For example, events will occur near the earlier date: when the spring is early; when May and June are hotter than average; or, when the location is south of Mille Lacs Lake.

In the forest, FTC defoliation usually does not affect tree health because FTC populations usually collapse before tree damage occurs. When the FTC eat all the leaves off a tree it will usually "reflush" new leaves by mid-July. Producing new leaves puts some stress on the tree because it must use more of its energy reserves. This can weaken the tree and make it more prone to other problems. But in most cases, the tree will survive the defoliation with no other problems. Your tree's best defense is for you to keep it in a healthy, vigorous condition between and during outbreaks by keeping it well watered and avoiding damage to its roots and trunk.

More information on FTC

map of FTC defoliation

During their peak, FTC can create an extreme nuisance to people living or vacationing in forested areas. Young caterpillars spin threads and fall from trees onto picnic tables, patios and people. Large, mature caterpillars wander widely in search of food and often appear to migrate across roads and open areas. Resting caterpillars commonly form large clusters of thousands of caterpillars on buildings, tree stems, campers, and other stationary objects.

During the first three weeks of June, FTC can be a downright nuisance. They don't cause a health risk to humans, but the presence of hundreds or thousands of caterpillars can be a real headache. Homeowners may want to adopt two basic strategies. First, identify the small trees, gardens, lawn furniture, buildings, etc. that you want to safeguard. Then work to protect the things you selected and ignore the rest (or at least try to). It takes a lot of time and energy to try to safeguard everything on your property. Second, be persistent. Some treatments may require daily monitoring and action.

  1. Before they hatch (any time from July to early-May) hand pick all the egg masses off of valuable plants. Destroy egg masses or dispose of them.
  2. Some people think FTC are for the birds. Lure in birds with bird feeders, especially the pine and evening grosbeaks, and they'll do some of the work by eating the caterpillars.
  3. In May and June, hand pick caterpillars off plants and dispose of them. Later, gather and destroy cocoons.
  4. Caterpillars can be brushed off the house, lawn furniture, etc. with a stiff broom or knocked down by a stream of water. Bag, bury or compost the dead caterpillars.
  5. Rig up a barrier around your garden, house or fruit trees (presuming you've already removed the egg masses from the twigs). Here are two popular methods:
    1. Use a wide band of masking tape, tree wrap tape or aluminum foil with a thick layer of Tanglefoot or petroleum jelly or a coating of vegetable oil spray. This stops the caterpillars from crawling up your tree until the band is full of them. (Then they just crawl over the backs of the caterpillars that are stuck.) This method needs at least daily attention and replacement of the goo. Remember to take the bands down by July 1st so the tree isn't injured by the tape or wrap.
    2. Stake up a 2 foot high plastic sheet and weigh down the bottom edge with sand or dirt. Then either apply a 2 inch wide band of Tanglefoot, petroleum jelly or vegetable spray near the middle or let the top 6-8 inches flop down to create a moving and unstable flap the caterpillars can't cling to. You may want to weigh down the flaps so caterpillars don't get flipped into the garden when the wind blows.
  6. Under some circumstances, you may want to spray an insecticide to protect gardens and trees at risk. The production from fruit trees, raspberries, strawberries and other fruit and vegetable crops will be greatly reduced or lost if the plants suffer moderate defoliation. Trees at risk include: newly planted woody ornamentals and tree saplings; trees recently damaged by construction, trenching, soil compaction, blacktopping, etc.; birches or oaks that have suffered 2 years of heavy defoliation or have active branch dieback; and, drought stressed trees.

    Insecticides have restrictions as to which plants and sites where they can legally be applied. If applying to shade and ornamental trees, the label should say it is for use on shade and ornamental trees. Microbial insecticides containing Bt, (bacterial products made of Bacillus thuringiensis ) are recommended to use for FTC control in the backyard because of their safety and the low toxicity to non-target organisms. Bt products are only toxic to caterpillars; they do not kill bees, flies, mosquitos, etc. However, Bt products are slightly slower to act since they must be eaten by caterpillars before they take effect. Apply Bt to the leaves of host plants; not to the bark or other non-edible materials. It is most effective on FTC when the caterpillars are small. Please read and follow label directions.

  7. FTC moths are attracted to lights during the nights in early July. Turning out your yard and exterior lights may reduce egg-laying on your trees and thus reduce next year's defoliation.
  8. The most important thing you can do for your defoliated trees is to keep them well watered. Supply 1 inch per week if you do not receive that much in rainfall from May 1 through September 1.
  9. Do not fertilize defoliated trees or use a weed and feed product on your lawn during an outbreak year. Fertilization encourages the tree to produce more leaves which puts an additional stress on the tree.

Winter injury

Winter has taken its toll on conifers this year. By this time you have probably noticed all the red needles on white pines and other conifers that are growing along highways. Although the needles look terrible, the buds, twigs and trees are not dead. The needles had a rough winter and they were discolored by winter injuries, but resist the urge to prune them away or remove discolored trees. Chances are very good that these trees are live and healthy beneath their mask of red needles. Buds are well protected during the winter and will grow once spring arrives. Winter injury was enhanced by strong, dry winds, many days of bright sunshine and by low relative humidity. Humans also added to the injury equation by applying road de-icing salts which are toxic to plants.

Undoubtedly the most evident damage occurred on white pines growing along highways and was caused by the application of de-icing salts. Earlier in the winter, each passing car sent up clouds of water with a little salt dissolved in it. This salty water settled on nearby objects, including the pines. Salt was absorbed into individual needles, accumulated to toxic levels in the needle tips and killed the needles back, starting at the tips. Trees within 150 feet of a highway can be easily reached by salt spray. De-icing salt damage usually occurs on the side of the tree closest to the road. Spring rains will rinse off accumulated salts, new shoots will develop in May and June and the dead needles will be shed so that by summer these trees will look reasonably healthy. Trees with thick wax layers on needles, large resinous buds and/ or with thick, robust needles are more resistant to salt spray damage. White and red pines are very susceptible to salt spray damage; Scots pine, Norway spruce, juniper and eastern red cedar are moderately susceptible and jack pine, Austrian pine, larch and black spruce are tolerant to salt spray damage.

Even during the dormancy of winter, tree needles need and use liquid water. When water, stored in twigs and needles, is gone, cells and tissues become progressively more dehydrated and start dying. Water is lost faster when the relative humidity is low, when dry winds are blowing and when warm, sunny days occur. Affected needles, turn red or brown from the tip down and, often have dark bands or a mottled appearance. In late winter, the needle discoloration intensifies and becomes more noticeable. Buds are usually not killed. Normally, snow cover prevents winter injury of young conifers by providing shelter from drying winds and from the glare of the sun. In some years it is common to see young conifers with a strong line of demarcation separating the brown, desiccated tops from the healthy, green branches that were covered by snow.

Some trees or groups of trees seem to get winter injury every year. In these situations, it is likely that the trees are under stress and do not have the resources to tolerate any internal desiccation and therefore suffer winter injury every year. For example, some clumps of roadside red pines show winter injury symptoms every year because they are growing offsite, either in soils that are too wet or in soils that restrict rooting depth. This stress predisposes the trees to needle desiccation and ultimately to repeated winter injury.

Winter injury is the most important factor limiting the northern range of conifer species. Temperate species, such as red and white pines, are much more vulnerable to injury than are the boreal species, such as black spruce and jack pine. Native tree populations are adapted to their locality. Moving them ( seeds or seedlings) 100 miles north or south of their site of origin can result in damage due to winter injury. Exotic species, like Austrian or Scots pine, should be planted in climatic zones similar to their site of origin in Europe.

Is there anything people can do to prevent winter injury? Here are some suggestions.

  1. When selecting trees to plant, choose species and cultivars that are adapted to your local growing conditions.
  2. Avoid planting white and red pines, balsam fir and white spruce within 150 feet of a highway to prevent salt damage.
  3. Avoid planting yew and arbor vitae on south or southwest sides of buildings or in sunny and windy locations.
  4. Erect temporary barriers around conifers susceptible to winter burn. They can be made of plywood, burlap, tar paper or plastics. Recycle your Christmas boughs and tree by propping them up on susceptible conifers. They will act as a barrier and also hold snow for more natural insulation and protection.
  5. Just after the snow melts and prior to bud break, rinse de-icing salts off both conifers and hardwoods.
  6. Reduce or eliminate the use of de-icing salts.
  7. Replace trees that have severe winter injury year after year. They are not in the right location and will only decline due to needle and twig loss over a period of many years.
  8. Keep conifers properly watered throughout the growing season and fall. Decrease the watering slightly in September to encourage hardening off, then water thoroughly in October until freeze-up. Watering only in late October does not help reduce winter injury.

Conifers growing in Minnesota have had a long, long winter with plenty of opportunities for winter injury. But in spite of their appearance, chances are good that your trees are live and healthy beneath their mask of red needles. Buds were well protected during the winter and will grow once spring arrives.

Oak tatters

Oak tatters is a recently identified condition that affects the first emerging oak leaves in the spring, causing them to appear lacy or tattered. The pattern of symptoms suggests internal injury to developing tissues inside the overwintering buds. Within a few weeks, a new flush of leaves will appear and they will be free of symptoms. Oak tatters can be confused with early season outbreaks of oak anthracnose or the effects of late spring frosts.

The first reports of oak tatters came from Iowa in the 1980's and, more recently, oak tatters has been observed in Minnesota and Wisconsin. In southeast Minnesota, oak tatters primarily affects bur oaks. Generally, large portions of the landscape have been affected. However, within the affected areas, a few symptomless trees can be found. The causal agent remains unknown.

The risk of continued oak tatters outbreaks is that eventually dieback will occur in the affected trees. Although the injury is in the early season, it costs energy to add a new flush of foliage. It's like going to the bank and continually draining down your account. There's a limit.

Please report any observations of oak tatters to your local DNR forester.