Community

May 17

Written by: Master Gardener Terry Engels
5/17/2012 2:46 PM 

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Be Watchful for Tent Caterpillars This Spring!
by Janelle Daberkow

If you had problems with tent caterpillars last year, be watchful for them again this year. Reports of the Forest Tent caterpillars started during the last week of April.

What do they look like?

Forest Tent Caterpillars are dark colored with bluish markings along their sides, and a row of footprint shaped white spots across their top side. They can reach a size of up to 2 inches.

Larvae pupate 5-6 weeks after hatching and feeding, usually in June. They pupate in folded leaves, bark crevices or other sheltered areas. Moths emerge 10 days later and live only a few days. They are buff colored and are about an inch in size and are often attracted to lights. Moths then lay eggs on the upper crown branches of trees encircling entire twigs as a glossy brown mass overwintering and hatching again in the spring.

 

Forest Tent Caterpillars defoliating an oak
Photo courtesy of Michelle Grabowski, UMN Yard and Garden News

These caterpillars prefer to feed on the leaves of birch, aspen, oaks, and lindens/basswood, but will feed on many other broadleaf trees. The caterpillars do not actually make tents, but do form a mat on a trunk or branch where they congregate, which is typically seen on the upper part of the tree. When trees are defoliated, older larvae move between trees and shrubs across lawns, driveways, or patios in search of another food source.

Outbreaks of this caterpillar usually last for three to four years, then subside. Populations of caterpillars across the state are proving to be less cyclical, and lingering for longer periods of time.

Reports of the Eastern Tent Caterpillar have been coming in throughout the area as well. These are the caterpillars that create the webbed tents seen in the crotches of trees in spring. The eastern tent caterpillar hatches at the time buds begin to open, usually mid-March-April.

After hatching, the caterpillars spin tents in trees to protect themselves from birds and other predators and from temperature extremes that occur at night or on cool or rainy days. Caterpillars emerge from their tents in warm, sunny weather to feed on leaves of trees such as apple, cherry, crabapple, plum and chokecherry trees, as well as many other deciduous trees.

A slightly different appearance

These caterpillars are bluish-black with a white stripe down their back and brown or yellow stripes along their sides. Larvae feed for 4-6 weeks anywhere from April to June depending on their emergence date. They then pupate in a cocoon about 1 inch long made of silk and located in protected areas, and may be attached to other objects like trees, garages or other structures. A reddish-brown adult moth emerges 3 weeks later to lay eggs. Eggs are laid in a band of shiny varnish-like material that overwinter on small tree branches and again hatch in the spring.

An Eastern Tent Caterpillar ‘tent’ on a young tree.
Many insects are inside this web for protection.
It does no good to spray the webbing with insecticide; it won’t penetrate to the caterpillars.
Photo courtesy of Jeff Hahn, UMN Yard and Garden News

Not a threat

Tent caterpillars are more of a nuisance than a serious pest of trees. Damage is usually cosmetic because of the unsightly tents in trees and because trees appear ragged from caterpillar feeding until they can replace lost leaves. Caterpillars are considered a nuisance, as they are found crawling over plants, walkways and roads.

Tent caterpillars rarely kill healthy, mature landscape plants. While caterpillars can nearly defoliate a tree, the tree will usually recover and put out a new crop of leaves. Both of these insects can be controlled without the use of insecticides.

How to get rid of them

Removal and destruction of egg masses from ornamental and fruit trees during winter, and removing the tents in early spring, can avoid the use of chemicals and defoliation of trees.

• In early spring, the small webs can be removed by hand. Caterpillars gather inside the tents overnight and during rainy weather. Gather the tents during early morning hours while the caterpillars are still inside them. Larger webs can be removed with a stick or broom or a heavy spray of water. They can also be destroyed at the point of pupation when caterpillars have spun their cocoons.
• Put the caterpillars in a pail of soapy water or crush, bury or burn them if it is permitted. Do not try to burn the web in the tree because you will cause more damage to the tree than the caterpillars would!
• It is not effective to spray caterpillars with an insecticide when they are inside of the webbing. The webbing serves as protection for the caterpillars, and sprays do not penetrate through the webbing.
• It is also not effective to spray for caterpillars if you have not seen any activity or feeding from them. Wait until a warm sunny day when the caterpillars leave the tent to feed on the leaves and apply the product to the foliage.
• As caterpillars grow, it becomes more difficult to treat for them. In large wooded areas, treatment may need to be done aerially, by a licensed applicator.

In times of heavy infestation, focus your treatments to young trees, as well as to ornamental and fruit trees. Larger deciduous trees will recover after feeding, even if feeding has occurred over several consecutive years.

• Bacillus thuringiensis var. kurstai (BT) is a bacterial insecticide that is very low toxicity and is effective if applied when the caterpillars are less than one inch in length; it can be found as Dipel, Thuricide or as Bonide Bacillus Thurengiensis.
• Other insecticide products that are effective against tent caterpillars include spinosad, acephate (Orthene), carbaryl (Sevin), permethrein (Eight), cyfluthrin, esfenvalerate and malathion.

Be careful of pollinators

If the tree is flowering, you are strongly encouraged to use a lower impact insecticide such as Bacillus thuringiensis or spinosad, neither of which harm pollinating insects that are active around flowering trees.

As always, use insecticides with care. Read the label before you purchase the product and again before you use it. Follow all directions and safety precautions.

For further reference, see this Extension publication:

http://blog.lib.umn.edu/efans/ygnews/2011/06/tent-caterpillars.html

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Thanks, Janelle, for the heads-up. Those FTC and ETC (they sound like swear words, which might be a temptation if you are invaded!) are such nasty pests.

Yard and Garden E-newsletter: http://blog.lib.umn.edu/efans/ygnews

UMN Extension website: www.extension.umn.edu

Call the Stearns County Extension office with your horticultural questions: 320-255-6169