In this Issue:


News from the Minnesota Grown Program


Vegetable Insect Pest Update

Salmonella Outbreak in Tomatoes


Strawberry Update

MDA’s Weekly Strawberry Pest Sampling Data


Tent Caterpillars and Apple Trees

Weekly Trap Counts

Apple Scab Infections

Degree Day Calculations

Field Day Invitation from Harry Hoch

Order: 2008 Minnesota Vegetable Guide

Insect, Pest Profiles

Vol 5 No. 5   June 13, 2008

Tent Caterpillars and Apple Trees

Thaddeus McCamant, Specialty Crops Management Specialist, Northland Community & Technical College

click to enlarge
Figure 1. Eastern tent caterpillar “bag”

Many apple growers have been complaining about bags of silk on bare apple limbs.  The bags are nearly always filled with caterpillars that crawl on the limbs and eat leaves.  The most common culprit is the eastern tent caterpillar, also called the “bag worm.”

Other caterpillars on the trees are forest tent caterpillars or “army worms”.  The two caterpillars look similar (Figure 2), but army worms don’t make the large silk nests.  Eastern tent caterpillars rarely leave the tree where the eggs were laid, while army worms crawl over the ground looking for new trees to eat.  In a forest, army worms only eat certain trees, such as aspens, while leaving trees like sugar maples alone.  Army worms love apple trees.  Army worms primarily live in northern Minnesota while eastern tent caterpillars are more common in southern Minnesota.  Both are found in sections of central Minnesota, where the picture in Figure 2 was taken.


Figure 2. Eastern tent caterpillars (left) and forest tent caterpillars (upper right) on an apple tree

Damage:  Left uncontrolled, both Eastern and forest tent caterpillars will defoliate apple trees.  In most cases, the defoliation occurs early in the summer, and leaves will resprout and the trees will survive, but defoliation will destroy two years worth of crops.  Any fruit set this year will abort, and the tree will be so busy recovering from the defoliation that it may not be able to form flower buds in August.  Tent caterpillars can defoliate small trees in one or two days after being first detected.  Army worm populations fluctuate from year to year, but even in years when the aspens escape defoliation, apple trees can still be hurt by a small population of army worms.

click to enlarge
Figure 3.  Egg masses of Eastern tent caterpillar on apple branches

Control: Eastern tent caterpillars are one of the few insect pests that can be controlled by hand.  During dormancy, remove any egg masses that you see on the tree (Figure 3).  In early summer, nests can be cut out of the trees and disposed of, or you can spray the nests with a number of different insecticides listed in the book IPM for Minnesota Apple Orchards.   (
Several apple growers have complained that broadcast sprays of insecticides do not control eastern tent caterpillars.  Sprays directed at the nests should work.

Army worms can be controlled with broadcast sprays.  There are a number of Organic Materials Review Institute approved insecticides labeled for army worms, including BT sprays and Entrust. 



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Co-Editors: Bill Hutchison (, Department of Entomology, University of Minnesota, Jeanne Ciborowski, Minnesota Department of Agriculture, Ag. Resources Management and Development Division, and Suzanne Wold-Burkness (, Department of Entomology, University of Minnesota

The Newsletter is published weekly from May through August, cooperatively, by the Minnesota Department of Agriculture (MDA) and the University of Minnesota (U of MN).  Reports are posted on the U of MN and MDA web sites on Fridays.  If you have suggestions and/or comments, please send your contributions by 4 p.m., Wednesday to Jeanne Ciborowski, 651-201-6217, , MDA, 625 Robert St. North, St. Paul, MN  55155.  You can access the Newsletter at the U of MN web site in htm format at: and at the MDA web site in pdf format at:

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