Predicting forest tent caterpillar defoliation

Predicting levels of defoliation by forest tent caterpillar

There are many reasons why people want to know if forest tent caterpillar (FTC) populations are going to cause defoliation in late May and June in the upcoming year. For example, a homeowners' association may be considering a spray treatment or a family is planning a reunion in June. There are two methods that can be used to predict FTC defoliation: local observations from last summer or egg mass surveys. Egg mass surveys are more accurate, so we consider this method the gold standard. Each method will be discussed separately below.

Method 1: Egg Mass Survey

If you would like to have a better idea of the potential severity of defoliation in your township or around your lake, you can do so by counting the number of egg masses on just a few trees. Here's how to conduct the survey:

1. Choose two or three sampling locations with small aspen trees growing in full sunlight. 
2. Get permission from the landowner(s) to cut the three trees down for each location. Bring a small handsaw, measuring tape, notepad, and pen with you. Wear a hard hat and gloves for safety.
3. Choose aspens with trunks (stems) that are two to four inches in diameter at 4½ feet above the ground (this is known as the diameter at breast height, or dbh).  Before you cut them down, find the dbh by first finding the circumference of each of the three trees. Circle the trunk with the measuring tape at 4½ feet above the ground. Record the number of inches.  Convert the circumference to diameter using the table.



Converting stem circumference to stem diameter Take stem measurements 4½ feet above the ground. Stem measurements are in inches.
If the circumference is:	6.3	9.4	12.6	15.7	18.8	22.0	25.1
Then the stem diameter is:	2	3	4	5	6	7	8




Fresh egg mass; some eggs not covered by bronze material

Old egg mass from previous year.

4. Now, search for and count the number of fresh egg masses in the tops of each tree. Be sure to check every branch and twig. You may find some old egg masses from a previous year. They are lighter in color, no longer covered by the brown material, and tiny holes will be visible on the surface. Don't count them.
5. For each tree, record the stem diameter and the number of egg masses you found. You are done with the field work.
6. For each location, determine the average diameter of the trees and the average number of egg masses found.
7. On the Predicting FTC defoliation based on egg mass survey graph, locate average tree diameter and average number of egg masses for each of your three locations.

Method 2: Trends in local observations

FTC populations typically follow a pattern that allows predictions to be made from local observations during the past growing season and trends over the past few years. Use the table below to compare population levels and number of years of defoliation with your observations in order to predict the level of defoliation next summer.