There are two methods that can be used to predict FTC defoliation: local observations from the previous summer and egg mass surveys. Egg mass surveys are more accurate.
Method 1: Egg Mass Survey
You can get a good idea of the potential severity of defoliation in your area by counting the number of egg masses on just a few trees. Here's how:
- Choose two or three sampling locations with small aspen trees growing in full sunlight.
- Get permission from the landowner(s) to cut three trees at each location. Bring a small handsaw, measuring tape, notepad, and pen with you. Wear a hard hat and gloves for safety.
- Choose aspens with trunks (stems) that are 2 to 4 inches in diameter 4½ feet above the ground (this is known as the diameter at breast height, or dbh). Before you cut them down, find the dbh of each as follows: Circle the trunk with the measuring tape 4½ feet above the ground. Record the number of inches. Convert the circumference to diameter using the table.
- 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 and no longer covered by the brown material. Tiny holes will be visible on the surface. Don't count them.
- For each tree, record the dbh and the number of egg masses you found.
- For each location, determine the average diameter of the trees and the average number of egg masses found.
- 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. Use the graph to predict the severity of defoliation in your area.
Converting stem circumference to stem diameter
If the circumference is:
Then the stem diameter is:
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.
Predicting defoliation levels caused by forest tent caterpillars
Observations from last summer
FTC population levels
Severity of defoliation
Caterpillars typically observed During a short walk, how many caterpillars did you see?
1 or 2
Cocoons typically observed Were cocoons observed on buildings or in shrubbery during a short walk?
Friendly flies typically observed During a short walk, how many friendly flies did you observe?
Nuisance moths observed How many dead moths did you sweep away on patios, sidewalks or driveways?
Egg masses found near well–lit areas Did you observe egg masses on trees or buildings?
1 or 2
Predicted FTC defoliation levels for the upcoming summer
Trace to Light
Moderate to High
High to Complete