# Predicting forest tent caterpillar defoliation

### Predicting defoliation

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:

1. Choose two or three sampling locations with small aspen trees growing in full sunlight.
2. 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.
3. 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.

4. ### 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.
5. 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.
6. For each tree, record the dbh and the number of egg masses you found.
7. For each location, determine the average diameter of the trees and the average number of egg masses found.
8. 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.

### 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 based on trends in local observations

Observations from last summer

FTC population levels

Very low

Low

High

Peak

Collapsed

Severity of defoliation
What percentage of the leaves on oak, basswood, aspen, and birch trees were eaten by FTC in late June?

Minimal

< 25%

Moderate

25-50%

High

50-75%

Complete

75-100%

None

0%

Caterpillars typically observed During a short walk, how many caterpillars did you see?

1 or 2

Several

Hundreds

Thousands

None

Cocoons typically observed Were cocoons observed on buildings or in shrubbery during a short walk?

None

Few

Many

Dozens

None

Friendly flies typically observed During a short walk, how many friendly flies did you observe?

None

None

Dozens

Hundreds

Dozens

Nuisance moths observed How many dead moths did you sweep away on patios, sidewalks or driveways?

None

None

Few

Many

None

Egg masses found near well–lit areas Did you observe egg masses on trees or buildings?

None

1 or 2

Few

Several

None

Predicted FTC defoliation levels for the upcoming summer

Trace to Light

Moderate to High

High to Complete

Complete
100%

None