Predicting forest tent caterpillar defoliation

If you would like to have a better idea of the severity of defoliation in your townships or Field Station, an egg mass survey could be done. Two or three sampling locations would be adequate. This survey should be done while the trees are leafless, late fall to early spring.

Choose the locations you are interested in and cut down three aspens. They don't have to be large trees but they do have to be at least codominants, that is, the crowns are medium size and receiving full light from above. We usually choose aspens that are two or three inches in diameter at breast height (dbh). Measure the dbh of the three trees that you fell.

Next, search for and count the number of fresh egg masses in each crown. Egg masses are sturdy, cylindrical structures that encircle fine twigs. They are 1/4 to 1/2 inch long and are made of a glossy brown material that protects the egg contents during our cold winters. You may find some old egg masses from last year. They are lighter in color, no longer covered by the brown material and tiny cavities will be visible on the surface. Don't count these older egg masses.

Find the predicted level of defoliation for your sample area by plotting the average tree diameter and average number of egg masses on the graph below. Actual defoliation will vary due to weather, size of the egg masses, number of egg parasites, etc. This survey however, gives a fairly good idea of what to expect. If you decide to do a survey, please send your results (legal descriptions, dbh's and counts) to the I&D Specialist in your Region.