Summer Shorts

Forest tent caterpillars: Where are they?

Egg mass surveys for forest tent caterpillar indicated that heavy defoliation could be expected in some locations especially across the Iron range from Nashwauk to Virginia. Up to 26 egg masses were found on small aspen trees in some locations. This is three times more that would be needed for complete defoliation. Yet, in many of these locations larvae are absent or scarce. What happened? In some egg masses a high percentage of eggs did not hatch, likely due to a build up in the number of egg parasites. However, even when eggs hatched larvae are scarce. This is likely due to the cool spring. FTC egg masses were hatching from Grand Rapids to Ely on May 6th just as aspen leaves were beginning to emerge. However, May was a cold month with frosts occurring every week. Aspen leaves across the Iron Range have some evidence of frost damage. Average weekly temperatures in Hibbing were from -6.3 to -9.1 degrees below normal. It may have been too cold for newly hatched larvae to feed and they may have starved to death.

Yet, intrepid caterpillar hunters are able to find caterpillars. Caterpillars in Grand Rapids were up to 1 and 1/4 inches long on June 21st.

Two-lined chestnut borers

Two-lined chestnut borer adults were being caught by June 14th on sticky traps placed on the trunks of red oaks that were damaged by the borer last year. The adults are expected to continue emerging and laying eggs through late July.

Red oak trees damaged by the two-lined chestnut borer last year were rated in September for the amount of crown dieback. Most trees rated as having 70, 80 or 90 percent crown dieback failed to produce any leaves this year.

It is too early to make any prediction on how the remaining live oaks will fare this year. Portions of northern Minnesota such as near Grand Rapids still have a moisture deficit. Even in a year with normal precipitation trees can suffer a moisture deficit in July and August due to their high amount of transpiration. Any moisture deficit through this summer is likely to favor a continuation of damage by the two-lined chestnut borer. So watch the weather and keep your favorite yard trees well watered to prevent stress.

Fall cankerworm

A small pocket of trees completely defoliated by fall cankerworm, Alsophila pometaria, was found in Itasca County in sec 27-T55N- R27W. Fall cankerworm larvae feed on basswood, ash, elm, oak, maple, fruit trees and a variety of shrub species. Widespread outbreaks of fall cankerworm periodically occur. Why are they called fall cankerworms, you might ask, if they cause defoliation in the spring? They are called fall cankerworms because they lay their eggs in the fall. This is in contrast to the spring cankerworm that lays its eggs in the spring. Just don't ask us why they are called cankerworms.


D. Green phase of fall crankerworm, E. Dark phase. G. Effects of larval feeding.

Fall cankerworms overwinter in the tree in the egg stage and hatch in early spring. Young larvae initially skeletonize leaves but later eat the entire leaf leaving only the main veins. Larvae usually complete their feeding by late June when they are about an inch long. They drop to the ground and pupate. The adults, gray moths emerge in October. The female is wingless. She climbs a nearby tree and lays her eggs on twigs and branches.

Fall cankerworm larvae are sometimes called loopers because of the way they lift up the center of their bodies when they crawl. They are also sometimes called "measuring worms" or "inch worms" (that would be a "2.54 centimeter worm" in metric, of course). Fall cankerworm larvae are variable in color. Some of the larvae in Itasca County were pale green color while others were darker with a wide black stripe running down their backs.

Jack pine budworm and fireworks

In spite of a cool spring, Jack pine budworm larvae are alive and doing well. As of June 15th, most larvae were ? inch long and moving out from the protection of the pollen cones to the elongating shoot. There they began feeding on the new needles, clipping them off and tying them to the twig with silk. Here they will find food and shelter while they continue to grow until about the 4th of July. The fireworks and festivities around them won't keep them from pupating quietly near their former home. Ten days later, small brown-mottled moths will emerge during the evening hours, mate and begin their short adult life stage. Some eggs will be laid on live needles near their pupation sites, but most eggs will be laid on trees miles away. Moths can be borne away on night winds and, as they are coming back down, they choose the tallest Jack pines that they can find for egg-laying sites. Eggs hatch ten to fourteen days later and the unnoticed larvae seek overwintering sites under bark flaps and needle bracts, where they will settle down to wait for spring next year.

In the meantime, the clipped needles are drying out, loosing their chlorophyll and developing an orange coloration that deepens into a brick red color by mid-July. If people haven't already noticed the missing Jack pine foliage, they won't fail to miss the bright orange/ red Jack pines. It's usually not as bad as it looks. If this is the first year of defoliation, you'll probably want to wait until September rolls around to evaluate defoliation and decide if there are forest management activities necessary for the defoliated stand. If this is the second year of defoliation, this is the extent of damage you can reasonably expect from this outbreak. Again, evaluate the damage in September.

There are three management options:

  1. Wait it out; do nothing; see what Mother Nature brings.
  2. Spray next year with a bio-rational pesticide (Bt) to keep most of the foliage green and not fed upon. This can be a viable option for young plantations, resorts and campgrounds and high value/ shade trees after one year of defoliation. Rarely is spraying used to protect foliage on older Jack pines (>50 years) in the forest.
  3. Pre-salvage the stand while the trees are still alive. Harvest these stands between September 1 to February 1, because neither pine bark beetles nor blue stain fungi will move into the cut products or the residual pines. For best results and to retain site productivity, delay harvest activities until after the ground is frozen but cease harvesting by Feb. 1 and remove cut products while the ground is still frozen.

Now, go out and enjoy the fireworks.

Bark Beetles

Phloeosinus canadensis, a red cedar bark beetle, was confirmed killing 20-year-old trees in Goodhue County last August. Field checked in May this year, the infestation had stopped. Last year's drought was taking its toll and significant bark beetle losses would have happened this year if not for the abrupt change in the weather pattern that occurred here in mid-April.

Rain! Speaking of rain, it would be nice to receive some in July and August this year. Spacing it out would be helpful. We have had enough for now!

Yellow-headed spruce sawfly

By late June, yellow-headed spruce sawfly larvae have usually completed most of their development and chewed the new needles off small spruce trees. Treatment to control the sawflies normally has to be completed in early to mid-June in order to protect some foliage. The cool weather in northern Minnesota has delayed sawfly development this year. Larvae were only 1/4 inch long in Aitkin County as of June 18th and had a couple of weeks to go before they would complete their feeding and drop to the ground to overwinter in a cocoon. Adult sawflies are active laying eggs about the time lilacs are in full bloom each year. Lilacs were past their prime but bushes with blooms could still be seen in mid-June across northern Minnesota. And if you head towards Grand Marais they haven't even started to bloom.

Larch casebearer

Larch casebearer damage to tamarack is again obvious across Aitkin County. Larch casebearer is an exotic pest first found in North America in the 1880's. Small larvae overwinter in a hollowed out portion of a needle, which they attached to a twig usually at the base of a bud. In early spring the larvae mine new needles giving the tree a brown or tan cast. Needle mining or defoliation by the casebearer has been widespread across much of the Lake States for the past few years, however, populations across much of the state appear to be declining.

Spruce budworm

On June 21st, spruce budworm larvae were from 5/8 to 3/4 inches long on white spruce in Aitkin County. Heavy defoliation was observed in plantations near Ball Bluff south of Jacobson. Heavy defoliation is also occurring on a white spruce plantation in southern Itasca County near Smith Creak where the budworm larvae were 1/2 to 5/8 inches long.