Last summer, aerial sketch-mappers were busy trying to find forested areas that didn't have forest tent caterpillar defoliation in northern counties. See map. There was a whopping 7.7 million acres of FTC defoliation; more acres than has ever been documented in Minnesota. Another two million acres of forest damage were mapped. See table below map.
Comparison of Aerial Survey Results for 2000 and 2001
|Agent||Acres for Year 2000||Acres for Year 2001|
|Forest tent caterpillar||2,039,919||7,759,807|
|Large aspen tortrix||63,942||none|
|Dutch elm disease||365||1,052|
TLCB is an opportunistic insect, that is, it can only successfully infest already-stressed oaks. Stress can come from drought, FTC defoliation, spring frost or any agent that triggers a refoliation. TLCB can also build up in oak slash, in windthrown oaks and in oak firewood piles.
Infested oaks have three really diagnostic symptoms:
- Upper branches leafless and dead; middle branches wilting and red; lower leaves green and alive. This is termed a "layer cake appearance", or, "Dead, Red and Alive".
- Leaves on branches that died during the summer year will be retained long after the rest of the tree shed its leaves.
- Small exit holes, where the adults have chewed their way out of the tree. These are small = 2- 5 mm wide and shaped like a capital D.
At Fr. Hennepin State Park and elsewhere along Mille Lacs Lake, a TLCB outbreak has started. Some is related to oak slash, but most is related to the effects of FTC defoliation. We observed that TLCB occurred throughout the Park in small groups of one to three oak trees. In September, some trees were already dead and leafless while others exhibited the "layer cake appearance".
Oaks that were more than 2/3rds green last fall may live. If a drought occurs or enough FTC defoliation to trigger a summer refoliation, then more oaks will be attacked and killed in 2002. Otherwise, expect oak vigor to return to normal and next year's mortality to be limited to already infested trees.
In Region 2, TLCB was active in Itasca County, especially in and around Grand Rapids where four pockets of infested and killed oaks were found. Both red and white oaks were involved.
Late season defoliation of red and bur oaks, along with understory hardwood species, was reported in scattered oak stands in central Hubbard County. Defoliation wasn't noticeable from a distance or from roadside observation but was readily observed when walking of offroad trails in some wooded areas. The defoliation was caused by variable oakleaf caterpillars, Heterocampa manteo, and by redhumped oakworms, Symmerista canicosta. Both species are members of a group of hardwood defoliators called the "Fall Defoliator Complex". Trees of all sizes were attacked by these insects.
We have not seen much of this particular group of caterpillars in Region 1 since the 1982-83 seasons when they caused widespread defoliation of oaks in Beltrami and Hubbard Counties. Unlike the defoliations in '82-83, the feeding in 2001 wasn't observed until early September. Damage to tree vigor is close to nil, since the trees would have been losing their leaves in a couple of weeks anyway. Except for being a nuisance to campers, hikers, and homeowners there is little harm done and infestations seldom last more than two years.
In Region 3, populations of fall defoliating caterpillars are collapsing. Very low numbers of orange-striped oakworms, red-humped oakworms, pink-striped oakworms and green-striped mapleworms were found in August and September.
The pine webworm, Tetralopha robustella, is a caterpillar which creates nests of clipped needles tied together with silk and brown excreta (frass particles). Webs of the pine webworm had increased to a noticeable level by mid- September in one jack pine plantation just north of Brainerd in Crow Wing County. Most of these webworms have little effect on the vigor of their hosts. On ornamental pines, hand removal easily controls this pest.
In Region 2, spruce budworm defoliated 17,200 acres in 2001. This is down from 27,800 in 2000. Egg mass surveys indicate that defoliation and populations in 2002 will be similar or slightly lower than those in 2001.
In Region 3, egg mass surveys for both species were conducted. No JPBW egg masses were found indicating continued very low population numbers. Similarly, spruce budworm populations will be very low, except in northeast Aitkin County, where they remain at moderate levels.