Large Aspen Tortrix

Large Aspen Tortrix Adult

Alaska Coperative Extension University of Alaska Fairbanks and U.S. Department of Agriculture Cooperating

 

Large Aspen Tortrix

The large aspen tortrix, a leaf-eating caterpillar, periodically affects large areas of quaking aspen in south central and interior Alaska. This insect is typical of northern tree defoliators; populations increase to epidemic proportions for 2 to 3 years then significantly decrease.

The first recorded tortrix epidemic in Alaska was in 1966 when over 5,000 acres were affected. In 1978, tortrix damage was found near Willow, Alaska and the following year infestations covered more than 26,000 acres. Total defoliation occurred on many sites (Figures l, 2).

The most common effects of heavy tortrix feeding are a temporary reduction in tree growth and occasional branch and top dieback. Healthy hardwood trees can generally recover from several years of moderate to heavy tortrix feeding. In a short amount of time they can produce new leaves.

Figure 1. Defoliated aspen trees near Willow, Alaska.

Figure 1. Defoliated aspen trees near Willow, Alaska.

 

When trees are not healthy and vigorous when attacked, the chance of death increases after two or more years of heavy infestation. This often occurs in areas of new building or road construction where trees are mechanically damaged and soil is compacted or excessive amounts of soil are placed on the root system.

Figure 2. Extreme aspen defoliation. Note the conspicuous webbing.

Figure 2. Extreme aspen defoliation. Note the conspicuous webbing.

 

Life History:

In Alaska the large aspen tortrix, Choristoneura conflictana (Wlkr.), has a one-year life cycle. Tortrix overwinter as small larvae (caterpillars) in bark crevices at the base of trees and along twigs. During the first 2 weeks of May, larvae become active and work their way into unopened buds. Leaves can be completely destroyed before the buds begin to open.

Older larvae, gray-green to black caterpillars with dark heads, feed more openly. In heavily defoliated aspen stands the understory plants, including spruce and birch, are eaten and often extensively "webbed" (Figure 3).

Mature caterpillars transform into black pupae, a nonfeeding stage between caterpillars and adult moths. The pupae are generally found within rolled aspen leaves. They can also be found on the webbed understory plants.

Adults emerge from the rolled leaves and are active from late June until early July. Tortrix moths are mostly gray with some brown markings on their forewings (Cover photograph). They generally deposit green egg masses on the upper surfaces of leaves.

Figure 3. Tortrix webbing on white spruce understory.

Figure 3. Tortrix webbing on white spruce understory.

 

Eggs hatch by the end of July, and the young caterpillars are active until mid-August. Within each rolled leaf, tortrix feed on the lower and upper surfaces. Only a skeleton of the leaf is left. A new generation of caterpillars migrates to protected areas, spins shelters, and overwinters to begin the cycle once again (Figures 4-6).

Figure 4. Tortrix egg masses. Note leaf skeletonization as a result of feeding by young larvae.

Figure 4. Tortrix egg masses. Note leaf skeletonization as a result of feeding by young larvae.

 

Guidelines For Reducing Damage:

Under natural conditions tortrix populations seem to cause little permanent damage. Artificially suppressing large numbers of tortrix on forested land is generally not warranted. They can, however, be more destructive in urban areas where human disturbances weaken aspen trees. Control alternatives in urban and suburban settings include a take-no-action approach and a chemical treatment.

Figure 5. Parasitized tortrix larva.

Figure 5. Parasitized tortrix larva.

 
Figure 6. Tortrix pupal cases.

Figure 6. Tortrix pupal cases.

 

Take-No-Action Approach:

An aspen tree that is healthy and growing well can withstand a few years of tortrix feeding. The tree's diameter and height will not be adversely affected; serious damage or death is uncommon. To assure good tree vigor, the following steps should be taken:

  • Avoid damaging the tree's trunk, injuring its roots, altering soil drainage patterns, or severely compacting the soil.
  • Spring fertilization helps promote tree vigor. The USDA Alaska Cooperative Extension recommends approximately one pound of fertilizer per inch of tree diameter. Any complete lawn or garden fertilizer high in phosphorus is adequate. Apply by making a series of holes, 8 to 10 inches deep, around the tree starting two feet from the trunk and extending a few feet beyond the dripline (Figure 7). Fertilizing should begin in the spring and continue throughout the summer, then be discontinued prior to fall dormancy. A feeding program may not be necessary every year. Fertilizer uptake soil type, rainfall, weather, and grass cover will all determine the time of reapplication.

Chemical Treatment:

Chemical treatment may be warranted if aspen trees show signs of extensive tortrix feeding for 2 consecutive years and if top or branch dieback is apparent. If used, insecticides can be applied from mid-May until early in June when aspen buds are open and new leaves are expanding. Young tortrix larvae are susceptible to chemical treatment at that time.

Presently, formulations of Bacillus thuringiensis (B.t.) are registered and could be used against large aspen tortrix. Several other insecticides for suppressing leafrollers, which are related to tortrix and have similar feeding habits, could be used. Always check with your local Alaska Cooperative Extension office for advice in selecting a chemical treatment.

Figure 7. Location and depth of fertilization.

Figure 7. Location and depth of fertilization.

 

Large Aspen Tortrix, by Edward Holsten, Entomologist, Forest Pest Management, State and Private Forestry, USDA Forest Service, Alaska Region.

Additional information on this insect can be obtained from your local USDA Alaska Cooperative Extension office, Alaska State Forestry office, or from:,

Forest Health Protection
State and Private Forestry
USDA Forest Service
3301 C. Street, Suite 202
Anchorage, AK 99503-3998.
Phone: (907) 743-9455

Forest Health Protection
State and Private Forestry
USDA Forest Service
2270 Sherwood Lane, Suite 2A
Juneau, AK 99801
Phone: (907) 586-8811

CAUTION: Pesticides can be injurious to humans, domestic animals, desirable plants and fish, orotherwildlife, if they are not handled or applied properly. Use all pesticides selectively and carefully. Since approved uses of a pesticide may change frequently, it is important to check the label for current approved and legal use. Follow recommended practices for the disposal of surplus pesticides and pesticide containers. Mention of a pesticide in this publication does not constitute a recommendation for use by the USDA, nor does it imply registration of a product under Federal Insecticide, Fungicide, and Rodenticide Act, as amended. Mention of a proprietary product does not constitute an endorsement by the USDA.

173/4-94/WV/1500 May 1994

The University of Alaska Fairbanks Cooperative Ex1ension Service programs are available to all, without regard to race, color, age, sex, creed, national origin, or disability and in accordance with all applicable federal laws Provide in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S Depar1ment of Agriculture, Hollis D Hall, Director, Alaska Cooperative Extension, University of Alaska Fairbanks.