Cherry Bark Tortrix
Life History: The Cherry Bark Tortrix (CBT) was first found in the Lower Mainland of British Columbia, Canada in 1990. CBT was subsequently discovered in the United States during 1991 at Peace Arch Park in Whatcom County. Of course, the person responsible for this discovery was WSU Master Gardener, Joe Massett. Master Gardeners are almost always the first to identify exotic pests entering the country. Since its discovery, CBT has spread southward and can currently be found South of Portland, Oregon. We believe that CBT came to North America from Europe and Eurasia. CBT is found throughout Europe, costal North Africa and as far West as Siberia. Curiously, CBT is rarely a problem in these areas and rarely requires treatment. In its native homeland, CBT is controlled naturally by parasitic wasps and other predators.
If you are so lucky to run across the inch-long adult moths, they are rather beautiful. The forewings are mottled with dark and light browns. Along the front of the wing, there are strips of orange, dark purple, and silver. The larvae are typical caterpillar-shaped and are usually transparent, with a pinkish lining in the gut. Mature larvae reach the lengths of one to 1 ½ inches. The eggs are very small, oval and salmon pink.
There is one generation of CBT per year in the Northwest. Adults fly and lay eggs from April to September. Females deposit eggs in cracks, crevices, wounds, crotches and lenticels (small natural opening of the bark) of trees. Eggs hatch after a couple of weeks and the first caterpillar stage forages for entrance into the tree bark. Once the larva gains entrance through an opening in the bark, the caterpillar will burrow deeper into the living tissue of the bark down to the cambium. Here the larvae feed though out the season until next spring. The caterpillars mine out winding tunnels in the bark. During mining, the larvae are constantly constructing a ‘frass tube.’ The tube is built on the entrance to the tunnel and consists of frass (digested food) and silk. The frass tube is a unique adaptation that offers: protection from predators gaining access to the tunnel, protects the caterpillars from the outside environment and a safe site to pupate.
Damage: CBT attacks practically all rosaceous trees in the Northwest. This includes apples, cherries, plums, apricots, almonds, peaches, laurels, quince, firethorns, photinia and hawthorns. In addition, CBT threatens many of our native trees such as black hawthorn, bitter cherry and Oregon crab apple. Damage to the tree is noticeable by dieback and premature wilting of the canopy. High populations can girdle and kill trees. More importantly, CBT damage opens up the tree to all sorts of dangerous mortality factors, leaving trees susceptible to bacterial and fungal diseases, frost damage, and other insect pests. All these factors can decrease the vigor, life span and fruit and flower production of infested trees.
Monitoring: You will notice CBT infestations by inspecting the trunks of your trees. The characteristic frass tube will be obvious due to its orange color. On heavily infested trees, you will see hundreds of these orange frass tubes along grafting sites and other wounds and openings. The tree, especially cherries, responds to the infestation by producing sap to flush out the larvae. Sometimes this is affective, but quite often I’ve seen CBT chew right through the rock-hard resin.
Management: Since the introduction of CBT to Western Washington, entomologist Dr. Lynell Tanigoshi has researched integrated pest management strategies for managing CBT.
Cultural Management- Do not purchase and plant susceptible trees. Mount Fuji oriental cherry trees (Prunus serrulata) are very susceptible to CBT damage, along with weeping cherry (P. subhirtella) and sweet cherry (P. avium). Pruning habits can also increase a tree’s susceptibility to CBT. Do not prune living branches from flowering cherries. Only prune out dead wood. On fruiting trees make good pruning cuts to allow for the collars to heal closed properly. Using pruning paint is not effective. Avoid any wounds to the bark; be careful with your weed-eaters and lawn mowers. Control weeds and hedges at the base of the trunk and keep it exposed. CBT will often lay eggs at the base of the trunk, if no other opportunities are available.
Chemical Management- When research began on managing CBT, little attention was given to chemical control. It would be impractical, expensive to try and manage the only free-living, exposed stages of CBT; adults and eggs are present from April until September. The larvae are protected underneath the bark and cannot be reached by insecticides. However, there is a weak link in the biology of CBT. The larvae of the CBT construct and visit their frass tubes with diligence in the spring and fall seasons. A well-timed insecticide applied in late September or early October can be very effective. The timing is import to insure that all live stages of CBT are inside the tree, adult flight and egg-laying is complete by late September. It only takes very little of mixed insecticides to control CBT. I use a liter-sized, cheap, hand-held atomizer sprayer. It only takes about a half liter to treat even the bigger cherry trees. Trees that may need an insecticide treatment are those that host many frass tubes that circumscribe the tree. Only treat areas of the bark with active frass tubes such as trunks and scaffold branches; do not treat the canopies. Apply insecticide until it runs off the bark. Visit Hortsense (http://pep.wsu.edu/hortsense) for the most current insecticide recommendation. It is important to note that there are NO chemical recommendations for edible, fruiting trees. Currently, Dr. Lerry Lacey, of USDA-ARS in Yakima, is exploring the use of entomopathic nematodes for managing CBT, and it appears successful so far. This will be a likely option for persons who that do not want to use synthetic pesticides and persons with home orchards.
Biological Control- A native Trichogramma wasp (about the size of a pencil tip) is very effective at parasitizing CBT eggs. To conserve beneficial parasitoids, offer small flowers in your gardens and reduce the amount of insecticides used in your yard. In Bellingham, we have recorded up to 98% parasitization by Trichogramma wasps. Dr. Tanigoshi and graduate students at WSU have worked on biological control campaigns using the miniature Trichogramma wasps and have found success. Seattle Woodland Park Zoo and other areas of Seattle and Portland have been able to recover released wasps from parasitized CBT eggs. Parasitized CBT eggs will appear as little metallic black bubbles instead of the healthy salmon-pink color. Of course, using a hand lens, you can often see a little hole chewed out the top of the egg where the adult wasp emerged from. Other endemic wasps are appearing to exploit CBT as a food source. I imagine that as CBT establishes the Pacific Northwest as its home, many of our native critters will find it a tasty meal and we will soon experience natural control of CBT as in Europe.
To reach Todd Murray please call (360) 676-6736 or e-mail him at: email@example.com