Monitoring and Biological Control of the Cherry Bark Tortrix, 2002
L. K. Tanigoshi and Koen G.H.Breedveld
Washington State University
Vancouver Research and Extension Unit
Vancouver, WA 98665-9752
tanigosh@wsu.edu
This will be another season in our efforts to gain insight in the biology and behavior of the exotic cherry bark tortrix, a small moth in the family Tortricidae, which now occurs in North America. Since the 1990’s its range has been expanding southwards from British Columbia to south of Seattle. The expansion seemed to have slowed for several years, however in the past two years, a small number of CBT adults have been trapped in Portland, Oregon.
General Biology
CBT causes damage to rosaceous trees, such as plum, almond, hawthorn, and mountain ash, but preferentially cherry trees. Caterpillars damage the phloem tissue, which transports nutrients to the roots. A large infestation of caterpillars can girdle a tree and ultimately kill it.
The adults are approximately 15-18 mm in length with golden-brown colored wings. They lay tiny eggs (~0.7 mm diameter) on the cherry tree trunk or the primary branches. Initially the eggs are creamy white, but color pink after several days as the caterpillar grows inside. Several days before the caterpillar emerges, the egg turns a dark salmon color. The caterpillar starts feeding on the bark and steadily eats its way into the tree. Once the caterpillar is underneath the bark, it feeds on cambium tissues where it forms galleries as a result of this feeding habit.
As the caterpillar feeds, it places its fecal pellets on the outside of the gallery entrance and forms a frass tube which protrudes from the bark. These frass tubes provide a visual character to detect CBT on a tree. Caterpillars feed throughout the summer and into the winter months. The following spring the larvae are full-grown and pupate in the frass tubes. First adults emerge in April or May. Their flight activity increases steadily into mid-July, when peak flight occurs for approximately two weeks. Flight activity declines quickly after mid-August and ends in mid-September.
CBT Control
Because CBT is not native to the Pacific Northwest it has very few natural enemies that specifically attack itand could keep them at low, non-damaging densities. This is the case in Europe where CBT is sporadically a minor pest on ornamental and commercial cherry and apple. In the PNW their populations have exploded and many ornamental cherry trees are infested to the point where they show signs of canopy dieback, gummosis or even death.
We know of one natural enemy that has been found repeatedly attacking CBT eggs. It is a tiny wasp (~0.5 mm) smaller than the size of a pinhead. The adults, known as parasitoids, run over the bark in search of CBT eggs. When they find a CBT egg, they will sting it and lay one egg inside it. The wasp egg will develop into a larva that emerges and starts feeding on the CBT egg content. As the wasp larva grows the CBT egg content is wholly consumed. The larva pupates inside the CBT egg and emerges several days later as a full-grown parasitoid. The cycle repeats itself over several more generations. One very interesting aspect of the wasp’s biology is that there are only females. This could prove extremely beneficial in our efforts to mass release these wasps for the biological control CBT.
This wasp scientifically belongs to the family Trichogrammatidae, a family that is widely used in the agricultural sector to control various moth pests. Its scientific name is Trichogramma cacoeciae and it occurs naturally at some locations in the PNW. Even though these natural populations are relatively high, they are localized and scattered and thus seem to have a small impact on areawide CBT populations. Our mass releases will hopefully directly decrease CBT populations at the release sites and facilitate dispersal of the wasps to other infested areas.
Wasp Releases
Similar to last year, Trichogramma cacoeciae will be shipped to the WSU REU station on a weekly basis from the USDA APHIS PPQ laboratory in Niles, Michigan. The USDA laboratory will rear the wasps on non-viable eggs from a different moth host. After the wasps have parasitized these eggs on a filter paper sheet they will be shipped inside specially modified one-half pint milk cartons. Each carton will contain approximately 5000 parasitized eggs. The carton shelters are sent by over-night courier to the WSU REU station and will be distributed to the various locations in Seattle by WSU researchers for further field releases by parks
The release cartons are placed high on the tree trunk (within arm’s reach) using wire or staples. The wasps will emerge out of the protective shelter and search nearby branches for CBT eggs. Running is the main way these weak flying wasps disperse and from last year’s observations we know that they disperse very well throughout the tree’s canopy within a week. To disperse to surrounding trees T. cacoeciae can fly on their own, but are more often randomly carried away by the wind. Because of these conditions and limitations for direct dispersal, augmentative field releases should be made at several locations around an infestation site.
The rearing schedule with USDA APHIS is timed so that the wasp’s pupae will emergence one day after their field release. If the cartons cannot be placed in the field within a day of receipt, they may be stored in a refrigerator (5-7 °C) for up to two weeks. However some wasps may emerge in storage, and the number of wasps surviving the storage decreases over time. We strongly recommend that the releases be made within a day or two of their arrival in order to maximize the numerical response, searching capacity and vigor of newly emerged adults. A release carton placed in the field can be removed 1-2 weeks after it is placed in the field. These cartons will be collected and shipped back to Niles, Michigan so they can be re-cycled for subsequent shipments.
CBT monitoring
Another aspect of our study is to monitor CBT flight activity. Similar to last year, sticky traps with pheromone lures will be placed in the field to attract the CBT males. This will give us more insight into the biology of CBT to determine the best timing of control methods.
The first traps will be placed in the field in April and monitored on a weekly basis. On the trap write: the date, location and ‘CBT’ (e.g., CBT, April 21, Green Lake site 1). The traps should be visited every seven days to count the total number of moths trapped. That number and the date should be written on the trap with a waterproof marker (e.g., April 28, 6 CBT). Also write the date on the pheromone
A trap is effective in attracting up to 70 to 80 adults, after which it should be replaced because there are too many moths on the sticky trapping surface. A pheromone lure will work for four weeks before it needs replacement. So, when replacing a full trap (70 or more CBT) transfer the pheromone lure to the new trap if it has not been used for 4 weeks. These traps will be collected at the various locations by WSU personnel at periodic intervals.