Article written by Madeline Wimmer, UMN Extension Fruit Production Educator
Images: Top left: Dogwood borer traps are designed for adults and best used with pheromones. Top right: Dead larvae (update: this is likely a secondary pest) were found underneath bark on a damaged rootstock shank and graft union region a few days after treatment of the bark. Bottom photos: Damage to the graft union and rootstock shank on dwarf apple trees can lead to vulnerable spots on the tree.
Introduction
Dogwood borers (Synanthedon scitula), also known as clear wing moths, are a pest that are native to eastern North America. Adults have primarily black bodies with yellow highlights on their abdomen and legs, and mostly clear wings. To the untrained eye, they can appear similar to a wasp, but they are entomologically classified in the moth order (Lepidoptera).
They are a pest of apple trees, usually impacting dwarf trees that produce burr knots, which are aerial roots that form on a tree in high humidity, shady conditions, and common on rootstock varieties like M.9. However, they can also attack trees with other vulnerable spots like damage to the rootstock shank and graft union. Females lay eggs within bark crevices and the larvae feed underneath the bark, damaging the tree. This is especially problematic because, during its life cycle, they can spend more than one year in this juvenile phase.
Dogwood borers do not directly affect fruits, but impact yield by contributing to tree decline and reduced vigor.
Dogwood borer spotted in SE Minnesota
This week, I’ve been working with an orchard in Southeastern Minnesota that was seeing apple trees decline over the past years in various parts of their orchard. Last growing season, they suspected dogwood borers associated with the decline after noticing larvae under damaged areas and ordered traps. This season, they have had a large number of adults trapped in their wing sticky traps with dogwood borer pheromone caps.
For management, they treated the trunks with insecticide, in which dead larvae were found under the bark after treatment; although they will likely need additional treatments depending on trap captures and label instructions as the adult egg laying season continues for a longer period of time. They have historically used black tree guards that may have led to higher humidity conditions during the summer, and are thus deciding to remove guards after one year of tree establishment moving forward.
Rapid/Sudden apple decline (RAD/SAD) and dogwood borer
In addition to humid conditions during the summer, stress to the graft union and rootstock shank can occur during winters with contrasting freeze thaw cycles. Even without dogwood borer infestation, damage to the shank and graft union has been an issue researched at various universities, including Cornell University, where professor Jason Londo has focused on the topic. Rootstock shank injuries have been observed on M.9 clonal rootstocks, but can also occur on other rootstock varieties, leading to rapid, or sudden apple decline (RAD/SAD).
Some thought as to why this occurs relates to the difference in deacclimation rates between the rootstock and the scion varieties. This type of damage may thus be exacerbated by late-winter and early-spring periods after the rootstock has entered ecodormancy and becomes increasingly responsive to the environment. Once the damage to the rootstock shank, graft union, or other parts of the tree has occurred, this can lead to a vulnerability where dogwood borer adults may enter and lay eggs.
Monitoring and mating disruption options
Tracking adults to monitor populations is a good first step to knowing when to take action. Winged sticky traps baited with pheromone caps (lures) were used by the orchard in the above example and commercially available. Inspecting the bark for frass is another sign of feeding.
Isomate DWB is a mating disruption product on the market that can reduce mating by making it challenging for adult males to find females. Isomate comes in the format of manual dispensers that should be hung at the rate of 100-200 dispensers per acre. It’s recommended to install mating disruption dispensers right before bloom somewhere between 4-6 feet from the orchard floor. Resources note that mating disruption is most effective when used multiple years in a row.
Management options
The Midwest Fruit Pest Management Guide lists Assail 30SG (page 56) as a main insecticide available to conventional fruit growers for tree and orchard management when infestations happen. In addition to this, preventing tree injury by reducing humidity in the orchard is helpful, and painting trunks with white latex paint can deter females from laying eggs.
Overall, the infestation is likely complicating already-occurring tree decline due tree stress from the environment and winter injury and rootstock susceptibility.
Do you think you’ve seen dogwood borers in your orchard?
Email at wimm0035@umn.edu or use our fruit crop reporting tool contact us directly.
Resources cited
Dogwood Borer (Synanthedon scitula) (Cornell University)
Dogwood borer injury to apple trees (Purdue University)
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