An association between invasive apple clearwing moth, Synanthedon myopaeformis (Lepidoptera: Sesiidae) and necrotic cankers on Malus domestica trees in British Columbia

Abstract

Sudden apple decline (SAD) is complex disease causing rapid mortality in apple trees across North America, typically associated with a necrotic canker near the graft union. To date, causal factors have been diverse and speculative, and infestation with Synanthedon myopaeformis (apple clearwing moth) was identified by producers in British Columbia as a concern related to the disease.  Assessments of declining orchards in 2019 showed an association between incidence of necrotic cankers and infestation of S. myopaeformis. Observations of tree stems 15 cm above the graft union showed that 100% of signs of S. myopaeoformis infestation (pupal casings, exit holes, frass) were in necrotic tissue. There was a positive correlation between necrotic canker size and count of S. myopaeformis signs.  It is unlikely that S. myopaeoformis infestation is the underlying cause of SAD, but this observational field investigation suggests a relationship between infestations and presence of necrotic cankers, which may be an additional stressor contributing to apple tree collapse associated with SAD.