From the Hudson to the Champlain Valley, its hard to find a single orchard of slender spindle high density apple on M-9 rootstock free of dogwood borer (DWB), Synanthedon scitula. It’s rare to find Dogwood borer on healthy bark or pruning wounds in apple trees, however, the adult will lay eggs on root initials or burrknot on certain rootstock varieties, giving rise to DWB adult egg laying and larval feeding within this susceptible area of the tree. The M-9 rootstock is quite susceptible to burrknots, very sensitive to fireblight and its shallow root system drought sensitive. Its near relative, Bud-9 has much less burrknot initials, having greater resistance to fireblight and Phytopthera & quite cold hardy. The detailed publication of the NC140 rootstock trial provides descriptions of rootstock and interstems selection strengths and weaknesses.
The American plum borer larva (APB), Euzophera semifuneralis (Walker), are also found on apple in habitats similar to those preferred by the dogwood borer. Borers can be monitored by checking under tree guards in the spring to locate active infestations as guards applied to newly planted trees encourage adult egg laying and larval presence. The larvae size of the two species are similar. DWB larva are white to cream-colored, with one row of crochets on the abdominal prolegs, while the APB larva is dusky purple to gray in color, and has two rows of crochets on the prolegs. Pheromone traps can be used to estimate the timing of peak flight in the Northeast. Adult emergence begins in early June and continues into early September, peaking in mid July. Traps should be placed at 4 feet above the ground for optimum captures.
If DWB larva are found in burrknots of young trees, trunk applications of Lorsban should be applied using a course directed application to tree trunks at the earliest window to reduce the present larval population and prevent further infestation by newly emerging adult egg laying as they take to the wing over the next few weeks.