Burr knot borers [Dogwood Borers, Synanthedon scitula (Harris), and Apple Bark Borer, Synanthedon pyri (Harris)] and related clearwing species

I. Introduction: A relatively new problem (1980's) involves larvae of clearwing moths developing in "burr knots" (adventitious root primordia) in above-ground portions of clonal rootstocks (Plate 79). The main species attacking burr knots is dogwood borer (DB), although apple bark borer (ABB) could cause also attack trees at such sites. Both are sesiids, or clearwing moths, the larvae of which are tree borers. DB has been called the most common sesiid in North America, although not of much economic importance except to pecan in earlier years. ABB, though capable of causing similar injury, has been much less common in recent years.

II. Hosts: DB has a wide host range (probably the greatest of any American sesiid), including apple, bayberry, beech, birch, black cherry, blueberry, chestnut, dogwood, hazel nut, hickory, mountain ash, oak, pecan, pine and willow. ABB feeds on the bark and cambium of apple, pear, mountain ash, hawthorn and juneberry (serviceberry).

III. Description: DB is a slim moth with a steely-blue or black body with two thin yellow abdominal stripes (the stripe on the fourth abdominal segment is heavier than that on the second segment). There is a rounded anal tuft on the tip of the abdomen (triangular or pointed in many other species). The forewings are transparent with a dark outer margin. The wing length is 2/10 - 3/10 inch (5-9 mm) (females in the upper part of the range); the total wingspan ranges from 6/10 - 3/4 inch (1.4-2 cm). Hind tibia are yellow with a dark band between spurs. Larvae are white with a light brown head, reaching a length of 6/10 inch (14 mm). The pupa is about 3/8 inch (10 mm) long and is usually found surrounded by a cocoon of silk and frass. ABB is similar to dogwood borer but has distinct red-orange spot located about 2/3 of the distance from the base of the forewing, and a wedge-shaped anal tuft (rounded anal tuft in DB). The abdomen is brown-black, with a yellow band on the rear edge of abdominal segments 2 and 4. The wing length is 2/10 - 3/10 inch (6-9 mm).  Larvae are differentiated by the head of the dogwood borer being as broad as long, while the head of the apple bark borer is distinctly broader than long.  (Another borer sometimes occurring in fruit trees is the American plum borer.  This is a dark colored caterpillar, the crochets of which occur in complete circles, rather than oval broken at either end, as in the Synanthedon species). (See related species under "Monitoring", below).

IV. Biology: DB is distributed from southeastern Canada and New England, west to Ohio and Minnesota, and south to Texas. This species probably has a single generation over most of our area, although with a prolonged period of activity. There is evidence for two generations in Virginia, Kentucky and Tennessee, but even in areas with one generation, adults are present throughout the summer. The beginning of activity in Virginia is in early to mid-May, climbing in mid-June; in the Virginia study, there was a large peak of flight activity in late July through early August. In Michigan, emergence starts in mid-June, peaking in early July, ending in August. In some areas a smaller second peak is present in early August. The second peak of DB is more likely to reflect infestation of apple and other alternate hosts, rather than dogwood. (This is supported by trapping data in different habitats in Virginia). Hence the large late peak of flight activity noted in our region may be of special concern. One New York study found that an average of 30% of trees were infested with DB (orchards ranging from 0-100% infestation). DB has been a serious problem in Michigan as well. This species overwinters as larvae from second to sixth instar.

Eggs are laid singly in wounds on the tree, and hatch in about 9 days. There are seven larval stages; larval development occurs in galleries beneath the tree bark. Larvae overwinter in these galleries and pupate in the spring. The pupal stage lasts about 25 days. Pupation occurs just below the surface of the bark. After the moth emerges, the empty pupal skin is left protruding from the bark surface. Adults are most active in dawn and twilight hours; most sesiids are active by day. Adult longevity is 6-8 days.

Eggs of ABB are deposited in crevices in the bark. The incubation period ranges from 4-8 days, averaging about 6. There is heavy mortality after egg hatch, with fewer than 25% of the larvae able to establish themselves. Establishment is more successful if eggs are deposited away from surface. Larvae are cannibalistic when confined. Some larvae complete development in 13 months, others require 20-22 months. 85% have one-year life cycle, the rest taking 2 years to develop. Larvae overwinter in silken cells under the bark. The pupal stage lasts 13-23 days, with an average of 18.5 days. Adults begin to emerge in mid-May; most emerge between May 10 and July 10. There are two peaks of emergence, the first in late May, and the other soon after mid-June. A single generation has been reported in the North Central states, with adults active in July, and larvae pupating in June. Adults are active mostly on hot sunny days.

V. Injury: DB is a wound invader on its diverse hosts. Both DB and ABB treat burr knots as wounds, with females ovipositing at such sites. Reddish frass at burr knots is a sign of infestation. While the burr knots themselves disrupt sap flow, girdling is enhanced once larvae are present. Furthermore, once established, feeding may extend beyond the burr knot into healthy tissue. Some scion/rootstock combinations may produce burr knots high on the trunk or at the base of limbs (e.g. `Gala' on M.25 or M.7a). These borers may exploit such sites (see photos of larva and exuviae). DB may also infest callus tissue at the rootstock/scion juncture, as well as gall tissue on hosts (e.g. black knot on peach). Chronic infestations will cause a decline in vigor, yield, and may rarely kill the tree. One study in Nelson County (Pfeiffer and Killian 1999) found an average of 7-13 aerial burr knots per tree in the lower six feet of trunk, with infestation rate by DB ranging from 7-20% of burr knots.

Healthy trees are rarely attacked by ABB. Heavy infestations of several hundred per tree can be very damaging; averages of 200/tree have been recorded in some blocks. An infestation usually starts in one of three types of sites: rough bark on trunk, crotch or lower branches (these are usually the most heavily-infested locations, but infestation here is most easily tolerated); bark around pruning wounds (infestations often increase rapidly and cause severe injury); or under scales of loose bark. Infestation produces rough bark that is conducive to further infestation. Infestations may occur in recently top-worked trees, females treating these as wounds. Females are attracted to dark grafting wax and tape.

VI. Monitoring: Pheromone traps are available for DB. Research in New York has shown that trap height is critical for DB. The optimal height is 4 feet (1.2 m); 1 foot (0.3 m) in either direction may reduce captures by 5-fold). Traps should be placed in mid-April, and monitored through July. Traps for this and related species are not very species specific, and many DB may also be captured on traps for lilac borer, Podosesia syringae (Harris). The latter species, as well as Paranthrene simulans (Grote), an oak borer, and Synanthedon acerni, the maple callus borer, may appear in high numbers in traps for DB or lilac borers. Peachtree borer may also be captured; this species is described in the peach section. ABB uses the same pheromone blend as DB. Trapping in Virginia has shown that lilac borer traps are more effective at trapping DB than are DB traps. Traps baited mainly with (Z,Z)-3,13-octadecadien-1-ol acetate (peachtree borer type), as opposed to (E,Z)-3,13-ODDA (lesser peachtree borer type), are likely to capture moths of this group. Descriptions of these species follow in order to reduce confusion when using pheromone traps for the more likely culprits: Lilac borer is a paper wasp mimic. Adults have a brown-black thorax with red-brown laterally and posteriorly. Antennae are red with black on the dorsal side. The forewing is transparent only in the basal portion; the rest is dark brown and opaque. There is a white or yellow streak near the base of the abdomen. Trapping in Virginia has shown that lilac borer adults become active in mid-April, with peak captures in early to late May. Adult activity declines through mid June, although occasional moths may be captured in July. Oak borer is a large species with bright yellow markings, causing the moth to mimic a yellowjacket. Maple callus borer has as a distinguishing characteristic a dark spot about halfway along the leading edge of the front part of the hind wing.  Male peachtree borer has narrow yellow bands on segments three to five-six

Burr knots may be examined for the presence of protruding pupal skins. Research on degree-day accumulation has been variable to date, and average dates are still better predictors than thermal units.

No thresholds are currently available. Researchers in New York obtained good control with a single spray timed for either first or peak egg hatch. In that study, first hatch occurred 9 days after first pheromone trap catch. Peak egg hatch occurred on 2 August, 20 days after peak catch.

See the UConn and NCSU Factsheets. WVU has prepared a crop treatment profile for DB.
Evaluating dogwood borer infestation in top-worked apple trunks.

Notice:  FQPA concerns have caused apple label losses for Lorsban, a supplemental label for Lorsban 4E was approved for dogwood borer control in states east of the Rockies, at the rate of 1.5 qt/100 gal.  The following restrictions must be observed: application must be made by handgun, limited to lower 4 ft of trunk, do not allow to contact fruit or foliage, maximum of two applications separated by at least 14 days. The preharvest interval is 28 days.  However, chlorpyrifos will likely be discontinued for use at the end of 2016.

Additional Reading:
This is taken primarily from a chapter by D.G. Pfeiffer, L.A. Hull, D.J. Biddinger, & J.C. Killian on apple indirect pests, reprinted with permission from Mid-Atlantic Orchard Monitoring Guide, published by NRAES, 152 Riley-Robb Hall, Ithaca, New York 14853-5701.

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