Contents: By Damage and Image
When Things Are Boring
The weeping trees await their destiny, an ignoble burial in the burn pile. Their fate in the pyre driven by their holy wounds, the tell tale signs of borers. Boring beetles and moths can drill a hole into many types of trees and wallets in nursery production. Given a greater understanding of the culprit species and their biology, growers in the Pacific Northwest will have more success in managing these borers. Research conducted in Willamette Valley nurseries by Rosetta, Altland, Cramer, Doane, and Elliott has given us useful information about activity of key borers in regional nursery systems.
Identification of the most common species attacking field and container-grown shade trees has been determined by using a combination of trapping and examination of infested trees. We have seen some strong trends concerning key borer species in nurseries. Perhaps the easiest way to categorize borers affecting shade trees in Oregon is by their general type. In our experience, ambrosia beetles, flat-headed borers, and clearwing moths cause the greatest number of losses in shade trees.
There are two species of ambrosia beetles that we most commonly find in our traps and trees in nurseries: Xyleborus dispar; and Xyleborinus saxeseni. The predominant damaging beetle is the European shothole borer, Xyleborus dispar, sometimes called the pear blight beetle. When autopsies are performed on infested bolts of shade trees, this is the villain most frequently found at the scene. Identification is made easier by the overwintering habit of the adult beetles, the tiny black beetle’s rear end projecting from the hole. This species can damage a wide variety of host trees, seemingly preferring the most expensive varieties grown. Hosts include: Acer, Aesculus, Alnus, Betula, Castanea, Celtis, Crataegus, Corylus, Cydonia, Fagus, Fraxinus, Juglans, Liriodendron, Magnolia, Malus, Platanus, Populus, Prunus, Punica, Pyrus, Quercus, Salix, Ulmus, and Vitis (Bhagwandin, 1992; Solomon, 1995).
After wintering inside a suitable host, the female X. dispar beetle takes advantage of the first warm day in late winter or spring to escape to a new adventure in another host, preferably a tree exhibiting signs of stress. The much smaller male is flightless, destined to be homebound in the tree in which it was born. The female’s initial attraction is to ethanol emitted by stressed trees. Once the female shothole borer finds a good host, she sends out invitations to her friends in the form of an aggregation pheromone. This volatile chemical is responsible for secondary attacks on trees, with hordes of beetles gathering at the housewarming party. Thus shothole borers and their social habits can leave trees riddled with entrance holes. It is thought that X. dispar has two flights per year in Oregon, in early spring (our trap catches show a peak in late March/early April) with activity into May and June.
The lesser shothole borer, sometimes known as the pinhole borer, X. saxeseni, seems to have an affinity for baited funnel traps. It has a well-earned reputation for showing up in large numbers in traps but has also shown up in nursery trees as well, though much less commonly as X. dispar. It also has a wide host range and can be found from Acer, Albizia, Arbutus, Betula, Carya, Cedrus, Celtis, Cornus, Diospyros, Fagus, Gleditsia, Ilex, Juglans, Liquidambar, Populus, Prunus, Quercus, Taxodium, and Tsuga (Solomon, 1995). There may be three flights of the lesser shothole borer in Oregon. Our traps captured adults as early as March but with distinct peaks in adult catches in April and June with minor activity in July and August.
The oak ambrosia beetle, Monarthrum scutellare , was the third most commonly captured shothole borer in our funnel traps. We have had few reports of damage associated with this beetle in nursery stock, however.
Another shothole borer found now found in Oregon is the black stem borer, Xylosandrus germanus.
The adult borers are dark reddish-brown, bullet-shaped beetles with copper colored spots on their wings (elytra). More often seen are the legless larvae with their large flattened “head”. Their real head is actually quite small. It is the large flat, slightly square prothorax next to the head that is most noticeable. The adult beetles are reported to fly in Western Oregon from May through August. They lay their eggs in bark crevices. The eggs hatch (eclose) in two to three weeks and the larvae bore directly through the bark and begin feeding in the phloem tissue of the tree. By early fall the larvae begin to move inward into the sapwood to form the pupal chambers in which they will overwinter as prepupa. They remain as larvae within the trees for one to two years depending on the elevation and or location in colder climates. In Western Oregon they are reported to be in the pupal stage from mid-March through mid-June.
For those growing conifers, the flatheaded cedar borer, Chrysobothris nixa, can be problematic. Host plants include Calocedrus decurrens, Cupressus macrocarpa, Cupressus forbesii, Cupressus nevadensis, Juniperus occidentalis, and Thuja plicata.
Although native to eastern Oregon, bronze birch borer, Agrilus anxius, has been detected in several urban locations including the Portland metropolitan area, Albany, Corvallis, and Eugene. Its spread to more areas in western Oregon is likely. These flatheaded borers are more slender than their cousins and tend to tunnel higher on the trunk. A. anxius feeds and girdles phloem tissue, causing dieback in small crowns of the upper canopy. Foliage begins to yellow in mid-summer, progressing to brown dead leaves. Raised, uneven cracking bark, dark staining and emergence hole on the trunk are also indications of an infestation. These borers are particular about their hosts, remaining in birches but species preference is noticeable. The extent of this pest’s impact in nursery production is yet to be determined.
While many shade tree growers are aware of this borer, we also have been seeing damage in English laurel, Prunus laurocerasus: ‘Otto Luyken’ and ‘Schipkaensis’. Damage is harder to see in shrubs as it can be covered with the lower foliage.
Those growing conifers may have experience with another clearwing borer, the sequoia pitch moth, Synanthedon sequoiae. Despite the name, we tend to see it more commonly in pine trees in nurseries in Oregon.
These are some of the most prominent borer species currently experienced in Pacific Northwest nursery production but that situation may change with the potential introduction of new exotic borer species. Hopefully armed with a little extra knowledge we’ll keep boring to a minimum.
Bhagwandin, H.O. 1992. The Shothole Borer: An Ambrosia Beetle of Concern for Chestnut Orcharding in the Pacific Northwest. Chestnut Growers of America
Schuh, J. and D.C. Mote. 1948. Insect Pests of Nursery and Ornamental Trees and Shrubs in Oregon. Agricultural Experiment Station, Oregon State College, Corvallis. State Bulletin 449. 163 pp.
Solomon, J.D. 1995. Guide to insect borers of North American broadleaf trees and shrubs. Agric. Handbk 706. Washington, DC: U.S. Department of Agriculture, Forest Service. 735 p.