Borers in the Landscape

Table of Contents

1. Introduction
2. Bronze Birch Borer
3. Locust Borer
4. Lilac Borer
5. European Pine Shoot Moth
6. References
7. Related Links

Introduction

The larvae of some beetles, and moths are called borers because they bore into buds, shoots, bark or wood of trees and shrubs. By boring into the plants, borers interrupt vascular tissue within the plant often causing die back above the infested area, as well as swellings in the trunks and branches where feeding occurs. Borer damage is also noted by dark, discoloured, dead areas with sap and sawdust-like borings (frass) clinging to the bark or littering the ground. The presence of borers is often not evident until it is too late to save the tree. Trees weakened by borers may be subject to wind damage. Generally borers will only attack trees which are under stress and maintaining plant health will reduce the chance of a borer attack.

Knowing which borer is to be controlled and applying chemical sprays when larvae hatch from eggs before they have bored into the wood, will provide maximum and effective control.

It has been noted that there is a link between flowering times of trees and shrubs and insect activity. References to the timing of chemical sprays according to bloom of some ornamentals to effectively control insects have been cited from "Coincide" by D. Orton. For suggested chemical controls, order OMAFRA Publication 383, Production Recommendations for Nursery and Landscape Plants.

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A. Bronze Birch Borer, Agrilus anxius

Host:
Paper birch, Betula papyrifera; European birch, B. pendula and cutleaf forms.
Resistant species: Monarch birch, B. maximowiczianna; Asian white birch, B. platyphylla szechuanica, B. platyphylla

Life Cycle:
Two year life cycle. Immature larvae overwinter in wood. In the spring, larvae complete development and pupate in late April and early May. The adults, which are about 6-12 mm long and metallic green in colour, begin to emerge in the beginning of June for about six weeks. Emergence holes have a characteristic "D" shape. Larvae, described as "flathead borers", are white with
an enlarged segment behind the head and two spines on the tail. Adults do feed on foliage but damage is insignificant. Eggs are laid singly or in groups beneath bark and hatch in about two weeks. Newly emerged larvae begin to bore into cambium.

Damage:
Feeding of larvae causes swollen ridges to appear on bark surface where tunnelling has occurred and can girdle branches and stems (Figure 1). The first symptoms to appear are sparse foliage, chlorosis of foliage, and dying branches usually followed by death in the second summer. Often a dead or dying crown leader is the first indication of damage (Figure 2).

Control:
Chemical controls should be applied when Spiraea x vanhouttei finishes bloom and repeat three weeks later to stop feeding of young larvae. Systemics are often applied too late, after damage has already occurred. Pruning out infested branches in spring, before adult emergence may help in reducing damage. Woodpeckers and chalcid wasps provide some biological control.

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B. Locust Borer, Megacyllene robiniae

Host:
Black locust, Robina pseudoacacia, may also attack honeylocust, Gleditsia and willow, Salix.

Life Cycle:
One generation per year. After overwintering as an immature larvae under the bark, feeding resumes about the time buds begin to swell. Sap oozing from the bark surface in the spring indicates the resumption of feeding, which continues until late July (Figure 3). After pupating in the tunnels, adults emerge late July to the end of August. The adult is about 18 mm in length and a jet-black, long horned beetle with bright yellow bands. Eggs are laid near wounded bark. Mature larvae are round headed borers, white, about 25 mm long and cylindrical. The adult borer seems to prefer trees that are at least four years old. Once trunk diameter has exceeded 15 cm, it becomes less susceptible.

Damage:
The feeding tunnels of the grubs weakens wood and trunks become swollen in areas where feeding has occurred.

Control:
Destroying infested trees during winter and spring will control overwintering larvae. Where possible probing holes with a sharp wire may destroy larvae. When Hydrangea paniculata "Grandiflora" is white or when Solidago sp. goldenrod, is showing colour to early bloom, chemical controls should be applied to control hatching larvae before they bore into the trunk. A second application should be made when H. paniculata "Grandiflora" blooms are fading pink and Solidago sp. are blooming with some unopened blossoms.

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C. Lilac Borer, Podosesia syringae syringae

Host:
Lilac, Syringa sp.

Life Cycle:
Larvae are creamy white, with a light brown head and about 2.5 mm in length. After overwintering as immature larvae, pupation occurs in the spring, followed by adult emergence late May to early June.

The adult is a clear wing moth, similar to a wasp in appearance, with a chestnut red to brown-black body. After larvae hatch, they feed on sapwood and eventually bore into heartwood. Eggs are laid near the base up to 1 m on the trunk.

Damage:
Boring of larvae causes enlarged swollen areas, and scars on trunks and branches. During the late summer and dry periods, terminals wilt (Figure 4). Foliage is dwarf pale. Continued attacks can weaken branches.

Control:
Newly hatched larvae are most vulnerable as they start feeding. This occurs about the time Spiraea x vanhouttei are in full to late bloom and Syringa vulgaris are in late to finished bloom. Chemical treatments should be applied at this time and repeated four weeks later. Borers may also be killed by probing holes with wires. Some natural parasites exist.

Figure 4: Dieback caused by feeding damage of lilac borer larvae.

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D. European Pine Shoot Moth, Rhyacionia duoliana

Host:
Red pine, Pinus resinosa; mugo pine, P. mugo; Scotch pine, P. sylvestis; Austrian pine, P. nigra, Ponderosa pine, P. ponderosa.

Life Cycle:
One generation per year. Insects overwinter as immature larvae in a feeding tunnel in the buds. As temperature rises in spring, larvae move to undamaged shoots to resume feeding. One larva may feed on more than one shoot or bud. Larvae can be found feeding under tent-like webs covered in resin (Figure 4). Hooking of developing candles indicates feeding of larvae (Figure 5). During late spring and early summer, larvae pupate and adults emerge two to three weeks later in June and July. Adults are rusty orange red moths with several irregular silver crosslines on the forewing. Eggs are laid in groups of two to ten at the base of buds, needle fascicles, twig tips or on bark of new and old shoots. Eggs hatch in about 10 days. Immature larvae spin webs between needle sheaths and twigs and begin feeding. By midsummer they have moved to buds. Feeding stops by late summer and larvae bore out buds for overwintering shelters.

Damage:
Larvae feeding kills the tops of terminal and lateral branches. Damaged shoots are crooked, distorted and may have a witches' broom appearance. Spring feeding of larvae causes the most damage.

Control:
As overwintering larvae become active in the spring and before they have bored into undamaged shoots, chemical controls should be applied. This is about the time that Magnolia x soulangiana blossoms begin to show pink. Newly hatching larvae may be controlled in early summer when Hydrangea arborescens "Grandiflora" and Catalpa speciosa are bloom. Control may also be achieved by removing crooked shoots in May and destroying them.

Summary:

There are many different borers in the landscape attacking shoots, twigs, bark and trunks of a wide range of ornamentals. In planning a control program it is important to know the life cycle of the pest in order to maximize cultural and chemical control strategies.

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References

1. Dirr, M.A.1983. Manual of Woody Landscape Plants. Stipes Publishing Co., Champaign, ILL.
2. Johnson, W.T. and H.H. Lyon. 1976. Insects that Feed on Trees and Shrubs. Cornell University Press, Ithaca, N.Y.
3. Orton, D.A. 1989. Coincide: The Orton System of Pest Management. Plantsmen's Publications. Flossmoor, Il.
4. Rose, A.H. and O.H. Lindquist. 1984. Insects of Eastern Pines Canadian Forestry Service. Publication 1313.
5. Anonymous. 1982. Insects of Eastern Hardwood Trees. Canadian Forestry Service. Forestry Technical Report 29.

Related Links

• Publication 383, Production Recommendations for Nursery and Landscape Plants

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