Apple Borers

Table of Contents

1. Introduction
2. Dogwood and Apple Bark Borer
   • Description
   • Biology
   • Damage
3. Round-Headed Apple Tree Borer
   • Description
   • Biology
   • Damage
4. Management of Borers

Introduction

The dogwood borer, Synanthedon scitula, and the apple bark borer, Synanthedon pyri, are both clear-wing moths in the Sesiidae family. They commonly attack trunks of apple trees on size-controlled rootstocks where burr knots have formed. Generally, these borers cause a slow decline and reduced yields over several years of infestation.

The round-headed apple tree borer, Saperda candida, is a long-horned beetle in the Cerambycidae family. It attacks healthy young trees and bores into the trunks often causing tree death. Once considered a major pest of apples (prior to 1950), this pest is rarely a problem in commercial apple orchards today. Occasionally injury is noted in apple orchards in eastern Ontario.

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Dogwood and Apple Bark Borer

Description

Adults are black and yellow striped clear-wing moths that resemble paper wasps on first glance (Figure 1). The 1.5-2.0 cm wingspan makes it about half the size of a related species, the peach tree borer.

The larvae are small (12 mm at maturity) and pale pink with brown heads. They are not readily observed since they feed in burr knots and adventitious roots near the soil surface. The pupa is small and brown and is observed as a pupal case partially extended from the adults exit hole in the trunk.

Figure 1. Adult dogwood borer.

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Biology

Borers overwinter as full-grown larvae in hibernaculae within feeding galleries under the bark. Pupation occurs in the spring and adults begin emerging in June. Emergence is extended, continuing through July, and often early August, with peak flight occurring in early July.

Moths are most active at dusk and dawn. Females lay eggs on the surface of burr knot formations. Once hatched the tiny larvae bore into the burr knots and begin feeding (Figure 2). As they feed, reddish brown frass is pushed towards the entry hole where it collects, held together by silken threads.

The two species have a wide host range, at tacking oak, dogwood, plum, black cherry, apple, hawthorn, willow, birch and other hardwoods.

Figure 2. Larva of dogwood borer.

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Damage

Both species bore into burr knots or adventitious roots just below the graft union. Burr knots are the result of many partially developed initials that form just below the graft union on some dwarfing and semi-dwarfing rootstocks. Rootstocks particularly susceptible to burr knot formation are M.9, M.26, and Mark. Feeding begins in the outer area of the burr knot (dead tissue), then progresses into healthy park, and eventually the cambium. The presence of the borers is indicated by the presence of reddish brown frass (droppings) on the surface of bored burr knots (Figure 3). Cutting away the outer layers of bark reveals discolouration of the underlying cambium (Figure 4).

Figure 3. Frass at base of tree, indicating borer infestation.

Figure 4. Bark cut away to reveal borer larvae and tunneling.

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Round-Headed Apple Tree Borer

Description

Adult beetles are light olive brown with two conspicuous white stripes running the length of the body (Figure 5). Length is just over 2 cm with the antennae almost as long as the insect. Larvae are pale yellowish white, approximately 2.5 cm long, with a dark brown head and black mandibles.

Figure 5. Adult round-headed apple tree borer.

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Biology

Unlike other cerambycid beetles, the long-horned apple tree borer attacks healthy, living trees. Hosts include a wide range of hardwoods including mountain ash, hawthorn, wild flowering crab apple, quince, plum cherry and apple.

The adult beetles are nocturnal and feed on leaves and occasionally fruit, but these are not considered of economic importance. They are active from early May into September, with females depositing eggs under bark and in small cavities in the tree trunk near the ground. Peak egg laying activity takes place in June.

Hatching larvae bore into the tree and begin feeding on cambial tissue. As they grow they eventually attack the heartwood as well. A single larva can kill a young tree. The life cycle requires two to three years before the larvae pupate and emerge as adults.

Presence of larvae is indicated by sawdust castings that are pushed from the tunnel and accumulate on the ground beneath the tunnel entrance.

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Damage

Larvae (grubs) feed on the cambial tissue and heartwood near the base of the tree by boring into the branches and the trunk (Figure 6). Girdling of young trees eventually results in a greatly weakened tree, often leading to death. Young non-bearing blocks of apple trees are particularly susceptible.

Figure 6. Round-headed apple tree borer damage to twig.

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Management of Borers

Several cultural practices can deter egg laying activity of these borers.

• When practical, select rootstocks with a lower propensity for burr knot formation to reduce attractiveness to dogwood and apple bark borer.
• If the graft union is close to the soil surface and dogwood borers are a problem, mounding of the soil around the trunk to cover the graft union reduces borer attack. Note, that this practice can invigorate dwarfing trees by encouraging additional root development.
• Keep the area around the trunk weed-free and do not mulch around tree trunks. Shade and high humidity encourage burr knot formation.
• Use wire mesh mouse guards rather than solid ones (suck as spiral wraps). Borers prefer habitat that is shaded and protected. Solid guards restrict air circulation and prevent bark hardening. This can predispose the tree to winter injury and provides good borer egg laying sites.
• Applying undiluted external white latex paint to the trunk area (by brushing, not spraying) deters female moths from laying eggs. The paint must be re-applied on an annual basis. If applied thick enough the paint payer may also suffocate borers already in the wood.
• Removal of adjacent wild hosts may be required where borers have become a problem near established apple orchards.
• Trees that have been severely weakened or killed by borer infestations should be remover and burned to eliminate insects still present in those trees.

Dogwood borer and apple bark borer can be monitored using pheromone traps placed in orchards in early to mid-June. Four traps per site, placed in low scaffold limbs, are recommended. Traps should be monitored twice a week and numbers of mother tallied and graphed.

Currently there is only one chemical spray registered for control of dogwood borers (a combination of Ambush (pyrethrin) plus Superior oil applied one or two times to the trunk area beginning at peak pheromone trap catch. See the Pest Management Decision Guidelines in Chapter Five of Publication 310: Integrated Pest Management for Ontario Apple Orchards, for further details on rates and timings.

There are no chemical controls registered for the round-headed apple tree borer. In late summer, trunks can be examined for presence of small entry holes lined with sawdust and frass. A small length of wire can be inserted into these holes to kill the boring larvae inside. This process needs to be repeated annually in blocks where the pest is a problem.

Natural enemies of apple borers include woodpeckers and nuthatches. Keeping weeds away from trunks encourages visits from these birds.

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