Excerpt from Publication 310, Integrated Pest Management
Table of Contents
The dogwood borer, Synanthedon scitula (Harris), apple bark borer, Synanthedon pyri (Harris), and recently introduced apple clearwing moth, Synathedon myopaeformis (Borkhausen), are all clearwing moths in the Sesiidae family. They commonly attack trunks of apple trees on size-controlled rootstocks where burr knots have formed. These borers generally cause a slow decline of tree health and reduce yields over several years of infestation.
The round-headed apple tree borer, Saperda candida (Fabr.), is a long-horned beetle in the Cerambycidae family. It attacks healthy young trees, boring into trunks and often causing tree death. Once considered a major pest of apples (prior to 1950), this pest is now rarely a problem in commercial apple orchards. Injury is occasionally noted in apple orchards in eastern Ontario. Hosts include a wide range of hardwoods including mountain ash, hawthorn, wild flowering crab apple, quince, plum, cherry and apple.
Clearwing moths are the most common borers seen in commercial apple orchards in Ontario.
Eggs are very difficult to see and are laid singly on the trunk of the tree. Larvae of all three species are dirty white with a reddish-brown head and thoracic shield (area behind the head) (Figure 4-38). Pupae are small, yellow-brown and sometimes observed as pupal cases partially protruding from the adult exit holes in the trunk or infested burr knots. Adult clearwing moths possess transparent wings with striking dark markings along the margins. They are daytime flyers and are often confused with wasps. There are numerous species of clearwing borer males which are attracted to and can be caught in commercial pheromone traps. Fortunately, they are relatively easy to distinguish from one another with little training.
Figure 4-38. Apple clearwing moth larva
Dogwood borers are the most prevalent borer found in commercial apple orchards. Adults are similar to peach tree and lesser peach tree borers but smaller in size. Adult males have a wingspan of 12-18 mm, and a black body with thin yellow abdominal bands and yellow legs (Figure 4-39). The tip of the abdomen appears as a rounded tuft. Female dogwood borers - sometimes observed when "calling" males - are similar in appearance, but have heavier yellow abdominal banding (Figure 4-40). Apple bark borer are very similar to dogwood borer, but have a wedge-shaped anal tuft and distinct orange mark on the wings.
Figure 4-39. Dogwood borer in pheromone trap
Figure 4-40. Female dogwood borer
Figure 4-41. Apple clearwing moth adult
Adult apple clearwing moths (both sexes) have a slender black body with a bright orange band across the abdomen (Figure 4-41). The wings are transparent with black margins and a wingspan of 20-25 mm.
Borers overwinter as larvae in hibernaculae within feeding galleries under the bark. Pupation occurs in the spring, and for all three species, emergence is often extended over several months.
Both dogwood borer and apple bark borer begin emerging in June, with flight continuing through August and early September. Peak flight typically occurs during July. Each species has one generation per year in Ontario. There is limited information on the life history of apple clearwing moth in North America. In Europe, development to adult may take one to two years depending on conditions. Data collected in Ontario (2007, 2008) indicate the moth has a single flight period, beginning in late May or early June, reaching a peak in mid to late July, and ending abruptly following peak activity. Apple clearwing moth was recently introduced and has very limited distribution in Ontario.
Moths are most active at dusk and dawn. Females lay eggs on the surface of burr knot formations, pruning cuts and wounded bark. Once hatched, tiny larvae bore into burr knots and create shallow, irregular, winding galleries. As they feed, reddish brown frass is pushed towards the entry hole where it collects, held together by silken threads. Clearwing moth species attacking apple have a relatively wide host range including apple, pear, oak, dogwood, plum, apricot, peach cherry, quince, hawthorn, willow, birch and other hardwoods.
All three species bore into burr knots or adventitious roots just below the graft union. Burr knots are the result of many partially developed initials forming just below the graft union on some dwarfing and semi-dwarfing rootstocks. Rootstocks particularly susceptible to burr knot formation - and attack by borers - are M.9, M.26 and Mark. Feeding begins in the outer area of the burr knot (dead tissue), then progresses into the cambium. Apple clearwing moth larvae tunnel under the bark anywhere on the tree where tissues have been previously damaged, including branches. Infestations are found below the graft union and under burr knots or around cankers above the graft union.
Clearwing borers cause a slow decline in tree health and reduced yields due to the girdling caused by larvae feeding in the cambium layer. Trees are increasingly susceptible to attack by other insects and may have reduced longevity. Sloughing-off of bark, crown die-back and general decline are symptoms of infestations in older trees.
The presence of the borers is indicated by reddish brown frass and pupal cases on the surface of burr knots (Figure 4-42). Cut away the outer layers of bark to reveal discolouration of the underlying cambium where larvae have fed.
Figure 4-42. Frass and pupal case, evidence of clearwing larva
Moths are observed resting on leaves during sunny days. To check for larval infestations, examine the bases of trees for 2-3 mm-wide holes and tunnels under the bark or in burr knots. Pupal cases and/or frass may be evident at the tunnel exits.
Place pheromone traps in orchards that had borer problems in previous years. There are several suppliers of pheromone lures for clearwing moths. Commercial lures are optimized to attract males of a given species, and often capture other clearwing moths, making identification critical for successful management.
Use pheromone traps to monitor for adult activity of these pests. Hang pheromone traps in the orchards before the first flight begins. For dogwood borer and apple bark borer, install traps mid June. For apple clearwing moth, install traps by mid May. Four traps per site, placed in low scaffold limbs, are recommended. Use a minimum distance of 50 m between traps, and monitor traps twice a week. Record the number of adults captured - graphing the results - to identify peak flight times. This information is used to time insecticide sprays.
The best way to reduce apple borers is to minimize burr knot development on trees. If possible, select rootstocks with a lower tendency for burr knot formation to reduce attractiveness to dogwood and apple bark borer. Some agricultural products such as naphthalene acetic acid (NAA) - used in thinning - increase the development of burr knots. Several cultural practices deter egg-laying activity of clearwing moths and round-headed apple tree borers.
If the graft union is close to the soil surface, and dogwood borers are a problem, mound the soil around the trunk to cover the graft union and reduce borer attack. Avoid mounding soil too high as this can result in scion rooting. Ensure the soil mound is wide enough to prevent freezing injury to the buried rootstock. 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 guards. Borers prefer shaded, protected habitat. Solid guards restrict air circulation and prevent bark hardening, predisposing the tree to winter injury and providing good borer egg laying sites.
Apply undiluted external white latex paint to the trunk area (by brushing, not spraying) to deter female moths from laying eggs. Paint must be reapplied annually. If applied thick enough, paint will also suffocates borers already in the wood.
Removal of adjacent wild hosts may be required where borers have become a problem near established apple orchards. Remove and burn trees that are severely weakened or killed by borer infestations to eliminate insects still present in those trees.
Another alternative is to insert a small length of wire into feeding holes to kill boring larvae inside. This process is very time consuming and must be repeated annually in blocks where the pest is a problem.
Natural enemies of apple borers include woodpeckers and nuthatches. Encourage visits from these birds by keeping weeds away from trunks.
Larvae feed in well-protected areas and are difficult to manage using insecticides. Pheromone traps may help time insecticide sprays used against clearwing moths. Refer to OMAFRA Publication 360, Guide to Fruit Production for information on recommended products, rates and timings.
The round-headed apple tree borer is generally considered a pest in neglected orchards or backyard apple trees.
Adult round-headed apple tree borers (Saperda candida) beetles are light olive brown with two conspicuous white stripes running the length of the body (Figure 4-43). Adult length is just over 2 cm, and antennae are 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 4-43. Round-headed apple tree borer adult
Hosts include a range of hardwoods including mountain ash, hawthorn, wild flowering crab apple, quince, plum, cherry and apple. Adult beetles are nocturnal and feed on leaves and occasionally fruit, but are not considered economically important. 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.
Unlike other Cerambycid beetles, the round-headed apple tree borer attacks healthy, living trees. Larvae (grubs) feed on the cambial tissue and heartwood near the base of the tree by boring into the branches and the trunk. Presence of larvae is indicated by sawdust castings pushed from the tunnel, accumulating on the ground beneath the tunnel entrance. 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.
Check for the presence of sawdust and signs of insect feeding in the heartwood. There are no established thresholds for round-headed tree borer.
No chemical controls are registered for the round-headed apple tree borer. Many cultural practices used to manage clearwing borers reduce infestations by round-headed apple tree borer.
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