Hydrangea Wood Borers tunnel through the inside of stems, branches, trunks and larger roots, damaging the vascular system that is essential for the transportation of food and water for the plant. This causes wilting and yellow of leaves as well as branch and stem dieback. In small numbers they are easy to control simply by pruning back affected wood to healthy tissue. In large numbers however they can cause extensive damage and are more challenging to control.
About Wood Borers
There are thousands of species of wood bores around the world that damage, even kill, everything from mighty shade trees all the way down to the small ornamental shrubs in our home landscapes. Typically, it is trees and shrubs that are under stress that are the target. When under stress whether it be from drought, damage, soil compaction, sun scald, disease, insect feeding, being newly planted, or just getting old, they chemically give off a certain odour that the borers are attracted to. Healthy trees and shrubs, besides being less attractive to bores, seem to be able to also ward off their attacks. This is largely due to their internal sap flow which helps to flush the eggs out and kills larvae (Nixon P., (2013)). There are 2 types of borers that affect woody plants. One is clearwing borers the other is beetle bores.
Clearwing Bores are the caterpillar like larvae of clear wing moths. These moths have a wasp like appearance and as the name suggests, have clear wings. They are active during the day in the summer months, where they feed on nectar, mate and lay their eggs singly in weak or damaged wood, roots, pruning cuts, in cracks and crevices in the bark and branch forks. The larvae measure about 2.5 to 6 cm (1 to 1 1/2 in.) in length and are usually a creamy or pinkish colour with a dark head. Clearwing borers undergo complete metamorphosis with most species over wintering in the wood. New adults emerge the following spring, leaving behind an empty pupae casing that protrudes from the bark. When the eggs hatch the larvae borer through the weak wood to feed. Feeding damage appears as wilting of the terminal shoots, yellowing and wilting leaves, plant growth stunted, along with dieback. Cankers, calluses or cracking may also occur as well as secondary damage created by birds pecking at the wood to consume the larvae. One big difference between clearwing borers and beetle bores is that clearwing bores push their frass out of their tunnels, through openings such as cracks and holes. Whereas beetle bore larvae pack their frass in their tunnels.
Two species of clearwing moths that have been found to be a pest of hydrangeas are:
- Dogwood Borer (Synanthedon scitula) is known to have one of the broadest range of hosts including “Hydrangea paniculata, viburnum”, (Macunovich J. and Nikkila S., (April 13, 2011)), flowering dogwood, ninebark, apple trees, mountain-ash, willow, Washington hawthorn, little leaf linden, beech, birch, northern catalpa, pecan, hickory, oak, chestnut, black cherry, elm, loquat, bayberry and other woody ornamentals. Stressed trees and shrubs are preferred. They can be found in the eastern side of Canada and the United States. The adults are wasp looking moths that appear blackish in colour with a yellow band on the second and fourth abdominal segments. They measure about 1-2cm (1/2″) in length. The adult moths begin emerging from the wood and laying eggs around June with new adults continuing to emerge throughout the summer. The eggs are laid in pruning cuts, damaged bark, burr knots, and weak wood. They hatch in about 8-9 days and the larvae begin tunneling into the wood to feed. They will pass through 7 instar stages in all and can be found at different stages of development. Larvae overwinter in the wood and resume feeding in the spring when temperatures rise above 7 to 10°C (45 to 50°F) (Gyeltshen J. and Hodges A., (2006)). Their life cycle takes between 1-2 years to complete, depending on local but in very warm regions it may be shorter.
- Peach Tree Borer (Synanthedon exitiosa) hosts include, oakleaf hydrangea, southern magnolia, little leaf linden (Potter D.A. and Michael F. Potter M.F., (n.d.)), all stone fruits such as peach, plum, prune, flowering cherry, apricot, purpleleaf sand cherry and cherrylaurel and many ornamental shrubs. Unlike other borer species, peachtree borers prefer healthy trees and shrubs (Frank S., (2013 Revised: Sept. 20, 2019)). They can be found throughout Canada and the United States, especially east of the Rocky Mountains. The adults are wasp looking moths, with males and females appearing slightly different. The females are slightly larger, dark metallic blue in colour with a broad yellow/orange band on the fourth abdominal segment; only their hind wings are clear, wingspan is approximately 3.5 cm. (1 3/8″). The males are more slender, a lighter metallic blue, and may have several very narrow, yellowish bands on their abdomen; both forewings and the hind wings are clear and span about 2.25 cm. (1″). The larvae are a creamy colour with dark heads and measure up to about 3.5cm (1 3/8″). Their life cycle varies according to local with colder regions like Canada, taking 2 years to complete their life cycle. In the United States they typically complete their life cycle in 1 year with warmer regions having 2 generations per year. Peachtree borers overwinter as larvae in the wood just below the soil line. They pass through 7 instar stages in all before pupating, which also takes place in the wood just below soil level. After about 20 days the adults emerge and begin mating and egg laying. Over half of the eggs will be laid on that first day with the remainder being laid over the next few days, before they die. The eggs are laid on the soil near the tree, in leaf litter, or in the lower bark of trees up to about 15 cm (6″). The moths are in flight from about late June to September. The eggs hatch in about 7 days and the larvae begin tunneling through the bark to feed.
Beetle bores are broken down into Roundheaded beetle borers and Flatheaded beetle borers.
Roundheaded beetle borers are often called long horned beetles because of their usually very long antennae, which can be as long or longer than their bodies. They are a large group of beetles with 1000’s of species. The adult beetles’ range in size from 1-8 cm (3/8 – to just over 3″). Their colour varies according to species, but are often brown, rust or black with some nondescript markings. They tend to camouflage well into their environment (although there are a few brightly coloured species). The beetles are nocturnal (active at night at night) and feed on leaves, stems, flowers, seeds, nectar and sometime fruit. Females cut notches in the bark to lay their eggs or lay them in bark crevices or other protected areas University of Alaska Fairbanks (n.d.). The larvae are creamy coloured grubs with a dark head and large mandibles (eating mouth parts). Their heads are rounded unlike the flatheaded borers, which helps to identify them. Many of these species prefer to feed on dead or dying wood and can often be found in firewood. Unlike the clearwing bores these larvae do not push their frass out of openings but rather pack it in their feeding galleries. Their life cycle is 1-3 years depending on species and climate. When scouting for this pest look for small round exit holes.
Flatheaded beetle borers are often called short-horned beetles due to their short antennae. The colouring of these beetles tends to be more metallic and they tend to be smaller than the longhorned beetles, ranging in size from just under 1cm to 2.5cm (1/3 to 1-inch long) (The Morton Arboretum, (n.d)). They also differ from long-horned beetles in that they are active during the day, loving to bask in the sun, make D-shaped exit holes and do not chew notches in the wood to lay their eggs. The larvae also differ in that they have a wide flattened thorax (the area behind the head) giving it the appearance of having a large flat head.
One beetle borer specie that has been found to be a pest of hydrangeas is:
Flatheaded apple tree borer (Chrysobothris femorata) which is actually an umbrella group consisting of 12 recognized species in all (Hansen J.A., Moulton J.K., Klingeman W.E., Oliver J.B., Windham M.T., Trigiano R.N., Reding M.E., (2015)). They have a broad range of hosts including oakleaf hydrangea, flowering crab apple, hawthorns, red maple (Potter D.A. and Michael F. Potter M.F., (n.d.)), American chestnut, apple, ash, basswood (aka linden), beech, black walnut, boxelder, butternut, common quince, downy serviceberry, elm, hackberry, hickory, hornbeam, mountain ash, oak, pear, quaking aspen, redbud, silver maple, sweetgum, sycamore, tulip tree poplar and willow (University of Massachusetts Amherst, (n.d.)); persimmon, peach, apricot, plum, currant, walnut, poplar, chestnut, serviceberry, horsechestnut (Bugwoodwiki, (last modified 19:29, 29 January 2010)) and others.
This specie is not to be confused with the Roundheaded apple tree borer Saperda candida who tends to prefer young healthy trees and occurs more in eastern North America. The flatheaded apple tree borers prefer stressed trees and shrubs, especially new transplants. They occur throughout Canada and the United States.
The adult beetles grow up to about 1.25cm in length (just over 1/2″), with a broad flat head, short antennae and flattened tear drop shaped body. Their wings are somewhat metallic and bronze in colour with pitted dull gray spots and discreet wavy bands. The top of their back is a metallic greenish blue colour, while the underside is more metallic bronze in colour. Beginning in late spring, the adult beetles begin chewing their way through the bark, producing D-shaped emergence holes that measure about 5mm (3/16″) (Bowers H. and Fulcher A., (2011)). They begin mating and the females begin ovipositing their eggs singly into bark crevices and weak and injured wood, often on the sunny side of the tree. The adult’s average life span is about 30 days. The disk-like eggs hatch in about 8 to 16 days (Bugwoodwiki, (last modified 19:29, 29 January 2010)) and the new larvae then chew their way through the back of the egg into the wood just behind the bark (the cambium) and begin feeding. As they feed and continue to chew their way through the wood, they pack their frass in the outer walls of their galleries. These galleries are either long, winding tunnels just below the bark surface or broad and circular. These feeding galleries may be visible on the outside of the tree appearing as winding ridges or cankers on the trunk or branches. Other signs of their presence may include cracking, sap oozing from cracks, sawdust, bark loss in the injured area, and as the tree continues to live and grow a callus roll may form around the injured area. The grubs themselves are creamy in colour, with a broad flatten thorax that tapers towards the abdomen and large black mandibles. When full grown they measure up to 2.5cm (1″). The larvae coexist at different stages of development. They overwinter in the tree by burrowing deep into the heartwood. Once full grown (which can take 1-3 years, depending on climate) they pupate in the heartwood and emerge as adults from spring through fall.
Visit EPPO Global data Bases website to view photos of flatheaded appletree borer damage.
Management and Control of Hydrangea Wood Borers
- Only buy healthy plants.
- When planting hydrangea locate them preferably in part sun to avoid plant stress.
- Keep your hydrangea well-watered and fertilized, healthy shrubs are typically less attractive to borers.
- Avoid injuries to the bark. Keep trimmers and lawn mowers well back from the base of shrubs.
- Avoid soil compaction in the area where hydrangeas grow.
- Ensure that the soil around hydrangeas drains freely, with no standing water.
- Prune out weak, diseased, broken, rubbing, and infected wood, during dormant season (March is usually a good month).
- Avoid tillage near the root zone.
- Regularly examine hydrangeas for signs of insect or disease damage and act quickly to reduce the stress to the shrubs.
Scouting: Try to identify as best you can the exact species of borer present. This will best equip you to deal with the pest. Learn what pests are common in your area. To assist you with this identification adhesive-coated purple prism traps or purple-painted multifunnel traps can be set out in the spring to catch the adult borer beetle for identification. Pheromone traps can also be used but you need to know the species of borer you are monitoring for.
Look for any of the following signs of borer activity.
- Drooping and yellowing leaves.
- Drooping terminal tips.
- Hollowed out branches and dead wood.
- Stunted growth.
- The presence of adult beetle borers or clearwing moths.
- Emergence holes either round, oval or D-shaped.
- Saw dust on the ground near shrubs or coming out exit holes.
- Frass coming from cracks and openings in the wood.
- Sap oozing from cracks and openings in the wood, often this sap will be mix with some sawdust.
- The presence of tunneling galleries under the bark.
- Cracked bark, cankers swollen areas or raised ridges of bark.
- Pupal cases hanging from side of bark.
- Birds peaking away at the wood.
Natural borer enemies:
- Braconid wasps parasitize clearwing moths and their larvae.
- Birds such as woodpeckers and red-breasted nuthatch.
Other control methods:
- Prune out borer infected and damaged wood back to healthy wood.
- Hand remove borers by fishing a hooked wire down the pin sized holes where the larvae first entered the wood. These holes are difficult to find, look for sap or sawdust to help locate them. you may need to slightly widen the hole with a knife in order for your wire to fit. Fish the wire all the way down to the end of the tunnel where the larvae will be trapped.
- The nematode specie Steinernema carpocapsae can be used for controlling wood borers (National Collaborating Centre for Environmental Health, (2012)). The nematodes are usually injected into the holes leading to their galleries.
- Horticultural oil can be sprayed on egg-laying sites.
- Hydrangeas with trunks or thick woody bases can be wrapped low down, to help prevent egg laying.
Look-a-like Insect Damage
Small carpenter bees (Ceratina sp.) can build their nest in woody stems, especially pithy stems like roses and the broken stems of weeds. These small insects’ look more like flying ants than bees. They range in size from about 6-9mm (1/4-3/8″) are shiny black in colour with bluish green, or blue overtones and sometimes some yellowish markings. They enter a newly pruned or broken stem or branch and borer downwards hollowing it out to make nest. The nest ranges from 1 ¼ to 12 inches deep (Baker J., (Jan. 8, 2017, Revised: Oct. 14, 2019)). It consists of a series of sealed off chambers, one on top of another, each housing one egg and enough food to last until adulthood. Secondary damage can come from birds who peck at the stem to access the insects. Unlike the wood borer larvae, small carpenter bee’s larvae do not cause feeding damage to the plant. The adults feed on pollen and nectar from the garden.
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