The scale insect superfamily (Coccoidea) includes several genera of sucking insects capable of damaging fruit and ornamental trees and shrubs. Depending on the species, mature scale insects may look like small rounded scales, miniature shells or small flat disks covered in waxy secretions. Despite their unusual appearance, these insect pests may go unnoticed because they usually remain stationary on the leaves, stems or fruit of their host plants, where they suck the sap. A number of species excrete a sweet, sticky substance (honeydew) on which a black fungus known as sooty mould develops. In small numbers, scale insects cause little damage, but large colonies can kill entire branches.
Host plants :
Various species of trees and shrubs, including alder, apple, ash, beech, birch, boxwood, catalpa, cherry, chokeberry, cotoneaster, crabapple, currant, dogwood, elm, euonymus, forsythia, grape, hawthorn, honeylocust, honeysuckle, lilac, linden, maple, mulberry, oak, peach, pear, plum, poplar, rhododendron, rose, smoketree, walnut, weigela and willow.
A few conifers: white cedar, false cypress, juniper, larch, pine, spruce and yew.
Signs and symptoms :
- At first, if there are only a few scale insects early on, they may go unnoticed. They are extremely tiny and usually remain virtually immobile, hidden beneath a waxy shell or encased in their secretions. Sometimes they are the same colour as their host plants.
- The appearance of mature scale insects varies depending on the species. Some look like small grey, brown, dark brown or reddish rounded scales, miniature shells or flat disks; others resemble tiny white spots or small cottony balls; and yet others are small powdery white insects, similar to pill bugs, covered in long waxy filaments.
- These tiny parasites mainly attack the tender, succulent parts of plants, feeding on the sap. They live in groups, attached to the bark on twigs, branches and trunks, hidden under the leaves, along leaf veins, at the base of stalks, on conifer needles and scales and also on fruit.
- Some species are highly prolific, quickly forming colonies so dense that they can form a waxy crust covering all of the bark. Certain species attack only a particular host plant, while others are attracted to a variety of hosts.
- On conifers, you may see any of the following: egg-shaped (Fletcher scales, spruce bud scales) or pear-shaped (pine tortoise scales) brownish scaly pustules on branches and buds; small circular whitish armour on scales (juniper scale); or tiny elongated white spots on needles (pine needle scale), making the trees appear snow-covered even in summer.
- On deciduous trees, you may see any of the following: small white popcorn-like balls on branches (cottony scale); tiny greyish or reddish-brown oystershell-shaped (oystershell scale) or egg-shaped (lecanium scales) bumps on branches; small golden disks buried in the bark of the branches and trunk (golden oak scale); small round bumps with reddish edges on fruit (San José scale); small oval insects with ridged bodies edged with waxy white filaments, similar to pill bugs, on branches and trunks (Comstock scale).
- Several species excrete a sweet, sticky substance (honeydew) that may cover the leaves and branches. This honeydew attracts ants and wasps and promotes the development of a black fungus known as sooty mould.
- In small quantities, scale insects usually cause little damage, but when branches become encrusted with huge colonies, the constant pumping of the sap weakens the plant.
- A severe infestation can cause the leaves or needles to wilt, dry out and drop off; infested fruit will remain small and drop prematurely; knots and galls may form at the base of young shoots and obstruct the flow of sap, drying up and killing badly infested branches. Young trees are more susceptible than mature trees.
- These insects may transmit viruses to plants.
Description and life cycle :
Scale insects are members of the order Hemiptera, which also includes aphids. In our climate, the most common scale insects are Diaspididae (armoured or hard scales), with hard armour; Coccidae (soft scales), with a leathery, waxy coating, but no armour; and Pseudococcidae (mealybugs), covered in waxy whitish filaments.
These insects undergo incomplete metamorphosis (hemimetabolic insects), which means that the larvae resemble small adults and go through several moults before reaching their final size. Males and females are different.
Eggs: They are tiny. A female (oviparous) may lay 400 to 1,000 eggs, depending on the species.
Larvae: Resemble adults, only smaller (0.2 mm) and translucent. Newly hatched larvae (1st instar larvae or crawler stage) have six legs and a pair of antennae.
Males: Resemble tiny flies (2 mm long), with legs and usually a pair of wings, but no mouthparts. They live for only a day or two and are rarely seen. They appear at a precise time of year and usually account for only a small percentage (1-2%) of the population.
Females: Small (1-12 mm), usually with no eyes, antennae or wings (apterous) and often no legs (apodal). The head has a long flexible tube (stylet), used for sucking sap. The body is usually covered with protective armour or waxy secretions.
In our climate, scale insects overwinter on plants as eggs, larvae or adults, depending on the species. In spring, when the buds burst (from mid-May to mid-June), the overwintering population comes to life and the eggs hatch.
After the eggs hatch, the young, mobile larvae emerge from their mother’s protective armour and move into new territory. This “crawler” stage lasts 48 hours. During this short time, the young insects are more vulnerable because they do not yet have a scale covering. It takes about ten days for them to secrete their first armour or protective waxy coating.
Among sedentary species, the young female larvae lose their legs during the first moult. They then settle. They moult several times to allow their armour to expand as they grow to the adult stage. This stage lasts about two months.
In late summer, after mating, the males die and the females lay their eggs under their armour or in a sac (ovisac) at the tip of their abdomens. Egg laying can continue until late August. Females in most species die after laying their eggs, but the eggs remain sheltered under their shells until they hatch the following spring.
Depending on the species and climate, these insects may produce one to three generations per year.
Favourable conditions :
High temperatures and the absence of predators or parasites promote the formation and development of colonies. The young larvae, which are very light, may be dispersed by wind or moved by ants, birds and squirrels. Handling and direct contact with other plants can be a major factor in spreading these insects.
Drastic pruning and heavy feeding with high-nitrogen fertilizer promote the growth of vulnerable, tender shoots.
Regular visual inspection is the best way to detect a problem. In winter, inspect the branches of fruit and ornamental trees and shrubs, especially those that have already been infested. In summer, the presence of honeydew, sooty mould, ants or wasps are good clues. Ants protect the scale insects and carry the young larvae to new sites. Large amounts of honeydew point to a serious infestation that has gone undetected.
- Avoid buying infested trees or shrubs: carefully check all young branches and the underside of leaves to avoid introducing these insect pests into your garden.
- Inspect all plants regularly. Early detection will allow you to target control measures and prevent serious infestations that are more difficult to control.
- Avoid damaging the plant and do not overfertilize, so as not to promote the growth of young shoots that are inviting to sucking insects.
- Disinfect all pruning tools regularly with a 70% isopropyl (rubbing) alcohol solution.
What to do :
Physical Control :
- If possible, isolate infested plants. Scale insects spread easily through handling and pruning tools.
- Start by pruning dead or seriously infested branches to reduce the insect population. Be sure to disinfect all pruning tools regularly.
- Remove all visible scale insects. Dead or alive, they may be sheltering hundreds of eggs under their armour or in their ovisacs. Live scale insects are harder to remove from bark, while dead ones come away easily.
- Trunk and large branches: scrub with a soft brush and soapy water, taking care not to damage the bark.
- Leaves and young branches: rub with a soft cloth or toothbrush and soapy water. Dab insects directly with a cotton ball soaked in rubbing alcohol to dry them out.
- Repeat the treatment as necessary to ensure that you have eliminated the entire population. A few overlooked eggs or insects are enough to rebuild a new colony.
- In the event of a very severe infestation, take cuttings (be sure to treat them) and destroy the mother plant. Gather and dispose of all infested branches, leaves and fruit; do not compost them.