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Scales (Indoor)

Ravageurs et maladies
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Mealybug.
Photo: Jardin botanique de Montréal (Pascale Maynard)

Onglets

Description

Summary

The scale insect superfamily (Coccoidea) includes several genera of sucking insects capable of damaging indoor plants. Most of them are tropical, non-hardy species, but they adapt very well to our homes. Depending on the species, mature scale insects may look like small rounded scales, small flat disks, miniature shells, cottony balls or tiny white insects covered in waxy filaments. Most scales remain stationary, attached to the underside of leaves or to stems and roots, where they suck the sap from their host. 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 cause plants to weaken and die. Finally, not all species are harmful: some species of scales are used to produce dyes, waxes and lacquers.

Signs and symptoms

  • If there are only a few scale insects early on, they may go unnoticed. They are extremely tiny and often remain immobile, hidden beneath a waxy shell or encased in their secretions. Sometimes they are the same colour as their host plants and lodge in the joints between the stems and leaves.
  • 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 attack mainly the tender, succulent parts of plants, feeding on the sap. They live in groups, attached on twigs, hidden under leaves, along leaf veins, at the base of stalks, and also on fruit and roots.
  • Several species attack one specific organ or host, but many of them are not particular. Some species are highly prolific, quickly forming dense colonies and a waxy crust on stems. Others may go undetected, forming fairly dense woolly colonies on the plant's roots.
  • Several species excrete a shiny, sticky substance (honeydew) that may cover the leaves and stems. This honeydew promotes the development of a black fungus known as sooty mould.
  • In small numbers, scale insects usually cause little damage, but large colonies eventually weaken the entire plant.
  • A heavy infestation can cause the leaves or needles to wilt, dry out and drop off; affected stems will appear misshapen, galls will form on them and they will dry out; infested fruit will remain small and drop prematurely. Young plants are more vulnerable than mature ones.
  • These insects may transmit viruses to plants.

Latin name (genus)

Aspidiotus, Coccus, Diaspis, Eriococcus, Lepidosaphes, Neolecanium, Nipaecoccus, Planococcus, Pseudococcus, Rhizococcus, Saissetia, Unaspis, Yceria, etc.

Host plants

Various indoor plants, including African violet, asparagus, avocado, bay, begonia, bougainvillea, bromeliad, cactus, cissus, citrus, coleus, crassula, croton, dieffenbachia, dracaena, eucalyptus, euphorbia, fig, fern, fuchsia, hibiscus, hoya, ivy, jasmine, Norfolk Island pine, oleander, orange, palm, philodendron, poinsettia and rhododendron.

Development cycle

Description and life cycle

Scale insects are members of the order Hemiptera, which also includes aphids. Most of the species in the order are exotic ones belonging to the following families: 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.

Many exotic scales do not hibernate. Some species may produce several generations a year, given suitable weather conditions. This means that you may see insects at all the different development stages (eggs, larvae and adults) at the same time.

The length of the life cycle depends primarily on temperature, while the eggs hatch faster when humidity levels are high. Warmer temperatures and higher humidity shorten the life cycle. Depending on the species, the life cycle may even extend over several months. Under optimal conditions, mealybugs complete their entire life cycle in about twenty days.

After the eggs hatch, the young, mobile larvae emerge from their mother's protective armour and move into new territory. This “crawler” stage lasts only about 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.

After mating, the males die and the females lay their eggs under their armour or in a waxy, white sac (ovisac) at the tip of their abdomens. The females in some species are able to reproduce without fertilization (parthenogenesis) or give birth to already-hatched larvae (viviparity).

The females in most species die after laying their eggs, but the eggs remain sheltered until they hatch.

Depending on the species and climate, these insects may produce 1 to 10 generations per year.

Prevention and control

Favourable conditions

High temperatures and humidity shorten the life cycle. The hotter and more humid it is, the shorter the life cycle. 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.

Identification

Regular visual inspection is the best way to detect a problem, especially on plants that have already been infested. A large quantity of honeydew and sooty mould indicates well-established colonies.

Prevention

  • Avoid buying infested plants: carefully check all young stems and the underside of leaves to avoid introducing these insect pests into your home.
  • 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.

Physical control

  • If possible, isolate infested plants. Scale insects spread easily through handling and pruning tools.
  • Start by pruning dead or seriously infested stems 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, while dead ones come away easily.
  • Leaves and stems: rub with a soft cloth or toothbrush and a solution of soapy water. Dab insects directly with a cotton ball soaked in rubbing alcohol to dry them out. Inspect plant regularly and repeat treatment as necessary.
  • Roots: remove the plant from its pot and clean the roots, removing scale colonies. Repot in sterilized potting medium and drench the soil with insecticidal soap.
  • In the event of a very heavy infestation, take cuttings (be sure to treat them) and destroy the mother plant. Dispose of all infested plant matter; do not compost it.

Biological control

In greenhouses, introducing natural predators and parasites (plant bugs, ladybird beetles, syrphid fly larvae, parasitic wasps) gives good results. This control method is difficult to apply at home, however.

Chemical control

As a last resort, use a low-impact pesticide with insecticidal soap as the active ingredient. Read the product label carefully and follow the manufacturer's recommendations.

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