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Magnolia scale

Ravageurs et maladies
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Magnolia scale females at maturity (swollen, with a pink hue)
Photo: Jardin botanique de Montréal (Pascale Maynard)
Neolecanium cornuparvum
  • Neolecanium cornuparvum
  • Neolecanium cornuparvum, overwintering nymph
  • Neolecanium cornuparvum
  • Neolecanium cornuparvum
  • Magnolia scale nymphs (Neolecanium cornuparvum) covered with a layer of whitish powder
  • Sooty mould fungus on leaves of Magnolia sp.
  • Neolecanium cornuparvum

Onglets

Description

Summary

Magnolias rarely have problems with diseases or pests, except for magnolia scale.

Magnolia scale is one of the largest scale insects in North America. Adult females can reach 12.5 mm. Infested branches appear to be covered with large tan or whitish lumps.

The insect often lives in dense colonies and is immobile for most of its life cycle. It feeds by sucking up magnolia sap. It excretes a sweet sticky substance called honeydew, which promotes the development of a sooty mould fungus on leaves and branches. Honeydew also attracts wasps, ants and other insects.

Infested magnolias are weakened and produce fewer flowers and leaves, which are also stunted. Repeated infestations can lead to branch dieback or even kill the tree.

Latin name (genus)

Neolecanium cornuparvum

Host plants

Magnolias, in particular cucumbertree magnolia (Magnolia acuminata), saucer magnolia (Magnolia x soulangiana), star magnolia (Magnolia stellata) and lily magnolia (Magnolia liliiflora).

Name of host plants

Development cycle

Description and life cycle

Magnolia scales overwinter as nymphs (immature instar) on one- or two-year old twigs. The dark grey or brown nymphs are oval, 1 to 2 mm long, with a waxy coating.

In spring, the nymphs become active again, start feeding again and moult.

Female nymphs gradually become larger and covered with a layer of whitish powder. Male nymphs are much smaller (about 35 mm), with flattish bodies that turn translucent white in early summer.  

Nymphs reach adulthood in early July (Montréal region). Mature females have oval, smooth and convex bodies. They can reach sizes of up to 12.5 mm. Males metamorphose into winged insects resembling tiny pink or yellow gnats. They mate with females and then die.

Females lay their eggs under their coverings. At this point they are very large and have often lost some of their whitish coat. They take on a pinkish colour before the nymphs emerge.

As the eggs hatch (mid-August to mid-September), the young nymphs leave the mother’s protective covering and move to new feeding sites. This “crawler” stage lasts about 48 hours. The nymphs then attach themselves to twigs where they will overwinter.

Newly hatched nymphs are a tan colour but turn darker once they start feeding. At this point, the young nymphs have no covering as yet. It takes about ten days before they secrete their first protective waxy coating.

There is only one generation of magnolia scales per year.

Prevention and control

Identification

  • Inspect your magnolia early in the spring, before the floral buds burst, and look for overwintering nymphs. They are about 1 to 2 mm long, dark grey or brown, and often clustered under twigs.
  • Between early August and late September, check your magnolia regularly. Pull off a few adult scale insects (females) and use a magnifying glass to see whether there are eggs or crawlers under their covering. At this stage the females are quite swollen and can measure up to 12.5 mm.

N.B.: Since the nymphs are tiny, they can be carried by other insects and birds. So you need to inspect your magnolia every year.

Prevention

  • Avoid buying infested trees: carefully inspect the tree first.
  • Inspect your magnolia regularly. Early detection will allow you to target control measures and prevent serious infestations that are more difficult to control.
  • Avoid drastic pruning and heavy feeding with high-nitrogen fertilizer, both of which promote excess growth of tender shoots attractive to scale insects.
  • Disinfect pruning tools between each cut with 70% isopropyl alcohol (rubbing alcohol).

Physical control

  • For highly localized infestations, you can cut off the most infested branches (ideally in the spring, or as soon as you notice the scale insects). Don’t cut branches more than 4 cm across, though, since the larger the cut surface the longer it takes for the wound to heal, and this increases the risk of infection.
  • If your magnolia is small, you can brush the infested trunk and branches with a soft brush (e.g. a toothbrush) dipped in soapy water. Make sure not to damage the bark. It is easier to remove the insects when they are swollen and pink coloured (adult females). If you cannot remove them with a soft brush, you can crush them before the egg-laying period.
  • On large magnolias, female scale insects can be dislodged with a heavy spray of water when they are swollen and pink. Be careful: adjust the strength of the spray so as not to damage the tree. It is best to test the spray on a branch first.

Chemical control

Horticultural oil

Early in the spring, before bud burst (during the dormancy period), you can use a low-impact pesticide with horticultural oil (“dormant oil”) as the active ingredient to eliminate nymphs that have overwintered on the tree. For the treatment to be effective, it is best to wait for daytime temperatures to reach 12 to 15°C for three to four days (at least two days, if the buds are about to burst). Applying the oil too early will not give satisfactory results. Spraying it too late, on the other hand (once the buds begin to burst or are already open), can damage flowers. Note that horticultural oil must not be applied if there is a risk of frost or rain within 24 hours.

Horticultural oil can also be sprayed in the fall (early October) if temperatures remain around 15°C for several days. Applying horticultural oil in the fall can sometimes damage foliage, but this generally has little impact late in the season. It is important to follow the directions on the label carefully.

Insecticidal soap

In late summer (about mid-August to late September), crawlers leave the mother’s protective covering. This is when a low-impact pesticide with insecticidal soap as the active ingredient can be used. The insects are much more vulnerable during this short crawler stage, before they have formed a protective waxy coating. You can spray insecticidal soap every seven to ten days, depending on the product instructions. Since crawlers emerge in successive waves, you will generally have to apply the pesticide several times.

Notes

If you can use only one treatment, it is best to use horticultural oil in the spring, when the plants are dormant.

Insecticidal soap and horticultural oil are contact insecticides. It is important to cover all surfaces of the tree, remembering the undersides of branches.

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