Magnolia scales, Neolecanium cornuparvum, are amongst the largest scale insects to infest shade trees, with some individuals reaching ½” in length. A conspicuous pest in mid to late summer, they cause damage to trees by weakening and girdling branches while producing sticky ‘honeydew’ on leaves and targeting stressed trees.
Trees at Risk
Star Magnolia (Magnolia stellata), Saucer Magnolia (Magnolia soulangeana), Cucumbertree Magnolia (Magnolia acuminata), Lily Magnolia (Magnolia liliiflora). It has also been reported on Daphne spp., Virginia Creeper (Parthenocissus quinquefolia), Tulip Tree (Liriodendron tulipifera).
Signs of Damage
Sticky honeydew on leaves and targets underneath the infested tree
Dwarfed or stunted growth on infested twigs
Branch decline on severely infested twigs
Presence of large, oval-shaped dark brown insects on twigs and branches, often in high population numbers in late spring/early summer
Magnolia scale can be a challenging soft scale to control. Reports from arborists and university scientists have yielded inconsistent control from both crawler sprays and soil-applied systemic treatments. The magnolia scales non-synchronized (crawlers hatch and settle for a prolonged period of time in late summer/early fall) life cycle makes the timing of spray products operationally challenging.
Scale insects are a serious threat to plant health.
Scale insects are small, highly modified animals that have little resemblance to most insects. Because of their small size and often cryptic appearance, large numbers may be present without being noticed.
While a few scale insects are little cause for concern, large numbers may be harmful to the host plant. By closely watching your trees and shrubs, we can often catch a scale infestation in its early stages and take appropriate action.
How do scale insects damage my trees?
Scale insects injure plants as they feed. Long, threadlike mouthparts are inserted into the host plant and used to suck plant sap from the tissues. If large numbers are present, the insects can remove so many nutrients from the plant that it does not have enough left over to carry on its own metabolic activities.
Large amounts of honeydew, a sugary waste product, may also cover leaf and other surfaces located beneath a scale infestation. A fungus called “sooty mold” will use the honeydew for food and can cover leaves, sidewalks, patios or other surfaces, giving them a discolored appearance.
Life Cycle of Scale Insects
Soft scale insects overwinter as young nymphs attached to the twigs of the host plant. The nymphs complete development, and the females lay their eggs during late spring. The eggs begin to hatch later in the year than those of armored scale insects; usually during late June and July.
Newly emerged scale insects are called crawlers. They receive this name because unlike mature scale insects, they have the ability to crawl and move from place to place. The newly emerged crawlers are in the dispersal stage of scale insects. The crawlers move to different portions of the host plant, insert their mouthparts and begin to feed. Armored scale crawlers then begin to build the waxy coverings over their bodies. The covering enlarges with each molt.
The crawlers of most soft scale species leave the twigs where the eggs were located and move to the leaves to begin feeding. The crawlers feed on sap from the leaves all summer, but return to the twigs to overwinter. They leave the foliage before the leaves drop in the fall. If they do not, they will fall from the tree with the leaves and eventually die.
Signs & Symptoms
Severe scale insect infestations cause:
Premature leaf drop and branch dieback
Undersized and sometimes yellow-mottled leaves
Black leaves, bark, sidewalks and other surfaces from sooty mold growth