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How to identify it
- Adults look like circular, elongate or oval bumps on bark, leaves or fruit, with no obvious head or legs; UC IPM puts the coverings at 1/25 to 1/4 inch long.[1][3]
- Armored scales are small and flat with a separate cover that can be pried off, leaving the insect body behind on the plant. UC IPM gives under 1/8 inch, Minnesota about 1/16 to 1/8 inch, and many match the bark so well they go unseen until damage shows.[1][2][3]
- Soft scales are larger, up to 1/4 inch, rounder and more humped. Their surface is the body wall itself, so flipping one removes the whole insect.[1][3]
- Newly hatched crawlers are tiny, flattened and usually yellow to orange, about the size of a printed period, and can be mistaken for mites or dust.[1][3][4]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| Whitefly nymphs and psyllids | These sap-feeders can resemble scales but have different biology and need different management.[1] |
| Diamond scale on palms | Not an insect at all: the blackish spots on palm leaves are fruiting bodies of a fungus.[1] |
| Mealybugs | Mealybugs are mobile, segmented and covered in white wax; UC IPM notes they are sometimes confused with cottony cushion scale and some soft scales, so look at the insect under the wax.[5][3] |
Where it lives
Scales attack trees, shrubs, other perennials and houseplants, and most species are highly specific to certain host plants. Armored scales are major economic pests and among the world's most invasive insects, and in cities such as Raleigh, North Carolina, scales build up more on street trees in the hottest spots.[1][6][7]
Biology and life cycle
Eggs are laid under the female's body or cover or in a cottony mass. They hatch into crawlers that walk over the plant, blow in the wind or ride on people, birds and other animals, then settle and start feeding within a day or two.[1][2][4]
Most armored scales have several generations a year and spend their whole lives feeding at one spot.[1]
Most soft scales have one generation a year. Crawlers move out to the leaves in summer, and the nymphs return to twigs and bark before leaf drop to spend the winter.[1][2][4]
Some soft scales are prolific: a cottony maple scale egg sac holds up to 1,000 eggs, and a cottony taxus scale female lays more than 1,000 before she dies.[4]
Heat speeds them up. On red maples in Raleigh, gloomy scale females laid about 14 more eggs for every 1°C rise in temperature, and warmer street-tree sites had over 70% more trees in poor condition than cooler ones.[8][7]
Damage and signs
- Heavy infestations make plants look water stressed: leaves yellow and drop early, and badly infested twigs and branches can die, sometimes keeping their brown leaves for a scorched look.[1][2]
- Soft scales excrete sticky honeydew that drips onto cars, walks and furniture and grows black sooty mold; armored scales feed on individual plant cells and don't produce honeydew.[1][2][4]
- Minnesota rates armored scales as typically more damaging than soft scales, and branch dieback is often the first sign: scales are easily overlooked until a twig is encrusted or dead.[2][4]
Health risk
Scales harm plants, not people. The nuisance is sticky honeydew, sooty mold and the ants it attracts, which annoy people even when the plant itself is fine.[1][4]
Control
Prevention
Keep plants well watered and suited to the site, inspect new plants for scales before you buy, and consider replacing plants that are infested year after year.[1][3]
Traps and physical control
Prune out heavily infested twigs and branches when the infestation is limited to a few parts of a small plant.[1]
Biological control
Parasitic wasps and predators such as lady beetles and lacewings often keep scales in check. Round exit holes in scale covers mean wasps are at work. Control honeydew-seeking ants, which protect scales from these enemies and can carry them to new plants.[1][2][3]
Pesticides
Horticultural oil or insecticidal soap aimed at crawlers controls most scales while sparing natural enemies; a dormant-season oil spray also works, but only on species that overwinter as crawlers or adults. Repeat sprays may be needed where crawlers hatch over a long period.[1][2]
Systemic insecticides differ: imidacloprid controls most soft scales but not most armored scales or cottony cushion scale, while dinotefuran reduces both soft and armored scales. Don't apply systemics to plants in bloom.[1][2]
UC IPM advises against persistent broad-spectrum sprays such as carbaryl, malathion and pyrethroids, which kill natural enemies and pollinators and can set off spider mite outbreaks.[1]
Timing sprays with tape traps
In spring, before crawlers emerge, wrap a few infested twigs with double-sided sticky tape and replace it weekly. Connecticut's station suggests black electrical tape wrapped sticky side out.[1][3][2]
Crawlers stick to the tape as yellow or orange specks; a hand lens helps tell them from pollen and dust. Shaking a branch over white paper also shows them.[1][2][3]
Unless another time is recommended for the species, spray after crawler numbers on the tapes have peaked and begun to decline.[1]
Where sources disagree
Overwintering: UC IPM says most armored scales overwinter as young nymphs and adult females, while Minnesota says armored scales overwinter as eggs under the mother's shell; Connecticut's station says it depends on the species. That matters because dormant oil works only on species that overwinter as nymphs or adults.[1][2][3]
Residual sprays: Minnesota lists pyrethroids and carbaryl among residual scale insecticides lasting one to two weeks, while UC IPM recommends against them in landscapes because they harm natural enemies and pollinators.[2][1]
Size: UC IPM puts armored scale covers under 1/8 inch and Minnesota at 1/16 to 1/8 inch; Connecticut's station gives 2 to 3 mm for armored scales and 5 to 10 mm for soft scales.[1][2][3]
Common questions
- Are the bumps on my branches alive?
- Flip a few with a thumbnail or squash them: live scales exude fluid, and scales with round exit holes have been killed by parasitic wasps.[1][3]
- Why is my car sticky under this tree?
- Soft scales and other sap-feeders excrete honeydew that drips onto anything below; check the leaves and twigs above for soft scales, aphids or mealybugs.[1][4][2]
- Do I need to spray for scale?
- Often not. Many scales are kept in check by natural enemies, and UC IPM says to check whether most scales are already dead or parasitized before treating.[1][2]
Sources
- University of California Statewide IPM Program, “Scales (Pest Notes, Home and Landscape).” ipm.ucanr.edu/home-and-landscape/scales/ (accessed 2026-11-11)
- University of Minnesota Extension, “Scale insects on Minnesota trees and shrubs.” extension.umn.edu/garden-and-home/yard-and-garden/yard-and-garden-insects/scale-insects (accessed 2026-11-11)
- The Connecticut Agricultural Experiment Station, “Scale Insect Pests of Connecticut Trees and Ornamentals (Smith, Cowles and Hiskes).” portal.ct.gov/-/media/caes/documents/publications/fact_sheets/entomology/scaleinsectpestsofconnecticuttreesandornamentalspdf.pdf (accessed 2026-11-11)
- Cornell Cooperative Extension, Nassau County, “Scale Insects (Home Grounds Fact Sheet).” ccenassau.org/resources/scale-insects (accessed 2026-11-11)
- University of California Statewide IPM Program, “Pest Notes: Mealybugs.” ipm.ucanr.edu/home-and-landscape/mealybugs/ (accessed 2026-11-11)
- Zootaxa, via PubMed, “Normark, Okusu, Morse and others (2019), Phylogeny and classification of armored scale insects (Hemiptera: Coccomorpha: Diaspididae).” pubmed.ncbi.nlm.nih.gov/31716328/ (accessed 2026-11-11)
- PLoS One, via PubMed, “Dale and Frank (2014), The effects of urban warming on herbivore abundance and street tree condition.” pubmed.ncbi.nlm.nih.gov/25054326/ (accessed 2026-11-11)
- Ecological Applications, via PubMed, “Dale and Frank (2014), Urban warming trumps natural enemy regulation of herbivorous pests.” pubmed.ncbi.nlm.nih.gov/29210225/ (accessed 2026-11-11)
- iNaturalist, “Coccus hesperidum taxon page (order and family).” www.inaturalist.org/taxa/330200 (accessed 2026-10-06)