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How to identify it
- The female's cover is round, flat and semi-translucent, about 1/16 inch (1.5 to 2 mm) across, lemon yellow to yellow-brown because her yellow body shows through, with a clear spot of shed skins at or near the center.[1][2]
- Male covers are smaller and oval.[1][2]
- With the cover lifted, the female often appears kidney-shaped.[1][2]
- It settles mostly on leaves, often beside the midrib or another large vein, and on fruit; it occasionally infests stems but is rarely found on bark or mature wood.[1][2][3]
- Yellow scale and California red scale are hard to tell apart by eye; telling them apart for certain takes a microscope and slide-mounted females.[3][2]
Look-alikes
| Look-alike | How to tell them apart |
|---|---|
| California red scale | Its covers are similar in shape but reddish-orange rather than yellow, and it infests all above-ground parts including bark and wood, while yellow scale stays mostly on leaves and fruit.[2][3] |
| Oriental scale | Another Aonidiella recorded in Florida. Its adult female is only slightly kidney-shaped at maturity and has pores around the genital opening that yellow scale lacks, so a microscope is needed to separate them.[5][2] |
Where it lives
Of Asian origin, it has spread to tropical and subtropical regions around the world. EPPO lists it in California, Florida and Texas; in California it has become rare and UC IPM says it has not been seen there in recent years.[2][6][3]
Biology and life cycle
Diaspididae of the World describes yellow scale as giving birth to live young, with a female producing about 150 crawlers; UC IPM, describing California red scale, whose biology it calls similar, gives 100 to 150 crawlers. The crawlers wander for a few hours, up to about a day, then settle for good and begin secreting their armor.[2][3][1]
Crawlers walk perhaps a yard on their own, but wind, flying insects, birds and the movement of infested plants carry them much farther.[2][3]
Settled scales pierce the leaf surface with thread-like mouthparts and inject saliva as they feed. Females molt twice more and never move again; they release a pheromone that draws males, which mate through a flap at the edge of her armor.[1][2]
Males develop under their own smaller armor and emerge as tiny, gnat-like, two-winged insects with long antennae and no mouthparts; they live about 6 hours.[1]
There are three generations a year in Turkey, and one generation took 64 days in southern California.[2]
It feeds on plants in more than 50 genera and 32 families. Citrus is the main economic host, and records include tea, peach, camellia, dogwood, euonymus, ivy, privet, southern magnolia, palms, poplar, rose and viburnum.[2][1]
Damage and signs
- Heavy infestations cause leaf discoloration, wilting and early leaf drop, and the tips of twigs die back.[1][2]
- On citrus, attacked fruit becomes pitted, discolored and stunted and may drop early or be unmarketable. Because yellow scale does not live on the wood, its overall damage is less severe than that of California red scale.[2]
- Scales are small, pale and stationary, so infestations are hard to spot until numbers are large.[2]
Health risk
It is a plant pest; none of the sources mentions any risk to people or pets.[1][2][3]
Control
Sanitation
Inspect leaves and fruit with a hand lens for flat, circular, yellowish scales; moving infested plants spreads the scale over long distances.[2]
Biological control
Parasitic wasps often keep yellow scale in check; NC State notes at least four species attack it. Diaspididae of the World says biological control made it rare in California's San Joaquin Valley, where it was a major citrus pest in the 1950s, and UC IPM says it is easily controlled by parasites where biologically based IPM is practiced.[1][2][3]
Controlling ants is probably relevant to protecting these natural enemies, as it is for California red scale.[2]
Pesticides
If natural enemies fail, NC State suggests horticultural oil, which armored scales should be susceptible to, using a lower rate during the growing season than in the dormant season to avoid injuring plants. Water plants well before treating and spray early in the morning so tender growth does not scorch. Systemic insecticides are another option.[1]
UC IPM calls oil the most selective pesticide for armored scales: it kills only the natural enemies it contacts, and its residue does not persist.[3]
Where it lives
Of Asian origin, it has spread to tropical and subtropical regions around the world. EPPO lists it in California, Florida and Texas; ScaleNet notes it was present in California's San Gabriel Valley as early as 1872.[2][6][4]
UC IPM says that in recent years it has not been observed in California.[3]
Where sources disagree
Eggs or live young: NC State says yellow scales lay up to 150 eggs, while Diaspididae of the World describes the species as giving birth to live young, about 150 crawlers per female.[1][2]
Status in California: UC IPM describes California red scale and yellow scale as found throughout the state's citrus regions but adds that yellow scale has not been observed there in recent years; Diaspididae of the World says it became rare through biological control and is being replaced by California red scale, while EPPO still lists it as present in California.[3][2][6]
Common questions
- Why does yellow scale look yellow?
- Its cover is semi-translucent, so the female's yellow body shows through.[1][2]
- Is yellow scale the same as California red scale?
- No. They are close relatives with similar covers, but California red scale is reddish-orange and also lives on bark and wood, while yellow scale stays mostly on leaves and fruit.[2][3]
- Do I need to spray for yellow scale?
- Often not: parasitic wasps commonly keep it in check. Treat with horticultural oil only when natural enemies fail to keep the scales at a tolerable level.[1][3]
Sources
- NC State Extension, “Yellow Scale Insect (PDIC Factsheet).” content.ces.ncsu.edu/yellow-scale-insect (accessed 2026-11-11)
- Naturalis Biodiversity Center (Diaspididae of the World, with USDA scale insect data), “Diaspididae of the World 2.0: Aonidiella citrina.” diaspididae.linnaeus.naturalis.nl/linnaeus_ng/app/views/species/taxon.php?id=113044 (accessed 2026-11-11)
- University of California Statewide IPM Program, “California Red Scale and Yellow Scale (Citrus Pest Management Guidelines).” ipm.ucanr.edu/agriculture/citrus/california-red-scale-and-yellow-scale/ (accessed 2026-11-11)
- ScaleNet (scale insect database begun at the USDA ARS Systematic Entomology Laboratory), “ScaleNet catalogue: Aonidiella citrina (Coquillett).” scalenet.info/catalogue/Aonidiella%20citrina/ (accessed 2026-11-11)
- ScaleNet (scale insect database begun at the USDA ARS Systematic Entomology Laboratory), “ScaleNet catalogue: Aonidiella orientalis (Newstead).” scalenet.info/catalogue/Aonidiella%20orientalis/ (accessed 2026-11-11)
- European and Mediterranean Plant Protection Organization (EPPO Global Database), “Aonidiella citrina (AONDCI): world distribution.” gd.eppo.int/taxon/AONDCI/distribution (accessed 2026-11-11)
- iNaturalist, “Aonidiella citrina taxon page (order and family).” www.inaturalist.org/taxa/1599938 (accessed 2026-11-11)