Hippodamia convergens and others
Ladybugs- Kind
- Wild — attract it
- Best for
- Twicestabbed lady beetle eats scales; conserve, do not buy
Pest
San Jose scale
Diaspidiotus perniciosus
The short answer
An armoured scale insect about the size of a pinhead that crowds onto the bark, twigs and fruit of apple, pear, peach, plum, cherry and other woody plants. Each scale injects a toxin as it feeds, leaving a reddish-purple ring on fruit and young bark. Heavy infestations thin the foliage, crack and kill branches, and University of Kentucky warns they can kill limbs or whole trees in a few years. It makes no honeydew. For a backyard tree the proven treatment is horticultural (dormant) oil sprayed thoroughly in winter or at bud break while the scales are in their overwintering black-cap stage; native lady beetles and tiny parasitic wasps help, but do not prevent damage on their own.
hero macro · San Jose scales crowded on an apple twig
Black-cap and adult Diaspidiotus perniciosus on an apple twig. Photo: pending.
Identification
The mature female scale is round, about 1/10 inch (2.5 mm) across, and white to grey with a raised nipple-like bump in the centre; in large numbers they crowd together into a crust on the bark. The male scale covers are smaller and more elongated. San Jose scale is an armoured scale: the cover lifts off, leaving the soft yellow insect attached to the bark, and unlike soft scales it produces no honeydew or sooty mould.
The crawlers, the only mobile stage apart from the tiny winged males, are bright yellow and about 1/100 inch long. They wander until they find a place to settle, secrete a white waxy cap, and the cap turns black — the "black-cap" stage in which the scale overwinters and is most exposed to oil. On fruit, look for small red spots, each with a scale at the centre, that fade to pink as the fruit grows.
Easily confused with
- Parasitised San Jose scaleBeneficial — leave itTiny emergence holes in the covers mean parasitic wasps such as Aphytis melinus or Encarsia perniciosi are at work; those scales are already dead.
- Oystershell scaleAlso a pestPurplish grey and shaped like a tiny oyster shell about 1/8 inch (3 mm) long; San Jose scale is round, white to grey, with a raised spot in the centre.
Damage symptoms
San Jose scale injects a toxin as it feeds, which discolours the tissue around each scale. Heavy infestations weaken the tree and, over a few years, can kill limbs or the whole tree.
- Small red or purple rings on fruit, each round a tiny scale; most obvious on green varieties
- Purplish-red discoloration on young bark and twigs
- A grey crust of overlapping scales on bark and twigs
- Thin foliage, poor vigour and small, poorly coloured fruit
- Cracked, dying branches and, in the end, a dead tree
Lifecycle & degree days
The scales overwinter as black-cap nymphs on the bark and resume growth in late winter. Winged males emerge in spring and mate with the immobile females. Crawlers follow: in April with a peak in early May in California; in Kentucky about a month after the male flight starts, first appearing between mid-May and mid-June; in Washington from early June, sometimes into early August. In North Carolina crawlers are active for much of the period from May to the first frost. Degree-day spray timing differs by source: UC IPM and Kentucky use 600–700, Washington State 400–450.
- Thresholds
- Lower 51 °F / 10.6 °C, upper 90 °F / 32 °C
- Per generation
- About 1,050 degree days (°F)
- Crawler spray timing
- 600–700 °F degree days after the first males are caught (UC IPM, Kentucky); 400–450 in Washington
- Generations / yr
- 2 in Kentucky and Washington; 4 in North Carolina; 4–5 in California
- Overwinters
- Black-cap nymphs on bark
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Host crops
Washington State calls it most destructive on apple and pear, and a serious pest of sweet cherry, peach, prune and other tree fruits. NC State notes it attacks a wide range of woody plants.
Monitoring & action thresholds
Action threshold
Red-ringed fruit or crusted bark
Extension sources give no numeric home threshold. Red spots on last season's fruit, or scale visible on the bark in winter, is the practical signal to use a dormant oil spray. Commercial programmes time crawler sprays with pheromone traps and degree days.
In winter, look at twigs and bark for grey crusts of scale. In spring, wrap a few infested twigs tightly with double-sided transparent tape (or black electrical tape smeared with petroleum jelly) and change it weekly: crawlers stick to it as yellow or orange specks. UC IPM advises spraying after crawler numbers on the tape have peaked and begun to fall; Kentucky says about a week after the first crawlers appear.
Beneficial
Biological control
Native natural enemies attack San Jose scale, but Washington State notes that although they destroy many scales, they do not give enough control to prevent damage. UC IPM names the twicestabbed lady beetle (Chilocorus orbus) and a small beetle, Cybocephalus californicus, and several chalcid and aphelinid wasps; Washington State records Encarsia perniciosi and Aphytis. Broad-spectrum insecticides sprayed for other pests kill these enemies, and UC IPM says scale populations can build up as a result. UC IPM rates petroleum oil low in toxicity to predators and parasites.
Aphytis melinus
Red scale parasite- Rate
- 5–10 per infested plant or 1–2 per ft² indoors; 5,000–10,000 per acre outdoors, every 1–3 weeks
- Temp
- Release at 60–85 °F (16–29 °C); no development below 55 °F (13 °C)
- Best for
- Armoured scale parasitoid; supplement to dormant oil
Cultural controls
Dormant oil is the core of backyard control, and coverage matters as much as timing: the University of Kentucky says both are critical. The rates below are from a home horticultural-oil label (Bonide All Seasons, EPA Reg. 4-80).
- Spray horticultural (dormant) oil while the trees are dormant or at green tip to delayed dormant: 2.5–5 tbsp per gallon of water on apples, 2.5–7.5 tbsp per gallon on stone fruit, on the label; UC IPM uses a 2% solution in early January in California
- Wet the whole surface of the trunk, main limbs and branches, with special attention to the undersides of limbs
- Spray only when the temperature is between 40 °F and 90 °F and the tree is not drought-stressed, and never with or after sulphur
- The label warns the oil is toxic to fish: keep it out of drains and water, and keep people and pets out until the spray has dried. UC IPM advises against letting oil drift onto flowering plants where bees forage, so spray before bloom
- Avoid broad-spectrum insecticides in summer, which kill the lady beetles and parasitic wasps that keep scale in check
Chemical, last resort
Oil only kills what it wets — cover every twig, and never mix with sulphurCheck any product before it goes on. Open the compatibility matrix →
- Horticultural oil (dormant)REI 12 h
Rating: the effect on this pest's natural enemies if a spray hits them or they feed on treated plants. REI is the re-entry interval on farm and greenhouse labels; home labels usually give none, so follow the label.
Sources
UC IPM Pest Management Guidelines: Apple — San Jose Scale (Wunderlich, text 03/09)
UC IPM Pest Management Guidelines: Peach — San Jose Scale (Hasey et al., text 09/15)
UC IPM Pest Notes 7408, Scales (Kabashima & Dreistadt, 09/2014)
Univ. of Kentucky ENTFACT-204, San Jose Scale (Bessin, rev. 1/26)
NC State Extension, San Jose Scale (Walgenbach, 2015)
WSU Tree Fruit, San Jose Scale (Hoyt, 1993)
Bonide Horticultural and Dormant Spray Oil label, EPA Reg. 4-80 (EPA-stamped 2024)
UC IPM, Relative Toxicities of Insecticides Used in Peaches to Natural Enemies (09/15)
Page status
Drafted from the sources below, then checked claim by claim against those sources on 23 September 2026.
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