Horticultural oil: what it kills and what it harms
Horticultural oil is a highly refined oil, nearly always petroleum, that is mixed with water and sprayed. It smothers the scale, mites, aphids, whiteflies and overwintering eggs it coats, then evaporates, so it leaves little behind for beneficials. 'Dormant' and 'summer' oil describe when it is used, not a different oil. The same product goes on stronger on bare winter branches and weaker on leaves. Keep it between 40°F and 90°F, and never use it with sulfur.
Also sold or listed as: paraffinic oil, summer oil, dormant oil, narrow-range oil, superior oil, mineral oil.
At a glance
- Active
- Refined mineral or plant oil
- Residue
- Short: hours to days (UC IPM). Oils dissipate by evaporation and leave little residue (Colorado State).
- Caution
- 4 of 6 beneficials
- Safe
- 2 of 6 beneficials
What it works on
- Scale insectsThe classic dormant-oil target. Oil smothers overwintering scale on bare branches, and the crawlers in summer. See also euonymus scale and brown soft scale. In an NC State trial, scale readily reinfested oil-treated plants, while systemic products kept it off for 133 days (Frank 2012).
- Overwintering aphid and mite eggsDormant sprays on fruit and shade trees, applied from dormancy through green tip to delayed dormant, as the label directs.
- Spider mitesA summer use. On greenhouse roses, 0.5 % petroleum oil every two weeks prevented infestations, but on roses already infested it only held mite numbers steady (Nicetic et al. 2001).
- WhitefliesOn the label as a summer spray. Colorado State lists whiteflies among the pests oils control during the growing season.
- MealybugsOn the label for trees, shrubs and fruit.
- Psyllids and adelgidsOn the label: adult pear psylla, immature psyllids, and adelgids, including hemlock woolly adelgid.
- Powdery mildewOn the label, at the lower summer rates.
Effect on beneficials
- Phytoseiulus persimilisOn greenhouse roses, P. persimilis numbers did not change after two petroleum oil sprays 7 days apart (Nicetic et al. 2001). But a sublethal oil deposit cut egg-laying when prey was scarce (Xue et al. 2009), and UC IPM rates predatory mites moderate. UC moderate → caution. — Nicetic et al. 2001, Exp Appl Acarol 25:37; Xue et al. 2009; UC IPM floriculture relative toxicities table (2022)Caution — moderate, 25–75 %
- Amblyseius cucumerisWe found no trial on A. cucumeris. UC IPM rates predatory mites moderate for narrow-range oil, based on western predatory mite. UC moderate → caution. — UC IPM floriculture relative toxicities table (2022)Caution — moderate, 25–75 %
- Chrysoperla carneaWe found no C. carnea trial. UC IPM rates general predators moderate for narrow-range oil. UC moderate → caution. — UC IPM floriculture relative toxicities table (2022)Caution — moderate, 25–75 %
- Orius insidiosusIn lab tests at the high vegetable label rate and at a tenth of it, by contact, ingestion and residue, horticultural oil had some lethal and sublethal effects on Orius eggs, nymphs and adults. UC IPM rates general predators moderate (UC moderate → caution). — Gaire & Amarasekare 2025, J Econ Entomol 118:2195; UC IPM floriculture relative toxicities table (2022)Caution — moderate, 25–75 %
- Aphidius colemaniMineral oil was rated harmless to A. colemani in side-effect tests of citrus acaricides (harmless → safe). UC IPM also rates parasites low for narrow-range oil (UC low → safe). A direct hit on a wasp can still kill it. — Urbaneja et al. 2008, Pest Manag Sci 64:834; UC IPM floriculture relative toxicities table (2022)Safe — low toxicity, under 25 % effect
- Encarsia formosaResidues from poinsettia cuttings dipped in 0.1 % mineral oil had little effect on Encarsia survival. UC IPM rates parasites low. A direct spray was not tested. — Buitenhuis et al. 2016, Insects 7:48; UC IPM floriculture relative toxicities table (2022)Safe — low toxicity, under 25 % effect
No data never means safe: treat an unrated pairing as caution and try it on a small area first. See every product side by side →
How it works
Oils mainly block the spiracles, the pores insects breathe through, so they suffocate. They may also act as poisons and disrupt feeding, which is why oils can slow the spread of some aphid-borne viruses (Colorado State).
Horticultural oils are refined to remove the compounds that injure plants. 'Superior', 'narrow-range' and 'supreme' are grades of that refining. Old dormant oils were heavier and unsafe on leaves, but modern oils are light enough for both seasons. 'Dormant' and 'summer' oil now describe the timing, not the product (Colorado State).
It works only on what it coats, and it evaporates quickly. UC IPM names oils among the products less toxic to natural enemies, and its natural-enemy table marks oil as directly toxic but with no residual toxicity. It will kill a beneficial it wets, but not one that arrives after the spray has dried.
Using it
- Dormant rate
- On bare branches, before buds show green. The home-use (mix & apply) directions give 2.5–7.5 tbsp per gallon of water for fruit trees, and 5–7.5 tbsp for roses, shrubs and shade trees. The label's concentrate directions give 2–5.4 fl oz per gallon for roses and shrubs and 2.5–4 fl oz for shade trees (Bonide Horticultural & Dormant Spray Oil, EPA Reg. 4-80; 2 tbsp = 1 fl oz). Wet all bark, especially the undersides of limbs.
- Summer rate
- On leaves: 1–2.5 fl oz (2–5 tbsp) per gallon on ornamentals, and 2–4 tbsp per gallon on tomatoes, once or twice a week. The concentrate directions add: do not spray ornamentals while they are in flower, and do not spray trees and shrubs more than once a week. Summer rates are well below dormant ones.
- Temperature
- The label says do not apply above 90°F (32°C) or below 40°F (4°C). Colorado State gives wider limits of about 100°F and freezing, and ties the heat risk mainly to drought stress. UC IPM says not above 90°F. Follow the label.
- Autumn
- Wait until trees have hardened off for winter, because autumn sprays can make them more prone to winter injury (Colorado State). The label cuts autumn dormant sprays to 5 tbsp per gallon and limits them to listed trees.
- Re-entry and harvest
- The agricultural box on the label gives a 12-hour restricted-entry interval. It sets no general pre-harvest interval, but some crop rows carry limits: table grapes, for example, must not be sprayed within 60 days of harvest.
Cautions
- Never with sulfur
- Do not apply with sulfur sprays or where sulfur is on the foliage, as it may cause leaf injury and leaf drop (label; Colorado State).
- Stressed plants
- Do not spray plants that are short of water, because 'shock' can cause leaf drop (label). Wet leaves, rain and humidity above 90 % also raise the risk (Colorado State).
- Sensitive plants
- Oil removes the blue bloom from Colorado blue spruce and Koster spruce. Use the lowest rate, or test first, on Cryptomeria, smoke tree, Chamaecyparis, juniper, Japanese holly and spruce. Red cedar and Douglas fir tend to be sensitive (label).
- Bees
- The label we read carries no pollinator statement, but it says not to spray ornamentals in flower. UC IPM's bee rating for narrow-range oil is II: do not spray flowering plants, except between sunset and midnight where the label allows.
- Fish
- 'Drift and runoff may be hazardous to aquatic organisms in water adjacent to treated areas' (label). Do not spray over water.
- People and pets
- The label's signal word is Caution: the product causes moderate eye irritation (Hazards to humans and domestic animals), and it should be stored out of reach of children and animals.
- Surfaces
- Oil can stain some surfaces, especially dark house paint (Colorado State).
Use instead
- Spider mites under glass: Phytoseiulus persimilis, with oil kept to the upper canopy if you use it at all. That is the approach that worked on roses.
- Aphids and whiteflies on soft plants in summer: insecticidal soap is another contact spray with no residue, though it has the same heat and water-stress limits. Under glass, use Encarsia formosa for whitefly.
- Mealybugs in a greenhouse: Cryptolaemus montrouzieri.
- Neem oil is a plant oil that works in a similar way. See neem oil for its bee warning.
Other products
Sources
Colorado State University Extension, Insect Control: Horticultural Oils, fact sheet 5.569
EPA label, Bonide Horticultural & Dormant Spray Oil, EPA Reg. 4-80, notification 19 Mar 2024
UC IPM, Relative toxicities of pesticides used in floriculture to natural enemies and honey bees (2022)
UC IPM Pest Notes: Biological Control and Natural Enemies of Invertebrates (toxicity table)
UC IPM Pest Notes: Aphids, UC ANR Pub. 7404
Urbaneja et al. 2008, Acaricide side effects on citrus natural enemies. Pest Manag Sci 64:834
Buitenhuis et al. 2016, Biopesticide dips for Bemisia on poinsettia cuttings. Insects 7:48
Nicetic et al. 2001, Petroleum spray oil and P. persimilis on greenhouse roses. Exp Appl Acarol 25:37
Xue et al. 2009, Sublethal mineral oil deposits and P. persimilis. Exp Appl Acarol 48:291
Gaire & Amarasekare 2025, Low-risk insecticides and Orius insidiosus. J Econ Entomol 118:2195
Frank 2012, Reduced-risk insecticides for scale in urban landscapes. Environ Entomol 41:377
Drafted from the cited sources, then checked claim by claim against those sources on 23 September 2026.
Always follow the label. Labels are legally binding; this page is not.