By Brenda Bolton, Mobile Master Gardener | [email protected]

Q: The shrubs in my yard have been attacked since last spring by whiteflies and scale. I read that spraying with horticultural oil would help, but when I went to buy the oil, there were so many choices that I couldn’t figure out which would be best to use. I hear that neem oil may be best for organic gardening. Which oil would you recommend?

A: Horticultural oil is petroleum-based or plant-based oil that contains an emulsifier enabling it to be mixed with water for spray application. When it comes to buying horticultural oils, there are many brand choices, but all oils function to control pests in the same way. All the oils, with a few exceptions, work by coating leaves, stems and any insects or insect eggs, pathogens or fungi on those leaves and stems.

The coating suffocates the insect or, in some cases, disrupts the insect’s food source, or stops spore development or germination. That is why it is necessary to completely coat the plant, including under the leaves where pests hide. An oil may also contain added insecticides or chemicals that poison insects. Neem oil, often advertised for organic gardening, is from the neem plant and works by a combination of suffocation and causing hormonal changes in the pests. Search “neem recall” at our website, aces.edu, for more information.

Horticultural oils, without chemical or poison additives, are considered safe treatments for various garden pests. Because oils work by mechanical means of suffocation and then evaporate rather quickly, you may need to make multiple applications to stop newly hatched insects. Though this means a bit more work, it is also the reason oils are safer to use, because the plants are not absorbing poisonous chemicals. Other than neem, oils are not systemic pesticides.

Oils are divided into two large groups. One is called dormant oil, and is applied by spray in cool temperatures when plants are dormant. Dormant oils are usually thicker and therefore coat stems and pests more thickly. Dormant oils are timed for use in cooler temperatures and are intended to coat over-wintering pests and eggs to reduce the spring pest population.

The other group is called summer, light, fine or supreme oil, and are lighter-weight oils that can be used year-round. Their instructions often give two different oil-to-water ratios, depending on the season or temperature. It is important to mix the oil according to those seasonal instructions. Plant-based oils may include sesame, canola, even cottonseed oil, but they all control by coating and suffocating.

Oils do not work well to eradicate hard-bodied scales or large, mobile insects that can escape being coated. Oils work best on small, soft-bodied insects, stationary or slow pests, and some fungi as the coating can keep fungal spores from germinating. The pests most easily controlled by oils are whiteflies, mites, soft-bodied scale and a range of insects when newly hatched and most vulnerable. Because immature pests are most susceptible to control, more success is achieved when the specific pest is identified and its life cycle is known, so the application can be timed to the pest’s most vulnerable stage. Oils provide control of powdery mildew, black spot and fungi by limiting spore germination. Citrus leaf miners can be controlled by use of oil sprays early in spring, but by the time leaf-miner damage is visible oils cannot treat the condition.

Oils should not be used on thin-leafed or fuzzy-leafed plants, such as azaleas, red maples, spruce and junipers. Any new spring growth can also be damaged by oils. Read the label carefully to see for which pests the oil is recommended, which plants can be damaged, how to mix the oil and water, and how often to apply.

Protect garden wildlife by applying oils in early morning, before the bees and butterflies rise to feed, and avoid sprays in late summer through October, when bees and butterflies and migrating hummingbirds are feeding. Oils should not be used in extremely hot temperatures, in windy conditions or when plants may not dry thoroughly before rain, watering or hot, direct sunlight. Always water plants thoroughly the day before spraying to be sure they are fully hydrated. Do not use oil on weak or stressed plants.

Don’t forget that you are also a life form, so protect yourself, too! Wear protective clothing and respiration masks.

If you have shrubs with whiteflies or scale year after year, or citrus with leaf miners every summer, it’s best to get a year-round oil and use it exactly according to label instructions on an annual spray schedule. November is the perfect time to use oil spray for overwintering pests or eggs, pathogens and fungal disease.

YOU ARE INVITED TO THESE UPCOMING GARDENING EVENTS

What: Lunch & Learn, Mobile Master Gardeners
When: Monday, Nov. 20, noon to 1 p.m.
Where: Jon Archer Center, 1070 Schillinger Road N., Mobile
Topic: Grafting Camellias, Vaughan Drinkard Jr.

What: Master Gardener Greenery Sale and MBG Holiday Market
When: Dec 1-2 (Friday, 9 a.m. to 4 p.m.; Saturday, 9 a.m. to 1 p.m.)
Where: Mobile Botanical Gardens, 5151 Museum Drive, Mobile
Deadline for pre-order: Nov. 15.
For order form, email [email protected] or go to https://mg.aces.edu/mobile/category/announcements/

Master Gardener Helpline: 1-877-252-4769 or send your gardening questions to [email protected].

Photo/Courtesy UF/IFAS Extension – Soft-bodied insects such as these whiteflies on a Florida-grown tomato plant are among the pests that can be controlled with horticultural oils.