Horticultural Oils have been used in the home landscape and orchard pest control for decades, but their use has increased in recent years as environmental awareness grows.
Years ago, horticultural oil was known as dormant oil and was only applied when plants were dormant. These older oils were very heavy and would cause damage to foliage if applied during the growing season. Today’s horticultural oils are much lighter and safer for plant foliage, but still must be used with caution to avoid burning foliage.
How Horticultural Oils Work
Insects don’t have lungs. They breathe by a different mechanism than mammals; a series of external openings on their body called spiracles. Horticultural oil kills insects by blocking these spiracles, which suffocates the insect. Horticultural oils will also disrupt the metabolism of insect eggs, preventing them from maturing into adults.
To be effective, coat the insects/eggs thoroughly with a generous application directly to the target pest. To avoid killing beneficial insects and pollinators it’s best to apply oils in the morning or evening when these insects are least active. Also, avoid spraying when plants are in bloom or you notice that beneficial insects are present. Oils are non-toxic and have no residual environmental effects, making them ideal for use on edible crops. Most can be applied up to the day of harvest.
When Not to Use Horticultural Oils
- Do not apply when temperatures are over 90°F as foliar burning may occur.
- Do not apply when humidity is high as foliar burning may occur.
- Do not apply in autumn as fall oil treatment may increase risk of winter damage.
- Do not apply within 30 days of applying a sulfur fungicide as the combination can be toxic to sensitive plants.