By Dave Meadows
Pioneer Columnist
Branching Out

Horticultural Oils: Part 2 of 2

We know from previous articles that mineral oils are highly refined complex mixes of petroleum hydrocarbons produced from paraffinic crude oil. We also know that pest attacks are a secondary stress factor and that the underlying problem is the key to a tree’s long-term recovery.

Horticultural oils can be an effective, low toxicity spray option for managing tree pests in the residential community. Today’s modern mineral oils are formulated with an emulsifier to make them easy to blend with water to make a spray solution. Other horticultural oils such as soybean, cottonseed, sesame, and neem are plant-based, less refined, difficult to blend and may cause plant damage when compared to modern mineral-based oils.

Dormant oil refers to the time of year in which it is applied. Usually in early spring before bud break. i.e., late April. At this time year daytime temperatures are warm enough to avoid freezing thwe spray solution and allows it to dry after application. One particular caution for early spring, dormant oil treatment, is to avoid spraying 48 hours before or after a frost. If the temperature is below + 5C the emulsion in the oil product can break down.

Dormant oil sprays target pests that have overwintering adults (scales and aphids) on bark, twigs, narrow crevices and bud scales. Insect pests also lay eggs on these parts of the tree. Eggs overwinter until temperatures are favourable for beginning a new generation. Dormant oil spraying also prevents these eggs from hatching. When spraying trees with horticulture oil it is recommended that the entire tree be covered evenly and thoroughly. Appropriate protective clothing, eyewear and masks are necessary to avoid contact and breathing when applying oil spray solutions. All oils must be used cautiously and ALWAYS consult the label for proper directions and precautions.

The more highly refined mineral oils are known as all season spray oils or summer oils which can be applied during the active growing season. These oils are safe to mammals (including humans) and most birds. However, mineral oils are non selective. Therefore, beneficial insects such as lady bug larvae are also killed. All horticultural oils work by suffocation when the spray forms a coating of oil on the eggs or insects’ body. When using mineral oil for spraying during summer, spray in the morning and evening to avoid contact with foraging bees.

Another benefit of horticultural oils is they can have repellent properties that act as antifeedant (discouraging feeding) for some leafrollers. This is encouraging as leafrollers, particularly on apple trees, have been a serious pest problem here in the valley over the last couple of years!

Using horticultural oils is an excellent, non-toxic, alternative to treating pests in the garden and landscape. With modern, mineral-based oils, application timing is extended from early spring through to late summer. Horticultural oils are easy and very effective in minimizing outbreaks of the usual suspects, such as aphids, scales, leafrollers, canker worms and leafhoppers.