Horticultural Mineral Oils
Horticultural mineral oils can play an important part in orchard pest and disease management programs during the foliar season. They are effective against scales, mites, and some other insects, as well as some diseases. All horticultural mineral oils must be registered by the EPA. Significant differences can exist in the chemical composition of horticultural mineral oils depending on source of crude oil and the manufacturing process. Efficacy and phytotoxicity can vary substantially with chemical composition and physical properties. The most important characteristics of a horticultural mineral oil to consider when assessing it for pest control are distillation parameters, molecular size and shape, unsulfonated reside (USR) measures, and viscosity.
Molecular shape: High concentrations of paraffinic molecules are desired since this fraction provides pesticidal activity. Paraffins are long carbon chains that interact readily with the surface waxes of mites and insects. Concentrations of paraffins in spray oils should be 60% or more to assure optimal pesticidal activity. The remainder is made up primarily of ring-shaped, naphthenic, and aromatic molecules. USR measures the aromatic component of the oil, which should be 92% or higher.
Compliance with FDA regulations on aromatics ensures that this fraction is safe to mammals.
Distillation and molecular size: Horticultural mineral oils available in the Pacific Northwest are either of the 415, 440, 455 or 470 type. Numbers denote the mid-boiling points (°F) of these oils when distilled under vacuum. The lighter horticultural mineral oils, 415 and 440 types, are suitable for dormant, delayed dormant and foliar (summer) applications. Use of horticultural mineral oils with larger molecules, 455 and 470 types, is best restricted to use in the dormant and delayed dormant periods. Viscosity is a measure of a fluid's flowability. It is used in many cases to identify a horticultural mineral oil, for example, as a 60- or 100-second oil. The viscosity of a horticultural mineral oil is by itself not a measure of performance.
The critical properties regarding pest and disease control as well as plant protection are governed by a crude source with high paraffinic content (shape related), distillation (size related) and processing methods to select for paraffins vs. naphthenics and to eliminate undesirable aromatics (shape related). For a more detailed and very informative discussion of the relationship between the biological activity and the properties of a horticultural mineral oil refer to Using Horticultural Mineral Oils to Control Orchard Pests, a Pacific Northwest Extension Publication (PNW 328, 1996) by M. Willett and P.H. Westigard.
Dormant or Delayed Dormant Use: Using horticultural mineral oil plus an organophosphate in the dormant or delayed dormant period is an important part of a good pest control program. This program provides the most effective control of San Jose scale, and is essential to early season control of European red mite and leafrollers. Many growers, however, have avoided the use of horticultural mineral oil or reduced the rate per acre below an effective level because of possible injury to trees, in some cases resulting in increased problems with these pests. Several precautions in the use of horticultural mineral oil should reduce the risks of using these products.
- Calibrate the sprayer before applying sprays to ensure proper gallonage and thorough coverage.
- Check agitation system as each tank of spray mixture is prepared.
- Test oil held over from last season for proper emulsification before using.
- If possible, use emulsifiable concentrate or flowable formulations of pesticides with horticultural mineral oil. If wettable powders must be used, add wettable powder or a slurry of wettable powder formulations to tank partially filled with water before adding horticultural mineral oil to ensure proper suspension in the spray mixture.
- Avoid applying horticultural mineral oil during cool (below 45 ̊F), damp, extremely dry, or windy weather. If subfreezing temperatures are forecast within 24 hours, discontinue spraying by mid-afternoon.
- Do not use horticultural mineral oil at more than the maximum label rate per acre. Various pesticides may have different maximum rates of oil for when it is tank-mixed with their product. Ground and aerial oil rates may vary. Reduce the rate of oil per acre below 6 gallons where concentrate applications are used. On young trees or close plantings where there is a good chance of double spraying, reduce the rate per acre.
- Do not allow the mixture to stand in the tank without agitation.
Foliar Use:The use of horticultural mineral oil after bloom for pest control is gaining in popularity. Horticultural mineral oils are primarily effective against pests through contact, making thorough coverage of the foliage very important. In addition to contact activity, there appears to be repellent activity for egg-laying adults (example: white apple leafhopper). As recommendations for the foliar use of horticultural mineral oils appear in this bulletin, use care in selecting products that meet criteria discussed. In addition, recommendations are going to be based on "dilute" spray volumes unless otherwise indicated. Avoid slow drying conditions and extremes of cool or hot conditions when applying horticultural mineral oils.