Dormant Spraying of Fruit Trees in the Garden

The authors are John E. Dibble, Entomologist Emeritus, U.C. Kearny Agricultural Center, and Richard L Coviello, Farm Advisor, Fresno County

Dormant sprays allow:

  • a wide period of time for application;
  • a broad range of useable materials;
  • maximum unobstructed spray coverage;
  • multiple pest targets; and because of
  • minimum risk of phytotoxicity and beneficial organism disruption, they become the most economically sound treatment of the entire season.

The properly applied combination of a selected insecticide and spray oil provides control of the peach twig borer (PTB) (found on all stone fruit and almonds), European red mite eggs, and San Jose scale (SJS) (the latter two occur on all stone, pome and nut crops). Other pests such as brown mite eggs, aphid eggs and certain diseases (with the addition of a fungicide) can also be controlled. Although often present on certain fruit and nut trees, over-wintering codling moths, oriental fruit moths, navel orange worms and two-spotted and pacific spider mites are not affected by dormant treatments. The only significant overwintering beneficial organism in stone fruit and almonds is the western (orchard) predator mite. There is some concern as to its well-being following dormant treatments, but over the years it appears to survive winter spraying.

The time for dormant treatment, which can successfully control much of the resident pest population in the garden, is from December until the latter part of February. Usually dormant means December through January. Delayed dormant means February through as late as the middle of March, provided pre-bloom bud cracking has not yet occurred and bee kill is not a problem. It is also generally felt that mite eggs are easier to kill in the delayed dormant period, as the shells become thinner when they approach hatching.

Plums and prunes occasionally tend to be susceptible to oil injury when treated in the dormant period. Experience, however, has shown that plums and prunes should be treated in the delayed dormant period, as injury seldom occurs during this time.

Walnuts are very easily damaged by dormant oil treatments. Therefore, if oil is used it should be applied at the very end of the delayed dormant or, better yet, at the time that local experience shows to be appropriate. Any deciduous tree suffering from lack of sufficient moisture may be more susceptible to dormant oil injury. Irrigate deeply before oil sprays and apply superior oils to eliminate this problem.

Although it is very important to control PTB early in the season, SJS is potentially the more serious pest. This scale is capable of building up over 1-2 years and actually killing large scaffold branches in the third year. Both of these pests can also be treated and controlled in early spring (usually the first or second week in May). However, time of application is considerably more critical than for dormant treatments, plus coverage is more difficult due to foliage.

There are four types of spray oils:

  1. dormant emulsive oil, a clear 98% oil;
  2. dormant oil emulsion, a white, mayonnaise-like 80% oil that is somewhat safer than dormant emulsive oil;
  3. original supreme type oil, a slightly safer oil that can also be used during the growing season; and
  4. superior or narrow range oils that are even safer (because of being purer) and can also be safely used throughout the growing season.

The old true dormant oils (1 and 2) are still available in some areas. The old Volck Supreme, now difficult to find, is the best example (3). The new Volck Supreme oil, which confusingly has the same name, closely fits into the No. 4 category. In recent years this latter group of narrow range type of oils has expanded its number of available label formulations which are mostly made from Exxon’s Orchex 796 and Unocal PGSO-2 base stocks. Other petroleum companies are also involved.

In summary, dormant spraying is strongly encouraged as being the most efficient and economical treatment of the season. The only reasons for not treating in the dormant season are lack of knowledge of its value, economics, or no pests present. Concerns about chemically disrupting any natural balance in the garden are sometimes expressed by homeowners. However, dormant treatments do not seem to be a factor and the overall benefits are great.

The following table may be helpful to applicators, as it shows dormant material efficacy and its relative performance. Thorough spray coverage is essential for good control.

Dormant Spray Control of Peach Twig Borer and San Jose Scale

Peach Twig Borer (PTB)Diazinon + Oil96%
San Jose Scale (SJS)Diazinon + Oil97%

WARNING ON THE USE OF CHEMICALS Pesticides are poisonous. Always read and carefully follow all precautions and safety recommendations given on the container label. Store all chemicals in the original labeled containers in a locked cabinet or shed, away from food or feeds, and out of the reach of children, unauthorized persons, pests and livestock. Confine chemicals to the property being treated. Avoid drift onto neighboring properties, especially gardens containing fruits and/or vegetables ready to be picked. Dispose of empty containers carefully. Follow label instructions for disposal. Never reuse the containers. Make sure empty containers are not accessible to children or animals. Never dispose of containers where they may contaminate water supplies or natural waterways. Do not pour down sink or toilet. Consult your county agricultural commissioner for correct ways of disposing of excess pesticides. Never burn pesticide containers.

To simplify information, trade names of products have been used. No endorsement of named products is intended, nor is criticism implied of similar products which are not mentioned.

EDITOR: Pavel Svihra, Horticulture Advisor, University of California Cooperative Extension, 1682 Novato Blvd., Suite 15013, Novato, CA 94947

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Contact Acorn Arboricultural Tree Service, Inc. with any additional questions or for a consultation using our online form or call us at 916-787-8733!

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