Dormant/Horticultural Oil Sprays

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Dormant Oil, also known as Horticultural Oil, is an oil that is mixed with water and a solvent such as soap and is sprayed on fruit trees and berry bushes and help to control the overwintering eggs of insects. This insects include red mites, spidermites, scale insects, pear psylla, aphids, white flies and other soft bodied insects. Oils kill exposed insects and mites by suffocating them or by penetrating the outside wall of the insects eggs and harming their internal cells.

Dormant oils are normally highly refined petroleum or vegetable based oils. These sprays are less toxic than most insecticidal sprays and insects are less likely to develop resistance to them. The sprays are safe to most mammals  and birds and are unlikely to harm beneficial insects because the spray is applied when they are not active on the tree. Dormant oils are quick drying, and do not leave a harmful residue.

Dormant oils should be applied to the trees before bud break in late winter or early spring. It is important that the oil be sprayed when there in no rain in the forecast for a couple days and at temperatures above freezing (40+F). Because the oil only kills insects and eggs by direct contact it is important to fully saturate the tree, paying special attention to crevices and spots where the insects are likely to be hiding. Dormant oils can be purchased at garden store for around 15-30 dollars or you can make your own dormant oil spray at home with a few ingredients!

The following are Dormant Oil recipes designed at Cornell University:
2 tablespoons of ultrafine canola oil
1 tablespoon of baking soda
1 gallon of water.

Nourishing Formula Containing: 
2 tablespoons of horticultural oil,
1 tablespoon of baking soda,
1 tablespoon of kelp and
1 tablespoon of mild dish soap mixed with
1 gallon of water.

Another dormant oil recipe contains
2 tablespoons of baking soda
5 tablespoons of hydrogen peroxide
2 tablespoons of castile soap — which is made from an olive oil base
1 gallon of water

Commercial sprays available for purchase:
Monterey Horticultural Oil
Bonide All season Dormant Spray Oil

Insect Pests controlled by Winter Oil Sprays
Winter applications of dormant/horticultural oil sprays can be helpful to control the following orchard insect pests.  Other means of control to consider during the growing season include attracting beneficial predator insects, physical removal with strong jets of water, and application of neem oil.

Mites 

Red mites are small red 8 legged insects who primarily effect apple, pear, and stone fruit trees. You will find adults on the under side of leaves and red overwintering eggs in groups in crevices and cracks, around buds and in rough patches of bark.These insects feed on the leaf tissue of your tree, which can cause the trees to produce less fruit buds. The mites also can cause leaf browning and make the tree unable to produce enough food to produce proper fruit.

Scale 

 Scale insects have no visible legs or antenna. They resemble individual fish scales,  attach themselves to the tree and feed on the sap and vital nutrients of the tree.

For more information on identifying scale insects:

Pear psylla 

These insects resemble tiny cicadas and have a dark spot on the top middle edge of both wings. They lay elongated yellowish eggs on or near fruit spurs and are barely visible without a magnifying lens. These insects feed on sap and produce a honeydew on the tree which can cause fungus and mold. This causes the skin of the fruit to become blackened and scarred. Pear psylla effect European pears much more than Asian varieties.

Aphids

Aphids have piercing mouthparts where they remove plant fluids from your tree. Aphids come in several colors, including green, white, and black, and can generally can be recognized by their small size, pear-like shape, and long antennae.  They are sometimes ‘herded’ by ants and so the presence of ants in a tree can be an indicator of aphid infestations.

Aphids harm the tree by removing plant fluid, spreading disease, and can stunt growth and cause deformed leaves and fruit. Aphids also secret a “honeydew” that causes mold to grow on your trees which can effect photosynthesis and attract other harmful insects.

This edition of POP TIPS prepared with assistance from 2015 POP Intern Sophia Taylor.  

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