As the weather warms a little, but before buds break and begin to show any color is a good time to dormant spray fruit trees.
Why spray? Many pests overwinter either as adults or eggs in crevasses in bark, under scales of buds, in mulch or fallen leaves under the trees. Dormant oil sprays, when properly applied, coat the eggs or insects and suffocate them before they become active. As the old adage states, an ounce of prevention is better than a pound of cure, and anything that makes this task easier means that it is more likely to get done.
For many years I mixed my own in a pump sprayer but found that this method was woefully inadequate because my rather basic equipment was not powerful enough to reach the top of many of my fruit trees. This resulted in me pumping like mad on the ground to get enough pressure and then climbing up a step ladder with the gallons of spray so that I could apply it where I wanted it. There must be an easier way!
Then I discovered hose-end sprayers where one simply attaches the top of a bottle to the end of a garden hose, and the mix is sprayed out with much better force. This avoids having to store and mix spray from concentrates. They have proved to be a real game-changer for me, and I feel that the extra cost is worth it. I easily got excellent coverage right to the top of my trees and into the nooks and crannies of the bark.
Of course I would rather not use any sprays in my garden and instead rely upon beneficial insects, but each year the new shoots on my peach trees would be covered with aphids long before the first ladybugs and syrphid flies arrived. The tips of the cherry tree would rapidly get coated with black aphids, and I never saw a ladybug anywhere on that tree. Another problem that I had was leaf blister mite on a pear tree which affected not only the leaves, but also the fruit. The first year that I used a hose end sprayer to apply the dormant spray I had almost none of these pests - only a few shoots on the cherry were affected fairly late in the season. The second year I had a similar result, but we also had a late freeze which might also have had some controlling influence on the pests.
If you garden organically, choose sprays that are OMRI listed for that crop. They may contain neem oil, mineral oil, copper or pyrethins because some are for insect pests while others are designed to deal with fungal spores, and some are combinations of both. Read the labels carefully and use only the one appropriate for your problem. UC IPM has recommendations for control methods on their website.
Choose mild, dry weather so that the plants dry off before nightfall, and do not use if it is windy, if frosts are expected, if the temperatures are high or if the buds have begun to open. Remember it's dormant oil. Using it on green, growing tissues may cause damage to the plant.