Q. I was traveling and missed the time frame for a dormant oil application to my fruit trees. Should I go ahead with it, or just follow the rest of the spray schedule?
A. Once trees start to leaf out and bloom, it is too late for a dormant oil application. The oil can severely burn the tender new foliage and flowers. Once the foliage matures and hardens off, a summer-weight application of horticultural oil can be useful to control soft-bodied insects such as aphids and scale crawlers (immature scale insects). A summer-weight application is 1-2 percent solution (1.28 to 2.56 ounces of horticultural oil per gallon of water). Avoid applying horticultural oils when trees are under drought stress, or when temperatures are over 80 degrees and/or humidity is over 80 percent.
Dormant oil applications are a useful tool to manage insects that overwinter on trees, even those that overwinter as eggs. Eggs have to exchange gases just as humans need to breathe. Dormant oil applications can suffocate eggs, and have a similar effect on immature and adult insects as the oil creates a film over their breathing pores (spiracles).
Dormant oil can also affect insects’ cell membranes and disrupt their functions, and perhaps create some toxins in the process. There is some evidence that horticultural oil also deters females from laying eggs on treated plants.
Summer applications are mixed at a lower concentration to avoid burning foliage and otherwise damaging the plant while maintaining the benefits of these less-toxic materials. Their main action is on insects present when the oil is applied. Once the sprays dry, beneficial insects and pollinators that visit treated plants are unlikely to be impacted by them.