Oil or No Oil Pre-bloom: What are the Consequences?

The primary goal of pre-bloom apple pest management is to suppress or control pest populations that overwinter within the orchard. Two of the most important are the European red mite (ERM) and San Jose scale (SJS). Both overwinter as immatures on bark of limbs and twigs – ERM in the egg stage often near buds and fruit spurs, and SJS as nymphs under waxy scales also on branches and the trunk. These sessile life stages make them very susceptible to pesticides compared to the mobile stages that emerge after bloom.

With more widespread use of pyrethroid insecticides for brown marmorated stink bug in recent years, problems with ERM and SJS populations have become more common. Pyrethroids are highly toxic to many natural enemies and have behavioral effects that can flare populations of both pests. Hence, the use of pre-bloom practices that can suppress overwintering populations of ERM and SJS can help to reduce the size of overwintering populations, and either reduce the need for or improve the performance of pesticides applied post-bloom.

Horticultural Oils: Refined horticultural oils have been used for more than a century for pre-bloom control of ERM and SJS on fruit trees. Dormant or delayed-dormant oil applications suffocate mite eggs and scale nymphs by preventing oxygen uptake and disrupting cell membranes. Because mite eggs and scales can be partially protected by bark, and because oil has no residual activity, application at high water volume is important for good coverage. Delayed dormant oil applications have proven to be highly effective at suppressing ERM and SJS populations and safe to most beneficial arthropods.

Insecticides: Since the 1960s, Lorsban (chlorpyrifos) was commonly included with oil to improve control of mites and scales. Its long residual activity on bark extended control so that the combination of oil + Lorsban often provided season long-control. The efficacy of pre-bloom Lorsban for scale began to erode by the time its registration was cancelled in 2020. Consequently, there was increased use of other scale insecticides, most commonly Esteem and Centaur.

Application of these materials can be pre-bloom with oil, or delayed until petal fall to first cover to target first generation crawlers. The post-bloom timing has become common, because the combination of delayed dormant oil and post-bloom insecticide extends the period of protection against multiple life stages (overwintering immatures and first generation crawlers). Two other insecticides with good scale activity are Assail and Diazinon, the later an organophosphate that is undergoing review by the EPA.

Control in the Absence of Pre-bloom Oil: While oil remains a highly effective management tool and its use is encouraged, its high cost is causing some to reconsider its use this year. In the absence of oil, growers should consider including an insecticide targeting scale both before bloom and between petal fall and first cover. However, the insecticide used at these different times should have different modes of action to reduce the potential for resistance development. Probably the most economical approach is to use either Esteem or Centaur before bloom, and Assail post-bloom, because Assail applied at first cover provides the benefit of controlling some key pests at that time, including rosy apple aphid and codling moth.

Learn more about southeastern apple insect pests at the Apple Insect Management page.

2023 Average Weekly Trap Captures

*Note that these averages illustrate only the timing of insect emergence and fluctuations in populations, and are not representative of population levels in any given orchard. The only way to have an accurate assessment of an individual orchard’s populations is to set up traps in that orchard.

2023 Accumulated Degree Days

About degree-day models