Many prune growers have told me they are considering skipping the dormant insecticide and oil spray and have asked me my thoughts on such a move.
The dormant insecticide and oil spray is used to control four primary pest of prunes; peach twig borer (PTB), San Jose Scale (SJS), European red mite (ERM) and prune aphids (mealy plum and leaf curl plum aphids).
The insecticide in the insecticide and oil treatment is particularly useful for PTB and aphid control.
In our research we have found that PTB has not been a problem in dried prunes after skipping three dormant spray seasons. If after skipping a dormant spray you are concerned about PTB, you still have control options such as Bt at bloom, May spray, in-season treatments. These options may be justified for fresh market prunes, but according to our research, should not be necessary for dried prunes.
We evaluated 37 prune orchards as a result of our series of winter meetings where we offered to examine fruit spurs to assess the SJS and ERM populations in prune orchards.
Treatment level For SJS we have established a very conservative treatment level of 10 percent of the spurs containing live scale. For ERM, the treatment level is 25 percent of the spurs containing ERM eggs - also very conservative.
Of the 37 orchards evaluated, 68 percent had scale levels below 10 percent and needed no dormant treatment for this pest, and 92 percent of the orchards evaluated had ERM levels below 25 percent.
Therefore, only about one in three orchard had SJS or ERM levels high enough to warrant treatment. These two pests can be adequately controlled with a delayed dormant oil treatment of four gallons per acre.
Prune aphids are our main problem with skipping the dormant insecticide and oil treatment. In our research, about 50 percent of prune orchards that do not get a dormant insecticide and oil treatment wind up with an aphid problem.
Partial control Aphids can be partially controlled with a "green-tip-to-full-full bloom treatment of four gallons per acre oil if full coverage is achieved. To get coverage, travel no faster than 1.5 miles per hour. We generally get 60 to 70 percent control of aphids with this treatment, which is enough in most cases.
Earlier applications of oil for aphids give poorer results.