Dunno. I'm thinking of skipping the treatment altogether, as I bought a new sprayer last fall with a pump that won't tolerate horticultural oil and I can't bear the thought of breaking out the old piston equipment for this or any other purpose. I'm thinking this may be a huge mistake, though. Misterbaby.

Scott,

Your logic seems sound to me. But I thought the old style dormant oil was pretty much non-existent anymore. With the new refining processes commonplace and all producing narrow-range oils, I thought just about all the horticultural oils now have labels for spraying during the growing season.

The thing is both scale and mites are more vulnerable from half inch green on so most people don't do full dormant spray anymore. What are you trying to control?

Three university horticulture depts said oil sprays have no residual effect and most people using them as dormant sprays do so too early in the season before the insects are active and exposed to the spray. They also said not to use within 30 days of any sulfur compound. Scott I think you use a copper spray that does not have sulfur as one of its compounds but others who use copper sulfate could kill their trees if they also use oil sprays within the 30 day period.
Dan

I guess what I am wondering is if the reason why people are now wanting to spray oil later is that everyone is using light oils and they don't work very well in full dormant periods. So, maybe the heavy oils actually would work well in full dormant, just as well or better as a lighter oil later. I looked around for studies of different kinds of oil compared and could not find anything (well I found one paper stating that heavier oils are more effective but just that sentence). There still is at least one heavier dormant-only oil being sold that is made by PBI Gordon.

Hey I just found an interesting old study, see below. It states that heavier oils work better, but it also points out that the rosy apple aphid wakes up later and that is the biggest aphid problem for apples showing the importance of a later spray. On apples the rosy apple aphid is about the only reason why I spray oil so it sounds like I should aim my oil spray at them only, and do the later spray with lighter oil. Harvestman, this may be the reason why you prefer the later spray. This study suggest lime-sulphur later because the light hort oils did not exist when it was written.

Overall, it sounds like for scale a heavier oil is better at delayed dormant, and for aphids a light hort oil later is better.

Scott

Here is a link that might be useful: study

I suppose the next question is....
Is the longer lasting dormant oil going to cause other trouble with not being able to use Sulphur fungicides?

It may be that they know there is a benefit to thicker oils... but that benefit is strongly outweighed by the fact that instead of 1-month of not using Sulphur... you gotta wait 3 months or some such...

Brown rot is a bigger problem on Peaches than mites... so maybe running a thinner, faster dissipating oil is a better trade off....

Thanks

I'm fairly certain the prohibition of using oil and sulfur containing compounds applies to foliage, not in the dormant season. The concern is burning of the foliage.

The recommendation not to use oil w/in 48hrs. of an expected freeze event applies to both dormant and non-dormant sprays, as it appears oil used close to a freeze can damage the bark, although I've no personal experience trying it.

Still dunno, but today I went out and sprayed the whole orchard with heavy oil and copper amonium with sticker-spreader. Everything now has a pretty blue sheen. Forgot not to treat the hawkeye's so they'll likely drop. Misterbaby. (Scott, I have the faintest recollection that the light oil was introduced so that rosarians could treat their plants in the spring.)

I would worry about the late oil spray if I were going to spray captan anytime soon after.

I have knocked out scale with hort oil applied even after petal fall. Never failed me yet. I don't know about RAA because it hasn't been an issue for me anywhere. I think there's probably no important advantage to the heavier oil in controlling anything because what you're doing is suffocating the buggers and how long can that take.

Research and real life are not the same. However if for you Scott the lighter oil applications have been unsatisfactory I can understand your quest for a better treatment but RAA is a huge problem in many areas so I'd think if heavy was much better it would be very well known.

I operate under the assumption that an application of dormmant oil in early spring before bud break would sufficate any eggs that were laid the previous season and over wintered. Prior to the oil I also apply Kocide followed
after petal fall with Immunox. After that I usually don't see insects until mid summer when the Jap beatles pay me a visit. I blast those with Immidan. My question would be why would you need additional application of oil if most of the insects would hatch in early spring and additional oil applications would be superfluous.

harvestman, for me RAA is my primary problem, I have not ever had any scale problems on apples. So I will be doing a later summer oil application on apples only from now on. I have found no data comparing a heavy oil at dormant time vs a light oil later for scale and maybe heavy oils died out for a good reason: the later light oil spray is just as good. The study I cited above was from an era before light summer oils existed. So maybe the primary conclusion is not to use summer oil products ever in dormant season, its always better to wait. If you have true dormant oil and not a summer oil it may be just as good to use that in dormant season (unless you have RAA).

Scott