Horticultural Mineral Oil- Horticultural mineral oils were among the first insecticides used in orchard pest management. Products have in recent years become more refined and quality controls tightened to reduce the potential for phytotoxicity. Pre-bloom use of oil is an important component of an apple IPM program. This use provides control of San Jose scale and European red mite (eggs) as well as suppression of aphids, all without negative impact on natural enemies. Summer use of horticultural mineral oils has increased as they have become safer. Typical concentrations of oil for summer use are 0.25-1%. Higher concentrations of oil have a greater risk of phytoxicity. There remain questions about the long-term negative impact of summer oil use on fruit tree vigor and fruit size. This is probably more of a concern for pear than apple but careful observation of any decline in tree vigor should be a part of any program using summer oil treatments. Season-long use of oil, especially for codling moth control in organic orchards, has been implicated in increased sunburn, poor fruit finish and poor wax deposition. Care must be exercised when using sulfur in orchards implementing an oil program to prevent phytotoxicity.
Lepidopteran pests
Codling moth – Horticultural mineral oils act against codling moth
by suffocating the egg. The best timing strategy is to allow oviposition
to occur and just prior to egg hatch make an oil application. Since oil
works only if applied topically to the egg, there is not residual control
of subsequent eggs. Thus, repeat applications must be made as more eggs
are deposited. A suggested protocol would be to apply the first application
at 200DD from biofix, with repeat applications at 400 and 600DD (approximately
every 14 days). This should cover about 80% of the first generation oviposition
period. Second generation applications should begin at 1200DD, with repeat
applications at 1400 and 1600DD. In orchards with high codling moth populations
the re-treatment interval should be shortened to 100-150DD intervals (or
every 7 days). Organic orchards that rely heavily on oil to control codling
moth should use this strategy in conjunction with mating disruption. If
codling moth pressure is moderate to high, then all available organic tools
must be used (including Entrust and granulosis virus).
Leafrollers – Oil used in combination with Lorsban in the delayed-dormant period is effective against leafrollers but it is the Lorsban that provides control. In one particular study, we used season-long oil applications to control codling moth and observed that leafroller densities were also suppressed. However, we have not been able to describe the exact mechanism for this action or even which life stage was affected. Oil can result in mortality of leafroller eggs and may deter oviposition by females. Oil is also a good adjuvant, improving uniformity of coverage and insecticide penetration through the leaf surface.
Lacanobia fruitworm – We have no information on the effect of oil on this insect.
Indirect/secondary pests
Oil is a toxicant to a broad range of secondary and indirect pests, and
often has an effect on soft-bodied insects. It suppresses mites fairly
effectively, although repeat applications may be necessary. One to two
applications per generation applied against white apple leafhopper nymphs
are effective in reducing populations. Oil is also an ovipositional repellent
to adult leafhopper females, but the effect is temporary. Woolly apple
aphids are also suppressed by oil applications. Oil at the delayed dormant
timing has been a standard recommendation for eight decades or more,
and is still effective in long-term control of the overwintering stages
of European red mite and San Jose scale.
Pear pests
Horticultural mineral oils are important tools in pear pest management.
At the very least they should be used for delayed dormant applications
and as adjuvants, and in some cases they can be useful as stand-alone
applications. Application of oil early in the season, in the dormant
and delayed dormant periods, is important in starting the pear psylla
control program. Additionally, these applications help control San Jose
scale and European red mites, if present. The early oil sprays inhibit
pear psylla oviposition; the mechanism of this activity is not known.
Nevertheless, this inhibition helps push peak oviposition back by one
to two weeks and helps synchronize the population development (there
is also some degree of ovicidal activity).
Using oils as adjuvants helps increase the level of control obtained from virtually every psyllicide it has been tested with. In some cases this is simply from the mortality induced by the oil, while in others there is synergism in activity. In our tests, we include 0.25% oil and find that it helps the chloronicotinyls, Agri-Mek, Neem compounds, and Applaud. This is also true of the new miticides. Caution should be taken when using oils with some materials, particularly emulsifiable concentrates, to reduce the possibility of fruit marking and other phytotoxic effects.
We have tested using oils alone for control of pear psylla, spider mites, and codling moth, and these studies have met with some success. It appears that repeated applications are capable of suppressing the pests; however, if damaging levels are present, the oils do not reduce the populations below eonomic thresholds. There are anecdotal concerns in the industry regarding the long-term use of frequent oil applications. Some have hypothesized that there could be significant reductions in yield and/or fruit size following several years of ‘intensive’ oil use. A preliminary two-year study in Bartlett did not find any negative effects; however future long-term studies need to be conducted to properly address this issue.
Beneficial arthropods
Oil can cause a reduction in predatory mite populations, but the effect
is fairly subtle, and has not provoked mite flare-ups.