Combatting Scale and Whitefly Indoors


I was down in the greenhouse this morning, and noticed as I was watering the Confederate Jasmine that the leaves had shiny, sticky spots on them, normally a sign that scale was present. Sure enough, a flip of the leaves revealed quite an infestation of these sucking insects. Years ago, this would have been a cause for concern, as scale infestations are particularly troublesome indoors during the winter, along with that other bane, whitefly. But no more. I now keep a handy little sprayer all ready with a solution of horticultural oil, and a single quick dose generally means the end of the problem.

While I had known about horticultural oil for years from outside applications on fruit trees, it had never occurred to me to use it indoors until I took a trip to the historic greenhouses at the Lyman Estate in Waltham, Massachusetts.  Marveling at the pest free interior, I asked the head gardener there what the secret was, and the was told that horticultural oil was the answer. The plants received a thorough spraying when they came into the greenhouse in the fall, and then as needed as infestations were noticed. That was it. I must admit I found the answer hard to believe, because at the time I was engaged in a several year running battle with whitefly that I simply could not win. I had tried everything, from Neem Oil to systemics like Isotox in an attempt to rid the greenhouse of this pest, all to no avail. It didn’t seem possible that a non-toxic oil, essentially refined mineral oil, would be effective where other far stronger pesticides had failed.

It was.

The greenhouse was whitefly free only several weeks later, thanks to an oil that works by simply clogging the breathing tubes of insects.

Gotta love it.

Obviously, using horticultural oil in a greenhouse setting is far easier than than using it indoors, but it’s equally effective, simply requiring a bit more work. If I notice any problems with my houseplants, I  take them outside on a warm day, hook up the bottle sprayer to the hose, and give them a thorough coating with the sprayer, making sure to flip the plants over to coat the underside of the leaves where the pests lurk. Once dry, the plants are returned indoors, and a single treatment is generally effective. Horticultural oil works well on almost any sucking kind of insect, including whitefly, scale, aphids, thrips, etc. Just be sure to follow the mixing directions precisely, and make sure you’re using one of the lighter “superior” oils, not a heavy “dormant” oil, which as the name implies should be used only on dormant plants outdoors. Finally, note that there are a few types of plants – African Violets and ferns – for example, that react badly to the oil; if you’re using the oil on a species you haven’t tried before, do a small test on a few leaves to make sure the oil doesn’t cause burning or discoloration several days before coating the entire plant.

Until next time, I’m Michael Weishan, for Old House, Old Garden.

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