One of the curiosities of indoor gardening is that many insect pests that are so hard to control indoors simply disappear when you put the plant outdoors for the summer. Aphids, mealybugs, spider mites, thrips, whiteflies and fungus gnats: all but scale insects seem to disappear when you move your plant into the fresh air for the summer.
I’ve never been able to discover what happens to these pests when you put the plant outside. Does a predatory insect move in and repress them? Is it the shock of suddenly being exposed to “real weather” that does them in? Is rain and overhead watering washing them off? That remains a mystery. But put a plant outdoors for a week or two and its pests are gone … from sight, at least.
It’s hard to tell whether the above is true of scale insects or not. Certainly, they don’t seem to proliferate outdoors, but the “shell” (their outer covering) remains on the plant. It may be harboring a live scale insect, but also a dead one. If you thoroughly clean up your plants as they go outdoors, going over their entire surface with a soapy toothbrush, at least it gets rid of all the obvious scale insects for the summer, and no, they then won’t come back while the plant is outdoors.
Not Gone, But Hiding
However, these insect pests are obviously lurking around somewhere, perhaps at the leaf axils or some other hidden spot, as they come back when you bring them indoors. Well, not right away, but eventually. In spite of preventative treatments, some always seem to return from whatever their hiding space was, usually in February or March, just as the plants starts a new season of growth, although earlier—late fall, early winter—in the case of spider mites.
This “return from nowhere” is especially true of mealybugs, scale insects, thrips, whiteflies and spider mites. They always seem to come back to the same plants, a sign that they were probably lurking there all along.
Aphids are unusual in that yes, they often seem to come back, but on different plants, so it’s not clear whether they’re descendants of last year’s aphids or entirely new ones having arrived from an outdoor source.
As for fungus gnats, well … they’re always sporadic, showing up here and there without rhyme or reason, going away for months, then coming back again. It’s hard to tell where they come from.
Prevent the Onslaught
To prevent the “return from nowhere”, give your plants a thorough soap and water treatment as described in Bring Your Plants Indoors Without the Bugs. It works well with most pests, although not mealybugs and scale insects. My experience with those two pests is that once a plant gets them, it will always have them. You may think you have finally controlled them, but they’ll be back! More on that in The Thirty Year Mealybug Infestation: a Horror Story. I’ve now learned to just dump any plant with mealybugs or scale insects: they just never seem to let go.
So, an insect-free summer outdoors? What plant isn’t going to enjoy that!