Houseplants Pesticides Plant pests

Longer Days Awaken Pests

You may not have noticed, but if you live in the Northern Hemisphere, many houseplant pests have been less visible of late than usual. That’s because many of them enter into diapause, a kind of semi-dormancy, starting in the fall. This is due to the influence of shorter day lengths. In fact, they are often so inconspicuous in late fall and early winter that you may even have thought you’d gotten rid of them entirely.

Whiteflies may be in diapause in mid-January, but by March they’ll be wide awake and hungry!

Other insects continue to remain active even when the days are short, but develop at a much lower rate than in summer and likewise may go sight unseen for a while. But when the days start to get longer and the increasing sun heats up your home or greenhouse just a bit more, the two groups are re-energized and begin to reproduce abundantly.

Even as early as late January, although it still feels like the middle of winter outdoors, days are getting perceptibly longer and gradually, depending on the species, the enemies of your plants will start get back to work. By early March, they are all active… and hungry.

The Culprits

Insects that are quiescent during short days (or almost so), but awaken as days lengthen include:

Aphids, fungus gnats and thrips, however, don’t seem to slow down much in winter. They are just about as active in January as in July! You have to keep your an eye open for these insects throughout the year.

Photo: Maja Dumat.

What to Do?

  • Inspect your plants at the end of January, looking especially at leaf axils and leaf undersides. A magnifying glass may be needed to see red spider mites, as they are very tiny. Subsequently, an inspection every two weeks is never a bad idea.
  • Set out yellow sticky traps: often flying pests like whiteflies, fungus gnats and winged aphids will be caught before the infestation even begins, nipping it in the bud.
  • Isolate infested plants so the pests can’t spread.
  • Treat infested plants.


Insecticidal soaps and horticultural oils are almost universal treatments against these creatures. Just follow the application details explained on their packaging. You can also spray infested plants with solutions of dishwashing liquid or other soaps, but make sure you test a few leaves first: they can be toxic de certain plants.

Another possible treatment is to spray a solution of 1 cup (250 ml) of rubbing alcohol in 1 quart (1 liter) of water to control scale insects, mealybugs, aphids and whiteflies. Warning: for your own protection, ventilate the room when applying rubbing alcohol on anything more than a limited scale. Its fumes can be toxic.

Keep an eye open and your finger on the spray bottle’s trigger. That way you ought to be able to stop pests in their tracks… and allow your plants to take full advantage of the lengthening days!

This text was first published on this blog on January 28, 2016. It has been revised and the layout updated.

1 comment on “Longer Days Awaken Pests

  1. In mild climates, we put some of the more resilient houseplants within sheltered situations outside for the end of winter after any threat of frost. In some regions, there is no frost. Not only to they get rinsed by gentle rain, but some of the wild predatory insects in the garden can tend to some of the insect pathogens that proliferate within the home where predatory insects do not like to go.

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