Came across this today.
During the winter months, when there isn't much to do except stare out the window or peruse seed catalogs, gardeners need to be on the lookout for a couple of tough pests that, once settled in, can be hard to control.
Fungus gnats, the near-microscopic flying insects that feed and breed within damp organic matter in potting soil, can actually be a problem any time of year. These tiny flying pests can spread pathogens, chew on roots and be a general nuisance. Fungus gnats' favorite hangout is the fungus existing in moist, organic soil, so the best way to battle these bugs is by letting your houseplants dry in between waterings. However, some overwintering houseplants — like those lovely amaryllis bulbs many of us are forcing during the holidays — like to remain somewhat moist. So...what to do?
A good drench of Neem oil through the soil is a good start, and a layer of sand on top of the potting soil works even better. This prevents the fungus gnats from reaching and subsequently laying eggs in the potting soil, but still allows moisture to reach the roots.
On the opposite end of the spectrum, spider mites can become a terrible problem for indoor plants when conditions are too dry, which is often the case during cold winter days when most of us are keeping toasty. If you notice a plant's leaves shriveling and dropping, or webbing in between stems and leaves, you've probably got a spider mite problem. Again, Neem oil is an extremely effective solution, and its low toxicity makes it safe to use around the house. The mites can be killed with several sprays of Neem. However, the humidity around the affected plant will need to be addressed to keep mites from returning:
Keep plants away from the hot blast of furnace output vents.
Spray sensitive indoor plants with water daily if possible.
Move all sensitive plants together and run a humidifier near them to keep air moisture levels high.