My houseplants have thrived outdoors during the summer enjoying the stronger sunlight, the humid conditions and good care. Before any frost touches them, though, they need to be brought back indoors. In returning them inside, insects can become a problem. A few always hide in the plants, no matter how careful I inspect them. A few weeks after the plants come in, things like mealy bugs, white fly, spider mites, and scale show up.
Here are some practices I use to help keep the invasion to a minimum.
First off is prevention or at least reduction. Several weeks before plants come indoors, they need to be cleaned on a weekly basis. Pick out debris that has accumulated in the pot, peel off dead foliage. Next I spray the upper and lower sides of the leaves with insecticidal soap. Not all plants tolerate this spray, so test it one week on one leaf before you use it on the whole plant. Most do fine. Before bringing plants indoors, spray with insecticidal soap one more time.
Soaps contain fatty acids that kill insects on contact, but have no residual effect to ward off new insects. They are not poisonous to animals or children, and don't remain in the environment. The soap can burn leaves if left to dry in the sun, or if the plant is sprayed when stressed from heat or drought, so be careful using it. There are good commercial brands of insecticidal soaps that come both in ready-to-spray premixes or in a concentrated form you mix yourself, but here are recipes for some homemade mixtures:
Soap-only Insecticidal Soap. 1-1/2 teaspoon soap (such as liquid Ivory) to 1 quart of water. Mix in a spray bottle. This recipe can be used as a general preventative. Use every two weeks, or once a month, spraying tops and bottoms of leaves, stems and soil surface.
Soap and Alcohol Mixture. Use this one on tough cases of scale.
Take above mix and add 1 cup of rubbing alcohol (isopropyl).
2 tablespoons of liquid dish detergent to 1 gallon of water.
Spray the plants in an area that won't be damaged by the soapy water, such as the bathroom tub, and let them dry. If the plant is infested with insects, spray once a week until they are gone. Then spray the plants on a monthly schedule. This not only kills the insects, it cleans the foliage of dust that collects on the leaves of indoor plants. Remember, plants react differently and soaps differ, so test spray a few leaves before covering the whole plant.
Some insects, such as scale, may still return. A cotton-swab dipped in isopropyl alcohol wiped on the insect kills it, or try the stronger spray recipe containing alcohol.
Don't bring in pests as you bring your houseplants inside.