Question:

What steps do you recommend as we transition our houseplants back inside and prepare patio plants for the winter?

- Dan G., Bosque Farms, Valencia County, NM

Answer:

This week we'll go into more detail about checking houseplants for bugs before they get too comfortable and their populations get out of control. For more tidbits on how to care for your patio plants and houseplants when you bring them inside for the winter, check out last week's column SWYG - Nov. 18, 2017.

I asked County Extension Agents from all over New Mexico to share the most common houseplant questions they receive. Responses were overwhelmingly pest-related. Dr. Carol Sutherland, NMSU Extension Entomology Specialist and NMDA State Entomologist, provided the skinny on a few pests you are likely to encounter and what to do about them:

Spider mites are nearly microscopic, 8-legged, wingless creatures that thrive on a tremendous variety of plants. When plants are moved indoors for winter, spider mites come with them. Spider mites have mouthparts small enough to puncture and kill individual plant cells. As mite populations increase, foliage will look increasingly dry and off-color. Dead leaf cells turn white or yellow; damage is cumulative. Older leaves may wither and fall as the mites move to new growth. Turn leaves over to look for the slow-moving mites, their pearlescent eggs, cast skins from molting, and fine silk filaments covering spider mite colonies. Lightly infested plants may benefit from periodic “shower baths” of water on their foliage (top and bottom), although this can be unacceptably messy and is not advisable for more delicate plant species. As plant health declines, spider mites can easily disperse to other plants by direct contact or by riding air currents created by the heating system or overhead fans. Seriously infested plants should be discarded.

Whitefly adults can billow out of infested outdoor plants like flying dandruff. Like many other common plant pests, whiteflies have a broad host range, including various popular houseplants. Although they look moth-like, whiteflies are very distant relatives of aphids, and both nymphs and adults have piercing-sucking mouthparts. Their two pairs of wings are dusted with fine white wax particles—the source of their common names. The largest adults are barely 1/8 inch long. Female whiteflies usually lay their tiny, stalked eggs in spiral patterns on the undersides of foliage. Toxins in whitefly saliva cause irregular growth in host plants as well as leaf curling, yellowing, and foliage loss. Honeydew, their sugary liquid waste product, is usually sprayed on host foliage, where it can leave a shiny, sticky residue that is also attractive to foraging ants.

Fungus gnats are those very tiny, blackish, mosquito-like creatures that fly in your face when you water your plants indoors. Female fungus gnats are attracted to the organic matter in your potting medium. Algae, fungi, and various microbial growths are potential food sources for the adults, while the moist organic matter in the medium is an attractive place to lay their eggs. Their minute, thread-like larvae feed mainly on this organic matter. It doesn’t take long for fungus gnats to find houseplants and become annoying pests. Try reducing the amount of water applied and putting a layer of clean builder’s sand over the exposed organic matter on the potting medium. Alternatively, mix Bacillus thuringiensis israelensis (i.e., Gnatrol®) with your regular irrigation water (read the label for rates!); this product is regularly used by many nurseries to kill fungus gnat larvae in growing media.

Mealybugs are the bane of existence for indoor plants. Distant relatives of aphids, mealybugs get their common name from the mealy white, waxy substance that covers the bodies of adults and nymphs. This body covering is exuded from glands in the backs of the insects. The wax protects these pests from water droplets as well as most topical insecticide treatments. Another problem is that mealybugs are mobile and tiny, 2–3 mm long at most for adult stages. They are excellent at hiding in natural crevices on the plant as well as on the lower stems and roots below the soil surface. It is nearly impossible to find and kill all of these pests even with diligent inspection and most non-systemic insecticides. Under these circumstances, surviving mealybugs can continue to feed on plant sap with their piercing-sucking mouthparts and reproduce without significant interruptions.

Organic methods of management: The best offense is a good defense. Carefully examine houseplants before bringing them into the house from outdoors or a nursery. Keep all newly acquired plants separate from your other houseplants. Continue inspecting new plants carefully for evidence of infestation over the next week or so before placing them with other plants in your collection. Then exhaust your available options using the “tool kit” provided by Integrated pest management (IPM) practices. Here is a link to an Extension publication on Circular 655: IPM For Home Gardeners.

Yellow sticky traps are available in most nurseries and are good indicators for the arrival of certain plant pests, especially in greenhouses. While they catch certain flying pests like thrips, aphids, whiteflies, fungus gnats, and others, sticky traps cannot “trap out” pest populations even indoors or in a greenhouse and do not affect pest reproduction on their host plants.

Organic treatments for insect pests on certain houseplants might include insecticidal soaps, spinosad Beauveria bassiana, certain horticultural oils from botanical or petroleum sources, pyrethrins, or azadirachtin. Read all labels carefully before applying them to your plant collection! Dish detergents are NOT labeled as insecticides; some may harm or kill plants, especially if usage rates are high. If particular plants are not included on the label, forego treating those plants with that product or contact the manufacturer for further information.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!