Q. What kind of pests do I have to look out for on houseplants? How do I prevent and/or treat for them? My last casualty was an English ivy that ended up with spider mites.
Houseplants can develop pest problems rather quickly since there are usually no predatory insects (like ladybugs, praying mantises, or lacewings) to keep the pest population under control. I once heard a story about a lady who had aphids on many of her houseplants, so she purchased a container of ladybugs and released them in her house. This took care of the aphid problem, but the ladybugs multiplied and soon her walls were covered with ladybugs.
There is an easier way to care for your houseplants.
If you suspect insect (or mite) problems, first look at the undersides of some of the leaves. Pay close attention to the tender new leaves since they are most attractive. Some pests are very tiny, but you can diagnose the problem by looking for certain signs.
Scale and aphids like to hide under leaves and will leave a sticky residue. Sometimes ants will be present since they harvest the sticky honeydew. Black sooty mold may even be present. Place ant bait at the base of the plant and treat with insecticidal soap or Neem Oil.
Spider mites are nearly invisible, but they leave very fine webbing on and between leaves. With a magnifying glass you may see tiny red specks – those are the mites. As they suck the sap, the leaves will take on a dull, rough appearance and will start to die and fall off the plant. Spider mites thrive in dry climates. They seem to really like ivy and schefflera. Insecticidal soap or Neem oil is the standard treatment.
Fungus gnats are annoying and their larvae can cause serious damage to houseplant roots. Yellow sticky traps will take care of the adult gnats, but the larvae must be destroyed as well. Treat the soil with Bacillus thuringiensis (var. israelensis) soil drench. This is not the same Bacillus thuringiensis that is marketed as a caterpillar killer and will probably only be available through a well-stocked garden center or mail order.
When dealing with an infested houseplant, you may have to decide whether it’s worth saving. In most cases, the plant may be so far gone that even multiple treatments may not be enough to save it. Sometimes it’s just easier to replace it. If you decide to treat, keep the infested plant away from unaffected plants.
Los Angeles County
mglosangeleshelpline@ucdavis.edu; 626-586-1988; http://celosangeles.ucanr.edu/UC_Master_Gardener_Program/
Orange County
ucceocmghotline@ucanr.edu; http://mgorange.ucanr.edu/
Riverside County
anrmgriverside@ucanr.edu; https://ucanr.edu/sites/RiversideMG/
San Bernardino County
mgsanbern@ucanr.edu; 909-387-2182; http://mgsb.ucanr.edu