Scale Insects on Houseplants

This is the second article on how to identify and treat insect infestations on houseplants. In all the years I maintained and managed interior tropical greenhouses, the most important factor in keeping the plants healthy is to make sure insects were held at bay. In each of the different areas of the eastern part of the United States where I worked many different greenhouses, I had to understand the yearly cycle of each greenhouse in the area I was working. It took me a full year to understand how the seasons affected the interior of the greenhouse and once I understood that, I knew when to expect the appearance of different insects. Because these greenhouses are retail, the use of highly toxic insecticides just was not an option. In the southern states where the temperatures and humidity are more consistent, the insect problem usually remained more constant than in the areas where the seasons are more distinctive.

Of the many types of scale, this type is a small hard-shelled little beast that is born from eggs laid and kept safe under the mother's body. When the eggs hatch, the little adult look-a-like is ready to go off on its own and begin feeding. If you look closely with magnification, you will notice that their shells are not complete yet and the body is visible. This is the crawling stage, when the insect is most susceptible to insecticides.

Because of their hard outer shells, insecticides need to penetrate it in order to kill the insect. I have tried systemic powders that one mixes in the soil of the plant, but it is not effective to an infestation that already has a hold on the plant. Besides having a terrible odor, the powder is completely ineffective on an infestation that has a strong hold on the plant. It works best as a preventive, acting as a deterrent to the many types of insects that attack houseplants. If this is a preferred method, mix the powder in the soil as soon as you purchase the plant and use it monthly. Be sure to follow the manufacturer's direction very closely. A warning, however, the smell is potent and the product is harsh. If too much builds up in the soil, the plant will suffer. The best liquid systemic insecticide is one that penetrated the foliage from the leaf surface. This type usually has more of an effect on an established colony, but is very deadly and should be used only outdoors and must be completely dry on the plant before bringing it back inside.

The hard shells of these little critters make the use of insecticides difficult. Because the breathing holes are on the insects' sides, horticultural oil is probably the most effect insecticide I have used on scale. It blocks those holes when coating the shell and the insect suffocates. It must hit the insect and coat the shell for it to be effective. This is one of the few solvents where the insect cannot develop immunity to the product. Dish detergents are very powerful chemicals and will often do more harm to the leaves than to the bugs. Isopropyl alcohol (rubbing alcohol) will not affect hard shell scale. It only works on mealy bug (soft shell scale). The only truly organic method of eradication is by squishing each insect with the fingers. This, however, is next to impossible when a plant is full of scale. Getting all of them will take longer than the insect can reproduce. It will be a constant battle that is not likely to have a happy-ending for the plant.

Scale, like mealy bug, secrete a sweet honey dew fluid that adheres to the leaves or any other surface near the plant. This sweet coating attracts some other things that are not welcome in our homes. Ants love this secretion and will often build a nest in the soil. On many occasions, people called me in mid January asking where the ant infestation is coming from that roams their house. In New England, there are no ants roaming except inside. If the plant is outside for the summer and gets scale, mealy, or aphids, at some point, the ants will find the infested plant and take up residence near their food source. Therefore, of course, by not putting the plants out for the summer, it helps to minimize the possibility of getting an infestation. (See the article Placing your houseplant outside for the summer.)

When one or two insects get on to a plant, it is much easier to get rid of them with less stress to the plant and you. Gently rubbing them off with the fingers will stop an infestation cold. The most important battle against scale is to prevent them from developing an infestation in the first place. I cannot stress the importance of a weekly check on your plants. If you are diligent and carefully inspect each plant on a weekly basis, you will not only spot any insects in the early stage, but will get to know your plants very well. Then if something is wrong, you will notice it right away, even if you are between inspections. Plants will react to insect attacks and show signs of stress almost right away, and if you ignore the problem in the early stages, it only gets worse.

The internet contains all kinds of information on everything imaginable. Of the sites on getting rid of insects on houseplants that I visited over the past several years, I noticed one thing; the site is vague about getting rid of the infestation and has very little information on how to prevent them in the first place. Some sites show pictures of the insects, which helps to identify the problem, but does nothing to specify how difficult it is to eradicate an infestation completely. The process is long, time consuming, and takes both diligence and persistence. The best option is to prevent an insect from establishing a colony on your plants.

If you are a bit familiar with plant physiology, then you know how a plant takes in carbon dioxide and along with sunlight and water, photosynthesizes food, and releases oxygen as a by product. This is why talking to your plants is actually beneficial because the release of CO₂ near the plant enables it to take it in quicker than through room circulation. Also touching the plant tends to stimulate growth. That is, gentle touching or rubbing, not constant petting or stroking helps to stimulate new growth. (Some plants react to constant stroking of their foliage by developing brown tips.) Thus, when checking for problems, handling the leaves to check for unwanted guests is a stimulant for the plant to grow.

If you are a plant lover like me, this preventative method of a weekly check is the most organic and safest way for both you and the plants for keeping them insect free. It is a win-win situation for all, and helps to keep your plants healthy, strong, and vibrant.

Plants that are most sustible to hard shell scale are:

Aglonemia (Chinese Evergreen), Coleus, Chlorophytum (Spider Plant), Marnata (Prayer Plant), all Ferns, all Hedera (Ivey), all Gesneriads (Africal Violets, Streptocarpus, Lipstick Vine {Aescananthus} Coumnea, Goldfish, Episcia) Most Cactuii, all Citrus, Dieffenbachis (Dumb Cane), Clivia, Coffea (Coffee Plant), Cyperus (Papyrus Plant), all Dracaena,
Dizygotheca (False Arelia), Fatsia (Japanese Aralia), Fasgedera (IvyTree), all Ferns, all Chlorophytum (Apider Plant), all Ficus, Gardenia, Hibiscus, most Herbs, Gynera (Purple Passion), Heptapleurum arboricla, Iresine (blood Leaf),all Jasmine, all Marenta (Prayer Plant), Calathea, Musa (Banabna Plant), all Orchids, Osmanthus fragrans (Sweet Olive), all Palms, Passifloria (Passion Flower), Pellonia, Peperomia, all Ohilodendron, Plectranthus (Swedish Ivy) Pittosporum )mock Orange), Podpcarpus (Budist Pine), Radermanchera, Saxifraga (Strawberry Begonia), Schefflera, all Scindapsis (pothos), Senecio (German Ivy), Spathiphyllum (Peace Lily), Stephanotis, Strelitzia (Bird of Paradise),all Succulents, Syngonium (Arrowhead Plant), Rhoicissus (Grape Ivy).