The downside to having plants is when they attract pests. And those little critters can be tricky -- they tuck themselves under the soil, come out only at night or try to hide on the undersides of leaves. Small groups of black bugs clinging to your leaves probably won't hurt your plant, but if you have large clusters of them, you need to treat your plant.
Many pests feed on plants. Some chew on the leaves, and others use special mouth parts to suck liquid out of the plant. The most common bugs on the undersides of leaves and fit the description of small black bugs are aphids and spider mites. Aphids are more commonly seen outdoors, while spider mites are more common on houseplants. Aphids are indeed bugs -- they are tiny insects that, along with black, also come in shades of yellow, green, brown and pink. Spider mites, on the other hand, are not insects -- they are related to spiders. Tiny and black or red -- they look much like moving dots -- spider mites are often recognizable by the webbing they leave around the leaves and stems. Both pests can be eradicated from your plant using the same methods.
Sick Plant Symptoms
A scattering of bugs is not usually a problem, and if you spot a few tiny black bugs but your plant seems otherwise healthy, you are probably fine just keeping an eye on the situation. On the other hand, if your plant is showing signs of distress -- wilting or yellowing, distorted or speckled leaves -- and you see large numbers of bugs, you need to treat the problem immediately to minimize damage to your plant.
Aphids and spider mites have many natural predators. In the case of outdoor plants these predators -- including lady and soldier beetles -- can help keep populations down. For this reason, it's best to first try nonchemical controls on plants affected by aphids and spider mites rather than using pesticides that can harm beneficial insects. One of the most effective ways of getting rid of those bothersome bugs is to hit your plant with a strong stream of water. Take houseplants outside first, and isolate affected plants from others so you don't knock the bugs onto a nearby plant. After spraying it down, set the plant out in the sun to dry. Repeat this process every time you notice bugs accumulating under the leaves. If you're in an apartment, use the shower in the bathroom or a spray attachment on the sink or wipe the bugs off with a damp paper towel.
Insecticidal oils and soaps are recommended for use on aphids and spider mites if infestations are severe. Consider the infestation severe if the plant looks sick. Coat the plant thoroughly, taking care to make sure all the undersides of the leaves are covered. For oils and soaps to be effective, they must come into direct contact with the bugs. More than one application will likely be necessary, and you can reapply after three to 14 days, depending on the specific brand and severity of the infestation. In general, most ready-to-use brands come in spray bottles and all you need to do is shake and spray. Always protect your skin and eyes when applying, and use horticultural oils and soaps only on plants that are not suffering from water stress. In addition, apply only on calm days when temperatures are below 90 degrees Fahrenheit. If you're using it on houseplants, take them outside to spray, if possible.