I have a bay laurel that I have been growing indoors for several years without moving it. This spring, I noticed that its new leaves are becoming sticky and secreting a sweet liquid that is settling on everything around the plant. I have never seen this before. Do you have any idea what might be causing this, and is there anything I can do to fix it?
Answer
If the leaves of your bay laurel (Laurus nobilis) are sticky, it is likely that it is infested with a sucking insect. The sticky substance you see is honeydew, a sweet liquid excreted by insects that feed on sap, such as scale insects (mealybugs or armored scale), aphids, and whiteflies. Honeydew sometimes attracts sooty mold, a black mold that covers the leaves and hinders photosynthesis.
Aphids and Whiteflies
These two insects are common on houseplants. Aphids form small colonies on young shoots, while whiteflies fly away in swarms as soon as the plant is touched. They multiply rapidly, especially in dry, warm air. To eliminate them, use an insecticidal soap, such as black soap or a ready-to-use product sold at garden centers. Spray the foliage generously (both above and below) every 4 to 7 days until they are eradicated. Avoid dish soap, which can damage plants. Isolate infested plants to prevent the pests from spreading to others.
Scale Insects
Bay laurel is particularly vulnerable to this insidious pest. These scale insects look like small, flat, brown or white bumps, often attached to the veins and stems. They can sometimes be detected earlier, when they are still translucent and stuck under the leaves. The problem is that they are very resistant to treatment. Even if you treat them with alcohol, insecticidal soap, or horticultural oil, just one survivor is enough for the infestation to return a few months later. And if they spread to other plants, the problem doubles in size.
Unfortunately, in many cases, the only reliable solution is to throw away the infested plant to prevent contamination of the rest of your plants. Then, thoroughly clean the surrounding area and monitor other indoor plants for several weeks.
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Umbellularia californica, California bay laurel has the same problem, but as a large tree. ICK! The lower foliage of many large trees typically has black sooty mold on it.