The hibiscus is believed to have originated in islands of the Indian Ocean, a warm, tropical locale that provided plenty of sunshine and warmth. Today, the hibiscus is grown in areas far from tropical, a situation that often results in damaged leaves. A wide variety of hungry insects also contribute to foliage problems on the plant.
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Pink Hibiscus Mealybug
Insect infestation, in severe cases, can be one of the causes of hibiscus leaves turning yellow and falling off. The pink hibiscus mealybug feeds on the sap of hibiscus and other ornamentals, causing the leaves to yellow and drop. This pest is new to the U.S., having first been spotted in south Florida in 2002. The insect possesses a pink body and secretes an orange substance that causes the plant to develop a black, sooty mold on the leaves. Entire defoliation of the plant is possible.
Other insects that can damage the foliage and flowers of a hibiscus include caterpillars, grasshoppers, snails, slugs, beetles, cutworms, leaf miners, scale, spider mites, aphids, whiteflies and thrips. Feeding by scales can cause yellow spots to develop on the upper surface of hibiscus leaves, eventually growing larger as feeding continues. Parasitic wasps help to naturally control the presence of scales. Whiteflies, an insect that resembles tiny, white moths, commonly feed on the upper surface of hibiscus leaves, bringing about a pale spotting on the flesh.
Hibiscus leaves turning yellow and falling off can also be a symptom of weather injury. The plant thrives outdoors through the winter only in USDA hardiness zones 9 and 10, an area limited to warm, tropical locales like south Florida, Hawaii and south Texas. Even in these zones, temperatures can routinely drop below 30 degrees F, a scenario that will cause cold injury to an outdoor hibiscus, damaging the leaves and resulting in their dropping off.
Developing yellow leaves that fall off is also a common problem when a hibiscus is brought indoors for the winter, reports the University of Illinois Extension service. The hibiscus is stressed due to less access to sunlight than it is accustomed to. Over-watering and over-fertilization contribute to this stress. During the winter months, the plant slows down biologically and requires less sustenance. Plants that are over-hydrated or given too much fertilizer may respond with a yellowing and dropping of their foliage.