When your plants don’t appear healthy and the leaves are sticky, sucking insects could be the culprits.

The stickiness is probably honeydew, and I’m not talking about the melon. Sucking insects insert their mouthparts, called stylets, into the plant tissue and suck out sap. Then they excrete what’s called honeydew, which is high in sugar and often makes leaves and stems of infested plants feel sticky. Heavy infestations could result in stressed plants with reduced vigor.

Question: It looks like there’s a powdery black mold on my citrus. What can I do about it?

Answer: The black sooty mold is a fungus disease that grows on the honeydew. A heavy covering of sooty mold can reduce the amount of sunlight reaching the leaves, affecting photosynthesis. Longtime Hawaii residents might recall the spiraling whitefly infestations in the late 1970s and early ’80s. At the time, there were few or no natural enemies to control the whiteflies, and they infested many types of plants, creating both honeydew and sooty mold. These not only created problems on infested plants, they also dripped onto vehicles and things under infested trees.

To get rid of the sooty mold, you have to first get rid of the sucking insects.

Q: How do I get rid of aphids?

A: Aphids are one of several types of sucking insects. All tend to live on the underside of the leaves, away from direct sunlight and predators. Soft-scale insects are often oval, and once they pass the crawler stage, they remain mostly immobile. Those with armored scales are covered with a waxy protection, and the insects resemble a speck of wax. Mealybugs are often yellow to gray with multiple protrusions along the sides and a white, waxy coating. Immature whiteflies resemble oval scales, and the adults look like tiny white moths.

Aphids are mostly pear-shaped with a pair of protrusions called cornicles on the rear of the abdomen. These sucking insects tend to stay together on the plant, so they often appear to live in colonies. To get rid of these insects, prune heavily infested branches. If infestation continues, spray the insects with insecticidal soap, making sure to spray the undersides of the leaves. The soap will smother the insects. It is usually effective on soft-bodied insects like scales and mealybugs but less so on hard-bodied insects like beetles, so it’s usually safe to use since it seldom affects the natural enemies of the sucking insects.

The state Department of Agriculture introduced several different ladybugs and parasitic wasps to control the spiraling whitefly. Some of these natural enemies are also controlling other whiteflies, scales, mealybugs and aphids. Rearing and release of these natural enemies of the pests are usually done by government agencies or university researchers. Gardeners are unable to buy natural enemies, and it’s not a good idea to import them from the mainland, as unwanted pests might come in along with them.

Some of the sucking insects, especially scales, are also controlled by diseases. A common fungus called the white fringe fungus infects scale insects and creates a white halo appearance surrounding the scale. The infected scales are overcome by the fungus.

Q: Is it true that ants "farm" aphids?

A: Yes. Ants feed on the honeydew, and therefore protect sucking insects, including aphids, from natural enemies like ladybugs and aphid lions.

Ants have even been known to move sucking insects like aphids to other parts of the plant. If gardeners can control the ants, the natural enemies have an easier time getting to the sucking insects.

Ant control is difficult, however, as spraying plants with pesticides is not effective for killing them. Instead, the colony of ants must be controlled with baits containing a toxin that foraging ants take back to the colony to share with others. Finding the right ant bait is a challenge, as not all ants will accept the baits being used.

One ant bait is a sweetened liquid similar to honeydew. For outdoor use, the liquid bait is placed in a container that keeps the rain out but allows the ants to enter. However, ants might feed on the liquid bait one day and ignore it the next. Another problem is you might get rid of one type of ant species, only to find that another kind has come along and moved in.

Richard Ebesu is a University of Hawaii extension agent and the Kauai Master Gardener coordinator. Email him at rebesu@hawaii.edu.