Check indoor citrus plants for scale insects to prevent sticky honeydew and sooty mold

Many people have a potted citrus plant or two that they keep indoors during the winter and move outdoors for the summer. It may be one of those dwarf trees that produce full-sized fruit. Or it could be a tree that their child planted from a grapefruit or orange seed.

Moving plants outdoors for the warm season helps to keep them vigorous. However, when potted plants live outside during the summer, they run the risk of being exposed to harmful insects.

Usually if the plants are in reasonably good health they can tolerate a moderate insect attack. Natural predators like lady bugs and lace wings will keep the detrimental insect population to a minimum.

When plants are moved indoors for the winter, they lose the protection of those natural predators. Dry air and low indoor light encourages insect development and weakens plants, which can result in a growth of the harmful insect population.

Scale insects are the ones I seem to have the most problem with. These are stationary insects that form protective coverings over themselves during their adult stage. Depending on the species, the covering can be hard as in the case of “armored scale”. Another type called “soft scales” produce a sort of fluffy, waxy glob as their covering.

Scales feed by poking their mouth parts into the plant and drawing nutrients from the plant juices. Like most other insects, they excrete waste. For scale insects, their waste is in the form of a sticky excretion called “honeydew” that can quickly build up on leaves. Aphids, another common type of sap-feeding insect, also excretes honeydew.

But how and why do scale insects produce so much sticky residue? The answer is that they pick their feeding spot very carefully. If you remember from middle school biology, plants have two basic types of tubes inside. Those that carry water from the roots up into the stems and leaves are called xylem. The other tubes, called phloem, carry nutrients manufactured by the leaves to the rest of the plant.

It is into the phloem where the scale insects like to poke their beaks. If they pierced a xylem tube, all they would get is mostly water and some dissolved minerals. The phloem sap contains sugars for energy, proteins for growth and other things necessary to sustain plant and animal life.

The scale can't use all of the sugar dissolved in the phloem juices so they excrete the excess sugar, which then falls all over the immediate area.

If you start to find sticky spots on the plant’s lower leaves, on nearby furniture or the floor, you know that the scale insects are ramping up their feeding. They can get out of hand quickly at that point and do some real damage to the plant, not to mention the mess they make.

Honeydew is sometimes mistaken for sap by some plant owners because it has the outward appearance that the plant is leaking sap all over the place.

If you've never seen an armored scale infestation before, it's not always easy to recognize since the feeding insects are stationary and are covered by a hard shell. They don’t have the typical appearance of what we think of as insect.

Soft scale, on the other hand, is easy to spot since the covering is white and fluffy.

Because honeydew is primarily sugar, it is water soluble and easily dissolves in water. Small plants can be rinsed off in the sink or bathtub; leaves and stems on larger plants can be wiped clean with a damp cloth.

A stream of water can knock off some of the scale insects from the plant, especially while they are in their “crawler” stage, the tiny nymph stage in the scale’s life-cycle. Crawlers hatch from eggs that the adult lays under its protective covering. Once the crawlers hatch, they crawl out from the protective shell and move to a to a new location on the plant before settling down to grow their own covering. That’s how a scale infestation spreads.

My citrus trees are way too big to rinse off in the bathtub and too heavy for me to move to the shower, so I use a damp cloth to rub off the scale from the leaves and branches whenever I find them. Citrus plants have smooth, hard leaves, which makes them easy to clean that way.

In the past I have been known to wheel my big potted citrus outside during a January thaw and spray them down with a garden hose, but it’s been quite a few years since the last time I did that.

Wiping off leaves can be a bit of a challenge on larger trees. However I find that if I do a few leaves or branches every day, I can usually keep up with the multiplying insect population.

It’s best to start any clean up early as soon as any signs of honeydew are found. If you wait too long, it can turn into a tedious, frustrating job.

Another problem with waiting too long to clean is the growth of “sooty mold”, a black powdery mold that grows wherever honeydew is on the plant and other surfaces.

All of that sugar provides food for sooty-mold fungus that will spread and cover the sticky surfaces with an unsightly coating of black powder that will easily rub off onto clothing.

So check your citrus plants, and other plants for that matter, for scale, and get them cleaned up before it becomes a big problem.