It's known as 'honeydew,' but it's not exactly sweet

By Tony Tomeo

Several years ago, while visiting a friend in Santa Monica, I parked my car under a carob tree while we took another car to San Diego. When we returned the next day, I found that an immense flock of wild parrots, which had been grazing in the tree, had dropped an incredible amount of carob shells and other organic matter all over my all too-cool Buick. I was not happy. I can certainly identify with those who are currently plagued with the sticky black mess under many of the holly oaks, olives, elms, tulip trees and various cedars. Those of us who want trees in our landscape expect at least some degree of foliar or floral mess at certain times of the year. Many tree species also drop fruit or seeds. However, this current epidemic of "black plague" is more than most plan for.

The black sticky substance, known as "honeydew," is actually not produced by the trees, but by either aphid or scale insects. As these insects feed, honeydew is excreted as a transparent syrup which later stains black from sooty mold fungus. During extreme infestations, honeydew and sooty mold can be very messy.

Near my home, the sidewalk under a very large deodar cedar was so sticky that Privet, the terrier, wanted to be carried until we reached clean pavement. Lighter infestations are actually much more common than most realize and usually do not present a problem. As with many gardening problems this year, the mild summer has enhanced this problem by minimizing the vigor of affected trees. Because aphid and scale are much more prolific on stressed trees, their populations have reached epidemic proportions, even where their presence is normally tolerable.

Fortunately, scale and aphid end their life cycles in autumn, leaving their eggs for next spring's generation. Most of the honeydew and sooty mold will eventually be washed off by rain. This is why insecticides which were not very effective during summer suddenly seem to start working. However, one must not forget that formerly infested trees contain the necessary ingredients for new infestation next year.

Although the problems will not begin to materialize until the trees break dormancy, it is helpful to know what to do about them prior to and as they occur. Scale and aphid are rather simple to combat in small trees and shrubs, which can be completely sprayed either with agricultural-grade oil to suffocate scale or soapy water to explode aphid. Of course, large trees need considerably more work.

Agricultural-grade oil is very effective at suffocating overwintering scale and their eggs as well as the eggs of any other insect or mite, including aphid. However, oil can only be applied to dormant deciduous trees, as the oil damages foliage. It cannot be used when trees are foliated during spring and summer, when insects are most active. Because oil must come into contact with the targeted pests, complete coverage is essential. If oil is not applied properly it may be ineffective and may damage the trees it is intended to protect.

Soapy water (and its commercially available equivalent) kills any aphid it comes into contact with, as well as washes off some of the honeydew and sooty mold. Unfortunately, it does not kill their eggs or scale, if also present. Although it is useful on foliated trees with active aphids, it is nearly impossible to apply completely to large trees, and professional pesticide applicators are often required. Even if the majority of aphids are killed, their populations recover very quickly, so multiple applications may be necessary. Because most of the damage is done in spring, and aphids normally finish before autumn, applications made after September are effective only at washing the trees.

One of the best controls for aphid and scale is actually very old technology. The same sticky barriers which prevent mites and scale from infesting citrus trees are also effective for large shade trees. Although there are products commonly sold for application as barriers, I find that simple axle grease works just fine except for its unsightliness. Scale cannot fly, so if tree trunks are not accessible and branches do not come into contact with unprotected trees or buildings, new infestation is not possible. Although aphids fly, they, as well as scale, have a symbiotic relationship with ants. To maximize the production of honeydew, one of their favorite foods, ants "farm" both species of insects and distribute them throughout affected trees and eliminate their predators. If ants are kept out, predatory insects reproduce at a rate proportionate to that of scale and aphid. These same predators, which cannot move as fast as ants, easily devour immobile scale or slow-moving aphids. These barriers are best applied in late winter, before active foliar growth, or better yet, immediately after properly scheduled application of oil or soap.

Flower of the Week: Canna

Even the least interesting species have impressive botanical names. However, canna is known only as Canna spp. This species was once very popular and is currently making a comeback. The large, lush green or bronze foliage is held on stalks as short as 1 1/2 feet tall or as high as 6 feet tall, depending on cultivar. The flowers, which appear in summer or autumn, range in color from red, dark red, coral, orange, pink, white, light yellow and bright yellow. Cannas require full sun and good drainage.

They perform well in large containers; but in the ground will tolerate almost any soil. They can easily be grown from tuberous roots obtained from other gardens in the neighborhood. Otherwise, their recent popularity has made them available in most nurseries or garden centers.

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Contact horticulturist Tony Tomeo at 358-2574.

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