SCALE INSECTS are the usual cause of sweet, sticky drippage (honey-dew) from trees that collects on surfaces below. APHIDS are often blamed, and sometimes contribute to the honeydew, but by far the most copious honeydew production is by soft scale insects described below. The good news is that we have methods to control the problem, no matter what insect is to blame.
COTTONY MAPLE SCALE is an insect pest of maples, especially silver maples and locust trees primarily, but also will infest several other species. The pest is very common in the tri-city area of Michigan at this time. Although the insect does not usually cause much damage to the tree, it is truly a pest to the humans who live below because of the copious honey-dew excretions of the insect. The honey-dew falls onto surfaces below and sooty mold proliferates in the sticky sweet substance.
Treatment of the scale is difficult, especially on large trees. Sprays are often ineffective and can worsen the condition if done in May and June, because beneficial predatory insects are killed while the adult scales are largely resistant to treatment by sprays. The white egg sacs burst open in July as the eggs hatch and the immature scales crawl out to the leaves. Sprays coating the underside of the leaves at this time can be effective, but thorough coverage is difficult.
Systemic insecticides applied by trunk or soil injection are the methods of choice but are more costly than sprays. The increased cost is justified because the method is VERY effective and may last two or more years, especially if "boosted" by the homeowner with soil applications of imidacloprid, (see below). The best thing about systemic treatment is that it does not kill the beneficial predatory insects (twice-stabbed ladybugs and tiny parasitic wasps).
LECANIUM SCALE is another "soft scale" insect which can be encountered in the tri-city area at this time. This scale is very prevalent on ash trees, sugar maples and oaks, but can infest many species of deciduous trees.
Like the cottony maple scale and other soft scales, the lecanium scales excrete large amounts of honey-dew with its attendant nuisance factor. However, these scales can also cause significant lower branch dieback and can be seriously stressful to the tree. We have seen ashes and sugar maples in Midland with severe lower branch dieback caused by this scale insect.
This scale's life cycle is slightly different that the cottony maple scale, with the eggs hatching slightly sooner in June or early July. Systemic insecticides have been shown to be effective against this insect if used with the right timing, and these treatments do not kill the beneficial insects.
We have had excellent results, achieving 80 to 90% reduction of scale populations and similar reduction of the honeydew and sooty mold problems. Our treatment of choice is soil or trunk injection of acephate or dinotefuran, usually supplemented by trunk or ground injection of imidacloprid. We often inject dinotefuran or acephate at the first visit, followed a few days later by an injection of imidacloprid. In spring treatments, the first treatment gives quick kill and the imidacloprid moves into the tree slowly and is present in the tree when egg hatch occurs and crawlers appear. We also suggest that HOMEOWNERS drench the soil each fall with Bayer tree and shrub care or other brands of imidacloprid (like Bandit) to "boost" the imidacloprid level in the tree. The tree may never need another injection. The cost for trunk injection is usually around $10 per trunk diameter inch. The rate may be reduced if several trees are treated in one yard or neighborhood.
EXPECTATIONS FOR RESULTS
The first product will rapidly spread throughout the canopy of the tree, usually within a few days. One limitation is that the chemicals do not necessarily go into 100% of the branches, so some branches may still have scale. The soil injected product may go into those branches and give more thorough control.
In most single stem trees, we see 80 to 90 % reduction of scale populations and honeydew production. In multiple stem trees, it can be difficult to get more than 60 to 70 % canopy coverage and scale control, but when the soil applied product spreads, control may improve.
If we inject in the spring, we usually see some control in May or June, and then excellent control by July and August when the new generation of scale hatches, begins feeding and are killed.
Injections can be given at any time in the growing season, when leaves are on the tree or in the spring just before leaves open.
For smaller trees HOMEOWNERS can apply soil drench of DINOTEFURAN for quick results supplemented with soil drench of IMIDACLOPRID for longer lasting scale control. Large trees with serious scale problems will need professional application of one or more insecticides by trunk injection and/or soil injection.