Honeydew fluid comes from sap sucking insects

Honeydew is a sugar-laden fluid excreted by certain plant-sucking insects. When these populations build-up, sticky honeydew may drip on nearly everything outdoors.

Scale and aphid on a honeylocust twig.
Scale and aphid on a honeylocust twig.

Scales, adelgids, and aphids – oh my! A range of tiny insects use specialized mouthparts to pierce leaves, needles, or even bark on trees and other plants so they can feed by sucking-up the sap of the plant.

Several of these sap-feeding insects are very particular about which kind of tree they use for hosts. Sometimes insects carry the name of that host tree, such as the balsam woolly adelgid, beech scale, or pine needle scale.

Scales spend nearly all of their life cycle in one place, protected by the “shields” they secrete. Depending upon the species, the shield can be woolly, soft, or hard. These shields help protect the scales from weather, predators, and insecticide sprays. Scale insects are only mobile for a few days after they hatch, when they are called crawlers. As the name implies, crawlers have tiny legs and can move about on the tree.

Crawlers are not protected by wax or wool and are, therefore, vulnerable to insecticides or sprays of horticultural oils and soaps. Once crawlers find a suitable location to feed, they insert their mouthparts into the tree and begin sucking sap. At that point, they molt. They become legless and remain attached to the tree for the rest of their life.

Magnolia scale populations blossomed in parts of southern Michigan and Wisconsin this year. In other areas, scale populations have grown large enough to draw many queries from homeowners. Lecanium scale (many species) that feed on maple, ash, and other trees have boomed in some parts of the state. Other common forest scales have colorful and descriptive names like the pine tortoise shell scale, oyster shell scale, and terrapin scale.

Soft scales, such as lecanium scale and magnolia scale, excrete lots of honeydew. Other scales don’t produce any honeydew. Some scales, such as beech scale (associated with beech bark disease), secrete a white waxy coating instead of honeydew.

Adelgids are similar, in some ways, to aphids, but they’re mostly immobile (like scales), have different life cycle details and body structures, and are associated with conifers. Adelgids insert long stylets into the host tree and feed on the sap during the entire stationary part of its life cycle. Spruce gall adelgids cause those horny growths, resembling cones, on spruce, particularly blue spruce.

The hemlock woolly adelgid is an exotic species that has devastated hemlock resources in eastern states. This pest is on Michigan’s “most unwanted” list. Observations of early infestations can often lead to successful eradication.

Aphids don’t produce a protective cover and are more familiar to gardeners, farmers, and horticulturists. Aphids can be important pests on the agricultural and horticultural crops. Young aspen have “herds” of aphids that are tended by ants. The ants feed on the honeydew and protect the aphids from predators. The woolly aphids can be particularly interesting because of their appearance. Some species will wave their “flags” of wool when disturbed.

Normally, scales, adelgids, and aphids don’t pose serious health threats to trees. Repeated, heavy infestation can reduce tree vigor and sometimes lead to declines. In some cases, these insects can allow pathogens entry to the tree, which can lead to a serious forest health issue.

More often than not, heavy infestations of these little sap-feeders cause problems for people. The last couple of years, in particular, have seen locally high numbers of scales and resulting sooty molds. This sticky, black material adheres to lawn furniture, house siding, cars, driveways, and most anything else kept under the canopy of scale-infested trees. These conditions are visually unattractive. Sooty mold can be removed by using a mild soap (about 3-4 ounces per gallon) in a sprayer. However, this does not eliminate the source of the problem, which is honeydew-producing insects and ubiquitous sooty mold.

Next year, the weather may be less favorable for these bugs.

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