When Americans think of honey, most of us assume it comes from floral sources—basically bees collect flower nectar, add enzymes, and evaporate moisture to produce the finished product. Also known as honeydew honey, the source is not floral but actually from the waste products of a number of sap-sucking insects including aphids, leafhoppers, and psyllids. Mealy bugs and scale insects are examples of psyllids and are known by gardeners as serious and pesky plant pests.
Although it may be off-putting to think of eating insect excrement, honeydew honey is prized in parts of Europe and New Zealand and often fetches higher prices than floral honey.
It is rich in mineral content, amino acids, and may have stronger antibacterial properties. When a psyllid insect or aphid ingests the plants sap, it digests the small amount of protein present and expels the rest of the water, sugars, tannins and other indigestible material as honeydew. If you have ever parked your car under a tree and found it covered in a sticky substance the next morning, chances are the tree is infested with sap-sucking psyllids or aphids.
For many other insects including ants, wasps, and bees, this is a valuable food source. In some locations, honeydew is excreted in such large amounts that a buildup of the dried material can form into cakes under the tree. Admittedly it is more appetizing to think of it as a gift from above rather than insect excrement. Some psyllids, notably the red gum lerp psyllid, produce a white coating of evaporated honeydew which forms a crystalized shell protecting the insect.
Finally, to complete your moment of zen, here is a gorgeous photo of one other thing that can happen to honeydew: consumption by a sooty mold fungus. On this eucalyptus, scale insects, another kind of sucking, pooping plant bug, are the guilty party. The sooty mold is the black film on the leaf at right.
Since it's eating honeydew, and not the plant, it's chiefly a cosmetic issue if you're a gardener. Those white scale insects are your bigger problem. Scale insects, ants, sooty mold, and honeydew invisible, but strongly implied by first three on a eucalyptus.
Creative Commons Bidgee; click image for source and license link. The views expressed are those of the author s and are not necessarily those of Scientific American. She has degrees in biology, plant pathology and science writing. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options.
Go Paperless with Digital. Now, lots of things can happen to all this honeydew. Here are a few options: First, the bug can fling it away with a special flicker not actual name, I'm sure. He she seems to be ever-so-considerately flinging his poo over the top of his her neighbor: But what if you could actually get something in return for making all that honeydew?
Note the ants. Weather conditions, especially high temperatures, can increase the potential for foliage burn. Do not spray dormant oils during the growing season. There is no residual effect so additional applications may be necessary. Fatty acid salts or insecticidal soaps are very good against aphids.
As with summer oils, they apparently work to disrupt insect cell membranes. They require direct contact with the insects and leave no residual effect. Nervous system insecticides, such as malathion, Dursban chlorpyrifos , and Orthene acephate , are labeled for use on many shade trees and ornamental plants for aphid control.
As with oils and soaps, coverage is very important and a follow-up application may be necessary. Be sure that the plant or crop that you are treating is listed on the product label. Sevin carbaryl is not effective against many aphids so it is generally not a good choice for control unless recommended specifically. In fact, applications of Sevin may reduce the number of beneficial insects, such as lady beetles, and increase the potential for aphid outbreaks.
Aphid control is most valuable for new plantings, where excessive sap removal is more likely to affect general plant vigor. Established and otherwise healthy plants can tolerate moderate to heavy aphid infestations, although affected leaves may wilt and turn yellow and there may be some premature drop.
Good cultural practices, such as watering and fertilization, will help to reduce stress by these insects. Problems with honeydew and sooty mold may develop but tend to be temporary and disappear after the aphids are gone. A few aphid species produce cupped or distorted leaves; these plants may lose some of their esthetic appeal for the season. Once the distortion occurs, the leaves will remain cupped and twisted until they fall off. Usually, the infestation is not noticed until the injury has occurred.
Insecticide applications often are less effective because the aphids are protected in the gnarled leaves. Plants that become infected with an aphid-borne virus may be severely stunted and may die.
Preventive sprays are rarely effective in keeping viruses out of plantings but they may reduce the spread within a group of susceptible plants.
Beneficial insects, such as lady beetles and lacewings, will begin to appear on plants with moderate to heavy aphid infestations. They may eat large numbers of aphids but the reproductive capability of aphids is so great that the impact of the natural enemies may not be enough keep these insects at or below acceptable levels.
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