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A Honeydew List

Jeffrey Hahn, Asst. Extension Entomologist

There have been various reports lately of trees weeping or dripping some kind of sticky substance. There have been different speculations about what causes this problem. Is it some kind of disease? Is it just sap? The answer: insects.

Photo: Typical aphids.  Jeff Hahn.

Aphids and certain scale insects feed in the phloem layer of plants using their needle-like mouthparts. They are not able to digest all of the sugars in the sap, and consequently, excrete a sugary liquid called honeydew. Honeydew is clear (can appear white) and is sticky. This material is not harmful to the health of trees but can be annoying when it coats deck furniture, cars, or other objects that are below infested trees. If this is a problem, try to remove it as soon as possible as honeydew can be very challenging to remove the longer it stays on.

Many ant species enjoy sweets and are attracted to honeydew. Some ants actually tend aphids to maintain their supply. Yellowjackets change their dietary habits during late summer and fall and are quite interested in the sugary content of honeydew. There can be so many yellowjackets attracted to a tree infested with aphids or scales that people may think that there is a nest in the tree.

Predators, especially lady beetles,may also be indirectly attracted to the honeydew. Of course they are interested in the aphids or scales that are producing the sugary liquid. One resident wanted to have a lady beetle larva identified. They were concerned because their tree was 'weeping' and these larvae were present. The owner was convinced that the larvae was causing the weeping and as a consequence started spraying them with an insecticide. Of course the lady beetle larvae were there to eat the aphids which were producing the honeydew.

Another consequence of honeydew is that it supports a fungus called sooty mold. Like the name suggests, it is black and sooty in appearance and is found on branches and leaves where honeydew is found. Although it is unsightly, sooty mold does not harm plants and should be ignored.

If you have a situation where you would like to reduce aphids, whether from the nuisance of the honeydew or from the insects themselves, there are several environmentally friendly options you can take. First, you can take a hose and direct a hard spray of water at infested leaves and branches. This knocks them off, effectively killing them. You can also apply a low impact insecticide, especially insecticidal soap or horticultural oil. These kill aphids while preserving natural enemies like lady beetles, lacewings, and syrphid fly larvae. Or just ignore them. As we approach fall, their activity slows down and they eventually stop producing honeydew.
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