Posted 29 May 04
HONEYDEW ... FRIEND OR FOE?
Reproduced courtesy of Apis-UK ... the free Beekeeping Newsletter
Ever since beekeeping began, beekeepers have varied in their opinion of honeydew. Some love it and others regard as a honey spoilant to be despised and thrown away. Much of the time this difference of opinion has been regionally based with for example the Germans having high regard for honeydew and the Americans and to a lesser extent the British despising it. (I know well how much the Germans regard this ‘forest honey’. In 1996 after 5 years of severe drought in Spain , the build up of aphids on the cork oaks and the accompanying lack of nectar in the flowers meant that almost my entire harvest was honeydew. A visiting German who purchased a jar of my honey returned and told me he would buy as much as I could produce. Ed). So what is honeydew? Where does it come from and why? And who wants it? This short fact file article is intended to answer these questions.
What is Honeydew?
Honeydew is the classification of ‘honey’ that refers to the sweet liquid collected by honey bees from the exudation of other insects such as aphids and scale insects. It is often found to be a more complex substance than honey due to the presence of enzymes etc deriving from the other insect involved in the production chain. Honeydews are normally high in fructose, low in glucose and have higher levels of the higher sugars such as maltose. The tendency to crystallise is low and in fact some honeydews never crystallise. Moisture levels in honeydews are usually lower than in honey and usually below 17% and fermentation is not of concern. Most honeydews have a high electrical conductivity arising from the a higher mineral content.
Why do insects excrete honeydew?
The feeding ecology of the insects involved such as aphids is interesting. Aphids for example feed from the phloem vessels by tapping into the vessels with their stylets. These are contained in the proboscis when not feeding and are thin and weak, but when used, a liquid secreted at the tips of the stylets which hardens them and forms a protective sheath around the stylets as they are slowly pushed into the plant. When the stylets reach the phloem tube, the aphid injects saliva into the plant cell which it is thought prevents the plant from resealing the puncture wound with their own special protective proteins. It can take an aphid from 25 minutes to 24 hours after starting before it gets a meal.
Plant saps are rich in sugars and low in amino acids or nitrogen. In order for the aphid to obtain sufficient nitrogen in their diet therefore, they have to take in a lot of the sap resulting in them receiving more sugar and liquid than they need. This then is excreted, collected by bees and ripened into honeydew. At times, if there is a build up of the aphid population this liquid can cover the leaves of trees and can fall to the ground. The liquid is fed upon by many insects apart from bees and also by yeast like fungus Thecla betulae which resembles a sooty mould.
Whatis the relationship between ants and aphids?
Who of us has not seen ants guarding aphids on a variety of plants? Honeydew is the reason that ants associate with aphids with some ants now almost dependant on aphids and some aphids will not excrete honeydew unless stimulated by ants. One particular aphid Paracletus cimiciformis is only found in the nests of the ant Tetramorium caespitum where it is fed and cared for by ants even though it now rarely if ever excretes honeydew. It has evolved in fact into a parasite and gains most of its food from the ants who offer it nectar. On plants, ants tend to herd aphids to the tops of plants which in fact has been found to render them more liable to predation. It has been found that ant attended aphids are 10 times more likely to be parasitised than unattended ones.
In some interesting research carried out last year by researchers at the University of Utah and reported in the May 9 2003 issue of the journal Science, it was found that some tree dwelling ants that were thought to eat other insects were actually plant nectar and honeydew eaters. Ants are extremely abundant in the rain forest canopy and the scientists now believe that they have previously underestimated the amount resource that trees are losing to the ants and their sap feeding associates. The ants are draining the water, carbohydrates and amino acids - the building blocks of proteins- out of the plants and the plants can die. So instead of being beneficial insects, eating pests and being rewarded with honeydew, it now appears that ants actively damaging the rain forests. One of the scientists noted that in areas super rich in ants, “you can just look around and see dead trees everywhere”.
Who wants honeydew?
In some countries especially in Eastern Europe and in some areas of the Mediteranean, honeydew producing trees and their associated insects are protected and valued. In Germany , honeydew from the Black Forest is known around the world as a prized product, and in New Zealand for example two species of beech tree, the Black beech Nothofagus solandri and the Red beech, N. fusca infested by two species of honeydew insect, produce New Zealand’s largest single exported honey crop.
So Friend or Foe? That depends upon who you are. If you are exporting tons of it to Germany for example, it is most definitely a friend. If small amounts of it are contaminating your wild flower honey and spoiling it for show purposes, then definitely a foe. The balance I believe must swing down on the side of Friend. (See also the historical note for a second opinion).