Plant lice that live on trees are also known as aphids. They spend the winter on the linden tree, and from spring, with the rise in air temperature, they begin to reproduce.
"One larva has grown up after five to seven weeks and then starts to reproduce itself," explains aphid expert Tomas Time.
A male is not necessary for the reproduction of these creatures - it also takes place through parthenogenesis, the development of a new individual from an unfertilized egg cell.
"Aphids are thus able to produce enormous quantities of offspring. British researchers counted hundreds of thousands of aphids on one tree in East Anglia."
They feed on the juice from the leaves
There are more than 5.000 species of aphids worldwide; there are about 800 of them in Germany. Basically, only two species live on linden trees. One has the Latin name Patchiella reaumuri and lives on the shoots at the base of the tree. Its second generation usually leaves the tree after winter and seeks habitat on other plants.
And the species that is actually "responsible" for the sticky honeydew that drips on cars is called Eucallipterus tiliae and usually lives on the underside of leaves. There, they feed on the sap found in the phloem, the conducting tissue of the leaves, by piercing it and practically letting the sap flow out into their stomach.
"What you're ingesting is a substance that's high in sugar and some amino acids," says Time. Aphids swallow a lot of juice, which they then excrete in the form of excrement. And it's actually honey dew that then drips onto the ground. Or on cars, where it dries and creates that irritating sticky layer.
The sticky mass can be removed relatively easily with warm water. But if it is not removed in time, it can become a problem. Because fungi like to grow in it, which turns the sticky mass black, turning it into a material that can damage the car paint.
"Forest Honey"
Honeydew is loved not only by fungi, but also by ants and bees, who happily feed on it. Some beekeepers specialize in the production of honey from honeydew, which, however, is usually called forest honey or honeydew honey. It is sold under that name "because many people", as Thomas Time says, "would not be thrilled if the label said: aphid droppings".
This expert says that aphids should not be viewed only as pests. Because Swiss researchers noticed that there are more aphids on trees if they are located in areas with higher air pollution.
The conducting tissue of the leaves of trees that are under stress due to pollution contains a greater amount of nitrogen bonds, and such trees have more aphids. In such cases, the amount of sticky honeydew is extremely large, but it also has one useful feature: it binds heavy metals from the air.
"This means," says Thomas Time, "that the heavy metals contained in the exhaust gases of motor vehicles cannot penetrate deep into residential areas." This is a fact that should be thought about carefully and that still needs to be well researched, says the expert.
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