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 These round bobbles can be found on beech quite frequently and are easy to spot because of their colour. They are caused by a gall midge (Mikiola fagi).

Also aphids can induce galls that look very different depending on the gall species and on the host plant. For example on spruce, Adelges abietis causes pineapple-like galls which are swellings of the needles bases.

Leaf curling like here on honeysuckle is another type of plant gall caused by Aculus xylostei-aphids.

Amazing bugs

... How insects make a plant to grow them the land of milk and honey

The natural world is full of surprises and mysteries that often only reveal themselves when we look closely enough. One of these mysteries became the topic of a study of mine which resulted in me carefully inspecting the leaves of young trees. Maybe, you have already wondered why plants sometimes have bizarre lumps, bumps or other weird structures on their leaves. But maybe, you have never noticed such a thing. Maybe, you did not know that this could even exist. So, what are these mysterious abnormal plant growths?

They are so-called “plant galls” and we can actually find them on any part of the plant, not just on leaves but also on buds, flowers, twigs, stems and even on roots. The galls can be large or small, round or spindle-shaped, they can be very conspicuous or unremarkable but in any case they are completely made of plant tissue. Why would a plant grow such structures?

One thing is sure, they do not do this just for fun. Instead, they are made to do this by insects. There are many insects that feed on plants. Caterpillars, for example, chew noticable holes into leaves before they turn into butterflies – if they find enough plants to feed on! And then, there are some plant-feeding insects that do not need to worry about whether there is enough plant material to feed on. They chose one plant and then they make that plant grow more and more tissue on which they can feed. And this is how plant galls come into existence.

Adult females of some beetles, flies or wasps do not only deposit their eggs onto the feeding plant of their offspring but also inject chemicals that cause the plant to grow extra tissue at that spot. When the larva hatches from the egg, it continues to release chemicals to keep the plant gall growing. Often, this newly-grown tissue encloses the larva at least partly. As a result, the larva lives in its own little private land of milk and honey inside the plant gall. There is an endless supply of food and the larva is sheltered from wind and weather. Pretty clever!

And the plants? They lose some energy by growing the galls but they usually continue to thrive, even if one plant hosts numerous galls.

The next time when you are outside, keep your eyes open and look for these abnormal plant growths. Galls have such recognisable forms that the causer can often be easily identified from the growth alone. You will be amazed by all the different forms, the little buttons, pointed tubes and spines.

by Anna Knuff (B3)