Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Edible Houses

Saturday, January 7, 2012

Think back to your childhood, when you saw your first gingerbread house. If you were like me, and most other children, your first thought was, “Wow! What I wouldn’t give to live in a house I could eat my way out of!” While this is only a wonderful dream for children, for many insects it is a dream come true - in fact, it is a matter of survival.

This is how it works. It begins when an insect irritates a plant. The irritant could be the female laying an egg in the tissues of the plant, or the young larval insect feeding on the plant. Whatever the irritant, it causes the plant to grow extra tissue - the gall - around the irritant or the irritator - the insect. Once the gall is formed, the insect is now tucked away safely inside. There it can continue to feed and grow in relative safety, not bothered by its enemies or the weather. When it gets hungry (being a larval insect it is always hungry since the function of a larva is to eat and grow) it nibbles on the walls of its house. As it grows, the chamber inside the gall also grows, since the insect is eating away at its house.

Since the plant grows the gall around the insect, getting in the gall is easy for the insect. Getting out of the gall when the insect has changed from a larva to adult is the tricky part. Since the gall is gallcomposed of mostly dead woody material, it is too tough for the adult insect to chew through. In fact, many of the gall making insects don’t have chewing mouthparts as adults, so they couldn’t chew their way out of the gall if their lives depended on it - which it does. Therefore, before the big sleep during which the larval insect changes into the adult, the larva must provide for its future, chewing an exit tunnel out of the gall and leaving the thinnest of possible doors concealing the exit. If the door is too thick, the adult will not be able to break out of the gall and will perish.

One wonders how the tiny larval insect knows to provide an exit tunnel it will need later in its adult life.. It was not taught this trick by an older, more experienced insect. It has never had the luxury of learning from one who has been there. In fact, sealed away in its tiny house, it has never encountered any living creature of any type. As far as it knows, it is alone in the world. And there is no trial and error period. If it doesn’t get it right the first time, it doesn’t get a second chance.
Insects, along with many other animals, seem to be born with all the knowledge necessary for survival. Since scientists don’t understand how this trait works, they have named it. It is called instinct. The parent insect doesn’t have to remain with its young to teach it survival skills - the young just “know”. Initially, this seems like a wonderful trait to posses - to be born smart. It does, however, have its drawbacks. It seems the stronger the instinct, the less the ability to learn new things. Studies have shown that many insects have almost no ability to learn anything new. If a situation arises that is not covered by instinct, then the insect is doomed.

There are quite a variety of insects that stimulate galls. Three quarters of them are either gall gnats (a type of fly) or gall wasps. Rounding out the gall producers are aphids, a couple of insect types that only entomologist would be familiar with, and mites - the only non-insect of the group. As well as these, galls can also be produced by fungi, bacteria and roundworms (nematodes). Of all these, it is the insect galls that are the most common and most noticeable.

While many types of plants are infested with gall insects, the preferred plants are oaks, daisies, roses and willows. Oaks alone account for 60 percent of all galls in the US. So if you are looking for galls, start with the oaks. You can find a gall on just about any part of the plant but most of them occur on the stems and leaves, making them easy to find.
One of the most noticeable and most familiar galls is the Oak Apple Gall. This gall is about 1.5 inches in diameter, of a dry papery texture and attached to the end of an oak twig. While up at the top of the oak tree, these galls are out of sight, but when the twig breaks off the tree and lands in the trail, the galls are very noticeable.
Since these galls are composed of large amounts of tannic acid, and since tannic acid is used as a natural dye, the galls can be used to make a high quality, indelible ink. Called Iron Gall Ink, it was the major type of ink used in Europe from the 12th century on. The main ingredients are oak galls, copperas (iron sulfate), gum arabic and water. If you would like to make your own ink, you can find the instructions by searching for Iron Gum Ink.

One note of caution. Before you grind up your Oak Apple Galls, make sure the tiny wasp has vacated the premises. If you see a neat little hole in the gall, then the wasp has left and the gall is up for grabs.

It is a fun project to see how many different types of galls you can find on your favorite property. Try making a collection. For those galls with the original occupant still present, a drawing of the gall counts just as much toward your total.


gall assortment