Gall wasps are mysterious insects whose way of life remains hidden from most people. The insects develop in a completely protected environment. From the outside, these development sites can be seen as rounded structures on the undersides of leaves. The inner workings contain exciting secrets.
What are gall wasps and are they harmful?
Gall wasps are harmless insects that cause galls on plants, especially on oak trees and roses. By laying their eggs in plant tissue and secreting hormones, growths are created in which their larvae live. Control is usually not necessary as they hardly cause any damage to the plants.
Gall wasps at a glance
Gall wasps represent the genus Cynipidae within the hymenoptera. There are more than 1,400 different species worldwide that are similar in their way of life. The product of the gall wasp is the so-called gall apple. This is created by fertilized eggs that are deposited by female animals on the underside of the leaf. Galls are products of various organisms, including bacteria or mites.
Other forms of galls:
- Markgallen
- Walling Gallen
- Hair or felt galls
- Roll or pouch galls
- Fold or mantle galls
Appearance
Gall wasps are between one and three millimeters long. They are inconspicuously marked and predominantly black in color. Males have one more antennal segment than females. When viewed from the side, the front body is usually very short and high, while the abdominal segment is round to oval in shape. Females of different species differ in their ovipositor stingers. This can be almost body-length or very short.
Confusion
Gall wasps can easily be confused with the sister family Figitidae. These hymenoptera should be viewed under the microscope so that they can be clearly distinguished from gall wasps. Only gall wasps form the typical gall apples. Figitidae species parasitize on other insects.
| Cynipidae | Figitidae | |
|---|---|---|
| upper torso section | microscopically small grain, therefore matt | shiny |
| Neckshield | inconspicuous | two side keels or striking plate |
| upper abdominal plate | third link longest | fourth link longest |
| Lifestyle | mostly plant-based | parasitic |
Gal wasp on oak trees
Gall wasps particularly like oak leaves. Even if the larvae of the gall wasps only cause minor damage, the oak defends itself by producing tannins that form the gall apple. This gall apple contains up to 60% tannic acid, which was primarily used in the past when mixed with ice s alts for tanning leather and as a dye. This so-called iron gall ink is still used today to sign state treaties.
Gall apples are a diseased growth on the tree to protect against the larvae of the gall wasp
Harmful or useful?
For most plants, plant galls caused by gall wasps do not cause any significant damage. The oak trees that are often affected in Germany regenerate very quickly from an infestation. While a non-native species may be considered a pest, some species are actually beneficial.
Gall wasps do no harm and do not need to be fought. But be careful when caring for a sweet chestnut tree in the garden.
Pest
Worldwide, the Japanese chestnut gall wasp is considered the most dangerous pest that can occur on the chestnut. If the tree is heavily infested, it develops fewer flowers and the harvest is lower. Individual observations have been made in Central Europe since 2002. The species has been occurring in Germany since 2013. Finds from Hesse, Baden-Württemberg and North Rhine-Westphalia are documented here.
Beneficial insect
There are few gall wasps that live parasitically on other insects. The larvae of these species usually have several small teeth or cutting edges and prove to be useful pest controllers. There are gall wasps that are considered natural enemies of the codling moth. The codling moth attacks pears and reduces the harvest.
Fight gall wasps?
Even if the leaves of a plant are heavily covered with galls, gall wasps do not cause any significant damage. Therefore, you do not have to fight the insects. If you find the growths bothersome, you can cut off shoots and leaves with sharp scissors and discard them.
Only the chestnut should you examine plant galls a little more closely. If the Japanese chestnut gall wasp is responsible for the growths, you should report the infestation. The effect of control measures is still being researched. There are hardly any results available about their effectiveness. Mechanical control is recommended in the early stages.
Tip
Encourage natural opponents who target the insects. The enemies include parasitic wasps and chalcid wasps. Liming the soil or applying preparations containing paraffin oil can prove effective.
Development and way of life
Females lay their eggs in carefully selected places. They focus on the developmental stage of the leaves so that their larvae are optimally supplied. The galls can also form on flowers and buds, twigs and stems or on the roots. Larvae pupate within the galls. The adult insect uses its mouthparts to make a circular hole in the shell and frees itself in this way.
Galle
Gall wasps trigger a growth in the leaf tissue when they lay their eggs. Females release hormonal substances through their ovipositors, so that the plant's growth programs are modified. A gall is created in which one larva develops. The insects eat a purely plant-based diet.
Each gall is shaped specifically for the species and consists of a hard shell and a soft inner tissue. Different numbers of chambers can be formed in the plant gall, in each of which a larva lives and eats the plant tissue. After she hatches, she ensures that the gall continues to develop.
The larva in the gall feeds on the leaf tissue, but the damage is rather minor
Where do gall wasps live?
Gall wasps are tied to certain climate zones, with many species now being introduced all over the world. They have specialized in host plants and cannot survive without them. In Germany, the native species show a very specialized way of life.
Dissemination
Gall wasps live in the temperate latitudes of the northern hemisphere. The majority of all genera and species are distributed in the Mediterranean region and around the Black Sea. There are no gall wasps in the tropics. Some species were able to spread into southern mountain regions. Only four genera have been described in southern America and Africa. About 100 species occur in the northern parts of Central Europe.
Host plants
Gall wasps develop on dicots. There is only one species in the world that lays its eggs on a monocotyledonous plant. Many species have specialized in certain plant species or genera. The original species occur on the poppy family, basket family and mint family. One line of development has specialized in the rose family. There are gall wasps that only occur on oak trees. A few species also colonize other deciduous trees such as maple, beech or willow.
- Oak: Common oak gall wasp and oak lentil gall wasp
- Rose: Common rose gall wasp
- Chestnut: Japanese chestnut gall wasp,
- Mornings: Diastrophus smilacis, North American species
Roses can also fall victim to the gall wasp
Species and galls
In Germany there are mainly species that cause gall formation on oak trees. The common oak gall wasp is the best known species, causing characteristic galls on the underside of oak leaves. They turn reddish in autumn, shortly before the insects hatch.
Oak lens gall wasps develop light-colored galls on the underside of the leaves that are arranged in a lens shape. The plant galls of the species Cynips longiventris are striking, as they are characterized by their spherical shape and irregular red stripes. The galls of the sponge gall wasp, which are called potato galls or oak apples, grow up to four centimeters in size.
Galls from the common rose gall wasp often appear on roses. These are called sleeping apples, rose apples or bedeguars and are found at the sprout ends of roses. They can reach a diameter of up to five centimeters and develop long hair-like growths. The interiors are criss-crossed by several corridors, each of which has a larva.
Are gall wasps dangerous?
Gall wasps are completely harmless to humans and pets. The insects cannot sting, even if the female's ovipositor appears frightening when extended. This only serves to penetrate plant tissue and lay eggs there. The insects' diet is predominantly plant-based, making gall wasps an exception within the partial order of legimen. The majority of these hymenoptera are parasitic and lay their eggs in animal organisms.
Frequently asked questions
Where do gall wasps occur?
The native species are mainly found on oak trees. They are responsible for the growths on the undersides of leaves. There are also gall wasps that specialize in roses. These species are preferentially found on wild roses, where they lay their eggs in the tissue of the shoot tips.
How do gall wasps live?
The female insects lay their eggs in plant tissue using an ovipositor. Secreted hormones ensure that the tissue proliferates. This creates a spherical protuberance that is protected by a hard shell. The larvae live inside the plant gall and feed on the tissue until they pupate.
Do I have to fight gall wasps?
Fighting is usually not necessary as the plants hardly suffer any damage. Even if a tree has tons of galls, you don't have to worry about the plant's vitality. Only the sweet chestnut can be severely damaged by gall wasps, so that the harvest is weaker. An introduced species that does not originally occur in Central Europe is responsible for this damage. Caution is also advised if the plant galls are not caused by gall wasps.
Can gall wasps sting?
These insects are unable to sting. They have nothing to do with actual wasps and are completely harmless to humans. Gall wasps have not developed any special defense mechanisms that could be dangerous to humans or pets.
Do all galls come from the gall wasp?
There are several organisms that can cause plant galls. In addition to gall wasps, bacteria, fungi, nematodes and mites can also be considered as gall formers. There are also other insects that also cause growth on plants. If you find a gall, you should identify it and determine the species.
