Gall wasps and their kin

13/07/2020

By Simon van Noort

Media images and interview requests, please contact:
Zikhona Jafta at mediaofficer@iziko.org.za

Gall wasps belong to the superfamily Cynipoidea and are found all over the world with a fantastically wide diversity of species that are classified into five families: Austrocynipidae, Cynipidae, Figitidae, Ibaliidae, and Liopteridae. Besides the well-known gall wasps (Cynipidae) that develop in galls formed on oaks, roses and herbs, predominantly in the Northern Hemisphere, the other four families contain many parasitoid species that are distributed worldwide. These parasitoid wasps attack other insects, some of which are pests of our agricultural and forestry industries, and in this process these wasps perform a critical natural control of insect populations and pest species. Some of the gall wasps (Cynipidae) are also used in biological control of invasive plants. Other species within the Cynipoidea are invasive pests themselves, or detrimental to biocontrol programs, so it is very important to be able to figure out who is who in the global natural zoo (of which we should all be responsible custodians), and what role they are playing in our diverse ecosystems. This group of wasps is extremely rich with about 3000 described species in the world, and many more (an estimated 20 000 species) still requiring documentation and description.

A user-friendly identification key to the Gall Wasps of the World including an overview of the various groups of cynipoid wasps and their biology has just been published in the journal Insect Systematics and Diversity. The scientific paper is freely available for download from this site, and online keys (including Lucid matrix versions) are available on WaspWeb. The key took eight years of development and testing by Matthew Buffington (U.S. Department of Agriculture-Agricultural Research Service), Mattias Forshage (Swedish Royal Museum of Natural History), and Johan Liljeblad (Swedish University of Agricultural Sciences) during numerous Hymenoptera Courses that were run for students. Matt Buffington laboriously photographed representative specimens for illustrating the keys, making them user-friendly and accessible to the lay-person, as well as to ecologists, foresters and agriculturalists needing to identify pest species or potential biocontrol agents. With the help of Chang-Ti Tang (George Washington University) and Simon van Noort (Iziko Museums of South Africa) the keys were refined and developed as online resources.

Identification keys come in two basic formats: traditional dichotomous keys where a choice needs to be made at each key couplet to continue, and Lucid matrix keys where diagnostic characteristics of the wasp in question can be selected randomly in the key until identification is achieved. The latter approach has the advantage that the selection process is not affected by an error early on in the key leading the user down a “red herring” path, which can happen with dichotomous keys as these continually fork in direction depending on a decision made at each couplet.

The northern hemisphere gall wasps make a huge variety of galls, comprising many strange and wonderful forms, on Oaks, Roses and herbaceous plants. We only have a few representatives of this Cynipidae wasp family in South Africa, but they are special African species.

The South African endemic (found nowhere else in the world) genus Rhoophilus contains the single species, Rhoophilus loewi. This species forms hard woody galls on indigenous species of Searsia (previously Rhus), modifying a primary gall instigated by a moth, and in the process killing the moth caterpillar. For more on this fascinating ecological interaction explore the Rhoophilus pages on WaspWeb.

There is also another special, endemic South African gall wasp, Qwaqwaia scolopiae, which is a gall inducer on the Red Pear tree, Scolopia mundii (Salicaceae) and is found nowhere else in the world. Like many of the gall wasp species, details of the biology of this species still needs to be figured out.

Some of the gall wasps are also useful for the control of invasive plants. A European species of Cynipidae, the Hieracium gall wasp (Aulacidea subterminalis) (top photo) has been used in the biological control of invasive weeds in both North America and New Zealand. The female lays eggs in the tips of the grass stolons of the invasive Hieracium Hawkweed species (bottom left photo). The resulting galls (bottom right photo) effectively prevent the vegetative expansion of the host plant, as the now damaged stolons normally would develop into new daughter plants.

The Chestnut Gall wasp, Dryocosmus kuriphilus, a species indigenous to China, has become a serious horticultural and agricultural pest species in many other areas of the Northern Hemisphere. The wasp forms deleterious galls on Chestnut tree species, resulting in fruit production losses of up to 80%. This pest gall wasp is in turn controlled by a chalcidoid species Torymus sinensis (Torymidae).

Besides the gall wasps, the Cynipoidea also include a diverse global range of parasitoid wasps that attack larvae of other insects, often flies and their maggots. Many of the host insect larvae are pest species and the wasps perform an extremely useful natural biocontrol agent role. The photo on the left shows a female Leptopilina japonica parasitoid wasp extending her ovipositor inside a raspberry to parasitize a spotted-wing drosophila fly larvae, Drosophila suzukii, which has become a serious pest of soft fruits worldwide. The fly maggots infest and feed on the fruit ruining the crop in the process, but by having the parasitoid wasp present these maggots are controlled by the wasp larvae. This is an example of classical biological control, implemented through the introduction of this indigenous Asian species to other parts of the world, thereby reducing the need for insecticide use in agroecosystems.

Another beneficial species, the Sirex ibaliid wasp, Ibalia leucospoides (Ibaliidae), native to the Northern Hemisphere, has been deliberately introduced to Australia, New Zealand and South Africa to control the invasive Sirex woodwasp, Sirex noctilio, a pest in Monterey pine plantations (Pinus radiata).

The Austrocynipidae is an endemic Australian family. The single species in this family Austrocynips mirabilis probably attacks moth caterpillars developing in the cones of Hoop pine (Auracaria cunninghami) effectively controlling the caterpillar population, which might otherwise have a negative impact on the reproduction of the pine trees.

Biology of the family Liopteridae is also unconfirmed, but they are probably parasitoids of wood-boring insects such as beetle larvae (Coleoptera: Buprestidae), or wasp larvae (Hymenoptera: Siricidae).

The Figitidae include a diverse range of parasitoid wasps that attack larva of flies (Diptera), or lacewings (Neuroptera), or are parasitoids of cynipoid or chalcidoid gall inducers, or hyperparasitoids of bugs (Hemiptera) through wasps (Braconidae or Chalcidoidea). South Africa is home to a figitid wasp that is the only known metallic species of Cynipoidea in the world – Pycnostigmus mastersonae. We know nothing about the biology of the entire subfamily Pycnostigminae, although based on assessment of evolutionary relationships with other members of the family Figitidae, we hypothesize that they may be attacking other gall forming insects, with their larvae developing as parasitoids of the gall inhabitant.

The Cynipoidea has always been a notoriously difficult group of wasps to identify. The publication of this user-friendly identification key has simplified the recognition of gall wasps and their kin, making the group far more readily accessible to researchers, conservationists, and biocontrol agencies. Identification keys provide an entrance into the classification system, which in turn provides information on the biology and likely lifestyle strategy of the wasp in question facilitating interpretation of the role the species is playing in a given ecosystem. A reliable identification key is thus an essential first step in the elucidation of this biological data.

The Cynipoidea are one group of the extremely diverse insect order Hymenoptera (ants, bees and wasps). Scientists at the Iziko South African Museum also conduct research on other groups of wasps and on ants:

Simon van Noort