Oak artichoke gall - AbsoluteAstronomy.com

Oak artichoke galls, oak hop galls, larch-cone galls or hop strobile
develop as a chemically induced distortion of leaf axillary or terminal buds on Pedunculate Oak

Pedunculate Oak

Quercus robur is commonly known as the Pedunculate Oak or English oak. It is native to most of Europe, and to Anatolia to the Caucasus, and also to parts of North Africa.-Taxonomy:Q...

(Quercus robur) or Sessile Oak

Sessile Oak

Quercus petraea , the Sessile Oak, also known as the Durmast Oak, is a species of oak native to most of Europe, and into Anatolia.-Description:...

(Quercus petraea) trees, caused by the parthenogenetic

Parthenogenesis

Parthenogenesis is a form of asexual reproduction found in females, where growth and development of embryos occur without fertilization by a male...

gall wasp

Gall wasp

Gall wasps , also called Gallflies, are a family of the order Hymenoptera and are classified with the Apocrita suborder of wasps in the superfamily Cynipoidea...

Andricus fecundator, which lays single eggs within leaf buds using their ovipositor

Ovipositor

The ovipositor is an organ used by some animals for oviposition, i.e., the laying of eggs. It consists of a maximum of three pairs of appendages formed to transmit the egg, to prepare a place for it, and to place it properly...

. The larva lives inside a smaller hard casing inside the artichoke and this is released in autumn. The asexual wasp emerges in spring and lays her eggs in the oak catkins. These develop into small oval galls which produce the sexual generation of wasps. A yew artichoke gall caused by the fly Taxomyia taxi also exists, but is unrelated to the oak-borne species. Previous names or synonyms for the species A. fecundator are A. fecundatrix, A. pilosus, A. foecundatrix, A. gemmarum, A. gemmae, A. gemmaequercus, A. gemmaecinaraeformis and A. quercusgemmae.

Galls

Young galls exhibit a tuft of long hairs protruding from the centre of the 'artichoke', the remainder being overlapping scales. The hairs are attached to the more solid gall which is released and drops to the ground from August onwards. After August the 'gall' is still visible, but it is really just the distorted leaf bud after the gall has fallen. A well grown specimen can be 2.0 cm long, dark green or russet, generally developing in June and reaching its full size before autumn.

The hairy catkin galls are oval, pointed, unilocular and unilarval structures, 0.3 cm long, changing colour from pale green to brown. The hairs are whitish in appearance. The wasps of the sexual generation are sometimes known by the synonym Andricus fecundator forma pilosus (DvL 1982).

The gall wasp which emerges from the gall chamber in Spring will always be an asexual female. But she will proceed to lay eggs on oak catkins, with a preference for Q. robur over Q. petraea, which develop into 'hairy catkin galls'. The Oak artichoke gall is more common on bush or scrub than tree oaks.

Life cycle

The wasps emerging from the oak artichoke galls will be female; and these females will go on to lay a solitary egg in the male flowers of the oaks, which will cause the formation of the 'hairy catkin galls.' The flies that arise form these galls are of both sexes and the cycle then starts again after they have mated and eggs are laid in the oak buds.

Once the oak artichoke gall has fallen to the ground the imago may leave the gall in the following spring or may delay the emergence for 2–3 years. Galls may persist and exhibit opened scales curving outwards.

Gall-forming insects

Some herbivorous

Herbivore

Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...

insects therefore create their own micro-habitats by forming usually highly distinctive plant structures called gall

Gall

Galls or cecidia are outgrowths on the surface of lifeforms caused by invasion by other lifeforms, such as parasites or bacterial infection. Plant galls are abnormal outgrowths of plant tissues and can be caused by various parasites, from fungi and bacteria, to insects and mites...

s, made of plant tissue but controlled by the insect. Galls act as both the habitat, and food sources for the maker of the gall. The artichoke gall is formed entirely from the bud

Bud

In botany, a bud is an undeveloped or embryonic shoot and normally occurs in the axil of a leaf or at the tip of the stem. Once formed, a bud may remain for some time in a dormant condition, or it may form a shoot immediately. Buds may be specialized to develop flowers or short shoots, or may have...

, composed of nutritious starch and other tissues. Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts. Galls may also provide the insect with physical protection from predators.

Predators, inquilines and parasitoids

Mature galls are sometimes broken open by vertebrate predators to recover the larva or pupa.

A number of insect inquiline

Inquiline

In zoology, an inquiline is an animal that lives commensally in the nest, burrow, or dwelling place of an animal of another species. For example, some organisms such as insects may live in the homes of gophers and feed on debris, fungi, roots, etc...

s live harmlessly within the oak artichoke gall and some of these, as well as Andricus itself, are parasitised by insects referred to as parasitoids. Some fungi may infect and kill the A. fecundator larvae.

Andricus curvator, the causative agent of the 'collared-bud gall' shows a marked preference for depositing its eggs on buds already colonised by A. fecundator. This may represent an early phase in the development of the inquiline mode of life.

Infestations

Removing and destroying oak artichoke galls before they dry and the wasps emerge may help to reduce the infestation. While fairly large, and sometimes present in quite large numbers on scrub specimens, they cause no measurable harm.

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