Not to be confused with the term gall in pathology, in which the word means a sort of lumpy sore caused by itching and rubbing, plant galls are formed by the expansion or increase of
the number of cells in the plant. Galls appear as irregular outgrowths, and can occur on the plant's roots, stem, leaves, buds, or even flowers. They can be formed by a variety of organisms such as fungi, bacteria, insects, and mites.
Some Plant Gall Causers
One of the most well known causers are the many species of Gall Wasps, also known Gallflies, a family (called Cynipidae) belonging to the order Hymenoptera.
A Bedeguar Gall Wasp (Diplolepis rosae)
Most adult Gall Wasps are very small and are rarely seen, and it is the actual galls themselves that you are most likely to see. Some Gall Wasps use a method of reproduction known as parthenogenesis, in which a male is not needed, however most species use the usual method of reproduction.
Once a female has created a gall by injecting a chemical that stimulates the plant to either create more cells or increase the size of existing ones, it lays its eggs inside it. These will hatch out as the tiny maggot-like larvae, which will feast on the nutritious layer of plant material inside the gall. A majority of Gall Wasp larvae pupate in the safety of the gall, before tunnelling their way out as an adult.
An image of the unusual Moss Gall on Dog Rose, formed by the Gall Wasp known as Diplolepis rosae.
A group of Spangle Galls on Oak, formed by the Gall Wasp known as Neuroterus quercusbaccarum.
Gall Mites, also known by the scientific name Eriophyidae, are a large family of small arachnids that can cause galls. Members of Eriophyridae are so tiny that they can pierce a single cell with their mouth parts, stimulating surrounding cells to enlarge and multiply, thus creating the gall. Most galls created by mites are known as erinea, and appear as mats of tiny hair-like projections, very different from the Moss Gall pictured in the Gall Wasp section. However, a few mites cause quite different galls, including the Bugle Gall, found on the leaves of Common Lime trees, and formed by the mite Eriophyes tiliae. The Bugle Gall, also known as the Lime Nail, takes the form of bright to dark red, tubular growths.
An example of a large group of Bugle Galls on Common Lime leaves.
A Maple Crimson Velvet gall on Maple, formed by the Gall Mite know as Eriphyes calaceris. This is a good example of an erinea gall.
A few species of Aphids can form galls and do so by piercing a single cell using their proboscis-like rostrum, and this, as in Gall Mites, stimulates the surrounding cells to multiply.
Pineapple Gall Adelgid
The Pineapple Gall Adelgid (Adelges) is a genus of Adeglid that is very closely related to Aphids and can create large pineapple-like galls on certain species of conifers, including the Norway Spruce. An interesting fact about this genus is that only females are known. The life cycle of the Pineapple Gall Adelgid is quite unusual:
Stage 1 - Nymphs hatch from eggs on host plant.
Stage 2 - The Nymphs then begin to feed on the leaves of the conifer, and their saliva stimulates the gall to form around them.
Stage 3 - They overwinter in the safety of the gall, then awaken the following year to lay eggs there. Once the eggs have been layed, they will die.
Stage 4 - The second generation of nymphs hatch in the gall, then develop there until July to September. At this point they emerge from their gall and undergo a finale moult on their host plant. When they have completed this, they will have formed wings.
Stage 5 - The winged adults of the second generation will fly to another tree and will lay eggs their, without the need of a male to reproduce. The next year, the young will emerge and the full cycle will be repeated . . .
Some Pineapple Gall Adelid nymphs.
A Pineapple Gall formed by the Pineapple Gall Adelgid
Of course, there's many other causers, I just wanted to mention a few of my favourites here.
The term inquiline comes from the Latin 'inquilinus' meaning lodger or tenant. It is used in zoology to describe a life form living in another's dwelling place, such as a hive, nest, etc. Some Gall Wasps have taken up an inquiline mode of life and can no longer even form a gall, instead they take up residence in the gall of another species. The lodger is almost identical to the previous owner of the gall, so identifying them can become tricky!
I hope that you've enjoyed today's rather long post on galls, causers, and inquilines. Galls certainly are fascinating little things, and worthy of a lot more attention than they usually get.
Until next time, keep on the wild side!