Gall rain

 

 

Vorsø 1975-87
The beautiful bedeguar galls on stems of dog roses (Rosa canina) are produced by larvae of the bedeguar gall wasp (Diplolepis rosae). Many larvae live in each of these galls. – August 23, 1981. (Photo copyright © by Kaj Halberg)

 

 

(September 1983)

 

In September, even on a bone-dry day, you can experience ’rain’ in the south-eastern part of the old western forest, Vesterskov, where an abundance of common oaks (Quercus robur) grow. Tens of thousands of tiny, lens-shaped items, measuring c. 5 mm across, silently fall to the forest floor, literally covering it. They are galls, which have been attached to oak leaves, produced by larvae of the cynipid gall wasp (Neuroterus quercus-baccarum).

On the forest floor, the larvae remain as pupae inside the galls during the winter, and in early spring, the now adult gall wasps gnaw their way out of the galls. The odd thing is that only female wasps hatch at this time. They now lay eggs in leaf buds or male inflorescences of oaks, dying shortly after. Larvae, hatching from these unfertilized eggs, produce galls, which look entirely different from the lens-shaped galls, being globular, and measuring up to 8 mm across.

Even though these eggs are unfertilized, they nevertheless produce both males and females, which hatch in early summer. They mate, and the females lay eggs in the nerves of oak leaves. Larvae from these eggs produce lens-shaped galls, and the circle has been completed. As females from the two generations look very different, they were formerly believed to constitute two species.

There are many other spectacular galls on Vorsø. On oak leaves, besides the lens-shaped galls, you find e.g. globular galls, red or yellow, produced by larvae of the common oak gall wasp (Cynips quercusfolii), and button-like galls, caused by larvae of the silk button spangle gall wasp (Neuroterus numismalis).

On dog rose (Rosa canina), you often find beautiful bedeguar galls, produced by larvae of the bedeguar gall wasp (Diplolepis rosae). Many larvae live in each of these galls. On willow leaves, oblong, red galls are sometimes produced, caused by larvae of the willow redgall sawfly (Pontania proxima). On the leaves of a large small-leaved linden (Tilia cordata), planted near the farm, you often see numerous galls, caused by larvae of the lime nail gall mite (Eriophyes tiliae). Stems of creeping thistles (Cirsium arvense) are often swollen, which is caused by larvae of the thistle-stem gall fly (Urophora cardui), formerly called Euribia cardui.

 

 

Vorsø 1975-87
Galls on a leaf of common oak (Quercus robur), produced by larvae of the cynipid gall wasp (Neuroterus quercus-baccarum). – August 17, 1983. (Photo copyright © by Kaj Halberg)

 

 

Vorsø 1975-87 
Galls on a leaf of common oak (Quercus robur), caused by larvae of the silk button spangle gall wasp (Neuroterus numismalis). In the background galls, made by larvae of the cynipid gall wasp (N. quercus-baccarum). – August 17, 1983. (Photo copyright © by Kaj Halberg)

 

 

Vorsø 1975-87
On willow leaves – here hybrid crack willow (Salix x rubens) – oblong, red galls are sometimes produced, caused by larvae of the willow redgall sawfly (Pontania proxima). – August 19, 1983. (Photo copyright © by Kaj Halberg)

 

 

Vorsø 1988-99 
On the leaves of a large small-leaved linden (Tilia cordata) near the farm, you often see numerous galls, made by larvae of the lime nail gall mite (Eriophyes tiliae). – October 10, 1992. (Photo copyright © by Kaj Halberg)

 

 

Vorsø 1988-99
Stems of creeping thistles (Cirsium arvense) are often swollen, which is caused by larvae of the thistle-stem gall fly (Urophora cardui). – July 25, 1988. (Photo copyright © by Kaj Halberg)

 

 

(Uploaded February 2017)