This post is mainly to inform you about a debate I have been having with a few people on ispotnature.org about whether the distinct lack of acorns this year is connected to the increase in galls, especially Oak galls and to share some facts on the most common Oak galls.
Firstly: what are galls?
Galls are made by an insect called a gall wasp. There are many different species of gall wasps and so a wide variety of shapes and sizes in the galls, from the ‘fluffiness’ of the Robin’s Pincushion Gall to the smoothness of the Oak marble gall. The purpose of the galls is to create their own microhabitat, in which the insect larva can develop (relatively) safe from predators and feed off the interior of the gall.
Surely you have noticed the lack of acorns in Britain this year compared to the abundance last autumn? Well we are debating whether the abundance of Oak galls is connected to the lack of acorns this year in one of iSpot’s forums. Here is the link to the debate so far: http://www.ispotnature.org/node/440563 (I’m jimmymac2).
A reason why there may be an abundance of galls this year is because of mild weather. We had a very mild winter last year, it didn’t snow at all where we live and the gall wasps will survive better in warmer conditions.
This might affect the amount of acorns because there is a species of gall called the Knopper Gall (Andricus quercuscalicis) that is made on (and completely covers) acorns. However, I haven’t seen that many Knopper Galls compared to the Oak Marble Gall (Andricus kollari) which grows on the twigs of the oak and therefore doesn’t affect the acorns at all.
The main species on Oaks
Oak Marble Gall (Andricus kollari)
This gall seems to be very common on Oaks right now, though doesn’t seem to affect the acorns, but grows on the twigs of the tree. They turn brown as they mature and from autumn a perfectly round hole can be found in the gall, where the gall wasp has exited. The females that emerge then lay their eggs in the buds of turkey oaks, which come out next spring. The generation of sexual males and females then make the gall and are ready for a new generation. The Wildlife Trust’s page on the Oak Marble gall says: ‘The Oak Marble Gall was introduced into the UK in the 1800s from the Middle East. Despite concerns about the Oak Marble Gall affecting populations of our native Oaks, there have not been any serious consequences yet.’
Common Spangle Gall and Currant Gall (Neuroterus quercusbaccarum)
The cynipid (gall) wasp has two distinct generations forming two distinct types of gall: The Common Spangle Gall is a disc-shaped gall found on the bottom of Oak leaves in early autumn. They are creamy-yellow with brown specks and there can be up to a hundred on each leaf, only one larva in each. They fall to the ground in autumn and there they continue to develop through the winter and they finally emerge as adults in April. These adults are the Currant Gall generation and the galls they make are smooth and round, green at first but then maturing to red, like a redcurrant. The next generation emerges in June from the Currant Galls and they create the familiar Common Spangle Gall so continuing the cycle.
Knopper Gall (Andricus quercuscalicis)
It is quite unmistakable in the field, it is a knobbly growth covering acorns, starting off red and green, but then turning brown when it matures. It is formed on Pendunculate Oak (and occasionally Sessile Oak), also called English Oak and is one of our native Oaks. The Knopper Gall drops off with the leaves in Autumn, to develop overwinter. After nine months within the gall the adult emerges in spring and has barely a month left to live. The females then lay their eggs on the catkins of Turkey Oak, from which the new generation of males and females create the Knopper Gall for another generation.The Wildlife Trust’s page on this species says: ‘The Knopper Gall arrived in the UK in the 1960s and spread rapidly. Despite concerns about the Knopper Gall affecting populations of our native Oaks (heavy infestations can cause poor acorn crops), there have not been any serious consequences yet.’
So far it seems that the abundance of galls and the lack of acorns is not connected, though if they were it would probably be down to the Knopper Gall, which completely envelopes the acorn it is on. I hope to gather more information in the future, so any comments will be greatly appreciated!