The Gall wasps are a member of the Cynipoidea family distributed through Europe and North America, typically varying in size between 1 to 8 mm. The wasp is entwined in a parasitic symbiotic relationship with the oak tree. They may infect multiple species of trees though
here we will be focusing on the oak tree. A single
oak tree can be infested with up to 70 different species of gall wasp which can
be harmful to the oak tree. (Askew, 1984)
In spring the female wasps search for sites in which to inject their eggs. The base of a successfully fertilised oak tree flower is the ideal location for a female wasp to inject her eggs; the female thrusts her ovipositor into the base of the flower and injects only one egg. The genetics of the acorn are affected by the young wasps as immature acorns grow into galls. The gall is a protective growth that is caused by the immature larval secretions and form a large array of distorted structures. Each species of wasp has characteristically different shape and sized galls which the young larvae feed on; this causes the galls to become shrivelled and brown.
In spring the female wasps search for sites in which to inject their eggs. The base of a successfully fertilised oak tree flower is the ideal location for a female wasp to inject her eggs; the female thrusts her ovipositor into the base of the flower and injects only one egg. The genetics of the acorn are affected by the young wasps as immature acorns grow into galls. The gall is a protective growth that is caused by the immature larval secretions and form a large array of distorted structures. Each species of wasp has characteristically different shape and sized galls which the young larvae feed on; this causes the galls to become shrivelled and brown.
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| Figure 1. Acorn infected with a single mature gall. Photographer: Justine Aw (2013) |
Autumn promotes the loss of the oak trees leaves and acorns along with the galls. The winter following brings harsh freezing condition covering the galls in ice until the beginning of spring. The ice melts and the adult gall wasps emerge after 9 months trapped inside and only live for a few weeks to seek another oak to lay eggs (Stone et al., 2002).
Another type of parasitic gall wasp Megastigmus trisulcus has evolved to take advantage of this phenomenon
and instead of producing its own galls it steals them from other species of
gall wasp. It is specialised with a highly equipped drill, the tip being covered in metallic zinc for a sharp cutting edge. It is then inserted into the
gall, directly into the larvae of the other wasp with a microscopic egg
injected into the host larvae. The parasitic larvae then eventually kills the
host and take its place in the gall for the duration of the seasons until spring
and emerges as the host would have (Flett et al., 2011).
This is a great example of evolutionary adaptation and
survival success. One can only imagine the kind of environmental pressures that
would have prompted such a specialised piece of equipment. It is clear that
these three species have a high dependency on the others for their own
survival.
References:
ASKEW, R. 1984. 8.
The Biology of Gall Wasps.
FLETT, H., AUSTRALIA, H. & BOARD, M. V. C. 2011. Establishment of Citrus Gall Wasp Parasites
in the Murray Valley Region, Horticulture Australia.
STONE, G. N., SCHÖNROGGE, K., ATKINSON, R. J., BELLIDO, D. & PUJADE-VILLAR, J. 2002. The population biology of oak gall wasps (Hymenoptera: Cynipidae). Annual review of entomology, 47, 633-668.
Figure 1. AW, J. 2009. Acorn infected with a single mature gall. Accessed 22/03/2015 from http://www.notcot.com/archives/2013/08/gall-wasp.php
Video with thanks to BBCWorldwide. https://www.youtube.com/watch?v=CzXccvoJThI
Very interesting. I thought it was amazing that each gall is species specific! Have any studies looked at whether different wasp species have specific tree preferences, due to the size and shape of gall produced? Is the “drill” of Megastigmus trisulcus a modified ovipositor? What do the oaks gain from this relationship, as this seems highly detrimental to the plant? I’m looking forward to the next post.
ReplyDeleteHi Tasmin, I have looked into your queries a found some very interesting information. There is host specificity in a large portion of species of gall wasps, with some particular species specializing on a single species. This has opened up gateways for methods of biocontrol. One example is in South Africa with the Trichilogaster acaciaelongifoliae and its host Acacia longifolia. The longifolia is native to Australia and occurs through the coastal regions of NSW, VIC, SA, though considered invasive in South Africa. The wasp was introduced in South Africa in 1982 and 1983 and reduced the reproductive potential of the Acacia by over 89 %, when only 50 % of the branches were infected with galls. The vegetative growth was reduced significantly when the vacated galls had desiccated. I believe they do have a preference when it comes to the size and shape of gall produced as different species infect different sites of the plant for example the branches, leaves, stemming buds etc. Megastigmus trisulcus drill is a modified ovipositor and is approximately the same length as the body. The drill is inserted into other species larvae through the gall and injected with a microscopic egg. The relationship is parasitic towards the oak, and does not receive any direct benefit from the relationship as I have found so far. I hope this answers your question, thank you for commenting.
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