Recently, we’ve been talking about galls here at Lakenheath. A gall is an outgrowth of tissue on a plant, induced and controlled by a parasite. The gall then acts as habitat, food source and protection for the parasite. Our gall discussion started a couple of weeks ago, while we were pulling ragwort. Volunteer Phil commented on a strange growth on a nearby thistle and wondered if we knew what it was.
Photo credit: Phil Hammond
A quick glance around showed us that this was no one-off either - quite a few thistles exhibited the same growth. Convinced it was a gall of some sort, Phil took a photo and later on that evening emailed me with his conclusion – the thistle gall fly. Now, this may seem too obvious an answer to be true, but I checked with a friend of mine who is particularly taken with galls and he agreed. It turns out that the thistle gall fly is fairly widespread in the south of England and it's quite a striking little fly. I've not seen one here myself though, maybe I haven't been looking hard enough because they look pretty distinctive...
Photo credit: Barbara Cooper - Thistle gall fly
The adult flies lay their eggs on the stem of creeping thistle and upon hatching, the larvae burrow into the stem and form the gall. The larvae pupate in early spring but remain in the gall, the fly then emerges June to October via a tunnel. I've also discovered that these flies have been introduced to North America to control creeping thistle, which is an introduced and invasive species over there.
This was an interesting topic in itself but Katherine then mentioned another gall we could see, just up from the visitor centre. I dashed out after her and she introduced me to the knopper gall:
Photo credit: Ali Blaney - knopper gall on acorn
This gall is caused by a species of wasp called Andricus quercuscalicis, no helpful common name I'm afraid and I'm struggling to locate an image! The wasps lay their eggs in oak buds and this distorts the growth of the acorn. The gall is sticky and red when it forms in August but becomes brown and woody in autumn. They then fall from the tree and the adult gall wasp emerges in the spring, through a vent in the top. These galls are apparently widespread so look out for them when you're next near an oak tree! It also turns out that these wasps have rather a complicated life cycle (which I won't go into here) and require two different species of oak in which to breed - penduculate and turkey oak. We are now trying to work out where our nearest turkey oak is....
The parasitic wasp theme continued just this morning, while Dave and I were working on the new raised plant beds outside the visitor centre. I spotted a very odd looking creature crawling along the floor near the workshop and on closer inspection we realised it was a large parasitic wasp dragging around a stunned caterpillar!
Photo credit: Dave Rogers - Sand digger wasp with prey
The sand digger wasp must be very strong, as it not only dragged the caterpillar over quite a distance on the ground but also started to carry it up the wall at one point! These wasps are solitary digging wasps that create a burrow underground in which to lay their eggs - this particular one preferring, yep you've guessed it, sandy areas. The Breckland area of our reserve is perfect! The wasp paralyses the prey and then drags it off to its burrow, where the prey will remain until the larvae hatch and consume it alive. A pretty gruesome end for the caterpillar (apologies for not identifying the caterpillar by the way - feel free to do so via a comment below.)
So it's been an intriguing week here at Lakenheath! It seems there is always something new to learn and it's always worth taking a closer look and asking those questions - why and what. Who knows where the path of discovery will lead you!