The Gypsy Moth, Lymantria dispar, is a notifiable pest listed by DEFRA, or at least it was when that document was published back in 1997. The insect was announced to be “a serious pest of trees and shrubs” and nurserymen and landholders were required to notify DEFRA or the PHSI HQ immediately.
Gypsy Moth caterpillar on birch trunk: blue warts at front, red warts at back. The black-and-white pattern may also be aposematic
It has arrived in the Gunnersbury Triangle with the hairy dark caterpillar larvae with blue and red warts on their backs all over some Birch trees. The infestation is rapidly defoliating them, and causing substantial damage to some Oaks too.
Lymantria means ‘destroyer’, quite a well-named genus. The caterpillars are aposematic, their hairs and bright coloration warning off predators; the hairs are irritant, containing diterpenes, complex organic ring compounds found in wood and plant resins for defence against microbes and fungi, and retained by the caterpillars for defence against predators.
It will be interesting to see how the trees cope. Oaks can generally recover even when thoroughly defoliated; the Birches may suffer more. People can hardly use pesticides in the nature reserve, even given the means to spray whole trees safely, but biological controls are imaginable. The caterpillars are parasitised by Ichneumon flies, which may well be keeping Gypsy Moths under some sort of control in Europe. There were no controls in place to halt the spread of Gypsy Moth in America, however, where the pest was accidentally introduced in 1869 from Europe by the amateur entomologist Étienne Léopold Trouvelot. He was hoping to cross-breed them with silkworms to improve their disease resistance; he is remembered instead for starting a disastrous continent-wide caterpillar plague which still continues. Attempts with other pest species to introduce their predators or parasites have often proved unsuccessful and sometimes disastrous in their turn.
A terrifying monster stalks the suburbs. Silently and with unerring accuracy, it scans the surface, using its advanced sensors to detect and identify targets buried deep below. Once a target has been located, the hunter drills down to find it, deposits the payload, and leaves in search of the next. It could be a cyber-borg or pilotless military vehicle. Actually, it’s Rhyssa persuasoria, the giant Ichneumon. And giant or not, it’s about 30mm long.
Rhyssa persuasoria, side view
Rhyssa is a parasitic wasp, a solitary hunter distantly related to the social, black-and-yellow striped wasps. Her prey are the larvae of other insects which burrow in dead wood for food and safety. Only when she is above, safety below is hard to find. For Rhyssa‘s weapon is as long as her body: her ovipositor is greatly elongated into a precision instrument that can drill deeply through wood and into the body of the larva. Once there, she lays a single egg down the ovipositor tube. The egg hatches inside the still-living larva, and devours it from the inside. The larva dies (so Rhyssa is a parasitoid, not a true parasite that avoids killing its host) and a young Ichneumon emerges.
Rhyssa persuasoria, dorsal view: in search of a host
Today at Gunnersbury Triangle we erected a new bench, to allow visitors to relax and enjoy a quiet moment in nature. It sounds a trivial task. If only. Two volunteers spent a day putting the “ready to assemble” kits together. All the supplied bolts were the wrong size, so they had to ream out all the pre-drilled holes to the larger size. Meanwhile, that day, I dug out the well-embedded MetPosts remaining from a previous bench. Then we dug 2 holes for the new bench: they promptly filled up with water, and the deeper we dug, the more the local weakly-cemented gravel (our local rock, when it isn’t sticky clay) collapsed into the hole, making it wider at the bottom. It was clearly hopeless.
So today we spent an hour prospecting for a drier place that would also be aesthetically pleasing, not harm the rare ferns nor disturb the nesting Blackcaps, and be close to an existing path. Then we started digging holes all over again. This time they didn’t fill up with water, much: just the bottom 10 cm or so. To keep the sides from crumbling, we avoided digging with spades: we lay on bin-bags, and wearing rubberised gardening gloves, scooped out handfuls of wet gravel. Then we levelled the two holes, cast a base of PostCrete in each, let it cure — at this point everybody disappeared for a cup of tea, leaving me in the wood guarding the site. I sat on a coppiced Willow trunk, and was approached by the giant Ichneumon when I least expected it. Luckily my little camera was not far away — you can readily imagine why I wouldn’t want the big camera with me while working.
The team reassembled, we gingerly lowered the bolted wooden creation into place, wedged it tight with broken bricks, and fixed it in place with plentiful PostCrete before cunningly sloping the top to shed rainwater. Needless to say, during this procedure we accumulated more and more tools all round the excavation site. If only it were as simple as drilling for an unseen caterpillar and laying an egg in it. But then, Rhyssa has the jump on us, with millions of years of evolution in her hunting technique.
While doing the Butterfly Transect at the Gunnersbury Triangle, I came across a fairly large Wood Wasp (in the sawfly family), about 25mm long. It was a bit tricky getting a photo, as these insects are distinctly skittish – they race about in the shadows, occasionally perching on a leaf’s upper surface. The breeze was wafting the branches gently, so patches of sunlight came and went. I shot several images with the miniature camera – it has two big advantages over my full-size SLR: one, it has a very short focal length, so it has a better depth of field than an expensive macro lens; two, it’s small and cheap, so I habitually carry it with me in my rucksack.
Wood Wasp, cf Sirex, on Ivy
The two images here show (left) the resting position with the wings over the body, the long antennae, the alarming-looking ovipositor, and the orange-brown legs; and (right) the plump black abdomen with white spots. Perhaps the two images show that it’s rather hard to get a single image which is suitable for identification. This one looks like a Sirex so perhaps that’s what it is. The eggs are drilled into wood.
The butterfly transect yielded the first definite Green-Veined Whites, i.e. I was able to get close enough to be sure; until now they’ve all been “Small/Green-veined” worse luck. There were some Speckled Woods and an Orange Tip, too; a Brimstone turned up after I’d put the clipboard away.
Sprouting Loggery
But the most curious observation of the day was this sprouting loggery. We ‘planted’ (more literally than we knew) the sawn Willow logs in the winter. They seem very happy in their new setting and are growing vigorously. It will be interesting to see how they get on.
Apologies to William Blake and the world in a grain of sand and all that.
Today, being a May Bank Holiday Weekend, it is very sunny and bright but the temperature has plummeted. I went shopping with my bicycle — wearing a thick fleece under a windproof jacket, and a ‘silk’ balaclava under my cycle helmet. So much for ‘Cast Ne’er a Clout / Till May be Out’. (I’ve never been sure whether ‘out’ means ‘May has come out’, i.e. it has begun, or ‘May has gone out’, i.e. it has ended. Whichever, it’s remarkably cold.)
After all the nature reserve visits lately, it’s time to look for wildlife closer to home. The blue tits have a lot of hungry little mouths to feed in the nest box above the kitchen door, and the adults flit in and out every minute or two. Sometimes one parent is still feeding when the other returns, whereupon the returner goes and perches in the Apple tree, calling softly, until the feeder flies out. There must be at least 40 feeding trips per hour, and it could easily be more. Remember that next time you’re wondering how much trouble kids are.
Ichneumon Fly on Currant leaf
The Blackcurrants are in full leaf now, and suddenly today each leaf seems to have an insect crawling over or displaying upon its upper surface. Some are certainly brief visits: smallish Ichneumon flies, about 10mm long and very slender, walk or run hastily about, on the lookout for caterpillars to parasitise with their eggs, a way of life disgustingly cruel enough to put Charles Darwin off religion for ever – leaving all the intellectual arguments aside, he simply found it sickening to imagine a loving creator doing anything so cruel. It’s interesting for such a careful scientist, able to spend 20 years marshalling arguments and evidence, that on a personal level, it was a visceral reaction that settled matters.
Harlequin Ladybirds mating
Other insects are clearly more like residents. Half-a-dozen leaves have a boldly coloured Harlequin Ladybird (or two: mating) in full view. It has been well said that insects fall into two camps: those that take good care not to be seen, and those that make sure they can be seen. Ladybirds, with their bold warning colours – red+black, yellow+black, red+black+white – are certainly in the conspicuous camp. This means they are signalling their unsuitability as food; the great pioneering zoologist E. B. Poulton coined the term ‘Aposematism’ (Greek ‘apo’ = from, ‘sema’ = sign, i.e. ‘warning off’) for this kind of warning coloration. In the case of the ladybird, they have bitter, foul-tasting or toxic chemicals in their bodies sufficient to make any predator gag, spit them out, and remember not to eat them again. This doesn’t necessarily save the life of the one they learn on, but it’s good for all the others. Each foul-tasting animal gets a better chance of not being tried out as a meal if it looks as much like other foul-tasting animals that predators may have had a bad experience with already. The result is Müllerian Mimicry (yeah, another famous Victorian zoologist) in which the vile imitate the vile as closely as possible. This is why bees look like wasps which look like bees: they all do better if they have the same obvious ‘don’t mess with me’ look.
A native 2-Spot Ladybird, on the same bush
The Harlequin Ladybird is big and bold, advertising itself fearlessly. It is spreading rapidly through Britain, having been unknown here not many years ago. It hasn’t totally displaced our native ladybirds: in fact, my Blackcurrant bush is also home to several 2-spot Ladybirds, much smaller and red all over but for one big black spot on each wingcase. The Harlequins are so called because they have many possible patterns, from much like a 12-spot Ladybird to almost entirely black (the odd red patch remaining), but they always have quite a lot of white on the head, which the natives generally don’t.
A nearly-black Harlequin ladybird… one species, whatever the pattern
As if that wasn’t enough, several small rather triangular true flies (Diptera) are displaying on the same bush; these are probably males waiting for a mate. They lack the ‘pictured’ wings of the Celery Fly – I’ve got those too, worse luck, though they are pretty little insects, and it’s curious to see them in a mating pile-up, as rival males fight to get the female. What she thinks of it, nature does not relate.
All you have to do to enjoy diverse insect life in your garden is … not to spray. In fact, the insects I’ve seen today are a good reason why spraying is a bad idea. The ladybird larvae are powerful predators of aphids, while the Ichneumon ‘flies’ (parasitic wasps) are valuable biological controls of many damaging species of moth, killing their caterpillar larvae. In short, they are the gardener’s friends.
That’s a lot of the world on a leaf. Or at least, a lot of evolutionary ecology for a May Bank Holiday weekend.
The English seem unemotional … except for their passion for nature
