Transformer

When our kids were small they collected toys called transformers, which were model vehicles that turned into menacing robots when you pulled, twisted and rotated their articulated parts. They were marvels of imaginative, miniature engineering but couldn't hold a candle to this wonderful natural transformer - a cabbage white pupa, which I found under the rim of a seed tray in the greenhouse yesterday. Insect metamorphosis is one of the most mind-boggling developmental transformations in the natural world.

Inside this pupal case the caterpillar - an insect eating machine that chewed holes in our broccoli leaves is turning into a 'soup' and reorganising itself into a delicate, winged, nectar-feeding cabbage white butterfly that will emerge next spring.

The caterpillar has pupated fairly recently - over the winter its green shade will turn to pale brown. If you look closely, about one third of the way down the image you can see the silken halter that the caterpillar spun to attach itself to the seed tray - it's last act as a caterpillar.

Click here to see a great YouTube movie of a large white butterfly hatching from its pupal case.

You can read about the biology of insect metamorphosis by clicking here

Butterfly dogfight - what happened next .........

The photograph in the previous post shows a female cabbage white butterfly escaping from the attentions of a pursuing male and if you look closely at that picture you can see that she's a well-worn individual with tattered wings.

This was even more apparent when she landed. Damage like this makes you wonder just how much wear-and-tear butterfly wings can sustain before the insect's ability to fly is seriously impaired.

As soon as she landed she returned to the serious business of egg-laying, although in this case her choice of location probably isn't a good one - she is laying her egg on a cabbage flower sepal, which will fall off in a couple of days, soon after the flower is pollinated and fertilised, so unless the egg has hatched by then it will end up on the ground.

Whenever the males harassed her the female cabbage white adopted this posture, with her abdomen raised at almost 90 degrees to her body. This is her signal that she has already mated and isn't available for mating again.

Here she is again, in her 'mate refusal posture', this time on a nasturtium Tropaeolum majus leaf. 

A large white butterfly has laid a cluster of eggs on this leaf, in the bottom-right of the picture. One way of separately large whites and small (cabbage) whites is that the former lay their eggs in clusters whereas the latter lay their eggs singly and well spaced out on the leaf.

Male cabbage whites seemed to be confused by females that adopt this tail-up posture. They try to mate but can find no way of doing so; this suitor is laying on his side.

The males are quite relentless in their pursuit of reluctant females, but with no success.

Life and death in the cabbage patch .....

This year we've had record numbers of small white Pieris rapae butterflies in the garden. At one point I counted twelve feeding on a single lavender bush - which is bad news for our brassica crops but does create some wonderful opportunities for watching insect behaviour.

Small whites lay their eggs singly on the underside of cabbage leaves ....

.... and the first thing that newly-hatched caterpillars do is to eat their own egg shell. At this stage the caterpillars are yellowish and translucent but as soon as they begin feeding .....

..... you can see the cabbage leaf that they've eaten travelling through their gut.

They are really feeding machines and begin on the underside of the leaf because the waxy leaf cuticle is much thinner there than on the upper surface, where the cuticle is very thick and probably too tough for an infant caterpillar's jaws. You can see here that it has only chewed through the leaf as far as the upper cutcle and hasn't bothered to eat all the way through, except for a small hole. but there's another good reason for stayinging under the leaf .......

.... the constant threat from parasitic hymenopterans. With their jerky movements and twitchy antennae, they are always on the lookout for a caterpillar to lay their eggs in ....

..... and this one found a victim, depositing its eggs inside the tail end of a caterpillar which was on the upper surface of the leaf and is now doomed to be slowly eaten alive.

This one took violent evasive action, dropping from the leaf and swinging on a silken thread was soon as it felt the presence of the parasitic wasp.......

.......... while this much larger one is feeding really dangerously, up amongst the flower buds on the bolting brassica - but maybe it's already parasitised anyway. [Thanks to Africa Gomez at BugBlog for identifying this caterpillar and the one above as the cabbage moth)

Much is made in the press about the effects of recent extreme climatic events on butterfly numbers but parasitism of butterfly caterpillars might be at least as important in determining fluctuations in butterfly numbers from year to year.

The inflorescences produced by the bolting brassicas are a good food source for hoverflies and also home to other visitors, like this harvestman spider - a scanenging omnivore which wouldn't be averse to eating the occasional butterfly egg or newly-hatched caterpillar.

Watching the (Insect) World Pass By...

Sometimes the best way to watch wildlife is to just sit still and let it come to you, and in our garden at the moment the best place to sit is next to this plant - dame's violet a.k.a. sweet rocket a.k.a Hesperis matrionalis. In addition to having a powerful carnation fragrance and being the food plant of the orange tip butterly caterpillar it's a real butterfly magnet and while I watched it was constantly visited by ....

... small whites and ....

... red admirals. Sweet rocket produces such an abundance of nectar-rich flowers that the butterflies are reluctant to leave and just clamber from flower to flower ..

... which gives you plenty of time to admire the intricate patterning on the underside of their wings.

After you've been sitting in one spot for a while you begin to notice things that you might never spot if you were on the move, like this unidentfied green caterpillar resting in the shade of a chive flower stalk.

Then other visitors begin to turn up - this is Tachina fera, whose larva is an internal parasite of noctuid moth caterpillars and which I've never seen in our garden before. It seems to have a real liking for forget-me-nots.

The sweet rocket is the current butterfly attraction in the garden but the focus of bee visits (particularly leaf-cutter bees) is the wood vetch Vicia sylvatica which attracts a constant stream of visitors. This is a very attractive plant that climbs through the surrounding vegetation and produces a large number of racemes of flowers over a long period.

If you sit in the sun for long enough - especially near the garden pond - then this hoverfly Helophilus pendulus almost always turns up. You can find some wonderful photographs of this species laying eggs - and pictures of its eggs and larvae - here