I feel rather ashamed that I lost control of the cabbage white caterpillars this autumn with the last of the calabrese and kohl rabi more or less ruined by their activity. The plants were pulled up and composted. But this lethargy on my part did result in a strange discovery. I noticed that quite a few caterpillars were crawling along the ground and then climbing the walls of the house. These late summer brood caterpillars have a different life cycle to the earlier broods. The early ones climb and make a pupa which takes about two weeks for the transformation from goo to butterfly but these late ones pupate for more than six months. It is a miracle that the transformation can happen at all but how the genetic code can know what time of year it is is beyond comprehension (of course it is probably temperature-related). The caterpillars, when fully fed, climb to pupate in a sheltered place on old stems, woodwork and crevices in buildings. And they emerge as butterflies in spring.
But those I saw climbing the walls did not pupate. Instead the skins split and there appeared to be a cluster of ‘eggs’.
This was the work of a parasitic wasp called Cotesia glomerata. The following is not for the squeamish. The tiny wasp detects the caterpillars and lays its eggs inside the caterpillar where they hatch and the larvae consume the caterpillar from the inside, avoiding the vital organs so they do not kill the host too quickly, which would, of course, ruin their plan. It is only when they are ready that they finally eat the last of the host and it splits open to reveal the pupae of the wasps. I am not sure how long these remain in this stage but if you look closely at my (poor) photo you can see an adult wasp which may have just hatched.
In a story worthy of the slickest David Attenborough doc’ things are not this simple because there are also parasites that parasitise the parasites (hyperparasitoids). They do this while the parasites are in the caterpillar. How they know they are there is beyond me. But it turns out that the plant itself tells the parasitic wasps that they need help.
As the caterpillars munch on the cabbage, chemicals are released that attract the wasps. I am not sure what these are. I know that plants produce chemicals called salvestrols when they are attacked and these are often bitter and make the plants less palatable. There is evidence to suggest that these are good for us and have disease-reduction properties when eaten. A logical extension of this is that organic produce, that is not sprayed to prevent insect attack, and gets the odd nibble, is nutritionally better for us – but I am not sure if there is reliable evidence to support this.
But the chemicals that the cabbage releases when it is chewed is like a scream of help and attracts the wasps. I suspect that it probably also attracts the butterflies though. There must be potential for the use of these chemicals to reduce caterpillar damage.
‘Just another example of the wonder of nature and the delicate balance that exists. And I don’t feel quite so slovenly about letting these caterpillars survive. And I am glad I didn’t spray them – though I suspect that it would have been a better death than being eaten alive.
On an allied topic, I have just made a bee hotel. I did it as one of my garden jobs for my weekly piece in Garden News. Bee hotels are designed to provide a winter home for solitary and mining bees and should be put up in spring really but making one is a good winter job. Some people take them down in winter and keep them in a shed to protect the insects and it is best to replace the ‘sticks’ every year or so to ensure there is not a build up of parasites.
You need to make a frame of untreated wood and pack the sections with hollow stems. Canes are the usual culprit but, since this is supposed to be helping nature, it seemed mad to cut up canes which have been imported. So I chose to cut up dead dock (rumex) flower stems and old allium stems, all from the garden – or over the fence. The tubes should be 7-12mm in diameter but I assume bees don’t carry rulers so I was happy to use a wide range of sizes as I cut them. The depth of the hotel should be more than 15cm (6in) so that the bees can get deep into the tubes and away from predators such as blue tits.
I did feel a bit guilty when cutting the dock stems because the dried, wrinkled leaves were home to some ladybirds but I made sure I placed the leaves in a sheltered place so they were safe. It took far more stems than I expected to fill the two sections with sticks so be prepared! I also drilled some holes in the timber for extra-fussy bees!
The hotel then had to be put up. Ideally it should be in a sunny spot, protected from torrential rain and 1-2m above the ground (I may move mine but I needed a neat background in case you spotted it is under the eaves of the shed! ). I hope the bees appreciate all the effort.