Doing the early rounds of the garden this morning I noticed big holes in some of the rhubarb leaves. The culprit was a large, hairy, black and orange caterpillar – the unmistakeable ‘Woolly Bear’ larvae of the Garden Tiger (Arctia caja). In the hedge a Sedge Warbler was belting out it’s scratchy song and in the distance a Cuckoo was calling. Now there’s a link between Cuckoos and Woolly Bears. As a youngster growing up on the outskirts of Birmingham I somehow developed an interest in birds, insects, plants and all things natural where my ‘sites’ were the patches of semi-natural wasteland, subsidence pools, old sand pits and fragmentary remains of farmland which surrounded most urban areas. Both Cuckoos and Garden Tigers were commonplace and during my insatiable reading of just about every natural history book in the local library I learned that only Cuckoos could cope with the densely hairy Woolly Bear caterpillars.
The long hairs are hollow and contain toxic chemicals that the caterpillar picks up as it feeds. It’s not a fussy insect and it feeds on a wide variety of plants. The chemicals are in the plants to protect them from herbivorous insects. Over a long evolutionary time period a number of insects have evolved a tolerance of these toxins and the chemicals are ingested and then dealt with metabolically or stored away. In the Garden Tiger caterpillar they are stored out of the way in those long hollow hairs. The toxins in Woolly Bear hairs are things like acetylcholine and histamine. These are the same substances that are in nettle stings. Those stings also contain things like oxalic acid that intensify and prolong the irritation caused by the nettle sting. One of the other things I learnt as a kid was that rhubarb leaves are poisonous as they contain lots of oxalic acid so if the Woolly Bear caterpillars takes up the oxalic acid from plants like rhubarb and stores it in its hairs along with the other chemicals then it’s surrounding itself with hundreds of long, chemically loaded ‘nettle stings’.
Cuckoos will swallow large hairy caterpillars and apparently the hairs end up embedded in the stomach lining and periodically are coughed up as a pellet. You quite often see birds bashing caterpillars against objects and this is thought to be a way of removing the gut contents. As we’ve seen already leaves contain all sorts of nasty chemicals and it’s probably a good idea for a bird to get rid of undigested leaves in the guts of their prey before its swallowed. I’ve seen Stonechats and Blackbirds in my garden taking big hairy caterpillars, such as those of the Garden Tiger, and then vigorously thrash them on the ground before eating them or carrying them off to the nest. Might this not just empty the guts but also bash off all those long nasty hairs? When they’re doing this, they do seem to be quite careful to grasp the caterpillar by the head to avoid the hairs.
Sadly Garden Tiger moth populations in much of southern Britain have decline catastrophically, down 92% according to: “The State of Britain’s Larger Moths 2013” report produced by Butterfly Conservation. In the same area there has also been a marked decline in Cuckoo populations. Census work for the BTO (British Trust for Ornithology) in England has shown declines of over 70% in Cuckoo populations since the late 1960’s. It’s likely that the reason for the decline in Garden Tiger populations is the same as for many other species of moth, butterfly and other insects; a combination of greater pesticide usage coupled with ever more intensified farming and the loss of those difficult, wet, boggy but flower rich corners of fields that farmers could never be bothered with. For Cuckoos it’s all of those and additional factors linked to climate change and problems along their migratory routes.
In about a month’s time the adult Cuckoos will already be heading back to Africa and my moth trap will start to catch dozens of adult Garden Tiger moths each night. As you walk along you are likely to see them in the grass or resting on the wall underneath the light outside your back door. It’s good to know that they are still doing well up here and that Cuckoos are still brightening our early mornings with that call that so epitomises spring. There are children growing up in the countryside in the south and east of Britain who will never hear a Cuckoo or see a Garden Tiger and I find that incredibly sad.