Imagine for a moment you are a Blue Tit with a hungry clutch of chicks to feed on a diet of caterpillars. When finally Spring arrives, why not go a stage further and take the Caterpillar Challenge? See how many you can find by simple observation, in your garden or during your daily exercise walk.
Caterpillars are the larvae of moths and butterflies. They are the growth stage in these insects’ life-cycles and, being juicy and nutritious, they comprise a major part of the diet that parent Blue Tits feed their growing young. Studies have revealed that by the time they fledge a typical Blue Tit clutch could have consumed 20,000 caterpillars, all provided by their parents of course. And that number does not take into account what the parents themselves eat during that period.
In the past two decades I have recorded more than 450 species of adult moths and butterflies in my relatively modest-sized, semi-rural garden – that’s just under 20% of the species found in the UK as a whole. Clearly there’s no shortage of adult Lepidoptera and yet I really struggle to find their caterpillars.
Over the years I have come across the larvae of 15 species of butterfly and perhaps 50 species of moths in my garden or close to home. But certainly not with any regularity or anything remotely approaching the numbers needed to feed a hungry brood of Blue Tits. And yet clearly there must be caterpillars in my garden and nearby because several Blue Tit families are successful each year. So where are the caterpillars hiding and why can’t I find them?
TOP TIP Many moth caterpillars are much more active after dark than they are in the daytime. Some remain inactive, hidden beneath leaves or clinging to stems during the hours of daylight. Others undertake a daily migration, hiding low down on the foodplant (or even disappearing underground) in the day and climbing up the stems at night to feed. Search for them after dark using a torch. LED torches are best, partly because they are so bright but also because the wavelength of light that they emit seems to pick out invertebrates particularly well. Of course, none of this explains how Blue Tits, which are active in the daytime, are so much more successful at finding caterpillars than humans, or at least this human. But then again, their lives depend on this ability.
Caterpillars have evolved all manner of ways to avoid being detected and eaten. Some are unpleasant to eat. Others employ defence strategies to deter predators. Many more prefer to avoid being seen in the first place and rely on camouflage, deception and entomological guerrilla tactics. Here is a selection of caterpillars found in my garden over the years, alongside the adult butterflies and moths they eventually become.
YOU ARE WHAT YOU EAT Some caterpillars take on the chemical signature of their foodplant and become toxic or unpleasant to predators. They make sure there is no mistaking who and what they are, and advertise themselves with striking markings and warning colours.
HIDING IN PLAIN SIGHT The caterpillars of some moth species are a perfect match for certain spots in garden. Two in particular wrap their bodies around twigs lengthways and a fringe of hairs eliminates any tell-tale shadows.
LOOK-ALIKES Some caterpillars have evolved the perfect shape, colouration and markings to match their surroundings. The deception can involve looking like a twig or a piece of plant debris, like a shrivelled leaf fragment for example.
COUNTER-SHADING In true military fashion, some caterpillars have markings and shading that serve to disrupt their outline when. Counter-shading can offset the effects of shadow and diagonal stripes mostly likely mimic the veins of leaves.
SAFETY IN NUMBERS The strategy works for some moth and butterfly species, the idea being that, statistically, any individual caterpillar is less likely to be attacked if a member of a group. The approach is sometimes combined with entomological special effects such as spiky bristles to deter predators.
IRRITATING BEYOND WORDS We all know that fibrous and bristly food can be hard to chew and swallow and the same is true in the insect world, where caterpillars are seeking to avoid being eaten by birds and other predators. Irritating hairs are a great way to put off would-be attackers.
HIDDEN FROM VIEW Some caterpillars migrate on a daily basis, and hide away low down the plant on in crevices by day, coming out to feed at night. Others have taken hiding to a new level and live inside the stems, branches or trunks of their foodplant.
DEFENCE STRATEGIES A number of defence strategies have evolved among moth species to avoid being eaten by predators. If danger threatens, some simply let go and drop the ground, which might explain why relatively clumsy attempts to search for caterpillars can produce poor results. Others use silken threads to abseil away from the threat. A few are more pro-active in their approach.