Who lays caterpillar eggs

Studies have shown that , and butterflies initially determine leaf choice by shape and size, but use taste and smell to confirm that the leaf is chemically "correct".

It isn't

In Peru females habitually alight on almost any available leaf except ferns when searching for the leaves on which they eventually oviposit. A female will reject several trees before selecting a particular one. Having found a tree that suits her she will then spend up to 30 minutes comparing dozens of leaves. She will settle on a leaf-tip, run up towards the base of the leaf as if measuring it, and then move on to another leaf. Eventually she narrows her choice down to just two adjacent leaves and then spends several minutes skipping back and forth between them before finally deciding which leaf to oviposit on. If she is disturbed part way through the oviposition process she will flee up into the canopy, but will return a few minutes later, relocate the leaf and complete laying the egg batch.

The factors that induce a female to lay on a particular specimen of tree or bush, or on a particular leaf, are often a mystery to human observers. always lays its eggs singly but I have counted up to 19 on a single leaf, and up to 100 on a tiny bush. These may have been laid by a single returning female or by several females in succession. These figures pale into insignificance however when learning about observations of various feeding species - In Kenya in 1926 Somersen estimated that a single 1 metre high bush of held about 57,000 eggs and young larvae of . In Sydney, Australia during a mass migration of Caper Whites , Waterhouse estimated that about 250,000 eggs were laid on a single 5 metre high Caper tree!

Butterflies usually lay the bulk of their eggs within the first few days of their lives. Older females lay smaller eggs, and the resulting caterpillars take longer to mature, making them more prone to predation and parasitism. This is probably part of the reason why and numerous other species avoid delay, and have evolved to lay all of their eggs in a single batch immediately after copulation. This strategy also ensures that as many eggs as possible are laid before the butterfly falls prey to a bird, reptile, wasp or spider.

egg batch under

Laying in batches improves the survival prospects for individual caterpillars. Wasps preferentially parasitize eggs at the edges of batches because they cannot easily reach those at the centre with their ovipositors. It seems that a few individual eggs around the edge of the batch are sacrificed to ensure that the bulk of them at the centre of the batch are left alone. Eggs at the centre are also less likely to be eaten by predatory insects, and are much better protected against desiccation.

As might be expected there are also negative factors involved when a butterfly "puts all it's eggs in one basket" - an entire egg batch could be deliberately eaten by a bird, snail, reptile or amphibian; or accidentally consumed by a grazing animal. To reduce the likelihood of this happening butterflies choose their egg-laying sites with great care.

Several Nymphalidae genera including , and lay their eggs in long vertical strands, dangling from the underside of leaves. sometimes lays in strands of up to 15 eggs long. It is not known what advantage the butterflies gain by adopting this strategy - perhaps the eggs at the end of the string are less susceptible to leaf mould?

egg batch of Marsh Fritillary , Wiltshire, England

egg batch of Small Tortoiseshell , Hampshire, England

It may seem surprising that something as small as a butterfly egg has its own parasitoids, but these cause high losses. The main parasitoids are wasps in the families Scelionidae and Trichogrammidae - as many as 60 of these can emerge from a single butterfly egg !