Nature on our doorsteps: Webs of caterpillars

Rosaleen Dwyer is the County Heritage Officer at South Dublin County Council – every week she gives us an insight into the natural heritage around us and the beautiful biodiversity of the plants and creatures

SPIDERS are not the only creatures to build web-like structures.

The caterpillars of butterflies like the Peacock and the Small Tortoiseshell, and moths like the small Ermine moths, spin large colony webs where their caterpillars can develop and avoid being eaten by predators.

Females lay large numbers of eggs on the leaves of their preferred foodplant.

On hatching, the young caterpillars work together to spin a large web around the whole group, while also incorporating some of the plant’s leaves.

The caterpillars then safely munch their way through these leaves.

When these are eaten, the web is simply extended to cover nearby fresh leaves and the colony moves over to the new feeding ground.

Peacock and Small Tortoiseshell caterpillars eventually leave the colony web and continue to feed either singly or in small groups.

Trees can be hosts for many webs of colony-forming caterpillars

The colonies of small Ermine moth species, however, remain together in their webs until they are mature enough to spin their own pupa and develop into adults.

Different species of small Ermine moth feed on Hawthorn, Apple, Cherry, and Spindle Trees.

Colony webs of the Bird Cherry Ermine moth can currently be seen on Bird Cherry trees in the Dodder Valley Linear Park.

The colonies are particularly noticeable when many separate wispy webs develop on a single tree.

The impact of the caterpillars is not thought to be too damaging to their host tree.

Once the caterpillars stop feeding and form their own individual pupae, the tree sends out fresh new leaves.

It can then begin to catch up on its own growing season.

Adult small Ermine moths also have a clever way of avoiding predators, like bats.

The moths make small continuous clicking noises which, to a bat, resemble the sounds that toxic moths make.

Bats therefore tend to avoid eating small Ermine moths.

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