Biology

Most soft scales, and members of other scale insect families, have a strange life cycle that involves spending almost their entire lives in a sessile - immobile - state.

The horse-chestnut scale is - by scale insect standards - relatively mobile. The nymphal and adult females have legs that are small but fully functional.

It is most noticeable in May and June, on the trunks and boughs of trees (see main photo), where each adult female secretes a large, white, fluffy ovisac into which she may lay up to 2,000 eggs.

Without the ovisac the insect is almost invisible against the tree bark (see photo).

Once the female has laid her eggs she dies, but remains attached to the ovisac. This helps protect the eggs.

Reproduction

The first-instar nymphs - crawlers - emerge from their ovisacs in early summer and migrate to the lower surfaces of the leaves where they settle down to feed on sap.

In the autumn the nymphs migrate back to the smaller twigs and branches, ahead of leaf-fall, where they remain until maturing and mating the following May.

In common with many scale insects the males have two wings and no mouthparts, so they have a limited time in which to mate.

The females’ final migration is to locate suitable oviposition sites - usually on trunks or main boughs.

Dispersal

Females have only limited mobility, so it is the crawlers that act as the main means of dispersal from one plant to another, or from one area to another. They are extremely light and readily carried by air currents.

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