Know your crop pests: Citrus swallowtail

This is a large butterfly with a wingspan of 8cm (females are slightly bigger than males).

An adult citrus swallowtail.
Photo: Paul Donovan

Description
This is a large butterfly with a wingspan of 8cm (females are slightly bigger than males). Adults have grey-black wings with white markings. The hind wings have two black, blue and reddish-orange ‘eyespots’. The caterpillar is a serious pest of many species of citrus. While young, it resembles a bird dropping, and can easily be overlooked. As it matures, it turns a uniform green.

The adult caterpillar can be up to 4,5cm in length. If disturbed, it rears up and displays a smelly, bright orange forked-like structure (the osmetrium) behind its head. This has given the caterpillar the common name ‘orange dog’. The caterpillar takes a month to mature. Three generations are seen every year – at spring, summer and autumn.

The caterpillar chews the leaves, reducing the plant’s ability to photosynthesise and stunting its growth. Well-established trees usually survive an infestation (unless it is very heavy). The caterpillars can harm younger trees and kill small trees, however.

Reproduction

The female lays individual eggs on the underside of leaves of a wide range of food plants, not just citrus species, although these are its favourites. The egg is round and whitish. Depending on the air temperature, it takes six or seven days to hatch.

Control measures
Use chemical insecticide only if there is a very large infestation of caterpillars and they are damaging the tree. Otherwise, simply pick the caterpillars off by hand or sprinkle diatomaceous earth on the leaves. The eggs and larvae of the citrus swallowtail is parasitised by a number of wasp species. The wasp larvae eat the caterpillar from the inside. Larger caterpillars may be eaten by birds.

Please note: this series is intended mainly as a guide to identifying pests. The control methods discussed are merely suggestions.

For help with area- or crop-specific measures, please consult your agricultural extension officer.

Paul Donovan is based in Botswana, where he advises farmers on controlling crop pests.