Caterpillars

Key Points

Caterpillar infestations can cause severe losses in many brassica crops
Second generation is often the most damaging
Best results are achieved when small caterpillars are treated
Apply Dursban WG in 600 to 1000 litres of water per hectare with a non-ionic wetter/spreader

Introduction

Cabbages, cauliflowers and many other brassicae can be destroyed by infestations of caterpillars. They are normally attacked by caterpillars of the Cabbage White butterflies — Pieris rapae (small white) and Pieris brassicae (large white) — and of the Cabbage moth — Mamestra brassicae — but can also be damaged by caterpillars of the Diamond-back moth — Plutella xylostella.

The most serious economic damage caused to commercial crops of brassicae is by caterpillars of the Small White butterfly. Eggs are laid on outer leaves but it is young leaves at the centre of the plant that are damaged to the greatest extent. The damage caused by caterpillars of the Large White butterfly can appear more dramatic as whole plants can be stripped of all foliage. However, only a few plants are normally attacked and these tend to be restricted to areas of sheltered headland and not throughout a crop.

Caterpillars of the Cabbage moth cause damage by feeding on the hearts of brassica plants. Attacks are less common in field-scale vegetable production. Feeding by caterpillars of the Diamond-back moth cause leaves to become silvery in appearance as they tend to feed solely on the underside of leaves without damaging the upper surface. These caterpillars will also attack sprout buttons and the hearts of brassicae plants. Damage normally occurs from the middle of summer onwards and can reach economically damaging levels very quickly.

Biology

Small White butterfly (Pieris rapae)

There are two generations of Small White butterfly each year. Pupae from the second generation overwinter and produce butterflies during the following March and April. Butterflies have white wings with black markings and females lay single eggs on the underside of leaves. Green, velvety caterpillars with a narrow yellow line down the back hatch from these eggs after about two weeks. Caterpillars grow to about 35mm before forming greenish-brown pupae on plants during June and July. Adults of the more damaging second generation emerge throughout July, August and September.

Large White butterfly (Pieris brassicae)

Adults of the first generation of Large White butterflies are present in April and May. They lay eggs in batches, on the underside of leaves, which hatch in around 14 days. Caterpillars remain in colonies, feeding and growing quickly to as much as 40mm in length. They are yellow-green with distinct black markings, yellow along the sides and a yellow line along the back. They are only sparsely covered with hairs. After pupating off the plant, a second generation of butterflies emerges in July and August. Again, caterpillars of the second generation butterflies are the most damaging to brassica crops.

Cabbage moth (Mamestra brassicae)

Shiny brown soil-borne pupae give rise to adult moths from the end of May and through June. Moths are grey-brown with white markings but are rarely seen as they only generally fly at night. Globular eggs are laid in batches on the lower surface of leaves of host plants. Caterpillars, which can grow to over 40mm in length, vary greatly in colour but often carry a dark brown, square marking at the rear end. The skin is smooth with few, short hairs. They take about five weeks to become fully grown, when they burrow into the soil and pupate. During the latter part of summer these pupae often produce moths which produce a second generation of caterpillars.

Diamond-back moth (Plutella xylostella)

Overwintering occurs as pupae and two or more generations are produced during summer. The first moths normally appear in May or June and can be found flying during the day and at night. They are small, only 6mm long, and brown and grey in colour with conspicuous white markings along the inner margin of each forewing. Favoured egg sites include all plants of the cabbage family. Pale green caterpillars, reaching about 15mm long when fully grown, are found in webs underneath leaves. After about three weeks fully grown caterpillars spin cocoons on stems and leaves of host plants. These become pupae, and after two weeks pupation, the next generation of moths emerges.

Product Choice

All the most commonly occurring caterpillar pests of brassica crops can be controlled with Dursban* WG. Applications should be made when damage is first seen, which is normally in early summer. All treatments of Dursban WG to control caterpillars in brassica crops should be made using a water volume of 600 to 1000 litres per hectare of water and should always include an approved non-ionic wetter/spreader.