Crops: Cole crops
Why is it a problem? Cabbage white butterfly is probably the most important of several moth pests that feed on Cole crops in Bhutan. It is a conspicuous pest that can be managed through regular monitoring and physical control, although insecticides are often used. It is mainly a problem in cabbage as market prices are reduced by the feeding damage to leaves, and the resulting frass (excreta).
Where and when is it a problem? It occurs, and requires at least some management, in all Cole-crop growing areas. Damage is normally limited early in the season, but then increases. It doesn’t normally reach very high densities, perhaps because of the wide-spread use of insecticides. The aggregated feeding behaviour can also limit the proportion of plants affected within a field, and therefore losses.
Eggs are orange and conical with longitudinal ridges and laid in batches on the underside of leaves. Caterpillars grow to about 25 mm and have longitudinal yellow strips with many black spots. All larvae readily regurgitate a thick, repellent, green liquid from their guts to deter predators and parasitoids. Larvae pupate on vertical surfaces. Adults are large (5-7 cm wingspan) and white with two black spots on the upper side of forewings (females) or underside of forewings (males). The day-flying adults are commonly seen flying in and around Cole crops and other host plants.
The aggregated feeding habits of these caterpillars can completely defoliate an individual plant leaving only the tough midrib and the veins in leaves. Attacks occur from seedlings through to the heading stage. Larvae initially feed on the first-formed outer leaves which often appear riddled with irregularly shaped holes. As caterpillars mature they feed on the centre of the plant. Their excretions can be found between leaves.
Larvae are large and conspicuous, feeding in groups on the underside of leaves. Groups are larger when larvae are young. If no action is taken whole plants can be attacked before larvae move on to adjacent plants.
Confusion with other pests: There are several moth species commonly found on Cole crops. Cabbage white butterfly larvae are relatively large and distinctive, and are the only ones that feed in aggregations. A second species, Pieris candida, has also been reported on Cole crops in Bhutan, but little is known of its importance. However, it would be managed in the same way.
Lifecycle: There may be 2-3 generations per year in Bhutan. Eggs hatch in 8-10 days, larval development takes about 30 days and the pupal stage lasts about 2-3 weeks in summer. Larvae are gregarious for most of their life, only becoming semi-independent towards the end of the final instar. They pupate on or under a protective surface off the ground, which can be some distance away from where it last fed. They over-winter as pupae.
Dispersal: Adults are active flies so can easily disperse between fields. Mass-migrations have been observed in India. Larvae can move between plants.
When can damage be expected? Populations are probably being suppressed by the prophylactic use of insecticides across commercially-grown Cole crops. Early crops are seldom injured because they mature before populations have built up sufficiently. Elsewhere, high temperatures and more sunshine hours, accompanied by low relative humidity and rainfall, has been found to favour population build-ups.
Hosts: It feeds on many plant species in the family Brassicaceae. In Bhutan, it has been recorded from Cole crops, mustard and spinach.
Cabbage white butterfly, and other moth pests, can normally be well-managed through regular monitoring, the physical removal of larvae and eggs when they are found, and crop hygiene practices. The current high dependence on insecticides for managing these pests could be greatly reduced or stopped if these practices were put in place.
Inspection for cabbage white butterfly presence and damage needs to be part of a regular (twice-weekly) crop monitoring of a wide range of Cole crop insect pests. The presence of the day-flying adults provides a good early-warning. Egg batches and larval aggregations (and damage) are conspicuous. Older larvae on older plants can be harder to find once they’ve burrowed into the heads.
Effect of variety
- Eggs and larvae (often in groups) are readily handpicked and destroyed when seen. They are very visible. Care should be taken not to pick eggs of lady birds which are also orange coloured and of the same size. Closer examination of the eggs will reveal the lady bird eggs to be smooth whereas the eggs of the cabbage white butterfly have longitudinal ridges.
- Practice crop hygiene: collect and destroy all stalks and stubble when harvesting the crop to prevent the build-up of subsequent generations.
Use of insecticides should rarely be necessary for cabbage white butterfly.
Image acknowledgements: NPPC.