How to recognise it
Usually noticed by ragged-edged foliage damage as the leaves are progressively eaten away until skeletonised. The larvae (caterpillars) are distinguishable by being hairy, yellow or green in colour with distinct black markings and with well developed mandibles. They can also be recognised by the ‘frass’ they excrete.
Most damage is done to mature plants in summer, autumn and early winter.
They undergo a complete metamorphosis, therefore are in the ‘Endopterygota‘ group of insects. Females lay batches of 20 to 100 yellow eggs on the underside of leaves in April and May. Within a fortnight they hatch into larvae which go through several developmental stages (instars) to reach around 25mm in length. The larval stage is the only one where the large cabbage white does its damage. In around June they pupate forming a chrysalis (the pupa) in the soil or attached into a crevice or woody stem by silk threads. The adults emerge in July/August and repeat the cycle (often causing an even more damaging infestation), with their offspring pupating in late summer or early autumn, to overwinter and emerge again in April/May.
Occasionally a third generation of adults emerges in September/October before the overwintering generation.
Why it’s a problem
They destroy the foliage and stems, particularly a problem with leaf crops such as cabbage.
Where you are likely to find it
On brassicas such as cabbage, cauliflower and brussel sprouts. Can also be found on wallflowers and the weed ’shepherd’s purse’ (Capsella bursa-pastoris).
How to deter it
Prevent the spread by fine netting crops before April and until late summer to prevent adults laying eggs on them.
How to get rid of it
Check the underside of leaves for eggs and larvae and pick off.
A biological control (suitable for enclosed areas such as glasshouses) is the small wasp Apantales glomeratus which lays its eggs inside the larvae. Alternatively the bacterium Bacillus thuringiensis can also cause a fatal disease in the larvae.
Birds such as starlings feed on the eggs and larvae, although can also damage crops in the process.
Severe infestations can be remedied using an insecticide such as pyrethrum, bifenthrin (and other synthetic pyrethroid compounds), acetamiprid and thiacloprid applied before the plant’s heart forms, but if some preventative action is undertaken then this isn’t usually necessary.
Is it good for anything?!
If we didn’t have caterpillars we wouldn’t have butterflies!
Other useful information
Large cabbage white butterfly adults migrate and can travel hundreds of kilometres to infest a new location.