Plants affected
All brassicas, including Chinese cabbage, some ornamental plants in the Cruciferae family, and nasturtiums.Symptoms
Holes of different sizes on the outer leaves of all brassicas
and often on the inner leaves of cabbages when the heart is cut through.
Caterpillars and their excrement are often found on the plants.
Cause
There are three common culprits: caterpillars of the large
cabbage white butterfly (Pieris brassicae) and small
cabbage white butterfly (Pieris rapae - pictured right), and the cabbage moth (Mamestra brassicae
- pictured above right). All three may be present at the same
time and tend to attack mature plants. Caterpillars of cabbage
moth and small white butterfly are more damaging as they bore into the
hearts of cabbages, whereas the yellow and black caterpillars
of the large white stay mostly on the outer leaves.
Control
Inspect plants regularly and pick off caterpillars when seen.
Spray with pyrethrum (Py Spray Garden Insect Killer, Scotts Bug Clear Gun for Fruit & Veg) or bifenthrin (Bayer Sprayday Greenfly Killer Plus, Scotts Bug Clear Gun or Doff All-In-One Garden Pest Killer). Both insecticides have a one-day harvest interval but use of bifenthrin is limited to one treatment during the growing season.
A pathogenic nematode, Steinernema carpocapsae, is available by mail order from some suppliers of biological controls. The microscopic worm-like nematodes are sprayed or watered on to the foliage, preferably in the evening when the leaves are likely to stay wet for longer. The nematodes enter the caterpillars’ bodies and infect them with a bacterial disease.