Vegetables Gardening - Growing Organic Vegetables

I think its just the caterpillar of the cabbage moth. Have you seen the
little white butterflys around your broccoli patch. They lay their eggs and
when those hatch they nibble through the green leaves. Usually from the
underside of the leaves. I have to hand pick them off in the morning when
its cooler they are active.

Good luck

Here's a link with a picture
http://www.gardenaction.co.uk/techniques/pests/cabbage_caterpillar_moth.htm

Cabbage Caterpillars and Moths
There are three common caterpillars which feed on brassicas:

Mamestra brassicae - Cabbage Moth
Pieris brassicae - Large Cabbage White Butterfly
Pieris rapae - Small Cabbage White Butterfly

Of the three above, the Small Cabbage White is the most common and does the
most damage.

small cabbage white - caterpillarIt is the caterpillars which do the damage
rather than the moths or butterflies. The damage occurs between April and
October because each of the varieties has two or more generations from
spring to autumn.

The life cycle is as follows. The adults appear in mid-spring and lay single
eggs on the leaves of cabbage, cauliflower, Brussels sprouts, kale and
kohlrabi. The eggs are yellow, oblong and ridged lengthwise. They hatch in
around 5 days depending on weather conditions. Initially they feed on the
outer leaves, but as they mature, they tend to go for the heart leaves. They
reach about 2.5 cms (1 inch) in length after a couple of weeks. Around this
stage they pupate into a chrysalis and it is from these chrysalis that more
butterflies and moths will emerge. The final generation of each year
over-winters as a chrysalis.

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