I HAVE had lots of trouble from cabbage white butterfly caterpillars this year and as I try to grow my vegetables organically I do not wish to spray them with chemicals. What options are there for organic control?

B MOORE

REG SAYS: There certainly seemed to be a lot of cabbage caterpillar activity this summer, they were crawling up the walls of my house at one stage in August.

As far as organic spray-on treatments are concerned there is only one option and that is the new natural biological nematode (Steinernema carpocapsae). In fact, this can be applied to the foliage using either a sprayer or a watering can but the caterpillars must actually be present at the time of application, otherwise there will be no food supply for the nematodes.

Also, bear in mind that they must come into contact with the caterpillars quickly as there is always the danger that these tiny beneficial creatures will dry out and die if exposed too long on the leaf surface. Similar to the nematode species used to combat snails and vine weevils, these microscopic worm-like creatures enter the body of the caterpillars and then release a bacterium which turns the caterpillars’ insides into a sort of soup.

Nourished by this soup the nematodes breed and spread to infect other cabbage caterpillars nearby and it is worth remembering that they are more effective against younger caterpillars so try to apply the control as soon as the larvae are spotted on the foliage.

These nematodes are available through some garden centres or by mail order.

You may also be interested to learn that this nematode can also be applied to the soil under apple trees in autumn in order to control codling moth larvae that have taken up residence in the soil after feeding in the fruits during the summer.

Any maggots that have turned into pupae will also be killed.

You may have heard of a disease of caterpillars (Bacillus thuringiensis) that was formerly sold as Dipel, among other names, as an organic control measure for cabbage white caterpillars but this has now been withdrawn from sale by the pesticide regulators as it was found to be an irritant to those having sensitive skin.

Another option would be to grow your brassica crops under a permanent cover of close meshed netting like Environmesh made by Agralan. This is very durable, lasting from seven to 10 years, and will also protect appropriate crops from carrot fly, cabbage root fly, birds, rabbits, wind and hail.