VAL BOURNE reveals the latest 'green' thoughts on how to deter pests

Some of you may have noticed that your favourite garden chemicals aren't for sale any more and you may have wondered why. It's all due to the EU Pesticides Review of 2002, which specified that all active ingredients had to be registered. Consequently, there's been a mass withdrawal of chemical products.

This has coincided with a resurgence of interest in vegetable growing and most people who choose to grow their own vegetables prefer not to spray them. As many of you know, I have always championed gardening without chemicals due to a mixture of Yorkshire thrift (some might say meanness), a sense of self-preservation and a love of wildlife in every form. I've gardened like this for 30 years, But I've often been publicly ridiculed for it.

But how times have changed. It was hoped that a lively debate would take place at this year's Malvern Spring Show - setting the green gardener against the chemical user. Peter Seabrook (a man I have a huge respect for) was going to defend chemicals and I was going to defend my green methods with lots of light-hearted banter. But Peter couldn't make the date and a trawl through the likely suspects failed to find anyone willing to defend chemical use in the garden.

I've found myself doing a series of six articles for the RHS magazine The Garden. The article on companion planting has proved enlightening, even to me!

In brief, companion planting is growing two crops together for mutually beneficial reasons. Over the years several combinations have been suggested. Broad beans and potatoes have been said to do well in each other's company. But these combinations has never been backed up by scientific research.

I had always believed that pungent plants (like lavender and sage) repelled pests like whitefly but new research by HRI Warwick (carried out by Finch and Collier) has proved that pungent plants do not deter pests. It seems, they simply confuse them. This is because insects feel through tiny receptors on their feet. They have to identify the same plant on four consecutive occasions in order to identify their host plant with certainty. So if your carrots are inter-planted closely with lettuces, for example, the chances are your carrot rootfly will fail its 'four-in-a-row' identity test and then fly on. Insects really do think on their feet.

This is a blow for me because I favour neatly hoed rows of carrots with a strip of brown soil on either side. Obviously, this arrangement presents no challenge to an insect who can quickly get its four consecutive readings through its feet. Interestingly, the market gardeners of Tudor London scatter-sowed mixtures of three different crops together. Many had their own combinations. Gardeners only began to sow rows after the invention of the seed drill.

There's further bad news for the vegetable grower. Insects have the ability to zoom in on green. When researchers painted the ground green, or covered the soil in green paper, the insects landed on the green soil and the green paper as often as the leaves. In other words, the insects couldn't pick up the scent of the foliage, they only went by colour. This is a new definition of green gardening.