Achieving a natural balance in the garden, where predators control pests, can be a sair fecht.

In the "wild", plants and animals fight an endless battle for survival and supremacy. There is an equilibrium of sorts between species, but it's constantly in flux as rival populations rise and fall.

Gardening and plant breeding cause two problems. Digging, planting, mulching and feeding create artificial and challenging conditions for plants, and by developing plants people want to eat and enjoy, scientists often remove the natural defence mechanisms of those plants. Pests enjoy "improved" specimen as much as we do, and the plant is often left vulnerable.

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Dale Walters, of the Scottish Agricultural College, writes: "In our continuous efforts to develop better yielding crop cultivars, very few species have retained the levels of pest and disease resistance present in their wild ancestors." For example, chicory and lettuce produce latex - the milky-like substance you see when snapping a lettuce stalk. This gums up the feeding parts of insects and has a bitter taste. It has largely been bred out of modern varieties, but you'll still see slugs ignoring the outer lettuce leaves, where there's still some bitterness, and slithering into the fresh, sweet hearts of the plant where defence mechanisms are weaker.

The modern cucumber developed from a prickly Himalayan ancestor with horribly bitter fruits. And, though cucumbers have been cultivated for thousands of years, some unpleasant ones persisted until the 19th century, when most were replaced by sweeter, more edible ones. In 1662, Samuel Pepys wrote: "Mr Newburgh is dead as a result of eating cowcumber." Even today, cucumbers that have been pollinated are bitter, but not fatal. In fact, many varieties have now been bred with female-only fruits.

You only need to glance at seed catalogues to see how tirelessly crop breeders have also been working to introduce varieties with resistance to certain pests and diseases. But, as Walters points out, these defences are often quite short-lived as pathogens manage to circumvent them. Look no further than the tattie crop to see how new strains of potato blight keep emerging to fell the latest blight-resistant varieties.

Gardeners need to protect crops that have lost their natural defences. Vigilance and prevention are our tools. Even voracious slugs won't fell an entire tray of seedlings at one sitting. At the first sign of trouble, there's nowt to beat a night patrol. Slug beer traps and organic slug pellets help, but nightly inspections pay dividends. You'll get a good haul by leaving one or two dead slugs on a path: their cannibalistic compatriots can't resist a free meal. During the day, you'll find them beneath a damp log or another piece of wood. Popping them in a bucket of very salty water kills them quickly.

At least we can see molluscs, but the larvae of root fly are invisible until a fine-growing broccoli collapses for lack of roots or a carrot riddled with larval tunnels is inedible. The solution is to prevent the adult fly from laying eggs near your plants. Covering either crop with fleece does the job, provided the fabric sits tightly on the soil. Collars are a less unsightly alternative for the cabbage family. Cut the bottom off a large yogurt pot, smear it with Vaseline to ward off slugs, upturn it and place it over a plant. And remember to protect young brassicas in modules. You don't want to transplant larvae with your plants. The adults are early layers.

Although achieving a "natural" balance is harder than you'd like, it works with green fly. It only takes a couple of years for the fly's natural predators - lacewing, wasps, hoverflies, ladybirds - to build up large enough populations to control the pest.