The news that millions of caterpillars of the Mottled Umber Moth have invaded a 2.5-acre area of woodland near Barnsley, south Yorkshire, stripping oaks and birches of shoots and leaves, will leave many gardeners reaching for the spray gun. Experts have blamed the invasion on the mild winter which, together with a shortage of natural predators in that area, have caused the caterpillars to thrive. Thankfully, gardeners shouldn't worry too much about a similar invasion on their patch of green. The Forestry Commission is calling the Barnsley incident a freak of nature and notes that no similar infestations have been reported in Britain. However, caterpillars can be a real nuisance in domestic gardens. There are many varieties of caterpillar in this country, but how do you tell if a caterpillar is responsible for the holes in the leaves of your plants? Obviously, it's a process of elimination. If you can't see any slime trails from slugs and snails, the chances are that caterpillars are the culprits. Sometimes you may also notice silken webbing, which is a common characteristic which caterpillars leave. Some caterpillars, such as those of Tortrix moths, actually produce webbing that sticks the leaves together in groups or folds the edge of the leaf over, so creating a safe haven for the caterpillar. It's best to hunt for caterpillars at night, when many of them do most of their feeding. Look out for tiny pellets of frass, their droppings. It's generally young growth which is worst affected as it is more tender and damage here can adversely affect the whole plant. If you see caterpillars, collect them up by hand or pinch off the leaves in which they are hiding and dispose of them. Insecticides are available as a deterrent, but if you don't want to go down the chemical route, try using the biological control based on Baccilus thuringiensis, a parasitic bacterium which attacks the digestive system but will harm nothing else. It can be purchased from biological-control suppliers. Caterpillars become infected when they eat the treated foliage and die within 24 hours. The treatment is safe to use on food plants. Of course, among the worst-affected crops are cabbages, if the large and small caterpillars which turn into cabbage white butterflies (Pieris brassicae and P. Rapae) take hold. The caterpillars are active from late spring to mid-autumn, while the cabbage moth (Mamestra brassicae) has one generation, with caterpillars feeding from midsummer to early autumn. The cabbage moth caterpillars bore into cabbage heads, while those of the large cabbage white remain on the outer leaves. Caterpillars of the large cabbage white butterfly are up to 4.5cm long and hairy, with yellow and black markings. Those of the small cabbage white have a velvety green appearance. Cabbage caterpillars also infest cauliflowers and Brussels sprouts as well as annual ornamentals including nasturtiums. You can deter them by covering crops with horticultural fleece. Pale green winter moth caterpillars are also common and invade many deciduous trees, fruit trees and roses, damaging blossom and fruitlets. One way to hinder them is by placing sticky grease bands at least 15cm (6in) around tree trunks in mid-autumn, to stop the wingless females from climbing up to lay eggs on the branches. Young caterpillars are easier to control than older ones, so hand pick them off if you can. | 
