Caterpillars are the larvae of butterflies and moths, although, the larvae of some other groups of insects, such as sawfly, are often called caterpillars. Caterpillars are often voracious eaters of plant tissue, although some burrow into fruits and pods. Heavy infestations can result in complete defoliation of plants if left untreated. Codling moth and Plum fruit moth are the main threat to orchard crops like apples and Plums. Moths usually start mating and egg laying in May in the U.K. It is important at this time to put pheromone traps like Just Moth trap out from May onwards. These will monitor and trap the male adult moths. If caterpillars can be seen on leaves or on plants, nematodes like those in Just Caterpillar Killer can be applied directly to them. They penetrate and kill the caterpillars without leaving any toxic poisons on fruit or plants. Applications can also be made to the soil, if the caterpillars are pupating in the soil during the autumn or spring. Glue band traps can also be put around tree stems.