Slugs and snails are terrestrial gastropod molluscs which typically chew leaves, stems, flowers, fruit and vegetable debris. Slugs differ from snails in having shells that are too small for the animal to retract inside; the shell-less state has evolved on multiple occasions during the evolutionary development of molluscs, so there is little taxonomic difference between the two, and both slugs and snails do considerable damage to plants. With novel crops being grown and with insect pests having been brought more under control by biological and other means, the damage done by molluscs becomes of greater significance. Terrestrial molluscs need moist environments; snails may be more noticeable because their shells provide protection from dessication, while most slugs live in soil and only come out to feed at night. They devour seedlings, damage developing shoots and feed on salad crops and cabbages, and some species tunnel into potatoes and other tubers. A weed is a plant considered undesirable in a particular situation; the term has no botanical significance. Often, weeds are simply those native plants that are adapted to grow in disturbed ground, the disturbance caused by ploughing and cultivation favouring them over other species. Any plant is a weed if it appears in a location where it is unwanted; Bermuda grass makes a good lawn plant under hot dry conditions but become a bad weed when it out-competes cultivated plants. A different group of weeds consists of those that are invasive, introduced, often unintentionally, to habitats to which they are not native but in which they thrive. Without their original competitors, herbivores, and diseases, they may increase and become a serious nuisance. One such plant is purple loosestrife, a native of Europe and Asia where it occurs in ditches, wet meadows and marshes; introduced into North America, it has no natural enemies to keep it in check, and has taken over vast tracts of wetlands to the exclusion of native species. In forestry, pests may affect various parts of the tree, from its roots and trunk to the canopy far overhead. The accessibility of the part of the tree affected may make detection difficult, so that a pest problem may already be far advanced before it is first observed from the ground. The larch sawfly and spruce budworm are two insect pests prevalent in Alaska and aerial surveys can show which sections of forest are being defoliated in any given year so that appropriate remedial action can be taken. Some pests may not be present on the tree all year round, either because of their life cycle or because they rotate between different host species at different times of year.