PLANT BUG

any member of two families of the insect order Heteroptera. The family Lygaeidae (see lygaeid bug) contains between about 3,000 and 5,000 species. One of the best-known members, the chinch bug (q.v.), is an important crop pest. The members of the family Miridae, which is one of the largest heteropteran families (about 8,000 species), are also known as leaf bugs. They are brightly coloured and feed primarily on plant sap, causing serious crop damage. Plant bugs occur throughout the world and have been found north of the Arctic Circle. They are soft-bodied and small, less than 10 mm (0.4 inch) long, and are easily identified by their four-segmented antennae and their beak. Among the important species is the four-lined plant bug (Poecilocapsus lineatus), which feeds mainly on blueberries, currants, and gooseberries. This yellowish bug has four longitudinal black lines along its back and is about 8 mm long. It feeds on plant juices, producing brown spots on leaves and causing them to wither. The female deposits clusters of six to eight eggs, which hatch the following spring, in plant stems; there is one generation per year. This pest is controlled by pruning or burning contaminated plants. The tarnished plant bug (Lygus pratensis), a well-known pest in North America, feeds on many plants, ranging from trees to grasses and cereals. It is about 6 mm long and is dark in colourwith yellow, black, and red markings. The use of insecticides and the elimination of hibernating spots (e.g., trash piles) help to control this pest. The apple red bug (Lygus mendax) is red and black and about 6 mm long. The front part of the thorax and the wings are usually red, and the posterior thorax and the inner edge of the wings are usually black. It is an important apple orchard pest that causes spotting of leaves and injures the fruit so that it is not marketable. The garden fleahopper (Halticus bractatus) is a small, shiny black jumping bug about 2 mm long. The forewings of this short-winged leaf bug lack a membrane and resemble the hard forewings of a beetle. The fleahopper sucks the juices from garden plants. There are usually five generations every season. An important cotton pest is the cotton fleahopper (Psallus seriatus). The oval-shaped adult is about 3 mm long and pale green in colour, with four black spots on its body. It passes the winter in the egg stage in the plant tissues of weeds. In the spring after the eggs hatch, the nymphs eat the weeds; they then migrate to nearby cotton fields to feed on the cotton plant. Later, the cotton fleahoppers return to the weeds. The life cycle is short, about 22 days, so that there may be seven or eight generations each season. The cotton fleahopper can easily be controlled by insecticide sprays. Helopeltis theivora is the tea blight bug of Southeast Asia. It is both common and highly destructive. Although most plant bugs are plant pests, some are beneficial. Cyrtorhynus mundulus of Australia feeds on the sugarcane leafhopper's eggs. It has been introduced into certain regions (e.g., Hawaii) as a control for this pest. Classification Critical appraisal In the classification above, only the major divisions and classes of living plants are listed, and a number of entirely extinct divisions are omitted. The classification outlined is somewhat conservative but is one that best conforms to available data and has gained wide acceptance. Biological classifications were initially mechanical or artificial; that is to say, they had no basis in evolution. This was followed by a period of natural system construction, whereby plants were grouped together on the basis of their overall similarities or differences, using as many characteristics as possible. Contemporary systems of biological classification are phylogenetic, which means that various plants are arranged together because they are thought to be related by descent from a common ancestor. As more evidence is accumulated, it is to be expected that classifications also will change to accommodate this new information. At the turn of the 19th century, the plant kingdom was frequently divided into two major groups, the cryptogamia (algae, fungi, bryophytes, and ferns) and the phanerogamia (gymnosperms and angiosperms). Subsequently, it was common practice among systematic botanists to group all vascular plants together under a single division, Tracheophyta. In contrast, all of the major individual groups of plants are now most often elevated to divisional rank, a view that interprets the individual major groups to be less closely related to one another than previously believed. Difficult and complex questions still exist in the definition and circumscription of certain groups. Although it is convenient to treat the Bryophyta (mosses, liverworts, and hornworts) as a single division, some authorities prefer to recognize three or five separate divisions for these plants, each representing an independent evolutionary lineage. Furthermore, the phylogenetic relationships, if any, of the bryophytic plants with primitive vascular plants remains unclear. The treatment of the Psilotophyta is controversial. Although here recognized as a separate division of lower vascular plants, some botanists prefer to include the group among the ferns (Filicophyta). The flowering plants have classically been recognized as composed of two distinct classes; however, the origin of each class and the phylogenetic relationships of their component taxa have yet to be satisfactorily resolved. William C. Dickison