Ecology of Common Insect Pests of Rice

Rice, the staple diet of over half the world's population, is grown over about 124 million hectares and occupies almost one fifth of the total world area under cereals. Classified primarily as a tropical and subtropical crop, it is cultivated as far north as 49° and as far south as 35°, and from sea level to altitudes of 3000 meters. It is established by direct sowing-broad­ cast or drilled-or transplanted, and under diverse water regimes: as an upland crop where there is no standing water and the rains are the sole source of moisture, or under lowland conditions whereby water, derived ei­ ther from rain or irrigation systems, is impounded in the fields. On slopes it is cultivated in terraces, and in valleys or other low-lying sites, floating rice may be grown in several feet of standing water. As many as 10,000 varieties of rice have been distinguished. The tradi­ tional tropical varieties are talI, leafy, and often lodge during the later stages of growth; whereas those grown in temperate areas are short, usual­ ly about 1 meter high, stiff-straw ed, erect-leaved and lodge-resistant. The plant characters of the latter group are commonly associated with high yields and are regarded as desirable. Low temperature is a major factor limiting rice cultivation. The opti­ mum temperature is about 30° C but during the flowering stage tempera­ tures of about 20° C inducc stcrility. Consequently, in regions of cool win­ ters only one crop a year can therefore be grown. In warm areas as many as three crops are common. Thus, although rice is grown under diverse cultural conditions and over a wide geographical range, it is essentially a crop of warm humid en­ vironments conducive to the survival and proliferation of insects. Of the more than 70 species recorded as pests of rice, about 20 have major significance. Together, they infest all parts and growth stages of the plant, are vectors of virus diseases, and are a major factor responsible for low rice yields-particularly in tropical Asia, the world's rice bowl. The insect problem is accentuated in multi cropping rice areas where the insects do not usually undergo a distinct diapause or dormancy but occur throughout the year in overlapping generations. The intensity of the insect problem in such areas can be illustrated by the experience at the Interna­ tional Rice Research Institute where, in 24 separate experiments conducted over the last six cropping seasons, plots protected from insects yielded al­ most twice as much as those left unprotected. Data from a large number of experiments from a number of countries which have average yields of