In temperate regions, rice is grown only during the summer months as a single crop and insect pest populations are suppressed by winter temperatures. In the tropics, where the expansion of irrigation systems prompted by the development of photoperiod insensitive varieties has not yet occurred and rice is grown as a single rainy season crop, the principal insect pests are striped, pink, and dark headed stem borers which have alternative hosts on other crops such as sugarcane, maize and sorghum, or the white stem borer which remains dormant in the rice stubble during the off season. The first monsoon rains signalling the beginning of the rainy season break the dormancy and the moths emerge during the start of the rice planting season. With modern varieties and dry season irrigation, rice is double cropped, and there is a shift in insect pest species in favour of those that feed only on rice and do not undergo a dormant period, but instead have dispersal powers to move to nearby fields being planted. The yellow stem borer is the most common species in tropical Asia, however its damage is tolerated because of the high tillering habit of the modern varieties. The rice brown planthopper and rice green leafhopper, both feeding only on rice and having robust dispersal powers, are prevalent throughout Asia and have produced many wide scale epidemics as vectors of virus diseases. Blast, a fungal disease, continues to pose serious threats to rice production, especially in areas where it is upland, highlands in the tropics, or temperate regions. Sheath blight, sheath rot, leaf scald and some other fungal diseases have threatened rice production in the humid tropics where agronomic practices have been improved to induce dense crop stands and succulent plants. Bacterial diseases, especially rice bacterial blight, caused epidemics in South and Southeast Asia in the 60's and are still threatening today's rice crop. In China, bacterial blight is a major threat to rice production.
The major weeds of rice in South and Southeast Asia are members of the [Echinochloa crus-galli] complex, [Echinochloa colona], [Cyperus differmis], [Cyperus iria], [Fimbristylis miliacea], and [Monochoria vaginalis]. Many of these are important in other areas of the world where rice is grown under flooded conditions. Wet-seeded rice is more susceptible to weed competition than transplanted rice and weed control with herbicides is more difficult because the rice and the weeds are at the same stage of development and as a result selectivity is often marginal. Wetseeding has increased in importance in some areas in recent years because of such factors as increasing in importance due to increasing production costs and the decreasing availability of labour.