Bilal Atta, Muhammad Rizwan, Arshed Makhdoom Sabir, Syed Sultan Ali (Rice Research Institute, Kala Shah Kaku, Punjab, Pakistan)
Rice is cultivated extensively in the most diverse ecosystems of tropical and sub-tropical regions of the world. It is the staple food for people in 39 countries, which include 2.70 billion people in Asia alone.
Rice production in Pakistan holds an extremely key position in agriculture and the national economy. Pakistan is the world’s 4th largest producer of rice. It is the second-largest export commodity after textile and placing Pakistan in the top four rice exporting countries of the world. Rice is the third-largest crop after wheat and cotton which is grown over 10 % of the total cropped area. Rice is a highly valued cash crop and it accounts for 6.7 % in value-added in agriculture and 1.6 % in GDP. Pakistan grows enough high-quality rice to meet both domestic demands and allow for exports of around one million tons per annum.
Among various biotic constraints of rice production, the insect pests are of prime importance and the warm humid environment of the crop is also conducive for their survival and proliferation. Among them, brown planthopper is a typical phloem sap feeder and one of the most serious and destructive pests of rice throughout Asia. It causes yield loss amounting to as high as 60% under epidemic conditions. It is difficult to monitor this pest and by the time plant damage becomes evident, significant loss in yield is inevitable. The only way possible for the management of this pest is the regular monitoring of the crop. Both nymphs and adults suck sap from the leaves and leaf sheaths, which results in yellowing of leaves, reduced tillering, reduced plant height, and an increase in the number of unfilled grains. Brown planthopper also causes the reduction in chlorophyll, the protein content of leaves, and photosynthetic rate, whereas severe attack of brown planthopper causes ‘hopper burn’ symptoms. Hopper burn appears initially as yellowing of older leaves but soon extends to the whole plant, which turns brown and dies. It also transmits virus diseases like a grassy stunt, ragged stunt, and wilted stunt.
Though many chemicals were recommended for the control of this pest, due to its feeding behavior at the base of the plant, the farmers are unable to control this pest effectively. As a result, farmers resort to the blanket application of insecticides which often disrupts the ecological balance of the rice ecosystem due to which this pest has already developed resistance against many insecticides in different Asian countries. Extensive chemical control of brown planthopper on rice can cause serious problems including toxicity to natural enemies of brown planthopper, increased total production cost, and possible long-term agro-ecosystem and human health damage.
The use of genetic resistance is the most effective measure for brown planthopper management. Cultivation of resistant varieties is an economical, efficient, and environmentally sound strategy for population management of insect pests. These varieties provide pest control at essentially no cost to the farmers and it is extremely important as today’s rising input cost has severely eroded the farmer’s profit margin. Varieties with various levels of resistance can be deployed for insect control in combination with other components of pest management. It also helps in the conservation of natural enemies and minimizing the number of pesticide applications. Resistant and moderately resistant varieties keep the pest densities below the economic threshold levels and best in combination with natural enemies. The restless behavior of insects on the resistant varieties also increases their vulnerability to their natural enemies. The use of these combinations brings the unrelated mortality factors and provides the density-independent mortality of insect pests.
Brown planthopper populations on rice have been categorized into four biotypes. The population in East and Southeast Asia is reported as Biotype-1, while Biotype-2 originated in Indonesia and Vietnam as the dominant biotype. Biotype-3 was produced in the laboratory at the International Rice Research Institute (IRRI) and in Japan, whereas Biotype-4 is found only in South Asia.
Thirty brown planthopper resistance genes had been identified from rice cultivars. The brown planthopper resistance genes from Bph1 to Bph9 are from O. sativa as gene source, whereas from Bph10 to Bph30 gene sources are from wild rice species (except for Bph17, Bph25, Bph26, Bph27, and Bph28). Among different methods evaluated to find brown planthopper resistance in rice, seedling screening is the most popular and high throughput method due to its rapid and efficient screening of rice lines.