By: J.A. Cheng
Zhejiang University, Hangzhou, PR China
Professor Cheng has been working on rice planthoppers since the 1970s and here he shares his analysis of the development of planthopper problems in China since the 1960s.
There are perhaps three stages in the development of planthopper problems in China in the last 40 years from the 1960s. During this period there were changes in species dominance, increases in frequencies and intensities of outbreaks, and all indications show that these problems are likely to worsen in the near future.
The mid-1960’s to late 1970s. This period corresponds to the Green Revolution (GR) in many parts of Asia. We often associate GR with the Philippines (Masagana 99), Indonesia (BIMAS), Bangladesh, India, Malaysia and Burma. Information from China was not readily available to the outside world then. China had her own GR to be self sufficient and during that time three planthopper species became major pests from potential pests. The small brown planthopper (SBPH) Laodelphax striatellus became most important in the mid-1960s and caused serious yield losses by transmitting two virus diseases, the Rice Stripe Virus and the Rice Black-Streaked Dwarf Virus. Around the late 1960s, the brown planthopper (BPH) Nilaparvata lugens was very serious. China had experienced BPH outbreaks a decade before the SE Asian countries did. The white backed planthopper (WBPH), Sogatella furcifera started to be more serious in the late 1970s. High frequencies of outbreaks, when average peak population per 100 hills was above 3,000 in insecticide-free plots, were recorded in the years 1968, 1973 and 1976.
The late-1970s to late 1990s. During this period, hybrid rice growing expanded rapidly as China strived for self sufficiency in rice. Farmers were urged to protect these crops vigilantly by routine spraying of insecticides like methyl parathion, BHC, methamidophios and BPMC. Most rices were attacked by two species of planthoppers, the BPH and WBPH, but WBPH was most dominant. Areas attacked by the WBPH increased as the area grown with hybrid varieties increased (see graph). Planthopper outbreaks occurred more frequently, often two years in succession, as in 1978-1979, 1982-1983 and 1987-1988. Losses were estimated to be about 1 million tons per year. The highest area infested with WBPH and BPH densities above thresholds was about 25 million hectares in 1991.
The new millennium 2000s. The is period is characterized by heavy infestations by all three species at the same time. Populations have surpassed those in the 1980s-1990s. The SBPH are not only causing yield loss because of the virus diseases they carry, but from direct damage from grain feeding by huge populations as well (see picture). The SBPH had not been known to cause direct damages in the past. Rice planthopper outbreaks in many rice areas occurred 4 years in succession, 2005 to 2008. In 2005, infestations were particularly serious which caused loss in production of an estimated 2.8 million tons and in subsequent years planthopper densities in insecticide-free plots had exceeded 20,000 in 100 hills. Population densities in the 1980s were less than 10,000 in 100 hills, indicating that planthopper problems from 2005 have grown worst substantially. The new millennium is also characterized by rapid development of insecticide resistance. The BPH has become practically immune to the compound imidacloprid introduced just 10 years ago. According to toxicological work done by Japanese scientists, WBPH has high tolerance to fiprinol, a compound generally used as a systemic insecticide.
China has been experiencing rice planthopper problems since the 1960s and has not developed a strategy to manage them. And lately these pest problems seem to have intensified. There are several speculations to explain the reasons behind these developments but there is insufficient research to fully understand cause-effects. Among the reasons postulated are heavy insecticide use have destroyed important regulatory ecosystem services, climate change resulting in elevated autumn temperatures, more intense and higher frequency of typhoons to displace the immigrant hoppers, changes in rice varieties particularly the high adoption of hybrid rice and insecticide resistance. Perhaps in 2005 when autumn temperatures were 2 or 3 oC higher, the abnormally high damages by planthoppers might be attributable to climate change. Changes of cropping system in southern China may have affected the immigration. The parallel increase in WBPH with increase in hybrid rice adoption indicates that this change might have caused the rise in WBPH. Perhaps the differential toxicity of fipronil to BPH and WBPH found by Japanese scientists might have also affected the dominance of WBPH. The high use of imidacloprid is obviously responsible for the rapid development of insecticide resistance. Imidacloprid is highly toxic to hymenopterans especially bees, and this might have contributed
to further deterioration of the hymenopteran parasitoid activities. All these factors and their combinations will need more ecological research to provide better understanding. Rice planthoppers are outbreak pests and such frequent and intensive occurrences of these outbreaks are clear indications of ecosystem breakdown. It is clear that rice production so heavily dependent on chemical inputs is not sustainable. The need for a paradigm shift towards managing rice insect pests is urgent but inadequate resources are devoted to explore for new approaches. It looks like unless there is a paradigm shift from the “silver bullet” approach to a more holistic approach and more emphases are placed on discovering “ecological solutions” rather than just “genetic solutions”, China will continue to suffer from the planthopper menace and losses in the near future.