International Rice Research Institute
Los Baños, Philippines
The term “biodiversity” is being used to describe the richness and variety of life on earth. It includes diversity at the genetic level, such as that between individuals in a population, diversity of species and the diversity at the habitat and ecosystem levels. The general assumption is that the biodiversity of monocultures, such as rice, is decreased and therefore unstable and prone to pest attacks and need to be “protected”. This thinking, not necessarily true (Way and Heong 1994), has persisted for decades and is still dominant among agricultural scientists and policy makers, has been the key driver of pesticide use in many agricultural systems.
Arthropod biodiversity and abundance are fundamental components of rice ecosystems that have resistance and resilience to pest attacks. They provide farms with ecosystem services, such as resistance to pest invasions and regulation of pest populations that prevent pest species from increasing to levels that can cause economic loss to farmers. Pest species at levels below the economic threshold have no yield consequences because of plant compensation abilities of the rice crop. In fact, they contribute toward the maintenance of a stable food web structure. At least 200 species of parasitoids and 150 species of predators in and around the rice ecosystems constantly provide the services that keep pest species at low levels. In addition, the aquatic fauna, often referred to as “neutrals,” play a significant role as food resources for predators (Settle et al 1996). Being an island, there is great potential of discovering new species of spiders, parasitoids and less mobile predators that are inherent in Hainan.
Rice production systems are ephemeral habitats in which rice is sowed and harvested over a period of 3 to 4 months. Many of the important rice pests are monophagous and r-strategists or opportunistic species and under normal circumstances cause little yield loss on rice farms with adequate biodiversity and ecosystem services. However, when r species, such as planthoppers, invade rice fields that are vulnerable with low biodiversity and ecosystem services, they tend to increase exponentially into outbreak proportions and destroy crops. In mainland China every year, planthoppers destroy at least a million hectares of rice (Cheng 2009) and, in years like 2005, when summer temperatures were elevated, 7.5 million hectares were destroyed.
In Hainan Island, about 30,000 hectares are affected annually by planthoppers, but records of losses are not available. In the 1960s and ’70s, when rice sufficiency was the primary national objective, policies and farmers adopted high pesticide routines. Today, such practices are still rampant in China and on Hainan Island as well. Research at IRRI and by Chinese scientists at Zhejiang University has now shown that routine insecticide sprays have very little economic gains but huge negative impacts on arthropod biodiversity, the food chain, and ecosystem services, which bring about planthopper outbreaks and rapid development of insecticide resistance. The important functions of biodiversity and ecosystem services had not been recognized or researched or taught in universities and had certainly not been factored into agricultural policies. It has been shown that when insecticide load is reduced arthropod biodiversity and ecosystem services can be restored (Heong et al 2007).
Cheng, J.A. 2009. Rice planthopper problems and relevant causes in China. Pp 157-178. In Heong KL, Hardy B, editors. 2009. Planthoppers: new threats to the sustainability of intensive rice production systems in Asia. Los Baños (Philippines): International Rice Research Institute.
Heong, K. L. , Manza,A., Catindig, J., Villareal, S. and Jacobsen, T. 2007. Changes in pesticide use and arthropod biodiversity in the IRRI research farm. Outlooks in Pest ManagementOctober 2007.
Settle,W.H., Ariawan, H., Astuti, E.T., Cahyana,W., Hakim, A.L., Hindayana, D., Lestari, A.S. & Pajarningsih (1996) Managing tropical rice pests through conservation of generalist natural enemies and alternate prey. Ecology, 77, 1975–1988.
Way, M.J. and Heong, K.L. 1994. The role of biodiversity in the dynamics and management of insect pests of tropical irrigated rice – A review. Bulletin of Entomological Research, 84, 567-587.