This example has become a classic illustration of the importance of understanding ecological interconnections. [Note that some people question its veracity; even if it isn't fully true, it is a great illustration of unanticipated consequences!] The World Health Organization sent pesticide to Borneo to kill mosquitoes. (The specific pesticide used was not referenced in my sources for this story -- most say DDT, and some say Dieldrin.) The pesticide was effective at killing the mosquitoes, but it didn't kill all of the roaches, which accumulated it in their bodies. Lizards living in the thatched huts ate the roaches. The pesticide that they acquired (in what process?) wasn't fatal to the lizards, but sickened and slowed them so that they were more easily caught by cats. Cats eating the pesticide-laden lizards died, having also aquired a significant dose of DDT from their habit of licking themselves. With the cats gone, rats increased, carrying a threat of plague. In addition, the pesticide killed a type of parasite that feeds on caterpillars, the caterpillars multiplied in the huts where they fed on the thatched roofs. The roofs stared caving in....The World Health Organization then had thousands of cats air lifted into Borneo to kill the rats!

Check yourself: What principles or concepts from previous portions in this document on pesticides in agriculture does this tale illustrate? If you wish, you can send me, by e-mail, a list of at least three of these concepts – this is strictly optional! (muirp@science.oregonstate.edu)

Let's look at a couple of additional case studies of pesticides' interference with natural systems of pest control:


Under the influence of green revolution varieties and practices, Indonesia became self-sufficient in rice production in 1984. The Indonesian government had encouraged greatly the use of pesticides as part of this effort, offering them to growers at only 15% of their market price, with the result that the growers used them abundantly.

Initially, the pesticides facilitated the yield increases, but they were most effective only when they were first used. As time went on, the farmers began having more and more trouble with pests – particularly with the brown plant hopper and a virus that it carries. In fact, rice production in Indonesia was about to collapse because of tremendous outbreaks of this and other pests.

Many of these were "secondary pests " as was the brown plant hopper. It rapidly developed pesticide resistance, and its major predators (e.g., spiders) had been killed by the treatments.

Surveys found as many as seven times more plant hopper eggs surviving in fields that had been treated with pesticides than in untreated fields, owing largely to the depletion of its predators.

Rice yields decreased so much that rice had to be imported for the first time in many years. Scientists were consulted, and convinced the government that excessive pesticide use was imperiling their future rice production. In response, President Suharto banned 57 of the 63 pesticides that they had been using and eliminated pesticide subsidies to farmers.

Since 1987, pesticide use in Indonesian rice has decreased by 67% (with a savings of $120 million in pesticide costs) and rice production has increased by 25%! Growers there continue to use some pesticides sparingly, but scientists also taught the growers how to scout fields for pest problems, spraying only when necessary, and also how to favor the natural predators of the rice pests. Basically, they assisted the farmers in developing a tremendously successful IPM (integrated pest management) strategy for their rice production. (IPM will be discussed in more detail when we focus on sustainable agriculture later in the term.)


This example doesn't focus on pesticides as much as the previous examples did, but it is a fascinating story of science and religion and pest control in agriculture. It takes place in Bali, one of the islands in Indonesia. Details on this and other stories as well can be found in the Stevens article (BioScience Feb 1994) referenced on the supplementary reading list for this unit.

For over 1000 years, farmers on Bali had been harvesting traditional rice varieties from a complex engineered terrace system. The system had been tremendously productive for centuries without ecological degradation. Irrigation of the terrace system was coordinated by the temples, whose priests had long ago devised a complex calendar of when each paddy should be irrigated. (When western scientists first learned of this system, they dismissed it as a "rice cult.")

When green revolution rice varieties were introduced, farmers were talked into abandoning this irrigation timing, as well as into growing new rice varieties and using abundant fertilizer and pesticides. The paddies were also kept in more continuous production than previously. Soon, the paddies were being infested by one pest after another (fungi and insects) and rice production plummeted

An anthropologist became interested in the historical irrigation system, run out of the rice temples. The anthropologist figured out that the traditional timing of water release into the paddies was incredibly orchestrated, as follows:

(1) The need for water and the need for controlling pests were intricately balanced in a system of planting and then fallow that required no fertilizers or chemicals.

(2) As soon as paddy is irrigated and rice begins growing, it becomes a food source for pests.

(3) If farmers plant one crop after another (without fallow times), pests multiply rapidly.

(4) Fallow treatments kill pests (from lack of food), and also are a time for natural fertilization: after the rice is harvested, the stalks are cut and left to rot, ducks are herded into the paddies to eat insects and provide fertilizer, and blue-green algae bloom and fix nitrogen .

(5) Once the pests have died in a given paddy, a new cycle of planting begins.

The anthropologist began working with an ecologist and computer programmer, trying to model an optimum system that balanced between the opposing constraints of pest control and water supply. The problem was this: if all paddies were planted at the same time and left fallow at the same time, pests would decrease, but the demand for water would exceed supply.

However, if they all planted at different times, while there may be enough water, the crops would be overwhelmed by pests, which can simply move from one paddy to an adjacent one. The trick was, how to juggle these conflicts -- pests and water supply.

They modeled seven potential management systems of 172 paddy groups in 12 different basins in two watersheds. The modeled systems ranged from having all follow the same cropping pattern to all following different schedules.

What was discovered was that the management system historically coordinated by the temples was the same as the computer model that optimized yields. The anthropologist and scientists have now convinced the Asian Development Bank that forcing changes in irrigation strategy was a mistake, and they are returning to the temple-controlled systems.

The next section (">>" at bottom of the page) examines the general concept of negative feedback systems, with illustrations using pest control and agriculture. Use "Navigate " for reminders on how to move about within and among these documents.