January 18, 2008

The pesticide paradox

Written by  Henry Sackville Hamilton
Entomologist K.L. Heong is a strong advocate of integrated pest management, which can dramatically reduce pesticide use. Entomologist K.L. Heong is a strong advocate of integrated pest management, which can dramatically reduce pesticide use. Photo: Ariel Javellana


If pesticides are supposed to control pests, why does an enormous reduction in use actually lower their numbers? Tests performed on the research farm at the Philippinesbased International Rice Research Institute (IRRI) have shown that, if pesticides are used less and less, then nature itself, in the forms of predators and parasitoids, will join the fight on the farmers’ side.

The research, performed by a team led by IRRI entomologist K.L. Heong,1 describes how, when IRRI farm operations were centralized in 1993, a new scheme for spraying pesticides was introduced. Instead of routine spraying once a week, pesticides would be sprayed only when pest densities in a field reached a certain level. Dr. Heong writes that “in most seasons, insect pest populations did not reach threshold levels and thus no insecticides were used.” After 14 years of the program, pesticide use on the farm has decreased by a staggering 87.5%. Insecticides, which are the main type of pesticides used on the farm, have fallen in use by 95.8%.

The study focuses on arthropods: invertebrates with a tough external protective layer (called a chitinous exoskeleton) and segmented bodies, and which make up more than 80% of all living animal species. For the paper, the arthropods were separated into four functional groups: herbivores, predators, detritivores, and parasitoids. Herbivores attack rice plants. Predators and parasitoids attack herbivores and detritivores. Detritivores eat detritus in the field.

Arthropods on the farm were surveyed in 1989, well before the introduction of the spraying scheme in 1993, and in 2005, well after it. Comparing those two surveys reveals some telling figures. In 1989, 46.2% of the arthropod population on the farm was herbivores. In 2005, when arthropods were next counted, only 11.2% was herbivores. The number of predators had risen from 40% in 1989 to 58% in 2005. Detritivores in 2005 formed 26.1% of the total arthropod density, up from 8.1% in 1989. Parasitoids experienced a smaller change: 5.6% in 1989 to 4.3% in 2005.

The reason for these swings is the unintended effects of pesticides. Pesticides can affect all creatures. Predators, parasitoids, and detritivores can be killed along with herbivores. In fact, because of their superior mobility, predators are more likely to come into contact with the poison and  thus are often more exposed to the toxins than herbivores. And, if predators are killed off, they can’t help suppress herbivore numbers.

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