Pesticides--Is There a Better Way?

Gypsy Chang　/　photos Vincent Chang　/　tr. by Mark Halperin

February 1986

中文

Here is the relationship between the T. Brontispae larvae and its natural enemy, the brontispa longissima, seen in three different stages. At up per left is the brontispa longissima and its cocoon. At upper right the T. Brontispae is in the process of laying eggs within the shell of the brontispa longiss ima. At lower center the T. Brontispae has already consumed its natural enemy, broken out of its shell, and gone.

Long before mankind made its appearance, insects inhabited the earth. Today, when several species of rare animals face extinction, the insect population remains well out of danger. According to Life On Earth by David Attenborough, not one insect species has ever been exterminated, despite the untold effort and money spent by man.

Man has many reasons for disliking insects. They are creepy, spread disease, but most important, have ruined crops for time immemorial. After years of research, biologists have yet to find a single plant species which is not prey to some kind of insect. In parts of Africa, up to three- fourths of the annual crop is destroyed by insect pests. In the U.S., the toll can reach up to one-tenth, while China is also no stranger to the effects of crop blight. Statistics show that over NT$4 million (US$100,000) is spent by farmers in Taiwan each year on pesticides to protect coconut trees alone.

A further look at coconut trees serves to better illustrate this problem. In the mid-1970s, the Pingtung County government planted over 600,000. Beginning in 1975, they were attacked by the Brontispa longissima, a beast measuring less that a centimeter which liked to hide in the unopened leaves and gnaw the young tender shoots. Its choice of location prevented it from being killed by pesticides and enabled it to stay until maturity when it could easily fly away. Many trees were also located near fish-raising ponds, and farmers were loath to spray pesticides lest they kill the fish. The damage toll reached 100,000 dead trees, with many more being unable to bear fruit.

Facing a problem that refused to disappear, county officials turned to Lai Po-liang of the University of Hawaii Agriculture Department. Lai suggested that they use pests to exterminate pests, which meant finding the animal's natural enemy in the food chain and introducing it to Taiwan. Up until this point, the animal, living apart from its predators, was free to breed unchecked and attack other plants and animals.

The T. Brontispae is the nemesis of the Brontispa longissima, using the chrysalis of the latter as both food and shelter, not hatching until it has consumed completely its enemy. Unable to cope with such a foe, the numbers of the Brontispa longissima decreased considerably, and the crisis eased.

The vast improvements in the ease and convenience of international travel have not aided the situation. Customs departments usually examine closely and often quarantine plants before allowing them to enter a country, but checking thoroughly each specimen naturally is impossible, and pests invariably find a way to slip in. Should they find themselves in an environment without natural enemies, their population often increases drastically, often at the expense of crops and other plants.

Crop blight has menaced man since the beginning of agriculture, and the invention of pesticides in the 19th century soon found widespread acceptance. Nevertheless, it was far from being a panacea. Insects are sturdy creatures, hatching in great numbers, with some species producing as many as thirty generations in one year. Such a rate of reproduction gives them the opportunity to adapt and develop immunities to pesticides, rendering them useless. The result is a type of "arms race" between insects and man which remains far from being concluded.

Pesticides have other drawbacks. Exterminating one insect can plant the seeds for another's unchecked expansion. Some kill not their target, but the target's natural enemy, aggravating the problem.

Recently, biologists have taken a page out of biotechnology and have begun to use pests to control pests. This approach appeals to many in that it creates no pollution and preserves the original ecology much better. There is nothing new about this technique, and farmers in the past have used all sorts of animals, such as shrikes and frogs, to keep their fields free from pests. The environment today, however, is far more complex, and scientists must resort to raising natural enemies on a large scale to restore an unbalanced insect population in the ecology.

Natural enemies can be divided into two types, predatory insects and parasites. Predatory insects generally will eat almost anything, which means it sometimes fails to accomplish the task that people had in mind for it. Parasites are generally favored, since they hatch on the eggs of their enemies and then consume them until there is no more to be eaten. Other considerations include the mutual suitability of the parasite to area's ecology and climate. For example, in addition to the Brontispa longissima, the T. Brontispae also feeds on a certain type of nectar. Failing to find a suitable substitute in Taiwan, many T. Brontispae died.

Natural enemies demand special treatment. The first shipment of T. brontispae sent to Taiwan from Guam spent too long in an improper environment, owing to the unavailability of direct flights from Guam to Taipei, and thus landed in Taiwan dead on arrival. The second shipment saw staff workers speeding with the jewels to the laboratory minutes after they arrived in Taiwan and placing them in safe quarters, where the boxes were opened and whatever germs present immediately killed. The T. Brontispae was put in a tiny beaker and observed closely for problems. Not until the birth of the third generation did people begin to relax.

The T. Brontispae later accomplished its task, limiting the spread of the Brontispa longissima and allowing the coconut trees a new lease on life. Despite its sensitivity to foreign travel, it proved to be economical, with total costs coming only to a few thousand dollars. Another mark in its favor was the ability of several T. Brontispae eggs to grow on only one Brontispa longissma, an important feature given the limited manpower of the Agriculture Council.

But most organizations must use simpler means to combat pests. The Taiwan Sugar Corporation spent several years trying to defend its products against the sugarcane borer. Success took a while but now the company itself raises parasites which are natural enemies of the sugarcane borer. Use of the "biological pesticide" has yielded an effectiveness rate of 70 percent.

Complete substitution of chemicals by biological pesticides still faces formidable obstacles. The number of pests is simply too many, and the funds and manpower available to raise and use natural enemies too limited to allow for wholesale use of this technique. Many farmers, moreover, feel more comfortable in using pesticides and are reluctant to change tactics. Some suggest a combined attack using biological and chemical methods. In order to prevent the former from killing the latter in the process, biologists first breed natural enemies which are immune to the pesticide before turning them loose in the field.

What biologists fear most, of course, is a pest without an enemy, and to obviate this threat, the U.S. spends $10 million annually on customs inspection and quarantine. Despite such efforts, pests can never be fully eradicated, and limiting their spread often creates unintended, harmful consequences. Though some may feel man has the upper hand in the war against pests, when one looks at the millions of pests man must confront in order to grow his crops, it is plain to see pests have little to worry about.

[Picture Caption]

Magnified, this coconut scale resembles a fried egg. Its actual size is less than three centimeters. The parasite has already laid eggs on its "skin," and its life hangs in the balance.

Predatory mites gorge themselves on the eggs of their vegetarian cousins. (Photo courtesy of Luo Kan-ch'eng)

Hurry up and grow! Workers at Taiwan Sugar raise natural enemies of the sugarcane borer.

Sugarcane borers turn the white pulp of the cane to a reddish tinge.

Germ-carrying citrus psylla eat away the buds of an orange tree.

"Pesticides? Sure, the more the merrier." Insects are well-equipped to defend themselves against pesticides.

At up per left is the brontispa longissima and its cocoon.

At lower center the T. Brontispae has already consumed its natural enemy, broken out of its shell, and gone.

Magnified, this coconut scale resembles a fried egg. Its actual size is less than three centimeters. The parasite has already laid eggs on its "skin," and its life hangs in the balance.