For generations, it has been conventional wisdom in the Western world that plagues of locusts are a major threat to the populations of Africa, warranting heroic efforts to combat them.

Now, according to a new report from the congressional Office of Technology Assessment, it looks as if the locusts are not a big threat and that, in any case, massive pesticide-spraying efforts don't do much good and may have harmed the environment and public health.

In the latter part of the 1980s, Africa experienced the largest outbreak of locusts and grasshoppers in 50 years. Donor countries, including the United States, spent $275 million to bomb the locusts with insecticides. The United States provided $60 million worth of aircraft, insecticides and technical assistance.

The latest outbreak ended abruptly last year. Some experts contend the war was won by human intervention and massive spraying. However, many locust specialists argue that the plague simply played itself out, perhaps aided by hotter and drier conditions that killed the locusts and their offspring.

"I would say the population went through a normal cycle and the spraying had little to do with ending the outbreak," said Dean Haynes, an entomologist at Michigan State University in East Lansing, who advised OTA. "It's hard to justify the costs of such an effort. There are serious insect problems in Africa and the locust outbreaks would be pretty low on the list of priorities."

The OTA report noted that many experts believe donor organizations, such as the U.S. Agency for International Development, do not deserve credit for ending the outbreak so much as blame -- "for the mostly uncounted financial, health and environmental costs of insecticide-based control programs; for using funds for emergency efforts that might have been better spent on long-term developmental efforts; and for focusing on a few insects that, while highly visible, do not cause crop losses as great as some agricultural pests."

Officials with AID stressed, however, that they were responding to what they perceived as an emergency and that while huge agricultural losses did not occur, in past outbreaks locusts destroyed far more crops.

Haynes and others argued, nevertheless, that Africa has enormous problems with mosquitoes, which transmit malaria, and tsetse flies, which have made large parts of the continent almost inhabitable. Dozens of agricultural pests rob African nations of more agricultural products than do the locusts.

One reason for such attention to locusts may be a mistaken impression, fostered perhaps by biblical tales, that the swarms cause famine. This perception among policy-makers in the donor countries and Africa may be enhanced by the fact that locusts often persist during periods of drought, when the land already is stressed.

Most of the time, the insects live as solitary individuals that fly at night. However, when local populations reach a high density and when the insects perceive that vegetation is ample, they begin to aggregate and roam the countryside in a swarm. While locusts are known to respond this way, so do some closely related species, called aggregating grasshoppers.

The OTA report suggested that a long-term commitment to Africa's insect pests could yield more benefits than sporadic and questionable attempts to knock locust swarms out of the sky. OTA, which advises the Congress on technical and scientific matters, also recommended that pest-control methods other than insecticides be favored.

Locusts can cause almost complete devastation. But the effects are localized, akin to those of a tornado. OTA reported that in 1986, the first year of the outbreak, locusts caused crop losses of less than 1 percent in nine most affected countries. In these cases, donor countries spent $40 million to save $46 million in agricultural production.