The Bush administration promised yesterday that a possible new combatant in the war against cocaine -- a caterpillar with a taste for coca leaves -- won't be deployed in South America unless local governments approve.

"We are not undertaking any biological war," said White House spokesman Marlin Fitzwater. "Neither troops nor caterpillars will go in without prior request and consultation."

A Peruvian Embassy spokesman said the ambassador met with administration officials and was assured no U.S. action would be taken without Peru's approval.

A Bolivian official said his country's environmental concerns have prompted it to reject the use of chemicals to destroy coca crops, and the same concerns probably will lead his government to reject the use of biological agents.

The administration's drug-budget proposal for the Agricultural Research Service for fiscal 1991 is $6.5 million, a $5 million increase over the amount to be spent this year.

The principal focus of that research is the malumbia, a white moth that, when it is still in its caterpillar stages, eats coca leaves, officials said.

Fitzwater emphasized that the insect research program is in the experimental stage. "The Department of Agriculture is studying not just coca but other drug plants as well to learn as much about them as possible," he said. "This research includes study of herbicide and natural enemies of these plants. . . . This program is experimental. Absolutely no potential tool will be considered for use until it is proven to be safe and effective."

Fitzwater said the subject of biological war against drug crops was not broached at last week's drug summit in Colombia.

Fitzwater also said the administration had "intelligence" about missiles and rockets in Colombia and "people with various plans" before President Bush's trip to the summit. But there was no confirmation of the reports, he said.

He was asked about Monday reports from Colombia that police had seized 10 shoulder-fired surface-to-air missiles from drug traffickers.

According to an administration source, the missiles were seized four days before Bush arrived in Colombia and were the reason for concern about a possible attack.

Meanwhile, Secretary of State James A. Baker III said the "most critical challenge" in the drug war is domestic consumption. "As long as the demand for drugs by Americans remains voracious, our nation faces an endless, uphill struggle to halt supply," he said.

In remarks prepared for delivery in New York at the U.N. Special Session on Narcotics, Baker said a U.N.-led anti-drug effort must entail concrete commitments and be fully implemented, if it is to succeed.

"Certainly, we the member governments cannot ask the United Nations to do things that we will not do at home," he said.