Night fell over the plaza. Thousands of tiny insects swarmed around the spotlights trained on Cuban President Fidel Castro, and 75,000 people listened nervously as he talked like a prophet of the "noxious plagues' that have descended on this island over the last two years.

African swine fever, sugar cane rust, blue mold on tobacco have damaged the economy and now dengue fever has killed 113 people and infected more than 270,000 others in the last seven weeks.

Castro did not claim to be certain of the source of these diseases but he noted that "many citizens are deeply convinced that these sicknesses, especially dengue, were produced in this country by Yankee imperialism."

Castro shared the suspicion. He said that the plagues "could have been introduced . . . by the CIA."

In other countries such an allegation might be greeted as a symptom of geopolitical paranoia. But to many Cubans they seemed probable, and as Castro warmed to the theme he abandoned the conciliatory tone toward Washington that marked some of his speeches earlier this way. The United States is once again the great nemesis of the Cuban people, according to Castro, and there will be no coming together.

Castro's speech on the 28th anniversary of the attack on the Moncada Barracks in eastern Cuba, which is generally regarded as the beginning of the Cuban revolution, was devoid of any discussion of Cuba's troubled relations with Central American countries. There was no mention of El Salvador, no mention of Guatemala. But the close relationship between Cuba and Nicaragua was apparent as Fidel Castro was introduced to the crowd by Humberto Ortega, defense minister of Nicaragua.

As Castro spoke and tremendous cheering and the waving of paper flags, he was careful to say that the Cuban economy is one the rebound after several years of slack production. During the first quarter of 1981, he said, most of the industrial and agricultural sectors of the economy produced more than the quotas that had been set for them. In some areas, such as dairy production, records were etablished, he said.

As he addressed the question of the various blights that have descended on his country, Castro was careful to lay out the background for his suspicions that the CIA might have been involved. He quoted from technical literature and testimony before the U.S. Senate on the uses and purposes of biological warfare.

Castro noted the well documented attempts of the CIA in the 1960s to kill him and to undermine the economy. He recalled attempts against his life with poisoned cigars and contaminated clothing, special guns and Mafia "mercenaries."

He said the intentions of the Reagan administration to "unleash the CIA" indicated a new turn toward the 1960s once again and he said that anything could be expected of "a government whose policy is characterized by its cynicism, its lies and its absolute lack of scruples."

"We summon the United States government to define its policy in this area and say whether the CIA will be authorized once again or not -- or is being authorized already -- to organize attempts against the leaders of the revolution and to use plagues against our plants, our animals and our population," Castro said.

[In Washington, a State Department spokesman denied Castro's suggestion, saying: "The Cuban government has always tried to blame the United States for its failures. . . . The Cuban revolution is a failure and it is obviously easier to blame external forces like the United State than to admit those failures."]

The Cuban president, who has been accused by the Reagan administration of interfering in Central American affairs by sending arms to revolutionaries there, was especially incensed at reports that Cuban exiles opposed to his government are training in southern Florida.As he put it, the Reagan administration has said "not one word" against them.

Castro's vehemence was a reflection of the depredations the disease has wrought on his people and his economy.

The sickness totally debilitates its victims. High fevers, excruciating headaches, vomiting, diarrhea and a painful rash, then a convalescence of several weeks are its trademarks. It is spread by the aedes aegypti mosquito, which also carries yellow fever.

Four stains of dengue fever are frequently reported in the Caribbean, but this one, known as Type 2, is new to Cuba and has spread more rapidly and with more deadly consequence than anything this nation has seen before. In children especially it can bring on fatal internal bleeding.

Whatever the source of the disease, Cuba's feelings of grievance against the United States have grown because of it. Castro said that Cuba tried to obtain quantities of malathion -- the same controversial insecticide being used in California to combat an infestation of Mediterranean fruit flies -- from Mexico. But Castro said that because components of the pesticide were made in the United States, Mexico is not permitted to sell it to Cuba, and as a result malathion had to be flown in from Europe at a cost of $5,000 a metric ton.

[A State Department spokesman said the Department of Commerce approved on July 17 an application from the Pan American Health Organization to export to Cuba 300 metric tons of ABATE, "a U.S.-made granular pesticide used to kill the mosquitoes that spread dengue fever." He said the pesticide would be shipped as soon as possible.]