July 1, 2016 10:22 a.m. ET
SÃO PAULO—A new decree by Brazil’s acting president that sanctions the use of aircraft to spray insecticides and larvicides to combat virus-bearing mosquitoes has raised concerns among some environmental activists and medical authorities just weeks ahead of the Olympic Games.

The dispute underscores Brazil’s continuing struggle to control the spread of the Aedes aegypti mosquito, which transmits dangerous diseases including Zika, dengue and chikungunya.

With the Olympics starting Aug. 5 in Rio de Janeiro, Brazilian officials are trying to calm athletes’ and tourists’ fears of contracting mosquito-borne illnesses.

The decree, signed June 27 by Brazil’s acting President Michel Temer, would permit aerial spraying to combat Aedes aegypti. Although the government hasn’t said which pesticides would be permitted, they reportedly wouldn’t include DDT, which is prohibited in Brazil.

The new decree quickly aroused opposition from environmental activists who said that aerial spraying would be harmful to humans and ineffective at stopping the spread of the mosquito. Opponents are planning demonstrations against the measure.

Dr. Artur Timerman, president of Brazil’s Society of Dengue and Arbovirus, who frequently has criticized the Brazilian government’s response to the Zika crisis, condemned the new plan.

“To use aviation in the fight against mosquitoes is ignorant,” Dr. Timerman said, adding that what he called aerial “fogging” would have no effect on mosquito larvae, but could pose serious health risks to humans.

In response to a request from The Wall Street Journal, Brazil’s Ministry of Health issued a statement Wednesday in support of aerial spraying “at strategic points to eliminate mosquito outbreaks.”

“The Ministry of Health points out that the use of insecticide must be done rationally, as a complementary measure to the prevention and control of disease,” the statement read.

Mr. Temer’s administration also drew fire this week for vetoing an import-and-export tax exemption, approved by Congress, on insect repellents, which many poor Brazilians cannot afford to buy at market prices. The Temer administration said the tax incentives didn’t meet federal guidelines.

Brazilian officials have been at pains to reassure visitors that the country can manage its mosquito problem, at least for the two weeks when the Olympic Games will take place.

On Thursday, the Brazilian government released updated figures showing that infection rates of Zika and dengue have fallen dramatically since earlier this year. The government attributes the decline to stepped-up eradication efforts. The cooler temperatures of Brazil’s current winter season also are likely a factor.

Through May 14 of this year, Brazil confirmed more than 1.2 million cases of dengue, only slightly fewer than the number for all of 2015, which set a record. Through May 28, there were 161,241 confirmed cases of Zika, which the World Health Organization has classified as a public health emergency. There have been more than 38,000 cases in Rio de Janeiro state, more than in any other state except Bahia in the northeast.

Zika has been linked by U.S., Brazil and global health authorities to a spike in Brazilian babies born with undersized skulls and brains, a condition known as microcephaly. The Zika virus also has been linked to Guillain-Barré syndrome, a disorder that can cause permanent nerve damage.

Olympics officials, the WHO and some leading medical authorities in Brazil have said that the Zika virus poses a very low risk to the expected 500,000 tourists and 10,000 athletes.

Still, a growing number of athletes have announced they won’t come to Rio out of concern about Zika, including golfers Rory McIlroy and Marc Leishman, and cyclist Tejay van Garderen.

Other prominent athletes, including basketball star Pau Gasol, also have considered not attending.

Julio Kampf, president of the National Syndicate of Agriculture Aviation, said in an interview that aerial pesticide spraying is used effectively in other parts of the world and is justified by the scale of Brazil’s mosquito crisis.

Mr. Kampf said that half of Brazil’s 2,200 agricultural aircraft could be deployed to combat Aedes aegypti.

“We must create a Brazilian protocol for, when necessary, an epidemic, using the aircraft in the war against the mosquito,” said Mr. Kampf, who thinks the government also should increase funding for public-health and basic sanitation measures to curb the spread of mosquitoes.

Mr. Kampf said environmentalists’ opposition to the use of pesticides already has hampered mosquito-control efforts in areas near the Olympics venues.

“As for ecological advocates, I wanted to see what they would say when you have someone in the family with microcephaly,” he said.

Write to Reed Johnson at [email protected] and Rogerio Jelmayer at [email protected]