By Caroline Stauffer

SAO PAULO (Reuters) - Genetically modified corn seeds are no longer protecting Brazilian farmers from voracious tropical bugs, increasing costs as producers turn to pesticides, a farm group said on Monday.

Producers want four major manufacturers of so-called BT corn seeds to reimburse them for the cost of spraying up to three coats of pesticides this year, said Ricardo Tomczyk, president of Aprosoja farm lobby in Mato Grosso state.

"The caterpillars should die if they eat the corn, but since they didn't die this year producers had to spend on average 120 reais ($54) per hectare ... at a time that corn prices are terrible," he said.

Large-scale farming in the bug-ridden tropics has always been a challenge, and now Brazil's government is concerned that planting the same crops repeatedly with the same seed technologies has left the agricultural superpower vulnerable to pest outbreaks and dependent on toxic chemicals.

Experts in the United States have also warned about corn production prospects because of a growing bug resistance to genetically modified corn. Researchers in Iowa found significant damage from rootworms in corn fields last year.

In Brazil, the main corn culprit is Spodoptera frugiperda, also known as the corn leafworm or southern grassworm.

Seed companies say they warned Brazilian farmers to plant part of their corn fields with conventional seeds to prevent bugs from mutating and developing resistance to GMO seeds.

Dow Agrosciences, a division of Dow Chemical Company, has programs in Brazil to help corn farmers develop "an integrated pest management system that includes, among other things, the cultivation of refuge areas," it said in an email.

Another company, DuPont, said it had not received any formal notification from Aprosoja. The company's Pioneer brand has been working with producers to extend the durability of its seed technology and improve efficiency since Spodoptera worms were found to have developed resistance to the Cry1F protein, it said in a statement.

The other two companies, Monsanto Co and Syngenta AG did not immediately respond to request for comment.

Tomczyk, who also spoke for Brazilian farmers during a dispute over seed royalty payments to Monsanto that ended last year, said Aprosoja encouraged the planting of refuge areas. But he said the seed companies have not given clear instructions.

"There are barely any non-GMO seeds available ... it is very uncomfortable that the companies are blaming the farmers," he said. Aprosoja hopes to reach a negotiated agreement with the seed companies, but if all else fails farmers may sue to get reparations for pesticide costs, he added.

Brazil is harvesting its second of two annual corn crops and expects to produce 78 million tonnes this crop year, slightly less than last season's record. Domestic prices recently hit their lowest in four years due to abundant supplies.

($1 = 2.223 reais)

(Reporting by Caroline Stauffer; Editing by Lisa Shumaker)