Brazilian farmers say their GMO corn is no longer resistant to pests, Reuters reported Monday.

The Association of Soybean and Corn Producers of the Mato Grosso region said farmers first noticed in March that their genetically modified corn crops were less resistant to the destructive caterpillars that “Bt corn” — which has been genetically modified to produce a toxin that repels certain pests — is supposed to protect against. In turn, farmers have been forced to apply extra coats of insecticides, racking up additional environmental and financial costs.

The association, which goes by the name Aprosoja-MT, is calling on Monsanto, DuPont, Syngenta, and Dow companies to offer solutions as well as compensate the farmers for their losses. In a release posted to the Aprosoja-MT website, spokesman Ricardo Tomcyzk said farmers spent the equivalent of $54 per hectare to spray extra pesticides, and that the biotech companies promised something they didn’t deliver, “i.e. deceptive advertising.” (via Google Translate)

But Monsanto, et al are unlikely to accommodate the farmers. According to Reuters, “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.”