GMO seeds are designed to allow application of pesticides, but after years of this the targets - weeds and insects - become progressively more resistant. This has begun happening with glyphosate (Roundup) and insecticide-resistant crops like Bt-corn. This forces farmers to both greatly increase the total quantity of pesticides applied, and reach back to older and less safe chemicals. This is the worry that they refer to, and why Monsanto continues to create new products.