Weeds can become resistant to roundup but this is not a serious issue. I can be easily solved by the application of alternative herbicides when a roundup resistant strain appears. The alternative herbicide will kill the roundup resistant weeds before they spread. Without weeds leaching nutrients the crops grow healthier and fuller so the world receives more food.
Insects are somewhat trickier because of their greater mobility and faster life cycles, but there are also procedures in place to handle this. These GMO crops are very different from roundup resistant crops in that they actually attack insects by producing pesticides. Every farmer who uses GMO crops that produce insecticides is required by contract and EPA regulations to also plant non GMO refuges within every field. These refuges allow non resistant insects a place to live so that in the highly unlikely event that an insect develops a resistance it has plenty of non resistant mates available and the resistance is lost in cross breeding. There are absolutely no documented incidences of insects developing Bt resistance when the refuge system is utilized. However, just in case there are new strains being developed which use up to two of the three effective pesticides. This means that in the really truly extreme unlikelihood that an insect becomes resistant to both insecticides they simply plant another strain that uses the third type and it kills the insects.
Peer reviewed citation:
http://www.nature.com/nbt/jour...
Simple diagram:
http://www.bt.ucsd.edu/learn/r...
There are mountains of evidence showing that GM crops produce more food per acre and have a very positive effect on feeding the world. They are also better on the environment because they reduce the usage of harsh herbicides and insecticides. There is no credible evidence that GMO crops have any negative effect on consumers. The few studies which claimed cancer links were proven false and have since been retracted. Continuing to make this work does require ongoing research and the development of new GMO plants. This research is expensive and that's why the seeds cost more. In the end the farmers still win because the increased output of the fields more than offsets the extra cost of the seed. In the process the entire world wins because we produce more food.
That is, until fear mongering groups who have no scientific proof fight to have laws put in place to prevent anyone from growing GMO crops. In their fight to "save" the world from GMO billions of people starve due to famines that could have been prevented.