There are no walls that stop modified pollen from spreading death.
Monstanto made terminator seeds that do not grow viable seeds. This is a clear and present danger to spreading a die off of accidental hybrid nearby crops. Of course Monsanto claims this is a myth.
http://www.globalresearch.ca/the-seeds-of-suicide-how-monsanto-destroys-farming/5329947

https://actions.sumofus.org/a/brazil-terminator-seeds

In 1970 the UK made a movie of a global die off of all grains called no blade of grass. They had some technical plot problems but the dangers are greater today.

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I have imagined giant fruit that was so good the dinosaurs made them go extinct. How about watermelon sized favorite fruits?