Aphids eat plants. It's what they do. To try to get them to top devouring plants that we want to eat, we can try to kill them off with chemicals, or we can just genetically engineer a new strain of wheat to emit aphid panic pheromones. It doesn't kill the bugs, but it does scare them away.
Researchers in Britain have taken some genes out of a peppermint plant, given them a tweak, and spliced them back into a strain of wheat to get the wheat to smell like panicking aphids. Apparently, panicking aphids smell just like green apples, but the extra smell that the wheat puts out is too faint for humans to pick up on. What can pick up on it are parasitic wasps, which (as a side-effect of the pheromone) will come and lay their eggs into live aphids and their young will later devour them from the inside. Pleasant.
Using genetic modifications to develop non-toxic methods of pest control is a good idea, but even if you forget about all the non-organicness or whatever of using chemical insecticides, it's a battle that we just can't win. Thanks to random genetic variations, targeted pesticides are never 100% effective, and even if they're 99% effective, the 1% you don't manage to kill will become the next 100% and all of a sudden they're all resistant to the chemical that worked just one single generation ago.
Also, just because a plant has been genetically modified to produce a chemical doesn't necessarily make that chemical any more effective than just spraying a plant with it. It's possible (even probable) that aphids will eventually stop getting scared by this wheat, although evolutionary selection for resistance will take longer since aphids that are susceptible to the chemical aren't immediately killed. (Read: So the susceptible ones can keep breeding, and it'll take longer for the resistant ones to become dominant in the population.) On the other hand, since this is an aphid pheromone, if they do stop responding to it, they'll also stop responding to the real thing, making it easier for predators to find and eat them.
There's no real long-term solution for agricultural pests, except for just trying to balance them out with natural predators that can evolve fast enough to keep the war going. But in the short and medium term, using cleverly engineered plants can help tip the balance in the favor of us hungry humans.