My understanding regarding GMO's is a little different - but I may not understand very well myself. It is not that gmo plants are resistant to pesticides, it is that the pesticides become incorporated into the plant and then kill or damage many species of insects and/or birds that are not pests but also vulnerable.

Some gmo plants have been developed that are resistant to herbicides, and then herbicides may be be applied much more heavily, also killing off diversity of plant species which provide food for insects, many of which are beneficial, and many of which, even if pests in the garden, are important food sources for other animals.

Mono culture has always been a threat to biological diversity, even pre-gmo. There is much evidence that diversity is not only the spice of life, it is necessary.

Here is an abstract of a study regarding decline of Monarch butterflies that appears to be attributable to increased use of round-up on corn and soybean fields made possible by gm corn and soybeans to be tolerant to round-up.
http://www.gmoevidence.com/dr-pleasants-roundup-ready-gm-crops-cause-monarch-butterfly-decline/.

Only slight thread drift. Common milkweed is a very tasty and nutritious wild edible plant. Young shoots, flower buds, very immature pods and the silk of still immature pods are all very delicious and nutritious. Do want to be absolutely certain, when harvesting shoots, that you have the right plant, Asclepias syriaca. dogbane and butterfly weed (another milkweed species) have very similar shoots but are poisonous, especially the dogbane. Kill off the milkweed and we are killing off a human food source, that we may well wish we had in the future.

As sg already noted, there is also the reality of the adaptability of fast reproducing organisms such as many insects to become ever more resistant to what controls are known to work. Just as the over-use of of antibiotics has resulted in ever more rapidly occurring resistant and dangerous strains of bacteria, often most common in hospitals. Think MRSA and C-dif.

Re pesticide, treated seed and some gmo seed - read the following. http://www.panna.org/blog/ge-corn-sick-honey-bees-whats-link. Granted this is an article with a strong bias, but I think the facts are pretty well researched.

Thing is, we humans are coming close to the edge in terms of using up the finite resources of this old world, and have pushed the earth to the point that nature is losing its capacity for resiliency in terms of what we humans need given our over-population that yearly worsens.
There are so many confounding factors, and agricultural methods are one piece of a much larger picture.

Mass food production is necessary (forget worrying about poor quality food and additives for a moment - that is really fluff in terms of the big picture) to provide enough food to sustain the current and near-term population of humans around this globe. Also ignore distribution for a minute. People are starving in large numbers around the globe because of issues around distribution of food and resources, but at this point, though not for long, we still are able to produce enough food to feed the world's population of humans. More sustainable methods of growing food are not conducive to mass production of food and are also dependent on local conditions. I am all for following sustainable food production methods. If everyone was, then some of us will be saved from evolutionary processes a bit longer. If there were a massive movement or a world dictator who insisted and enforced long-view sustainable methods of producing food and slowing of global warming we humans will likely survive much longer as a species, but short term, many will starve more quickly than otherwise. If there were a mass movement or a world dictator who insisted and enforced food production methods that would guarantee, in the shorter term, that all should have food and no one starve because of lack of world-wide ability to produce enough food, then more of us live longer short term, but we speed up our evolutionary demise. We humans have proven so adaptable at adapting the world to us and our wants and needs that we have outpaced ourselves.

I hope that doesn't sound gloomy and pessimistic. I don't see it that way. I see it as simply realistic and as presenting all of us with some hard choices and exploration of values.