Okay, i will go over the process of creating mutant insects that just might cripple anything plant related (if you have the time and money to make an army of them ;) ).
NOTE: This is for information purposes ONLY!!!! It's not my fault if a farmer blows your head off or if you shorten your lifespan, get sued, etc.
That being said, let us mutate!!!

Step 1: Get a Cage of Some Sort.

Like the step says, get a cage. I recommend a steel crate; like the ones used for holding dogs and chickens n stuff. Then line it with some screen wire, the kind used for screen doors. Epoxy, tape, burn, weld, spit; do whatever to make it stick. Line it completely so no insect will escape or be left behind.

Step 2: Obtain Pesticides

Grab every pesticide type, model, and brand you can get your paws on!!! Even a one legged drunk sailor can do this! The best idea, but it's very time consuming, is to go to a farm and ask every crop duster pilot and farmer what kind of pesticides they use. Then, take note of the dosage that you would normally use, and get a spray bottle with about a gallon of each pesticide/insecticide. Something like the bottle of witch's brew i put up.

Step 3: Obtain a Whole Lot of Insects, and Play the Waiting Game

Get a lot of insects of any type (locusts and grasshoppers are the prime candidates here), and let them "repopulate", until the population has doubled in size. Heck, if you want to make things REALLY cramped, let them triple, or quadruple in population.

Step 4: Spray Pesticides

Spray less than the recommended dosage of pesticides/insecticides. Spray from each P/I bottle; enough to kill off half the population.

Step 5: Let Them Rebreed

This might be a repeating cycle... After you massacre half of them, let them repopulate again. For the cage, i suggest a door for half the space. One half for females, one for males. Open the door to let them breed. Or, since they'll be all jumbled up, then just let them....uhhh........do what ever they've gotta do.

Step 6: Repeat the Process Until None of Them Die

Use the force, and repeat steps 3-5. I recommend getting something to pass the time, like looking at instructables ;).

Step 7: Insect Power!

Now take your cage full of battle hardened insects, and release them over a farm! Soon, every farm will be crippled, and insects will reign supreme power (until they meet mr. fly swatter). Hope you've liked it, and check the next step; there's been some controversy over what the P/I does to the bugs.

Step 8: Possible Scientific Explanation

Well, i'm no scientist, but here's an explanation as to why they become immune. Referring to a previous comment on the intro part, "the insecticides break down the chitin in the insects". Now, for those of you that don't know what chitin is, it's a polysaccharide, a long chain polymer of N-acetylglucoseamine, derived from glucose, better known as plant sugars (look at a plant cell, you'll know what i mean). This forms the shell of insects, and creates the oh so yummy sounding crunch when you step on them. In it's pure, undiluted or modified state, it's rather leathery. -Note- shiitake mushrooms' fiber is 80% chitin, according to a bag of them..........is anybody other than me feeling sick right about now? moving on... When calcium carbonate is produced in the insect's body along with chitin, it becomes hard, like armor. I wish we could secrete armor =( especially when you're getting PUMPED at the gym duuuude! you'd be like godzilla!!!! Anyway, that's what chitin is. The purpose of P/I is to break it down to soften and weaken the bug, then sucker punch them nasty chemicals when they're weak. That's cold blooded! It's like kicking a dude in the crotch, then shooting them through the lungs so they slowly die. Life sucks as an insect. So, the purpose is to attack them with these chemicals to weaken them, and whoever makes it through is hardened to it, much like humans with the common cold/ fever/ flu/ AIDS =/. Plus, now the chemicals are entwined with the bug's cell structure, so these chemicals that kill others, become bio-shields to them. Then, they have hmmhmm and when the baby comes out, it too has chemicals entwined in it's cell structure. I hope i've deconstructed science to everyone's liking.