Long story short: Insects only attack dead or dying plants, and fertilizers do not make plants healthy.
Back in 2021, we were hit with a "150 year" drought here in NE Texas. We normally get something like 40" of rain every year, and the previous year was exceptionally wet.
That was also the first year I started doing regenerative ag through rotational grazing on my pasture. The previous owner probably sprayed herbicide and pesticide and fertilizer everywhere then planted ryegrass, because what I had was neatly planted rows of ryegrass up to my armpits.
Grasshoppers ate pretty much everything to the ground. I owned a patch of dry clay soil and a few weeds, along with some spots of bermudagrass that was completely brown.
The next year the grasshoppers were back, of course. There weren't enough chickens in the world to eat all of them. But curiously, in one spot of pasture, there were no grasshoppers, no weeds at all in fact. The bermudagrass had come in so strong an thick that everything else died. It couldn't compete. The area was completely free of grasshoppers too.
Then in 2023, even fewer grasshoppers. Now I saw legumes like clovers and hairy vetch and all sorts of flowering plants mixed in with the grass. The only areas the grasshoppers seemed to do well were the areas that were weak. The grasshoppers were tiny too.
And in 2024, I could barely find any grasshoppers at all. My wife liked to collect grasshoppers in soda bottles to feed the chickens, but that year she was only able to find a few.
In order to understand what is going on, let me introduce you to some powerful concepts. Concept #1 is that a high BRIX score (BRIX measures the sugar content of the leaf) means that insects won't touch it. This is because insects simply can't handle sugar. Concept #2 has to do with how the plant grows. Roughly, there are primary processes that produce sugars and starches and cellulose. These make the plant grow big and green. The secondary processes make the nutrients we expect the plant to have, the flavors and also things like wax and natural insecticides. Fertilizers overstimulate the plant to focus on primary processes. Dead soil can't support the secondary processes.
See, the minerals and nutrients that the plant needs are fed via the roots by bacteria, fungus, and even tiny insects. In healthy, living soil the plant and the soil communicate. The plant will actually send some of the sugars down to its roots. These sugars will create colonies of bacteria and such, and there is an entire lifecycle that happens, eventually leading to the nutrients the plant needs being made available for the plant.
There has already been tons of research into what exactly is happening in the soil, but it seems the more we learn the more questions we have. Right now, the "state of the art" is to stop messing with the soil, let the weeds grow, let the insects run free, and get Nature to do her thing. By using a light touch, we can encourage certain things to grow, or discourage other things. IE, by grazing animals grasses get a boost and weeds take a hit. But ultimately we can't fully control the entire process, and by mucking things up by throwing fertilizers in the ground, we are making all sorts of problems that we barely understand.
Why are your plants being devoured by insects? Simply put -- your plants are sick. They are dying. It may look green and leafy and tall, but if those secondary processes aren't happening, then not only are you denying yourself of valuable nutrients, but the insects are able to move in. You need to stop throwing fertilizer everywhere. Heck, even manure and compost can throw things out of balance, so be very careful.
Your focus as a farmer, a gardener, a rancher, whatever you do with nature, should be the soil. Do those things to build soil up naturally, don't mess with stuff you don't understand. Study nature, observe the patterns, and figure out how to use things to your advantage, but don't think that by trying to kill everything you are going to create thriving life.