Way back in the day, hundreds of years ago, people planted crops all the time for food. How do think they kept away all the pests that would ruin their plants, without the help of the chemicals we have now?
Removal by hand was at least one method - which is far more feasible with smaller fields particularly when they are being worked by many more people per acre than is currently done.
Large machinery capable of rapidly covering acreage is a relatively new development; in fact widespread farming with tractors is a fairly new development itself (from what I've seen it wasn't until after World War 2 that tractors started to outnumber horses for field work) --- and really brand spanking new when compared to the long history of agriculture.
...and this site looking at the economic history of tractors: Economic History of Tractors in the United States ...of a very interesting note is Figure 1 and the paragraph discussing it as it shows the change over from farming with horses to farming with tractors.....
The Native Americans relied on companion planting for the most part. Their agriculture practices were vastly different from the monoculture fields we see today. There wasn't a need for so many pesticides If a bug got to the tomatoes, it probably wouldn't be interested in the cabbage. If it got the cabbage, that was fine because there was a predator bug lurking on the peppers.
I believe the early farmers did use a version of pesticides. They were derived from roots, herbs, and other natural things. Over time these stopped being effective because the bugs naturally evolved to become resistant. More severe pesticides came along and the same thing happened eventually. It's an endless cycle.