Firstly, there has been a tendency in recent years to limit the use of pesticides due to the danger of the pesticides themselves.
The Bodek website notes:
Since the imposed ban on the pesticide DDT, because of its carcinogenic properties, was implemented, infestation in produce has increased exponentially. It’s no longer a matter of periodic infestation, because insects have developed resistances to many of the milder pesticides. Now, vegetables that are not properly cultivated and inspected can be subject to gross and severe infestation.
Secondly, the agricultural industry is most concerned with insects which damage crops and insects which are dangerous to health. It is not concerned with other insects.
A publication of the United States Department of Agriculture lists “food action levels” of allowable insect infestation. Quantities of insects below these levels are permitted (and no “action” need be taken). These levels are, for example, forty thrips per 100 grams in asparagus; sixty aphids, thrips and/or mites per 100 grams of broccoli, and so forth. Quantities below these levels are acceptable to the United States Department of Agriculture—but they are not acceptable in halacha.