While the consumption of insect locusts was not completely unheard-of, it was most uncommon and highly unusual, relegated to last-ditch-options in stark survival situations. He certainly could have eaten those bugs in their short season, occasionally, whenever locust larva would pupate, mature into adults, and then emerge above ground en masse, eat, sometimes swarm, mate (after which the males die), and lay new egg cases in the ground, after which the females also die.
It is quite possible that events surrounding the references in Matthew and Mark coincided with a time (a short season) when grasshopper-like locusts were available to John in abundance. Yielding him a bonanza of high protein sustenance, for many adult and larval insects are a wonderful source of "survival-chow".
And, there is always the very realistic possibility of God's Providence in the provision of sustenance, and the possibility that John The Baptist was not out in the wilderness, living on locusts and wild honey, for a long period of time but just for a short season.
While, in the Israel region, there are always some locusts available, it is during and shortly after the rainy seasons when they are seen in any abundance. Throughout the year, any available locusts are well scattered among the few lush, green, well-watered areas where either natural or agricultural vegetation predominates. The majority of the lands in that region are dry, rocky, barren and void of vegetation - and locusts do not hang-out in the sandy deserts.
Under normal climatic conditions, locusts are solitary creatures. But occasionally, in times of scarce vegetation, whether caused by too many locusts - by a lack of rain - or by a combination of the two, locusts become agitated, begin flying about, then they begin to uncustomarily congregate in large masses. The release of chemical precursors, pheromones, induces a massive coordinated hatch of additional locusts, which further agitates the ever-growing mob of insects. And they begin to swarm, joining together with any other mobs, inducing each to also swarm. And although locusts can be agriculturally problematic whenever their numbers are great, they truly become a "Plague" when they swarm and produce ruin throughout entire regions. A plague of locusts will cause famine after they have ravaged flourishing crops, other plants and trees throughout a region.
Locust swarms are not even a dependable ANNUAL event, and if John The Baptist were so observant or so lucky as to discover an area where female locusts were, en masse, laying their egg cases in the ground, he could harvest the spent, dying, adult female locusts. But he would have to wait many months to dig-up and harvest any locust larva or pupae until after they had progressed from embryos into an edible stage of development. And, that assumes he observed an area where egg cases were being laid and knew where to later dig, expending much energy to cover a few acres.
Instead of feasting on insects, it is much more likely John The Baptist survived on a "bean-feast" of locust pods since he was out in the wilderness for a great length of time.
Leguminous evergreen tree (Ceratonia siliqua) native to the E Mediterranean region and cultivated elsewhere. It is sometimes known as locust, or St. John's bread, in the belief that the "locusts" on which John the Baptist fed were carob pods. The tree, about 50 ft (15 m) tall, bears compound, glossy leaves with thick leaflets. Its red flowers are followed by flat, leathery pods that contain 5-15 hard brown seeds embedded in a sweet, edible pulp that tastes similar to chocolate.
Source: The Columbia Encyclopedia, Sixth Edition. Copyright © 2002, Columbia University Press.
LOCUST - lo·cust, n.