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Locusts are among the most well known of all insect groups, and this notability is largely due to their association with biblical-sized swarms that are said to become large enough to blot out the sun in the middle of a sunny day. While there is much debate concerning how large locust swarms can truly become, it is true that massive locust swarms have stripped entire crops of food in the past. Such swarming behavior is definitely strange, especially considering that locusts are solitary insects, and not social insects that swarm naturally. This distinction is important, as only social insects, such as bees and wasps, exhibit swarming behaviors. Since swarming is only possible with group cooperation, why is it that locusts swarm? As it turns out, researchers have only recently learned that an individual locust transforms from a solitary insect into a social insect, and sometimes back again. Or a locust may never transform at all, in which case it remains solitary.

For most organisms, being solitary is the most advantageous way to live, as being solitary means only having to consider one’s own well being. In fact, evolutionary biologists have long been puzzled by the existence of social behaviors among animals, including humans, as altruistic concerns can be in direct conflict with an individual’s welfare. When it comes to social insects, scientists are even more puzzled, as it is hard to understand how altruistic behaviors can be advantageous from an evolutionary perspective. Of course, it is best to alternate between self-interested modes of behavior and altruistic modes of behavior. After all, in many cases, social cooperation is of the greatest advantage to both an individual and the group. And this is precisely why solitary and social modes of behavior have survived in locusts over the course of evolution. Locusts, much like humans, alternate between solitary modes and social modes of behavior in order to extract advantage in changing circumstances. Locusts are born solitary, but when their environment grows to contain more locusts and becomes more crowded as a result, social cooperation becomes the greatest advantage, so they convert to being social insects that indulge in swarming behavior. If a locust’s environment remains small and spacious, they remain solitary. If, after converting to sociality, a locust’s environment becomes bereft of other locusts, that locust can convert back to being solitary in order to optimize its chances for survival in a mostly non-social environment.

Can you think of any other arthropod species or groups that demonstrate swarming or other social behaviors despite being largely solitary?