MALE CICADA KILLERS DON'T STING

by Whit Gibbons

July 26, 2015

Last week I wrote about the females of an enormous wasp native to the United States. Known as cicada killers, these wasps do not live in nests; instead, each female deposits her eggs in the ground on full-grown cicadas that she has paralyzed and carried to that spot for the developing larvae to feed on. The males, meanwhile, concern themselves with other activities.

Like many animals, including humans, these wasps have powerful territorial instincts. A clever study has shown that when boundaries are clearly marked, they have fewer problems with trespassers. The research findings also illustrate once again that any animal species, even a lowly insect, can have fascinating behavior.

These warlike black-and-yellow insects look like they might deliver a serious sting but are generally harmless to humans - the females because they only sting people if they are picked up or stepped on; the males because they do not have stingers.

Female cicada killers spend most of their time looking for cicadas. The males spend their efforts preparing to meet and mate with females. Adult males emerge from the ground in summer before the females and set up mating territories, which they defend against other males. The territories are established in areas where female cicada killers will be emerging from the ground. Upon emergence, the females normally mate with a territorial male.

Male cicada killers confront other males who enter their territory. Using intimidation as their primary weapon, males will sometimes even defend their territory from other animals, including people who enter the area. Despite their imposing appearance, if a male cicada killer chases you simply stop and enjoy its antics. Or if you prefer such sport, run, and it will chase you.

Researchers conducted field experiments with cicada killers to confirm previous anecdotal reports that naturally occurring landmarks are used by some species to define territorial boundaries. To test the importance of visual landmarks in territorial behavior, investigators caught individual males and painted unique patterns of colored dots on their bodies. After releasing the insects in a flat, grassy area with no obvious landmarks, they determined by field observations the size and shape of each defended territory, generally occupied by a single male that was identifiable by its paint pattern.

The researchers then placed 30 3-foot-long wooden dowels horizontally on the lawn in a random pattern to serve as tangible landmarks in the otherwise homogeneous habitat. Upon remapping the territories of the wasps, the investigators found that by the next day, 42 out of 62 territories had been redefined within the study area, with the dowels being used as boundary markers. No male's territory crossed a dowel into the territory of another male, indicating that these landmarks were observed as the boundaries by males on either side.

Further observation revealed an interesting phenomenon among wasps that were defending boundaries where a dowel could be used as a landmark on two sides and where no clear landmark was available on the other two sides. These males spent more than six times as long during the day defending the sides with no landmarks as they spent defending the sides where dowels were present and could be used as landmarks.

In the experiment, dowels represented natural landmarks. One conclusion offered by the investigators was that the use of obvious natural landmarks to define territorial boundaries could have evolved because of the reduction in costs of territorial defense. That is, establishing a territory in a homogeneous habitat is not energy efficient for a cicada killer because of the constant patrolling and vigilance necessary. When boundaries are distinct, the wasp can be more resourceful.

Presumably cicada killers are not more intelligent than politicians in Washington, D.C., or in Mexico. Nonetheless, they seem to have developed better coping mechanisms for dealing with the issue of establishing, defending and respecting boundaries. Maybe there's something to be learned from these territorial insects by people on both sides of the border.

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