Scientists are trying to understand what caused the cicadas, insects with an especially long life cycle, to mature four years earlier than expected

The 17-year cicada is one of the most fascinating insects in nature. Its nymphs develop underground over 17 years, as its name suggests, after which the adults emerge together to the surface - billions of individuals over a short time span. They enjoy a few intense weeks of mating, at the end of which the females lay their eggs that start a new cycle of 17 years.

However, in 2017, entomologists (insect researchers) from the US were surprised to discover adult cicadas belonging to the species that was only supposed to emerge in 2021, four years earlier than expected.

Chirping and burrowing

The cicada is a very common insect, known mostly for its characteristic chirping that can be heard during the spring and summer in open fields, which sounds a bit like sawing. The cicadas look like a large fly – their size can reach up to five centimeters – but they belong to the Hemiptera order (true bugs), which also includes fleas and aphids. There are more than 1300 known species of cicadas, all belonging to the suborder Auchenorrhyncha. Of these, almost 300 species can be found in Israel.

The mating season of the cicadas is spring: following mating, the female lays eggs on plants, and the nymphs that hatch from them burrow in the ground at a depth of two or more meters, and suck xylem sap from plant roots. These insects have a “missing” stage in their lifecycle, and do not go through a pupal stage. The nymphs are generally similar in shape to the adult form, and occasionally undergo moulting – that is, they shed their exoskeletons to make room for their next and larger stage. The fifth and last stage of the nymph burrows its way out, moulting from nymph to adult shortly after emerging from the ground.

The life cycle of most cicadas lasts between two and five years. In the springtime the new adults emerge from the ground, mate, create the new generation and die after only a few weeks of adult life. But there are North American species (Magicicada) with a life cycle of 13 years, and some even 17 years.

Initial defense

The adult cicadas are defenseless in the face of predators, and their strategy of survival seems to be reliant on their extraordinary numbers. When billions of cicadas emerge almost at once, no predator can annihilate them, and enough individuals survive to ensure the prosperity of the next generation.

Researchers believe that the long life cycle of cicadas offers them an advantage against predators, most of which do not live for such long periods. The evolutionary advantage of specifically 13 or 17 years, is due to the fact they are both prime numbers. For example, if the cicada had a life cycle of 12 years, its life cycle would be synchronized with those of predators with a life cycle of two, three, four or six years.

Researchers from the US discovered that there was a decrease in the population of birds that prey on the long-life cycle cicadas, specifically in the years that the cicadas emerge in multitudes. The researchers ruled out the option that the presence of cicadas causes the decrease in the number of the birds. They could not prove that the cicadas disrupt the breeding cycles of the birds, but they also could not rule out such a possibility.

The 17-year cicada. | Photo: Shutterstock
The mechanism enabling them to time their development is still unclear. The 17-year cicada. | Photo: Shutterstock

A changing climate

Recently, entomologists looking for cicadas which are supposed to emerge after 13 years, also found individuals of the species with the 17-year cycle, which, as mentioned earlier, were four years early. It was only a relatively small number of insects, but it is still a deviation from the orderly timetable that the cicadas have adhered to ever since humans started monitoring them.

Researchers don’t really know what the cause of the surprising change is. The mechanism enabling the cicadas to develop into their adult form over the span of 17 years – more than any other insect – is not completely clear, so it is difficult to understand what exactly went wrong. Naturally, fingers are pointing mostly to a prime suspect that has already been involved in similar offenses: climate change.

Researchers believe that cicadas living underground have a mechanism for counting hot days or hot weeks. Global warming has likely caused a change in this cycle, at least locally. The fact that the weather is getting hotter and hotter during recent years might affect developmental processes of the cicadas’ nymphs, and accelerate them. An additional mechanism that could explain the process might be a change in the composition of the xylem sap of plants due to climate change. Researchers from California were successful in shortening the life cycle of the 17-year cicada, after artificially changing the seasonal cycle of peach trees, the roots of which the cicadas were feeding on.

In the meantime, it is difficult to know if the early appearance of the cicadas in a few areas in the eastern US is a local and temporary phenomenon, or a real and fundamental change in the biology of these fascinating insects – such as a transition from one life cycle to another. One of the problems with an insect with such a long life cycle is the amount of time required to study it. Time will tell if most of the cicadas of this species remained true to their original biological clock, but much more information will only be available after the completion of the next cycles, 17, 34 or 51 years after the end of the current cycle.