Bengaluru: Billions of cicadas, belonging to a cohort called Brood X (or Brood 10), have started to emerge from underground to moult (shed their skins), chirp, feast, mate and die, across the states along the eastern part of North America.
These insects have a life cycle of either 13 or 17 years depending on species, and they mostly live underground in the early stage of their development. They emerge after 13 or 17 years, and remain above the ground for a few weeks. The ones emerging now have a life cycle of 17 years.
The phenomenon is of scientific and cultural significance in the US and concerns have been growing of late about the declining cicada population as well as early emergence of individuals in some locations, signalling a major and expected change in their behaviour due to the human impact on the environment.
Cicadas come in over 3,000 species and are found on all continents except Antarctica. While the ones emerging now are periodical cicadas, there are also annual cicadas that emerge every year.
These insects are conspicuously loud, with calls being produced exclusively by males for courtship and mating. Their life cycle is divided into the stages of eggs, nymph stage, metamorphosis (moulting) and adulthood. They have an adult lifespan of two months at most.
When an egg hatches in leaves or barks, the immature form of the insect, called a nymph, drops to the ground and burrows deep into it with its strong front legs. Cicada nymphs are thought to live at depths below 2 metres, after having dug exit and aeration tunnels for survival and emergence.
When they are ready to emerge, they climb up on trees or plants and start to moult or shed their skin. The abandoned skin — an exoskeleton — remains stuck to the tree.
Cicadas moult three to four times, before reaching their final adult stage, when they are fully above ground and are considered to have emerged and are ready to fly. They have weak hind legs and are not good jumpers. They also do not move well on their feet, preferring to fly even short distances.
Both nymphs and adults drink plant sap, and nymphs drink directly from the roots by tapping into the xylem (plant tissue), which transports water and nutrients from the roots to leaves.
Adult cicadas are marked by a loud chorus as males congregate and wait for females or fly to seek them. Once she has mated, the female cicada carves a groove in leaves or twigs or branches where eggs are deposited. The leaves or branches eventually brown, wilt and die.
While cicadas do drink from roots, damage leaves and cover plants with their exoskeleton, they are not considered a major threat to vegetation, crops or gardens when they are just starting to emerge.
However, they can damage plants during the egg-laying stage after mating. While mature plants can withstand some damage to leaves and branches, younger plants could be susceptible and require protection.
Also read: The 6 pests coming to eat our crops, thanks to climate change
Brood X and concerns
Brood X, also called Brood 10 or the Great Eastern Brood, is one of 15 broods of cicadas that appear periodically in the eastern US. Brood X contains the largest individual number of cicadas, which also cover the greatest range.
There are other broods as well. Different broods have different life cycles and emerge at different times. They are highly localised to specific geographical regions in their habitat range.
The emergences of Brood X have been documented since 1715. The one before this year’s was in 2004, and the next one is expected to occur in 2038.
Entomologists have been increasingly concerned with random emergences of individuals from Brood X, with some emergents appearing four years early both in 2017 and in 2000.
According to experts, climate change plays a contributing role, as cicadas tend to emerge when the ground temperature becomes about 17-18 degree Celsius.
Also read: Insects are declining at alarming levels. This is bad news for human beings
Survival strategies
The curious case of some cicada species’ life cycle has been the subject of much research.
One of the reasons why the periodical species, like the genus Magicicada found in North America, have such long life cycles, is thought to be a response to predators.
Cicadas are eaten by most animals that eat insects including birds, rodents, bats and spiders. When adults emerge, they are also eaten by other animals like fish, reptiles, and even humans.
Some predators specialise in preying on cicadas like the praying mantis or cicada killer wasps. These specialist species are a higher threat to a cicada species. However, since these predators tend to have a typical life cycle of less than two years, they can’t rely on cicada populations for prey.
Another theory for why periodical cicadas have such long life cycles is that they evolved this life cycle during the ice ages or glaciation periods to survive periods of deep freeze. As the Earth warmed, predators who adapted similar strategies started to emerge more regularly and peak in numbers periodically. To prevent their own populations from peaking when other insect and predator populations peak in cyclical numbers, cicadas are thought to have evolved to peak in intervals of prime numbers.
Cicadas have also evolved a number of anti-predator protections.
They are well camouflaged and blend into the barks of trees because of their coloured bodies and transparent wings. They are able to both play dead as well as engage in deimatic behaviour or startle display, where they perform flashy displays with their coloured hind wings to startle or scare predators.
The ones that are not camouflaged to blend in often tend to be aposematic or unusually brightly coloured. These colours attract predator attention and advertise that the animal is toxic. Since cicadas are not actually toxic (or scary) and their bright colours (or startle displays) are only an imitation of a warning signal, these behaviours are a form of what is known as Batesian mimicry.
Since cicada emergence attracts the brief, opportunistic attention of nearly every predator around, the insects combat their loss of numbers by raising their numbers. This is called predator satiation and is a reproduction strategy where even after every predator has had its fill, there are enough cicadas to carry on mating and perpetuating the species.
(Edited by Rachel John)
Also read: Male or female, mating is equally risky for these insects, finds IISc study