The effects of climate change on insect life cycles

The clock begins ticking for a female fig wasp as soon as it leaves the fig it was born in. In just 24 hours, it has to reach another receptive fig that can provide shelter to lay eggs in exchange for the pollen that she brings. It’s a win-win situation, but this mutual agreement took a lot of fine-tuning. Researchers believe that the initial chapters of this agreement were inked between 70 and 90 million years ago, while dinosaurs were still roaming the Earth.

“If I were to think of an icon for the plant world, I would think of the fig tree,” says Renee Borges, Honorary Professor at the Centre for Ecological Sciences (CES). Her lab has worked extensively on decoding the fig/fig wasp relationship. “Most of us know that there is a mango season, so we don’t get mangoes in the monsoon [or] in the winter. But there is nothing like a fig season because the figs have to be available throughout the year.” This makes the fig tree a reliable source of nutrition for animals and insects even when other fruits are scarce, thus qualifying as a ‘keystone species’ (one that has a huge impact on its natural environment relative to its abundance) in the tropical ecosystem. 

But with changing climate patterns, some clauses of the original agreement seem to be falling apart. “Experiments have shown that fig wasps die at temperatures above 35-37°C,” Renee adds. Those temperatures have now become quite common in most parts of India, even exceeding 40°C during summer. In recent years, fig wasps have figured out a strategy to beat the heat: they have started leaving their birthplace earlier in the morning and flying higher up above the ground, navigating their way to the host fig by sensing chemical signals. 

Does this amendment salvage the agreement? Temporarily, yes. “[But] if they are unable to fly because the temperatures are unsuitable for them, the wasps will die right where they are or on the way to their destination,” Renee says. “The reason we don’t ‘see’ them dying is because they are small and there are other things that may eat them up very quickly.”

The threat of climate change hangs like the sword of Damocles over other insects, too, like bees, which play a crucial role in pollinating crops like oil seeds, sunflowers, and pulses. Insects are especially vulnerable to a changing climate because they cannot regulate their body temperatures, Renee explains. “What do we do when we are hot? We turn on the fan. Where are the fans for the animals that are living outside?”

Her concerns are echoed by Saskya van Nouhuys, Associate Professor at CES,  whose lab is deciphering the impact of interactions between species on their population dynamics. “Flying generates a lot of heat, so insects can be passive and just sit back when it’s too hot to avoid overheating.” This might save individual insects, but such behavioural changes can have severe effects at the population scale. 

Saskya points out that extreme climate events can reduce the abundance of plants that are required for insect survival, consequently leading to a decline in insect populations. This can have a domino effect on the populations of birds feeding on insects, too. 

Saskya’s lab studies parasitoid wasps. In a study that her team conducted in northern Europe, they found that the synchronisation between the development of the parasitoid wasp and the butterfly (the wasp’s host) was intricately dependent on the temperature in the spring. She explains that in northern Europe, plants don’t grow much in the winter, hence the butterfly caterpillars go into diapause (similar to hibernation). During this period, the caterpillar uses its stored fat to stay alive. In some cases, when the winter extends for a longer time than usual, the caterpillars die from starvation. The parasitoid waits for the right moment – spring. If everything goes according to plan, the caterpillar starts growing during the spring, and so does the parasitoid. However, if the spring is warmer, the caterpillar outpaces its parasitoid and becomes a pupa quickly, leaving the parasitoid without a host. Such warm springs can wipe out a complete generation of parasitoids.

But some scientists think that not all aspects of an insect’s life cycle may suffer so deeply from climate change. Rohini Balakrishnan, Professor at CES, is one of them. She says that acoustic communication – how insects speak to each other – may not be affected as much. Her lab’s work on insect communication is rooted in the long evolutionary history of insects, starting from the members of the order Orthoptera, the first terrestrial creatures on Earth to communicate acoustically. Because these insects have evolved over such a long time, they might have better adapted to various climatic conditions. “The fluctuation in temperatures is enormous, especially for temperate species, so this is not a problem they have not seen,” Rohini says. 

Since insects are poikilotherms (organisms that cannot regulate their body temperature), their muscle contraction rates change with temperature, which can affect the rhythm of their chirps, she explains. Even then, this might not be as badly affected by climate change as other aspects of an insect’s life, such as its life cycle, physiology, and interaction with other species. “They have already evolved mechanisms to deal with that kind of problem,” she says.

Given that climate change is largely human-driven, many of our endeavours have shifted many win-win relationships to win-lose scenarios for many of the coexisting species on this planet. 

Renee and Rohini also point out how habitat fragmentation is a major culprit in disturbing the ecological balance, especially for insects. Habitat fragmentation makes it difficult for the insects, which are relatively smaller in size compared to birds and animals, to reach their desired locations. Since the insects’ life cycles are so tightly regulated on short time scales, the effects can be especially magnified. Imagine the fig wasp frantically racing against time to find a receptive fig, only to find a tree just moments before the wasp closes its eyes forever. 

“We are getting lots of reports on heat waves [around the world] because of which humans are unable to work outside,” Renee says. “But pollinators are [also] doing all their work outside. There is no umbrella for them.”