How rising CO2 levels threaten insect reproduction

Climate change is altering life at every scale, from ice caps to insect wings. But beyond the well-known shifts in temperature and weather patterns, there is another, less obvious change underway.

A new study published in the journal National Science Review reveals that rising levels of carbon dioxide (CO2) are scrambling the reproductive instincts of one of the world’s most destructive agricultural pests – the cotton bollworm (Helicoverpa armigera).

By throwing off these insects’ ability to select the best egg-laying sites, the shift could ripple through ecosystems, affect food security, and rewrite pest management strategies.

Why CO2 matters to insects

Insects have survived mass extinctions, ice ages, and volcanic winters thanks to their adaptability. Yet their fine-tuned sensory systems make them vulnerable to environmental shifts. Many species use chemical cues to locate food, mates, or egg-laying spots.

For H. armigera, CO2 is not just background atmosphere – it is a guidepost. Female moths can detect tiny differences in plant-emitted CO2, homing in on younger leaves that produce slightly more of the gas. These leaves tend to offer better nutrition for developing larvae.

From 1750 to 2023, atmospheric CO2 climbed from 278 parts per million (ppm) to about 420 ppm, largely due to human activity. Scientists now project levels could reach 1000 ppm by 2100. The concern is that such a jump may overwhelm insects’ CO2 “signal,” replacing a clear guide with confusing noise.

Young leaves give off more CO2

The research team – led by experts at the Chinese Academy of Agricultural Sciences – began by confirming the bollworm’s preference for young cotton leaves.

In semi-natural enclosures, mated females deposited the majority of their eggs on these tender leaves rather than older ones.

The choice is not cosmetic. Larvae feeding on young leaves had higher survival rates, longer bodies, and greater weights after a week compared to those on older leaves. Measurements showed young leaves emitted about 200 ppm more CO2 than mature leaves – a signal strong enough for moths to track.

This makes evolutionary sense: laying eggs where food is fresh and nutritious boosts the odds of offspring survival, a fundamental driver of insect behavior.

High CO2 confuses moths

Researchers grew cotton and bollworms in normal and high CO2. At high CO2, females lost their strong preference for young leaves.

Instead, moths laid more eggs on older leaves – less nutritious sites that could compromise larval health.

“This disruption is akin to confusing a key olfactory cue from a GPS system,” said study co-author Professor Guirong Wang. “Without accurate CO2 signals, the insects struggle to find ideal egg-laying sites, which could affect pest population dynamics and agricultural damage.”

Only mated females reacted to CO2 in experiments. Virgin females and males detected it but did not act on it, hinting that hormonal or brain-level changes shape how the signal is used.

Moths have three CO2 sensors

The bollworm’s CO2 sense comes from three gustatory receptor genes – HarmGR1, HarmGR2, and HarmGR3 – expressed in a small sensory organ called the labial palp. From there, nerve signals travel to a specific processing center in the brain called the labial pit organ glomerulus.

While many insects use just two CO2 receptors, H. armigera needs all three. The team used CRISPR/Cas9 to delete each receptor individually. Any loss broke the insect’s ability to respond to CO2, both in lab tests and in naturalistic settings.

Interestingly, receptor loss did not change the total number of eggs laid – just where they were placed. Mutant moths spread their eggs more randomly, ignoring the young leaves they would normally target.

The trimeric receptor setup may give bollworms finer sensitivity to different CO2 concentrations. This could be an evolutionary edge in complex environments where atmospheric and plant-emitted CO2 levels fluctuate.

Insects use more than CO2

Even without CO2 detection, mutant moths showed a faint preference for young leaves. That suggests other cues – perhaps smell, taste, touch, or even visual signals – help guide oviposition. The redundancy makes sense; in nature, relying on one sense can be risky.

Still, CO2 appears to be a major driver. Its disruption could throw insect reproductive strategies out of alignment with the environments their larvae need to thrive.

Ecological stakes beyond cotton fields

H. armigera feeds on over 200 plants and causes huge crop losses. Lower reproduction might reduce larvae, but pests can adapt or spread, making outcomes uncertain for farmers.

On a larger scale, altered oviposition behavior could affect the balance between pests, their predators, and pollinators.

Changes in plant chemistry under high CO2 – such as shifts in volatile compounds – could compound these effects, influencing not just bollworms but entire insect communities.

Blocking CO2 sense in insects

For decades, scientists have explored ways to manipulate insect senses to manage populations. CO2 is already used to lure mosquitoes and soil pests into traps. The bollworm’s reliance on CO2 for oviposition opens a similar door.

“By targeting the CO2 receptors, we can explore novel, eco-friendly pest control strategies,” said Dr. Qiuyan Cheng, first author of the study.

One promising approach is RNA interference (RNAi), which can silence specific genes. Disabling CO2 receptors could make bollworms misplace their eggs, reducing their impact without pesticides.

Such strategies would need to be carefully tested to avoid unintended consequences for non-target species and ecosystems. But as chemical pesticides lose effectiveness due to resistance, sensory-based controls are gaining appeal.

Climate change as a sensory disruptor

This study adds to a growing body of evidence that climate change does not just push species poleward or alter breeding seasons – it can fundamentally interfere with how animals perceive their world.

For insects, whose survival often depends on chemical cues, this could be especially disruptive.

As atmospheric CO2 heads toward unprecedented levels, the signals insects evolved to follow may become unreliable. That means adaptation will be essential – for insects, ecosystems, and agriculture.

For the cotton bollworm, a scrambled CO2 compass might lead to lower reproductive success, but it could also drive unpredictable shifts in where and how it attacks crops.

For farmers and ecologists, understanding these sensory changes will be key to anticipating future pest pressures.

The study is published in the journal National Science Review.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–