Why predators attack adult moths instead of caterpillars

While age may bring wisdom for people, it appears to bring peril for white-lined sphinx moths. A recent field study shows that birds, rodents, and other predators are far more likely to peck, bite, or maul these insects once they emerge from the pupal stage and spread their wings.

Led by the University of Texas at El Paso (UTEP), the research lends experimental weight to a long-standing evolutionary idea. Complete metamorphosis may protect youthful stages from predation, allowing individuals to reach reproductive maturity before facing the most dangerous chapter of life.

Caterpillars versus adult moths

Biologist Brett Seymoure, principal investigator and assistant professor at UTEP, studies holometabolous insects – species that pass through egg, larva, pupa, and adult phases.

The prevailing theory suggests that larvae enjoy a relative sanctuary from predators, giving them a chance to grow before adulthood brings mobility, exposure, and heightened risk.

“There are multiple hypotheses about why these insects go through so many stages before reaching adulthood,” Seymoure said.

“Our research set out to test the prediction that caterpillars are less likely to be attacked by predators, enabling the insect to survive to adulthood and reproduce before it has to face the most dangerous part of its life.”

Creating life-sized decoys

Directly observing countless predator attacks on real moths would be impractical. Instead, graduate student and lead author Braulio Alexis Sanchez created convincing clay replicas of both caterpillars and adult moths.

To mimic the wing patterns of Hyles lineata accurately, he printed photographs on special paper calibrated for precise color matching, then attached the wings to the clay bodies.

Because clay records punctures and impressions, each model would serve as a forensic logbook, revealing encounters with sharp beaks or probing mandibles.

Moths meet predators in Texas

The team installed pairs of decoys – one caterpillar, one adult – on creosote bushes scattered across the rugged Indio Mountains Research Station in West Texas, a property managed by UTEP.

Spacing the models closely ensured they faced identical environmental conditions. Over three consecutive days, researchers visited sites at dawn and dusk, inspecting every replica for new damage, counting beak marks, tooth gouges, or missing segments.

“Out of 70 replicas that were attacked, 50 were adults and 20 were caterpillars, and most of the attack marks were consistent with birds,” noted Sanchez.

In other words, adult-shaped models were assaulted two and a half times more often than larval shapes. Because both forms were equally available to predators, the data point toward active selection: visually oriented hunters seemed to prefer the adult silhouette.

Predators prefer winged moths

The results bolster the notion that larval life provides a safer developmental window. If predation focused primarily on caterpillars, populations might struggle to produce enough breeding adults.

“If moths in the caterpillar stage were more heavily attacked, it would be much more difficult for moths to reach adulthood and reproduce,” Seymoure noted.

Instead, the danger spikes only when the moth takes wing – precisely when reproduction becomes possible.

A reminder of insect importance

Beyond theoretical interest, the study highlights insects’ ecological centrality. As Sanchez put it, “Without insects, there are no humans, trees, or plants.”

Insects pollinate crops, decompose organic matter, seed food webs, and cycle nutrients. Yet global surveys report worrying declines.

Understanding at what life stage insects are most vulnerable can inform habitat management, pesticide timing, and conservation priorities.

Identifying the moth’s predators

While bite impressions suggested birds were the main culprits, the researchers plan to broaden their surveillance.

Nocturnal moth predators – bats, rodents, even wandering spiders – might attack under cover of darkness.

Motion-activated cameras, night-vision optics, and ultrasonic detectors could capture real-time predation events and complement the static clay evidence.

The Chihuahuan Desert hosts unique avian assemblages and sparse vegetation, potentially exaggerating visual hunting cues. To test generality, Seymoure’s team hopes to replicate experiments in woodland, grassland, and urban settings, and with different moth species.

If adult-biased predation proves universal, it would underscore metamorphosis as a widespread risk-management strategy in insects.

Maturity invites greater danger

Complete metamorphosis remains one of evolution’s most ingenious inventions, separating growth and reproduction into distinct anatomical phases.

The new findings suggest an additional benefit: by bottling youth within a relatively inconspicuous caterpillar, nature minimizes early-life losses.

Once it stockpiles resources and unfurls its wings, the adult embarks on the perilous business of mating and dispersal. For Hyles lineata, growing older quite literally means stepping into the firing line.

Focusing on climate and scent

Future studies may integrate chemical cues – some predators use scent rather than sight – or assess how climate change alters predator-prey dynamics.

For now, the clay decoys at Indio Mountains recount a simple story: in the desert sky, an adult moth is a conspicuous, protein-rich morsel, while a plump caterpillar clinging to creosote is more likely to pass unnoticed.

Age, it seems, is more than just a number for these shapeshifting insects; it marks the difference between relative safety and a sudden, predatory strike.

The study is published in the journal Insects.

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