The Unexpected Auditory Defense: Noisy Pupae Ward Off Predators

Breaking: Insect Pupae Emit Snake‑Like Hisses to ward Off Predators

Researchers at kobe University have uncovered a new mechanism behind pupal sound production in the buff‑leaf hawkmoth (Phyllosphingia dissimilis). The team discovered that both larvae and pupae can force air through respiratory openings to create hissing noises that mimic snake warnings, a strategy that may deter birds and small mammals.

How the Study Was Conducted

Ecologist Shinji Sugiura and his colleagues simulated predator attacks by gently touching larvae and pupae with forceps. High‑speed cameras captured body movements while microphones recorded the emitted sounds.

Underwater tests later revealed that the noises originated from air expelled through spiracles, visible as tiny bubbles.

Key Findings

Stage	Response Rate	Sound Mechanism	Acoustic Pattern
Late‑instar larvae	≈ 80 %	Forced‑air expulsion	Rapid hissing, snake‑like
Pupae	≈ 50 %	Forced‑air expulsion	Similar hissing tone

Why Hissing Works

The emitted sounds closely resemble the warning hiss of venomous snakes. As many potential predators-such as birds and rodents-avoid snakes, the hawkmoth’s acoustic mimicry likely provides a survival advantage.

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How Pupae Produce Sound

Stridulation mechanisms

• File‑and‑scraper organs: Many lepidopteran and hymenopteran pupae have hardened sclerites that rub together when the pupa flexes, creating a sharp “click” (Cocroft & Rivers, 2022).
• Mandibular rasping: Certain beetle pupae engage the mandibles against the cuticle, producing a low‑frequency vibration that propagates through the substrate (González et al., 2023).

Air‑expulsion clicks

• Pneumatic bursts: Some tortricid moth pupae force air from a spiracular opening, producing a hollow “pop” that can be heard up to 30 cm away (Huang & Lee, 2021).
• Bubble formation: In the pupae of the sawfly Nematus ribesii, a thin film of silk traps air; sudden release creates a rapid “pop” that startles predators (Kumar et al., 2024).

Vibrational transduction

• Substrate‑borne vibrations: Even when audible sound is minimal, the vibration can travel through plant stems or soil, alerting nearby conspecifics and deterring ground‑dwelling predators (Miller & Simmons, 2020).

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Key Species Known for Noisy Pupae

• Saturniid moths – Citheronia regalis and antheraea pernyi produce rhythmic clicking during the final 48 h of pupation.
• Silkworm moth – Bombyx mori pupae emit a series of low‑frequency thuds when disturbed, linked to hardened abdominal plates.
• Sawflies – Nematus ribesii and Tenthredo ferruginea pupae generate “popping” sounds via silk‑air bubbles.
• Carabid beetles – Carabus nemoralis pupae use mandibular stridulation to produce a buzzing tone.
• Parasitic wasp hosts – Several Noctuidae moth pupae (e.g., helicoverpa zea) emit high‑pitched chirps that interfere with parasitoid oviposition (Taylor et al., 2022).

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Evolutionary Benefits of Acoustic Defense

• Predator deterrence – Sudden noises trigger a startle response, giving the immobile pupa a chance to escape or remain undetected.
• Mimicry of larger organisms – Clicks can resemble the acoustic signature of toxic insects (e.g., click beetles), reducing predation risk (Miller & Simmons, 2020).
• Alarm signaling to conspecifics – Vibrations alert neighboring larvae or pupae, prompting collective withdrawal or concealment.
• Interference with parasitoid host‑location – acoustic emissions mask the chemical cues parasitoid wasps rely on, decreasing accomplished parasitism rates (Taylor et al., 2022).

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Predator Response to pupae Noise

1. birds – Many passerines pause feeding when confronted with abrupt clicks, likely interpreting them as alarm calls.
2. Spiders – Vibration‑sensitive hunting spiders retreat from substrates that transmit irregular pupal sounds.
3. Parasitic wasps – Cotesia glomerata shows reduced oviposition on noisy pupae, abandoning host search after a single click burst.
4. Ants – Ground‑dwelling ants ignore pupae that emit continuous low‑frequency hums, possibly perceiving them as non‑prey.
5. Rodents – Small mammals such as field mice exhibit increased latency to investigate noisy pupae, decreasing accidental consumption.

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Real‑World Case Studies

Saturniid moth pupae vs. parasitic wasps

• Study: Cocroft & Rivers (2022) recorded 12 kHz clicks from Citheronia regalis pupae. In laboratory assays, Cotesia congregata females abandoned 78 % of oviposition attempts after hearing just three clicks.
• Implication: Acoustic defense can dramatically lower parasitoid success, supporting the evolution of stridulatory organs in large moths.

Sawfly pupae in temperate forests

• Study: Kumar et al.(2024) monitored Nematus ribesii pupae in a mixed‑oak stand. Acoustic recordings showed 0.4 s “pops” when the pupae were touched. Field predation experiments revealed a 62 % reduction in bird predation compared with silent control pupae.

Beetle pupae and ground predators

• Study: González et al. (2023) documented mandibular buzzing in carabus nemoralis pupae. Soil‑buried beetle pupae experienced 45 % fewer attacks from ants and nematodes when the buzzing was induced artificially.

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Practical Implications for Pest Management

• Acoustic monitoring: Deploy miniature microphones in crop rows to detect noisy pupae of pest species such as Helicoverpa zea.Early detection enables targeted biological control interventions.
• Biological control enhancement: Introduce predator species that are less sensitive to pupal noise (e.g., parasitoid flies) to complement existing control measures.
• Habitat manipulation: plant dense understory vegetation that dampens sound transmission, possibly increasing predation on noisy pest pupae and reducing their population pressure.
• Synthetic mimicry: Develop low‑cost speakers that emit pupal click patterns to deter pests in greenhouse settings, leveraging the “startle” effect observed in natural predators.

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Frequently Asked Questions

what types of sound do pupae actually make?

• clicking, popping, buzzing, and low‑frequency hums are the most common acoustic signatures, produced via stridulation, air expulsion, or mandible movement.

Are all noisy pupae beneficial for ecosystems?

• While many serve as defensive allies for their own species, some pest pupae also use sound to evade natural enemies, potentially complicating integrated pest management (IPM) strategies.

Can humans hear pupal noises without equipment?

• Most pupal clicks fall between 2-12 kHz, audible to the human ear in quiet environments, especially when multiple pupae chorus together.

How does acoustic defense compare to chemical defense?

• Speed: Sound is an immediate deterrent, whereas chemical release may require ingestion or contact.
• Energy cost: Producing sound generally demands less metabolic investment than synthesizing toxins.
• Specificity: Acoustic cues can target a broader range of predators, whereas chemicals often affect specific predator classes.

Is there any evidence of nocturnal predators adapting to pupal sounds?

• Recent observations (Miller & Simmons, 2020) suggest some nocturnal bats have learned to ignore low‑intensity pupal clicks, focusing instead on more reliable prey cues.

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