Why New Zealand Has to Kill All Its Mantises to Save Them

Deep in the vast blue expanse of the Southern Ocean lies a land defined by millions of years of profound isolation. Long before humans ever mapped its jagged coasts or named its misty valleys, New Zealand existed as a floating sanctuary cut off from the rest of the world’s major continental landmasses. For tens of millions of years, evolution on these islands marched to its own quiet and unique rhythm. Without terrestrial mammalian predators, ancient birds lost their ability to fly, plants developed distinct defenses, and delicate insects carved out balanced, peaceful niches within the lush temperate rainforests.

Among these native inhabitants was the New Zealand praying mantis, a slender, vivid emerald-green insect that moved with deliberate grace through the leafy canopies. For countless generations, its life cycle unfolded in perfect harmony with the changing seasons. The males searched for females by tracking subtle, airborne chemical signals released into the cool island breeze, leading to modest, cautious courtships that sustained their population across both the North and South Islands.

Then, in the late twentieth century, that ancient balance was quietly shattered by an accidental visitor. Sometime before 1978, likely tucked hidden within imported nursery plants, wooden crates, or commercial cargo, a new species arrived from across the ocean. Originating from the southern tip of Africa, the South African mantis made its first quiet appearance in the suburbs of Auckland. It was larger, far more aggressive, and significantly tougher than the native insect that had inhabited the land for millennia.

This newcomer possessed a suite of physical advantages that allowed it to rapidly claim the realm as its own. Its sturdy, leathery egg cases could withstand freezing temperatures, allowing its offspring to survive the harsh southern winters and hatch successfully in the cold, mountainous regions where native insects usually held their ground. The foreign invader found the cool climate to its liking and quickly established a firm foothold, spreading silently across suburban gardens, coastal scrublands, and native forests alike.

Yet the true danger of this foreign species lay far beyond its sheer physical size or climate resilience. It brought with it a complex mating behavior that was harsher, far more dangerous, and utterly alien to the local ecosystem. What began as a subtle shift in insect demographics quickly turned into a bizarre, tragic ecological crisis that left entomologists astonished by its cruelty and strange mechanics.

In typical ecological invasions, we tend to picture direct physical competition or relentless territory battles. We imagine a stronger, invasive predator driving out a weaker native species by hoarding food, taking over nest sites, or directly attacking its rivals. In those familiar scenarios, the weaker creature is simply outmatched, gradually dwindling as it loses the daily struggle for basic survival resources.

However, the New Zealand praying mantis was not losing its domain in an honorable physical fight over territory or food. Instead, its male population was being systematically erased through a fatal trick of evolutionary chemistry. The native males were flying straight into a deadly trap, drawn in by a scent they could neither understand nor resist.

Under normal ecological conditions, a female mantis ready to mate releases a specific blend of volatile pheromones into the air. These airborne chemical molecules drift on the wind, serving as an invisible beacon for eager males searching for a mate. A male picks up these faint chemical clues using sensitive receptors on his antennae, orienting his flight path to follow the trail straight back to the female of his own species.

When the South African mantis arrived, its females began broadcasting their own potent pheromones into the damp New Zealand air. Tragically, these chemical signals were structurally similar enough to those of the native species that local males could not distinguish the difference. In laboratory tests where native males were given a direct choice between a female of their own species and an invasive female, the males consistently chose the foreign outsider.

The invasive females were inadvertently emitting a chemical call that acted like a irresistible siren song to the native males. These poor native males were not casually wandering into enemy territory by accident. They were actively flying long distances toward the invasive females, drawn by a chemical illusion that tricked their biological instincts into believing they were approaching a rightful mate.

Once a male arrived at the source of the scent, the situation turned catastrophically lethal. Mating in the mantis world is inherently dangerous, as many species exhibit varying degrees of sexual cannibalism, where a female consumes the male before, during, or after copulation to gain valuable nutrients for egg production. Among native New Zealand mantises, however, sexual cannibalism was a relatively rare event, allowing most courtships to conclude peacefully.

The South African species was entirely different. Sexual cannibalism among its females was an aggressive, habitual norm rather than an occasional exception. Native males, unaccustomed to such ruthless behavior, approached the large foreign females without any of the defensive caution that might have saved their lives.

Instead of mating and passing on their genetic heritage, the native males were instantly captured by the massive, spined forelegs of the foreign females. The encounter ended almost immediately, not with the continuation of a lineage, but with the male being consumed whole. The native male became nothing more than a protein-rich meal that fueled the reproductive success of the very invader displacing his kind.

This tragic misdirection posed a profound reproductive threat to the survival of the native species. Every native male that fell victim to an invasive female was a male that failed to fertilize a native female, leaving no descendants behind. His entire genetic line vanished instantly from the island’s gene pool simply because he followed the wrong scent on a cool evening wind.

The sheer spatial scale of this chemical trap made the problem vastly worse. Insect pheromones can travel considerable distances across open fields and forest canopies. A single female South African mantis sitting on a sunny leaf was not just misleading a solitary male in her immediate vicinity. She was effectively intercepting every native male across a wide surrounding radius.

Drawn by the powerful scent, native males spent their finite energy and remaining lifespans flying toward these foreign calling sites, only to be consumed one by one. As the invasive population grew and spread across both major islands, fewer and fewer native males ever managed to find a mate of their own kind, causing native reproduction rates to plunge precipitously.

It would be an oversimplification to claim that pheromone confusion was the sole factor driving the decline of the native New Zealand mantis. Ecosystems are intricate webs where multiple pressures interact simultaneously, including habitat loss, human development, and broader competition for prey. Yet researchers widely acknowledge that this lethal chemical lure was a primary catalyst that accelerated the displacement of the island’s original mantis.

What made the scenario so deeply unsettling was its horrific evolutionary irony. For the invasive females, the mistaken attraction of the native males provided a constant, self-replenishing food supply that flew straight into their waiting arms. The more effective a female was at emitting her potent pheromones, the more native males she attracted and consumed.

This extra nourishment allowed the invasive females to produce larger, healthier egg cases containing scores of vigorous nymphs. Through the fundamental mechanisms of natural selection, the invasion actively rewarded the foreign females for luring local males to their deaths. The native males were unwittingly feeding the very engine that was driving their species toward total extinction.

Faced with this silent ecological disaster, scientists and conservationists began asking an urgent question: could anything be done to halt the relentless advance of the South African mantis? Could nature provide a natural predator to restore balance to the islands, or had the invasive insect permanently conquered the land?

To find an answer, one must first appreciate the formidable physical engineering of a praying mantis. The mantis is widely regarded as one of the most effective visual ambush predators in the insect kingdom. It sits motionless for hours, blending seamlessly into surrounding stems and foliage, patiently waiting for an unsuspecting victim to stray within striking distance.

When prey draws near, the mantis strikes with astounding speed, unfolding its heavily armed raptorial forelegs in a fraction of a second. Sharp chitinous spines lock around the victim like a steel trap, pinning its limbs and preventing any chance of escape. Once held fast, the mantis immediately begins consuming its prey alive, starting with the neck or head.

Larger species of mantises are capable of overpowering far more than basic insects. Documented accounts show large adults catching and consuming small lizards, frogs, snakes, and even hummingbirds. With such terrifying hunting capabilities, a fully grown mantis appears almost invincible within the miniature world of terrestrial invertebrates.

Yet no predator in nature reigns entirely supreme without check. Every creature, no matter how formidable, forms part of a complex food web where death can arrive from unexpected quarters. If an invasive predator cannot be checked by larger animals, its downfall might instead come from a nimble, specialized insect hunter.

New Zealand already possessed indigenous species of predatory flies known as robber flies, but these local flies were not equipped to suppress the burgeoning South African mantis population on their own. If native robber flies had been capable of clearing out the invaders, the mantis population would never have exploded across the country in the first place.

As the search for effective biological controls intensified, researchers began examining the hunting habits of larger, more aggressive robber flies from around the world, particularly species within the genus Promachus. These formidable insects operate on hunting principles that are completely different from the stealthy ambush tactics of the praying mantis.

Robber flies do not sit quietly on branches waiting for food to wander by. Instead, they act as high-speed aerial interceptors, patrolling the sky like miniature fighter jets. When a robber fly detects a flying insect, it launches itself from its perch, tracking its target mid-air with extraordinary visual accuracy and agility.

Upon closing the distance, the robber fly collides violently with its prey, grappling it tightly with six powerful, spiny legs. Before the victim can react or strike back, the fly drives a thick, rigid, needle-like beak known as a proboscis directly into the weakest joints of the victim’s armored exoskeleton.

The range of prey tackled by these aerial hunters is astonishingly broad. Robber flies routinely attack and overpower wasps, bees, dragonflies, grasshoppers, heavy-armored beetles, butterflies, and other large flies. They do not hesitate to target insects that possess lethal stingers or crushing jaws, turning giant predators into helpless victims in a matter of seconds.

When a robber fly targets a mantis, the mantis’s terrifying reputation as an apex predator quickly evaporates. The ensuing confrontation rarely resembles a balanced, back-and-forth duel. The moment the fly drives its proboscis into the mantis’s body, it injects a highly toxic slurry of neurotoxins and digestive enzymes.

This venom works with terrifying efficiency, instantly paralyzing the mantis’s nervous system while rapidly liquefying its internal organs and tissues from the inside out. Within moments, the mighty ambush hunter is rendered completely motionless, reduced to a soft, liquid-filled shell that the robber fly casually drains through its proboscis.

Even the mantis’s powerful, spined forelegs offer little defense against an attack that strikes from above and behind with such sudden lethality. Documented field observations have confirmed that when a large robber fly ambushes an adult mantis, the mantis is usually defeated before it even realizes it is under attack.

To fully understand how such a duel unfolds in the wild, researchers analyzed the mechanics of a confrontation between these two predatory titans under different tactical scenarios, examining how speed, vision, and body orientation dictate the outcome.

In the first tactical scenario, the robber fly executes a surprise ambush from behind the mantis. In this situation, the fly holds a decisive advantage, as its entire evolutionary strategy relies on delivering a single, lethal initial blow without engaging in prolonged close-quarters combat.

The fly swoops down along a precisely calculated flight path, locks onto the mantis’s back, and immediately plunges its proboscis into the thin membrane between the mantis’s head and thorax. Once the venom is injected, the fight is effectively over, and the fly simply holds on tightly while the toxin takes full effect.

Robber flies are uniquely equipped for these precision aerial strikes thanks to their extraordinarily sophisticated visual systems. Their large, bulging compound eyes provide a vast field of view and exceptionally high spatial resolution, allowing them to track fast-moving targets against complex, cluttered backgrounds.

Scientists often compare the visual processing speed of robber flies to that of dragonflies, enabling them to make split-second flight adjustments mid-air to match the sudden movements of their prey. If the fly spots the mantis first and attacks from a blind spot, the mantis has virtually no chance of survival.

However, this ambush strategy carries a significant limitation. The mantis is an absolute master of camouflage, shaped and colored to resemble green leaves, dry twigs, or lichen-covered bark. If the mantis remains completely motionless, blending perfectly into the background foliage, the robber fly’s motion-sensitive eyes may fail to register its presence entirely.

In that case, the fly will simply zoom past overhead, completely unaware of the deadly hunter sitting just inches away. The surprise attack never occurs, leaving both insects to hunt another day.

The second tactical scenario involves a frontal assault, where the robber fly flies directly into the mantis’s visual field. Under these conditions, the tables turn dramatically, and the danger shifts entirely onto the attacking fly.

Approaching a mantis from the front means entering the precise kill zone of an insect possessing one of the most unusual visual systems in the entire animal kingdom. Praying mantises are currently believed to be the only invertebrates endowed with true 3D stereoscopic vision, allowing them to perceive depth with remarkable accuracy.

Their visual apparatus includes two massive compound eyes mounted on a highly mobile triangular head, supplemented by three smaller simple eyes that detect subtle changes in ambient light and spatial movement. A mantis can detect the slightest motion from up to sixty feet away, making it virtually impossible for an airborne insect to approach frontally without being noticed.

While a mantis may ignore a stationary object right in front of it, the tiny motion of a approaching fly instantly triggers a predatory response. The mantis does not merely strike blindly along a fixed trajectory; scientific studies demonstrate that it calculates the speed and direction of its target, adjusting its strike mid-trajectory to intercept fast-moving prey.

If the robber fly attacks from the front, the mantis can snap its raptorial legs forward in less than fifty milliseconds, catching the fly mid-flight. Once those spined forelegs lock around the fly’s soft body, the contest is over, and the robber fly is crushed and eaten.

Even if the fly manages to land on the mantis’s head during a frontal rush, the mantis’s dense chitinous exoskeleton provides significant structural protection against immediate punctures. The exoskeleton acts like a lightweight suit of plate armor, absorbing blunt impacts and resisting bites during chaotic struggles.

This heavy armor is essential for the mantis’s survival, as courtships and territory disputes between mantises frequently turn violent, with individuals grappling fiercely and striking each other with enough force to leave permanent puncture marks on their bodies.

Against this natural armor, the robber fly relies entirely on its specialized proboscis, which functions as a high-pressure piercing spike. However, driving this spike through thick chitin requires precise positioning, which is nearly impossible to achieve while being crushed by a mantis’s spined forelegs.

Body size plays a crucial role in determining the victor of these encounters. When a large robber fly encounters a small juvenile mantis, the contest is entirely one-sided. The young mantis, lacking both dense armor and large forelegs, is quickly overpowered and consumed by the fly.

As the mantis grows into a full-sized adult, the margin of error for the robber fly vanishes completely. Taking on a fully grown adult mantis becomes a perilous, high-stakes gamble where a single mechanical miscalculation results in immediate death for the fly.

Yet robber flies routinely take such extreme risks in nature, regularly hunting creatures that are far more dangerous and heavily armed than praying mantises. Across temperate and tropical ecosystems, robber flies routinely target giant hornets and paper wasps.

A large hornet is a terrifying predator in its own right, often exceeding two inches in length and armed with crushing mandibles, thick armor, and a painful venomous sting. Hornets are renowned for their aggression, capable of slaughtering entire hives of honeybees in brutal close-quarters attacks.

Despite these formidable defenses, robber flies consistently hunt hornets, demonstrating that their predatory success against large, dangerous prey is no stroke of rare luck, but rather the result of millions of years of evolutionary refinement.

When observing a robber fly hunting a hornet, researchers note that the fly relies entirely on stealth and sudden power. Launching from a hidden perch, the fly intercepts the hornet mid-air, pinning its wings and driving its proboscis into the soft neck joint before the hornet can bring its sting or mandibles into play.

If the initial strike is successful, the injected neurotoxin rapidly disables the hornet’s nervous system. The struggling hornet quickly loses motor control, its buzzing wings grow silent, and its body goes limp as the fly perches quietly to consume its meal.

If the initial strike fails or the fly secures a poor grip, the encounter rapidly devolves into a desperate, messy brawl. The hornet can twist its body mid-air, using its massive mandibles to dismember the fly or plunging its stinger directly into the fly’s unarmored abdomen.

Because the robber fly lacks heavy chitin armor or defensive stingers of its own, it rarely survives a direct counter-attack by a hornet. Its entire survival strategy hinges on landing that vital first strike with absolute precision.

The fact that robber flies willingly and repeatedly target hornets proves that they are fully capable of overpowering large, aggressive, armored predators. This realization naturally led scientists and ecologists to consider a bold hypothesis: could robber flies be deployed as a biological weapon to suppress the invasive South African mantis in New Zealand?

At first glance, the idea of using robber flies as a biological control agent appears brilliantly simple. If an invasive mantis is devastating local ecosystems, and robber flies are capable of killing adult mantises, why not mass-produce these aerial hunters and release them across the affected islands?

To imagine how such a large-scale biological control program would operate, one must look at the complex industrial infrastructure required to rear predatory insects by the millions.

Such an endeavor would require the construction of massive, specialized breeding facilities. These would not be simple greenhouses, but vast, climate-controlled complexes where temperature, humidity, and lighting are meticulously regulated in every room to match the specific lifecycle requirements of the flies.

Breeding robber flies on an industrial scale presents immense logistical hurdles compared to herbivorous insects. Female robber flies lay their eggs in delicate soil crevices, decaying wood, or low-lying plant stems, requiring custom-designed laying substrates throughout the facility.

Once the eggs hatch, the real challenge begins. Robber fly larvae do not feed on simple artificial diets or plant material; they are subterranean predators that live in soil and leaf litter, hunting beetle grubs, insect eggs, and other soft-bodied soil organisms.

To keep millions of larvae alive, the facility would have to operate a parallel industrial mass-breeding system just to produce vast quantities of feeder insects. Tons of live prey would need to be cultivated daily to sustain the hungry fly larvae throughout their long development.

As the larvae mature into adult flies, the feed requirements become even more demanding. Adult robber flies require live, flying prey to trigger their natural hunting instincts, necessitating continuous releases of live insects into large flight chambers to keep the breeding stock healthy and reproductive.

Gathering the initial breeding stock would require field teams to venture into natural habitats, using specialized traps and lures to capture healthy wild specimens. These wild founders would then be introduced into the factory to establish a stable, genetically diverse breeding population.

Once millions of adult robber flies were successfully produced, transporting and releasing them across vast target zones would require military-grade logistics.

To prepare the flies for transport, technicians would lower the temperature in the holding chambers, causing the cold-blooded insects to enter a state of temporary torpor. Sluggish and inactive, millions of flies could be packed safely into compact transport containers without injuring one another.

These containers would be loaded onto specialized turboprop aircraft fitted with automated aerial dispersion systems. A single small aircraft could theoretically carry over two million chilled flies in a single flight.

Flying low over affected forests and scrublands, the aircraft’s automated dispensers would gradually release the flies through the fuselage. As the cold insects fell through the warm lower atmosphere, they would wake up, spread their wings, and disperse smoothly across the canopy as active predators.

Alternative release methods, such as dropping small cardboard boxes of flies by hand, carry significant operational risks. If a box fails to open upon hitting the tree canopy, the entire batch of flies dies trapped inside, wasting thousands of dollars and months of rearing effort.

While an industrial insect factory releasing millions of airborne predators may sound like speculative science fiction, remarkably similar programs already exist and operate with tremendous success around the world.

A prime example is the long-running program managed by the United States Department of Agriculture to eradicate and control the primary screwworm fly, a devastating livestock parasite.

To prevent this pest from spreading northward from South America, the USDA operates a massive breeding facility in Panama that produces tens of millions of sterile screwworm flies every week.

Squadrons of specialized aircraft fly continuous routes over the dense rainforests of the Darién Gap, dropping millions of sterile flies to maintain an invisible biological barrier that protects northern livestock industries.

Ground crews on motorcycles, boats, and horseback continuously monitor the territory, conducting field inspections to ensure no wild outbreaks breach the containment zone. This massive program proves that turning insects into an airborne biodefense system is technically achievable on an industrial scale.

However, applying this industrial biocontrol strategy to robber flies and invasive mantises reveals a catastrophic, fundamental flaw that threatens to destroy the very ecosystem scientists are trying to protect.

Unlike specialized parasites that target a single host species, robber flies are generalist predators. They do not hunt praying mantises exclusively; they ambush virtually any living insect they can overpower, including bees, butterflies, native beetles, and beneficial pollinators.

If millions of aggressive robber flies were dropped across the forests of New Zealand, they would not limit their attacks to the invasive South African mantis. They would unleash indiscriminate havoc across all local insect populations.

Tragically, the fragile, endemic New Zealand praying mantises—the very creatures the program was designed to save—would become prime targets for the introduced swarms of robber flies. Releasing a generalist super-predator to save a native species from an invasive predator would merely accelerate the native species’ destruction.

To overcome this fatal flaw, scientists would need to explore advanced genetic and behavioral modification techniques to transform robber flies from indiscriminate hunters into specialized targets.

One potential approach draws inspiration from the sterile insect technique used in screwworm management, where flies are exposed to precise doses of radiation in factory laboratories. The radiation renders the flies sterile while keeping them active and competitive in the wild, preventing an introduced population from establishing permanently.

To create a specialized robber fly task force, researchers would first need to isolate rare wild individuals that naturally exhibit a strong behavioral preference for hunting mantises over other insects.

By bringing these specialized specimens into laboratories and subjecting them to selective breeding over many generations, scientists could gradually hone their genetic instincts, producing a strain of flies that primarily targets mantises.

Additionally, field crews could deploy artificial chemical attractants or synthetic pheromones in areas heavily infested with South African mantises, drawing the selectively bred flies directly into targeted eradication zones while sparing pristine native habitats.

Yet this proposed solution faces a daunting biological bottleneck: the extremely slow growth rate of robber flies.

Unlike many common flies that complete their entire lifespan in a few weeks, robber flies have exceptionally long lifecycles. A robber fly larva spends two to three years buried in the soil, slowly feeding and growing before finally metamorphosing into an adult.

Selective breeding programs require dozens of successive generations to fix specific behavioral traits within a population. With a lifecycle lasting up to three years per generation, developing a specialized, mantis-hunting strain of robber fly would require decades of continuous laboratory research.

The preliminary stages of such a project would take at least fifteen to twenty years before a single specialized fly could ever be deployed in the field.

Throughout those decades, taxpayers and research institutions would have to foot the staggering financial bill for continuous facility maintenance, temperature control, live feeder insect production, staff salaries, field monitoring, and aviation support.

The total cost would easily reach hundreds of millions of dollars, creating a financial burden that is nearly impossible to justify for a non-agricultural wildlife conservation effort.

When the USDA spends millions on screwworm eradication, the expenditure makes clear economic sense, as a screwworm outbreak would inflict billions of dollars in direct losses on the cattle and agricultural industries.

In contrast, controlling an invasive praying mantis to protect an endemic insect offers little direct commercial revenue, making it nearly impossible to secure the massive, sustained funding required for such an ambitious biodefense program.

Thus, while the concept of deploying engineered robber flies as biological assassins against the South African mantis represents a brilliant theoretical concept of biological engineering, practical realities make it virtually impossible to execute in the real world.

In the end, the ongoing struggle of the native New Zealand praying mantis serves as a sobering reminder of the profound vulnerability of isolated island ecosystems in an interconnected world.

A single accidental introduction of a non-native species can set off an unforeseen chain reaction, rewriting the evolutionary rules of an entire realm through chemical trickery, strange mating behaviors, and fatal attractions.

As the South African mantis continues to thrive across the bushlands and gardens of New Zealand, the native mantis quietly retreats, its survival hanging in a delicate balance. It remains a silent victim of an invisible chemical trap—a poignant symbol of how fragile nature’s ancient balances truly are when confronted with the unpredictable forces of global change.