Green lacewings produce a notoriously foul odor when handled or threatened, earning them the old common name “stinkflies.” The smell comes from paired glands in the thorax that release a cocktail of volatile chemicals, the most widespread being a compound called (Z)-4-tridecene. Some species add skatole to the mix, the same molecule responsible for the smell of mammalian feces. The stench is no accident: it serves as a chemical shield against predators, and it is just one layer of a surprisingly rich chemical life that spans defense, mating, and even disguise.
Where the Smell Comes From
Adult green lacewings carry a pair of prothoracic glands, small sac-like structures tucked behind the head on the underside of the thorax. When a lacewing is disturbed, squeezed, or grabbed by a predator, these glands release a fluid loaded with volatile compounds. Researchers who have dissected and analyzed these glands across species from North America, South America, Australia, and China consistently find (Z)-4-tridecene as the dominant ingredient.1PubMed. Prothoracic gland semiochemicals of green lacewings On its own, (Z)-4-tridecene has a waxy, slightly unpleasant odor. But in many species, especially those in the genus Chrysopa, the glands also contain skatole (chemically known as 3-methylindole), which is intensely malodorous even in tiny quantities. Skatole has been identified in green lacewings from genera across multiple continents, including Ceraeochrysa, Chrysopa, and Plesiochrysa.2Revista Brasileira de Entomologia. Responses of green lacewings to semiochemicals: species- and sex-specificity
The chemical blend varies by genus and sometimes by species. For example, Chrysoperla species, which include the common green lacewing (Chrysoperla carnea) sold commercially for pest control, produce (Z)-4-tridecene but have not been found to produce skatole so far.3Revista Brasileira de Entomologia. Responses of green lacewings to semiochemicals: species- and sex-specificity That distinction matters if you are trying to figure out why one lacewing encounter left you gagging and another was merely unpleasant: the species with skatole in their arsenal smell dramatically worse. Other chemical variations include the substitution of different carbon-chain-length alkenes and the addition of unusual amides, which create subtle but measurable differences in the scent profile from species to species.4PubMed. Prothoracic gland semiochemicals of green lacewings
Why They Smell Bad on Purpose
The prothoracic gland secretion is a defensive weapon. When the synthetic version of (Z)-4-tridecene, the main compound from Chrysoperla carnea, was tested against predatory ants, the ants showed clear avoidance behavior, backing away from the chemical rather than approaching.5PubMed. Prothoracic gland semiochemicals of green lacewings The glands are a last resort, a chemical equivalent of a skunk’s spray. The lacewing does not walk around oozing the stuff; it releases the fluid only when physically threatened. This is why you rarely notice the smell from lacewings sitting on a window screen at night, but the moment you try to pick one up or accidentally crush it, the stink hits you.
The defensive chemistry is not limited to adults. A green lacewing species called Ceraeochrysa smithi coats its eggs with a liquid containing fatty acids and aldehydes. When tested against ants and cockroaches, this fluid proved potently irritant and protected the eggs far better than the uncoated eggs of a related species.6PubMed Central. Chemical egg defense in a green lacewing (Ceraeochrysa smithi) Chemical defense, in other words, starts before the lacewing even hatches.
Do Lacewing Larvae Smell Too?
Lacewing larvae look nothing like their delicate-winged parents. They are stubby, alligator-shaped predators with oversized jaws, and they earn colorful nicknames like “aphid lions.” Whether they produce the same kind of defensive stink as the adults is a different question, and the answer is more complicated.
Larvae lack the prothoracic glands that adults use. However, many lacewing larvae in the genus Chrysopa are famous for another chemical trick: they pile the carcasses of dead aphids and other debris onto their backs, creating a sort of trash heap that rides along with them. For a long time this was assumed to be physical camouflage, hiding the larva from visual predators. Research over the past two decades has shown that the disguise is also chemical. The waxy surface compounds on aphid carcasses closely match those on living aphids, and ants that tend aphid colonies use those waxy compounds to tell friend from foe. Larvae carrying untreated aphid carcasses were attacked far less often by aphid-tending ants than larvae carrying carcasses that had been washed to remove surface lipids.7PubMed. Do aphid carcasses on the backs of larvae of green lacewing work as chemical mimicry against aphid-tending ants? Essentially the larvae are wearing a chemical disguise, smelling like the aphids the ants are programmed to protect rather than like the predator the ants would attack.
This strategy turns out to be surprisingly sophisticated. Analysis of the surface hydrocarbons of the North American woolly alder aphid, the ants that tend it, and three different predator species (including a green lacewing, Chrysopa slossonae) found that each predator’s hydrocarbon profile was chemically closer to the aphids’ profile than to either ant species. The predators’ surface chemicals were essentially a subset of the aphids’ own compounds.8Ecological Entomology. Convergence of chemical mimicry in a guild of aphid predators The fact that unrelated predators from three different insect orders have converged on the same chemical-mimicry strategy suggests that ant-guarded aphid colonies are a powerful evolutionary pressure. The lacewing larva does not just happen to smell like an aphid; its chemistry has been shaped by generations of selection for a convincing impersonation.
Whether any of this smells bad to a human nose depends on context. Aphid hydrocarbons are not volatile enough to register at a distance, so you would not notice the disguise unless you were handling the larva closely. What you might notice is a faint, musty smell from the debris packet itself, which is essentially a pile of decomposing insect parts.
The Smell That Attracts, Not Repels
Not everything green lacewings produce is designed to drive other creatures away. Males of several Chrysopa species manufacture a pheromone called iridodial to attract mates. In field trapping experiments, traps baited with iridodial pulled in large numbers of Chrysopa nigricornis males during summer and early fall, with a few females also responding in early fall.9PubMed. Male-produced pheromone of the green lacewing, Chrysopa nigricornis Iridodial belongs to a family of compounds called iridoids, which show up across the insect world in contexts ranging from defense to communication.
What makes the lacewing pheromone story particularly interesting is where the raw materials come from. Males in the genus Chrysopa apparently cannot synthesize iridodial from scratch. Instead, they need to acquire chemical precursors by feeding on specific prey or plant materials.10Annual Reviews. Chemical Ecology of Neuroptera Lab experiments have confirmed this: when Chrysopa oculata males were fed nepetalactol (a compound related to the active ingredient in catnip), they converted it to iridodial with high efficiency.11PubMed Central. Pharmacophagy in green lacewings (Neuroptera: Chrysopidae: Chrysopa spp.)? This dietary dependency means that a male lacewing’s ability to produce its courtship signal hinges on what it has been eating. A male that cannot find the right chemical precursors may simply be unable to attract a mate, linking diet quality directly to reproductive success.
Why They Show Up at Your House
If you have encountered the green lacewing smell, it was probably because one or more adults wandered indoors. Green lacewings are strongly attracted to lights at night, and in late summer or early fall, some species seek sheltered overwintering sites. Your porch light and an open door can bring them streaming inside. The species most commonly encountered indoors in North America and Europe belong to the Chrysoperla group, which, as noted earlier, lack skatole. Their smell is still unpleasant but noticeably less horrific than their Chrysopa relatives.
The smell you notice when a lacewing lands on you and you reflexively swat it is the defensive secretion being released all at once. If you leave the insect alone, it will not voluntarily emit the odor. The practical upshot: if a lacewing gets inside and you want it gone without the stink, gently guide it onto a piece of paper or a cup and carry it out. Crushing it, even lightly, is what triggers the gland release. The odor dissipates within minutes in a well-ventilated room, but on skin or fabric it can linger for an hour or more, a testament to the stickiness of the fatty-acid-rich secretion.
Lacewings in the Garden and the Smell Trade-Off
Green lacewings are among the most widely used biological control agents for aphids, mealybugs, and other soft-bodied pests. You can buy them as eggs or larvae from garden suppliers, and their voracious appetite as larvae makes them genuinely effective. But many gardeners who release lacewings are caught off guard by the smell of the adults that eventually emerge.
Researchers have been working to exploit the lacewing’s chemical ecology to make biological pest control more reliable. A ternary blend of plant-associated volatiles, including phenylacetaldehyde, has been shown to attract both female and male Chrysoperla carnea lacewings at field sites across southern, central, and northern Europe.12PubMed. Optimization of a phenylacetaldehyde-based attractant for common green lacewings (Chrysoperla carnea s.l.) In dose-response experiments, dispensers releasing this blend produced significantly more lacewing eggs and larvae compared to untreated plots, with numbers rising as the dose increased. More lacewings meant more aphid suppression.13Basic and Applied Ecology. Semiochemicals and habitat manipulation to support green lacewing activity to reduce aphid infestations in agroecosystems Separate work in corn and soybean fields using volatile attractants also increased trap catches of green lacewings, though researchers caution that attracting adults to an area is only the first step; the adults need to lay eggs, and the larvae that hatch are the ones that actually eat the pests.14Journal of Insect Science. Volatile Semiochemicals Increase Trap Catch of Green Lacewings (Neuroptera: Chrysopidae) and Flower Flies (Diptera: Syrphidae) in Corn and Soybean Plots
Plant chemistry also plays a role. Experiments with Cnidium monnieri, an agricultural companion plant, found that specific plant volatiles attracted Chrysoperla sinica lacewings. Females were especially drawn to certain green-leaf volatiles, while males preferred a different set of terpenes, suggesting the sexes respond to different chemical cues when foraging.15Elsevier / Biological Control. Volatile chemical cues emitted by an agricultural companion plant (Cnidium monnieri) attract predatory lacewings (Chrysoperla sinica) For gardeners, the practical message is that planting certain flowering species can help attract and retain lacewings without relying on commercial releases. The trade-off, of course, is that more lacewings in the garden means a higher chance of occasionally encountering that prothoracic-gland stench when weeding or harvesting.
How Lacewing Larvae Sense the Chemical World
While the adult lacewing’s chemical output gets most of the attention, the larvae are also intensely tuned in to smells around them. Scanning-electron-microscope studies of lacewing larval heads have identified specialized sensory structures on the antennae, including smooth-walled sensilla with terminal pores that are consistent with a chemosensory function.16Authorea. Ultrastructure on head and trunk of green lacewing larvae associated with debris-carrying behavior These structures are tiny, measured in single-digit micrometers, but they allow the larva to detect chemical cues in its immediate environment. This chemical sensitivity is essential for a creature that hunts by touch and smell in the dark, navigating leaf surfaces to find aphid colonies it cannot see.
The same sensitivity likely helps debris-carrying larvae select the right materials for their camouflage packets. If the chemical disguise depends on wearing prey whose surface compounds match those of living aphids, the larva needs some way to evaluate what it is piling on its back. Whether larvae actively choose carcasses with the best chemical match, or simply grab whatever is nearby, remains an open question. But the presence of chemosensory equipment on the antennae makes active selection at least plausible.
When Chemical Camouflage Fails
The aphid-carcass disguise is effective but not foolproof. Experiments have shown that the success of the chemical mimicry depends in part on whether the ants have recently learned the scent profile of the specific aphid colony they are tending. Ants that were familiar with a particular aphid species were better at distinguishing real aphids from predators wearing aphid-scented disguises than ants encountering an unfamiliar aphid species for the first time.17PubMed. Efficacy of Chemical Mimicry by Aphid Predators Depends on Aphid-Learning by Ants In other words, the disguise works best against ants that are new to a colony. As ants spend more time with a particular aphid group, they get better at spotting imposters, creating something of a chemical arms race played out on the surface of a single plant stem.
This finding has implications for biological control. If you release lacewing larvae into a garden bed where ants have been tending an established aphid colony for weeks, the ants may already be skilled at detecting and attacking the larvae despite their camouflage. Managing ants separately, through barriers or bait stations, can improve the survival rate of released lacewing larvae and let them do their job as pest predators more effectively. The chemistry of smell, in the lacewing world, is not just a nuisance for the human who picks one up; it is the currency of survival at every life stage, governing who eats, who mates, and who gets eaten.