Eyespots on the wings of Polyphemus might startle a would-be predator. M. J. Raupp
How can you not love Greek mythology? Polyphemus, a terrible, one-eyed giant that enjoyed feasting on men, notably Odysseus and his crew. Fitting that to escape from the cave of Polyphemus, the crew blinded him with a spear and snuck out of the cave with a herd of sheep. Giants also populate the world of insects. Polyphemus is the name given to one of our largest moths that has not one eye, but four on its dorsal wings. These are not true eyes like the compound eyes on the head of the Polyphemus moth. These false eyes or eyespots are an evolutionary masterpiece created by hundreds of colored scales arranged in patterns resembling the eye of another animal like a bird, mammal, or snake.
This montage begins with a very pregnant female Polyphemus moth that decided to lay her eggs on a lawn chair. Hopefully, the caterpillars made their way to an oak or maple tree like this one. Here is what this giant silk moth caterpillar looks like as it ascends the tree. Watch as smaller and larger caterpillars munch on one of their favorite foods, the leaves of oaks. One kind human found this female Polyphemus marooned inside a parking garage after a storm. With a little assistance, this magnificent moth took flight. Here’s looking at you Polyphemus. Video and image credit: Nancy Koran, Peihan Orestes, and Jenny Milward.
Several species of insects employ clever patterns of coloration resembling eyes on parts of their body where eyes really do not occur. For example, in a previous episode of Bug of the Week, we met the larva of the swallowtail butterfly adorned with two sinister eyespots on its thorax. These false eyes helped create the illusion of a serpent for a larva that is really a tasty caterpillar. Scientists believe that these eyespots aid in defense of insects and other animals in several ways. Eyespots may resemble the eyes of a potential predator’s own predators. Moths and butterflies are tasty fare for many birds, but in turn, birds are meals for larger winged predators such as owls. Eyespots and color patterns on the wings of some moths resemble the face of an owl. Imagine the terror of a bird about to eat what appears to be a harmless moth, when suddenly the hungry bird confronts the face of an owl. A second way that eyespots may aid in defense is to direct an attack away from vulnerable parts of the body. Some predators attack the head of the victim where maximum damage results. False eyespots on less critical parts of the body such as wings may steer a first strike away from a lethal spot and provide time for the intended prey to escape.
The beautiful Polyphemus caterpillar is one of the largest caterpillars in North America M.J. Raupp
Last week, I received images of a very pregnant female Polyphemus moth that had settled on a patio and proceeded to lay batches of eggs on a lawn chair. I know not what became of the eggs or the moth, but with luck the eggs hatched and the tiny caterpillars, which don’t eat lawn chairs, found their way to sustaining leaves of elm, oak, walnut, or more than a dozen other woody trees and shrubs. These marvelous caterpillars will gain more than a thousand times their birth weight before they spin cocoons of leaves and silk and change into pupae. With luck, in a few weeks, we will have dozens of four-eyed moths returning to the wild to scare the daylight out of would-be predators.
Acknowledgement Special thanks to Nancy Koran for providing inspiration and images for this episode. Cool camera work by Peihan Orestes and Jenny Milward helped create this story. The fantastic Caterpillars of Eastern North America by David Wagner was used as a reference.
Amidst a colony of brown ambrosia aphids death awaits in the from of predatory midge larvae of Aphidoletes aphidomyza. These fierce mini-monsters may consume dozens of aphids during their development. Circles mark the locations of predatory midges. Photo by Paula Shrewsbury
Last week we met voracious flower fly larvae as they ravaged populations of brown ambrosia aphids on my silphium cup plant. This week things have gone from bad to worse for the pesky aphids as populations of sneaky predatory aphid midges arrived in force and rained carnage on the aphids. In past episodes, we visited many wild predators like firefly larvae, tiger beetles, and spiders that employed powerful jaws or fangs to slice, dice, and consume hapless prey. Today we meet a predaceous midge, Aphidoletes aphidomyza, with a fondness for the legs of aphids. Hold on, a fondness for legs, what’s that about? Here’s the story. Larvae of predaceous midges are many times smaller than those of the large flower flies we met last week. In fact, they are much tinier than the aphids on which they feast. These diminutive predators sneak up on the victims and with persistence and luck, they attach themselves to the joints on the aphid’s leg. After they are plugged in, they inject paralytic venom, likely produced in their salivary glands, into the blood stream of the aphid. In a matter of minutes, the paralytic action of the venom takes hold immobilizing the aphid. As I watched one of these attacks, I saw a small midge larvae intermittently strike and bite the left foreleg of a small aphid. As the movements of the aphid subsided, the midge slowly slithered beneath the aphid and attached itself to the right hind leg of the moribund aphid. Half an hour later the midge moved to the left hind leg. With its victim motionless, the predator finally settled on rump of aphid for its meal. After subduing their victims, midge larvae may move to meatier parts of the aphid’s body such as the thorax where they may be joined by other larvae. Some accounts report that these tiny assassins will kill more aphids than they consume. Not sure how to explain that. The aftermath of the midge’s carnage is blackened hulks of shrunken exsanguinated aphids.
Aphids dance away from a predaceous midge larva searching for a victim. This one snared the left foreleg of a small aphid and is injecting paralytic venom. Next it moves to the right hind leg of the moribund aphid. Then on to the left hind leg. With the aphid motionless, the larvae settles on ‘rump of aphid’ for dinner. Nearby, two midge larvae feed on the head of an aphid. When these tiny monsters are done, all that remains of the aphid colony are shriveled black carcasses.
These brown ambrosia aphids are not entirely defenseless. Watch as one drop kicks a predatory midge right off a leaf. Here’s the instant replay at one tenth normal speed.
In the killing field of Aphidoletes aphidomyza after the feast, all that remains are black shriveled carcasses of aphids. Photo by Mike Raupp
If you are feeling a bit sorry for the poor aphids in all of this, I understand. Their demise seems gruesome, meeting death by mouth hooks and venom producing flies. But remember that aphids are major pests of our crops and landscape plants causing billions of dollars of loss worldwide. Predaceous midges are key players in mitigating losses to pests in natural and managed ecosystems. They are produced commercially and regularly used in greenhouses as biological control agents for aphids. To learn more about Aphidoletes as biological control agents, please click on the links below.
Adult predacious midges are small delicate flies resembling their cousin, the boxwood leafminer. Photo by Mike Raupp
Acknowledgements: Bug of the Week thanks Dr. Shrewsbury for spotting predaceous midges in the aphid outbreak on the cup plant. The informative articles “Entomo-venomics: The evolution, biology and biochemistry of insect venoms” by Andrew Walker, Samuel D. Robinson, David K. Yeates, Jiayi Jin, Kate Baumann, James Dobson, Bryan G. Fry and Glenn F. King and “Aphid Predatory Midge” by Jim Walgenbach were used as references for this episode.
Amidst a horde of brown ambrosia aphids, a syrphid fly larva attacks its next victim. M. J. Raupp
One of the best performers in my flower bed is a raucous native plant known as cup plant, Silphium perfoliatum, a premier attractor of insects to the garden. Extravagant floral displays provide nectar and pollen to wide variety of flies, bees, butterflies, and wasps. Nutrients coursing through vascular vessels support several species of sucking insects including leafhoppers, treehoppers , and aphids. And where there are abundant juicy prey items, there are predators, lots of them. During spring and early summer, populations of brown ambrosia aphids have exploded on my cup plants. Like many of their kin, in spring and summer these gals are parthenogenic. They are an all-female society reproducing without the assistance of males. As aphids feed, they excrete a waste product called honeydew. Honeydew contains volatile organic compounds (VOCs), the aromas of which act like a dinner bell ringing “come and get it”. The more aphids and honeydew on a plant, the more likely it will be discovered by flower flies. Once the infestation is detected, the females fly lays a small white egg near the colony of aphids. The egg hatches into a wriggling larva (a.k.a. maggot) whose sole purpose is to hunt and eat soft-bodied prey. With no true eyes, this blind assassin discovers its victims by swinging its head to and fro, searching for prey with sensory structures located on the front end of its fleshy head. When it bumps into an aphid, the flower fly larva snares the aphid with its mouth hook [it looks like Captain Hook’s hook]. Mouthparts pierce the aphid’s cuticle, and the larva sucks the aphid’s blood.
When in bloom, cup plants are dynamite attractors of pollinators. But pre-bloom, cup plants often generate fantastic populations of brown ambrosia aphids. Volatile odors released by the aphids serve as a dinner bell for squads of hungry predators and parasitoids. Not long after aphid populations exploded, adult flower flies deposited eggs near colonies of aphids. From these eggs hatched fierce larvae laser focused on hunting and eating aphids night and day. Watch as one of these tiny terrors makes short work of a misguided aphid. The flower fly larva snares the aphid with its mouth hook. It looks like Captain Hook’s hook. It then pierces the aphid and sucks out its blood. Flower fly larvae make short work of aphids on plants. Along with gangs of spiders, lady beetles, lacewing larvae, predaceous midge larvae and parasitic wasps, aphids on my cup plants will soon be history.
It’s easy to see why another name for the flower fly is hover fly. Flower flies deposit white eggs like these near colonies of aphids. Eggs hatch and blind flower fly larvae hunt by casting their head two and fro. Prey like this aphid are snagged with a mouth hook. Once captured the contents of the aphid are consumed. Sometimes hapless aphids blunder into fly larvae. You can see the dorsal heart of the larva beating as it feeds. Little wonder that aphid populations can collapse when flower flies and other predators and parasites arrive.
Flower fly maggots have prodigious appetites. In the laboratory, I have watched these predators consume more than 25 aphids in a day. Reports of aphid carnage in the literature puts the casualty figures at more than 200 aphids during development for each maggot. In some agricultural systems, flower flies are believed to provide 75% to 100% control of aphids. In my experience with aphids, flower flies, with a little help from lady beetles and other predators, can entirely wipe out populations of aphids in a matter of weeks. So, before you reach for the aphid spray, carefully look to see if the maggot brigade and company are at work. While the brown ambrosia aphids put a minor beat down on my cup plants, they are generally good news for my garden. The aphids have become a factory for many species of predators including spiders, lady beetles, lacewing larvae, predaceous midges and parasitic wasps that will move to other plants in my landscape once the brown ambrosia aphids are kaput, all part of Mother Nature’s plan for a more sustainable landscape.
This little Cycloneda lady beetle has her jaws wrapped around a juicy brown ambrosia aphid. M. J. Raupp
Acknowledgements
We thank Dr. Jeff Shultz for identifying the cool male lynx spider and Dr. Paula Shrewsbury for planting silphium, identifying the pretty polished lady beetle and providing inspiration for this episode. The fascinating account of defensive behaviors in aphids entitled “Collective Defense of Aphis nerii and Uroleucon hypochoeridis (Homoptera, Aphididae) against Natural Enemies” by Manfred Hartbauer was consulted to prepare this episode.
The underside view of an adult firefly reveals the whitish light organs where photocytes, cells that produce light, are located. Large eyes help fireflies find the glow of mates at night. M.J. Raupp
Happy 20 years Bug of the Week
This June marks the celebration of 20 years of Bug of the Week. We want to thank all our viewers in more than 200 countries worldwide for your continued support. Last year we set a record with more than 370 thousand visits. Thank you so much. This week we call on one of Mother Nature’s most fabulous creations to help us celebrate. Three cheers for fireflies.
Much of this episode comes courtesy of Dr. Paula Shrewsbury, Entomologist at the University of Maryland, College Park, who created this great article for her “Beneficial of the Week” in the IPM Alerts – Landscape & Nursery.
“I saw my first fireflies of the season in Sharpsburg, MD on May 17th; and this week I saw fireflies flashing in Columbia, MD. It looks like it is going to be another good year for fireflies based on the amazing display of flashing lights so far.
Fireflies, also known as lightening bugs, are really neither bugs nor flies. They are characterized as soft-winged beetles in the order Coleoptera and the family Lampyridae. There are over 2,200 known species of fireflies, of which about 165 species have been reported in the U.S. and Canada. Fireflies are found in temperate and tropical regions and in the humid regions of the Americas, Asia, and Europe. In the U.S., the abundance of fireflies is greater east of the Great Plains than in western states. Interestingly, fireflies that produce light are uncommon in western North America. Some firefly species are diurnal, and therefor have no need to create light. These species are known as daytime dark fireflies and they use chemical pheromones for mate attraction. Although the adults do not light, the larvae do glow at night, similar to eastern species.
Most flashing species occur east of the Mississippi River, are about ¾” in length and are active at dusk and night. Adults and larvae of many firefly species exhibit bioluminescence – they glow in the dark! Many organisms such as bacteria, fungi, jellyfish, algae, fish, clams, snails, crustaceans, and of course insects [including some click beetles] exhibit bioluminescence. Firefly species have special light organs that make the underside of their abdomens light up.
To help celebrate twenty years of Bug of the Week, let’s enjoy one of Mother Nature’s finest light shows courtesy of fireflies. How do they produce light? See the white segments at the tip of the firefly’s abdomen. These segments contain photocytes, cells that produce light. I asked this little guy to show off his stuff. Look at him go. Don’t worry, he was released nonplused but unharmed. Male fireflies perform aerial displays to impress females watching from vegetation on the ground or on branches. If the she firefly likes the performance, she may signal back with her flash. Watch as a male firefly searches for his mate on a cluster of leaves. Eventually he finds her and their union is consummated. During June and July, take a moment at twilight to visit a lawn, meadow, or park to enjoy these beautiful and illuminating creatures.
How do fireflies make light? The light emitted by a firefly is actually a chemical reaction in the beetle’s abdomen. The light organ has special cells that contain a chemical called luciferin. An enzyme called luciferase combines oxygen with luciferin in these cells to create light. Scientists actually do not know how fireflies regulate their lights to turn them on and off. You might have also noticed how “cold” the light looks. This is because no infrared (or heat) or ultraviolet frequencies of light are emitted. Among the light-producing fireflies, lights are yellow, green, or pale red.
Firefly larvae called glow worms have luminescent organs on the underside of their abdomen. M. J. Raupp
Why do fireflies bioluminescence? The purpose of this bioluminescence varies. It is believed that the flashes are part of a signaling system for attracting mates. Both males and females emit light intermittently or in specific flash patterns. The rhythmic flash patterns produced are specific for each species of firefly and vary by sex within a species. The flashes that we see are from the males that are attempting to attract a mate. For example, males of the common eastern firefly (Photinus pyralis) flash every six seconds. Females watch the light “show” and if a display from a specific male is particularly attractive, she will flash a response but only if it is from the male of the same species. The male descends to that location to mate with her. In addition to transferring sperm to the female during copulation, the male offers a nuptial gift of rich protein, which the female uses to provision the eggs that will soon start to develop in her ovaries. Interestingly, in one species of firefly, Photuris pensylvanica, the female mimics the flash pattern of another species, Photinis pyralis, to attract the male of the other species to her. When the male of the other species arrives thinking, he has found his mate – she eats it to obtain defensive compounds used to protect her eggs. A bad surprise for that male.
Why are fireflies considered beneficials? Well, the soil active firefly larvae or glow worms are voracious predators of soft-bodied invertebrates and known to feed on slugs, snails, worms, and other soil-dwelling insects. Glow worms use their mandibles to inject prey with a paralyzing neurotoxin, making it defenseless, and then secrete digestive enzymes that liquify the prey making it easier to consume. Firefly larvae or glow-worms are believed to glow as a warning signal telling predators not to eat them as they are mildly toxic and taste nasty. It is not well known what all adult fireflies feed on but some feed on pollen and nectar and some are reported not to feed at all.
A pair of firefly larvae snack on a hapless earthworm. M. J. Raupp
Although the larvae of fireflies are referred to as glow-worms, technically this is not quite correct. Glow-worms are a type of firefly where the adult female is flightless and maintains the appearance of a larva and she emits a long-lasting glow, similar to larvae. The males have the appearance of an adult firefly. To make it more confusing, other insect larvae that glow, are sometimes called glow-worms too. Since most fireflies that produce light are in the Eastern U.S., it makes the nightly light shows we encounter here something special to behold for a few weeks during spring and early summer. Be sure to help young people you know, and others, enjoy the experience of observing and collecting fireflies. Be certain to release the little lights when you are done!”
Light pollution and fireflies
Over the past few years, many have been concerned about dwindling numbers of lightning bugs in our region. While hard data on this issue are difficult to come by, one important study conducted by scientists at the University of Virginia suggests that light pollution caused by brightly lit homes and buildings has disrupted the normal ecology and behavior of these remarkable creatures. By adding artificial light to nocturnal courting grounds, normal courtship behaviors and mating success of fireflies were compromised. The authors suggest these reductions in mating success could lead to fewer fireflies in locations with light pollution. The development of natural areas and destruction of habitat are also thought to contribute to reductions in firefly populations. Others believe that widespread use of residual insecticides to treat lawns may have contributed to the lightning bug’s decline. Perhaps unfavorable weather cycles or a dearth of food for predatory lightning bug larvae, which live on the ground, may have suppressed their numbers in years past.
What can be done to help our fireflies and other nocturnal insects? Reducing sources of artificial light at night (ALAN) by using motion detectors to trigger security lights, timers and dimmers to regulate intensity and timing of illumination and simply turning off unnecessary lights can help. Outdoor lighting along pathways can be shielded from above to reduce light scattering that might attract flying insects. To learn more about light pollution and some solutions to ALAN, visit DarkSky.
During these glorious days of June and July, take a moment at twilight to visit a lawn, meadow, or park to enjoy these beautiful and illuminating creatures.
Acknowledgement
Bug of the Week thanks all our viewers over the last two decades. Our F2s, Eloise, Abby, and Jackie provided the inspiration for this episode and Dr. Shrewsbury provided much of the cool content. The interesting articles “Experimental tests of light-pollution: Impacts on nocturnal insect courtship and dispersal” by Drs. Aerial Firebaugh and Kyle Haynes, “Flash Signal Evolution, Mate Choice, and Predation in Fireflies” by Sara M. Lewis and Christopher K. Cratsley, “Silent Earth” by David Goulson, and fascinating studies of Dr. Sara Lewis and Dr. Thomas Eisner and their colleagues, served as resources for this Bug of the Week.
