Hyphantria cunea — “Fall webworm”

Hyphantria cunea, also known as the fall webworm, is an American species of tiger moth that originates from from North America, Canada, and Mexico. However, Hyphantria cunea was also introduced to Europe and Asia by humans, and has become a very destructive invasive species in these regions.

They are notorious defoliators that can consume entire forests when the populations are at peak levels; larvae can cover entire trees with the communal silken webs that they spin. Despite that, they are generally not considered to be pests within their native range – in fact, this species has an important ecological role, for they are preyed upon by a great number of invertebrates (wasps, flies, spiders) and vertebrates (birds) that rely on it as a food source. Populations of Hyphantria cunea show large fluctuations and in some years the moths appear to be rare while in other years they can become over abundant. Between outbreaks where larvae and moths become abundant and seem to cover entire forests, populations are suppressed by  predators and also various pathogens such as fungi and bacteria; population ecology dictates that the carrying capacity of their environment regulates the number of individuals.

Hyphantria cunea moths

The adult moths of Hyphantria cunea have a variable appearance. The majority of them appear to be all white, except for the bright yellow femora on their legs. However, the moths also have variable black pigment. The black pigment is often expressed as seemingly random black spots and may vary from just a few hardly noticable black dots to heavy dark spots and stripes, in some cases similar in appearance to some species of Spilosoma moths.  In extremely rare cases adults have been found that appear to be almost completely grey or black.

One of the most notable features of Hyphantria cunea is the high degree of polyphagy of the caterpillars. In the wild, the larvae have been recorded to feed on over 600+ types of plants and shrubs, making it one of the most polyphagous moths in the world (in reality perhaps thousands). Despite that, they do show some degree of favoritism; hickory/pecan (Carya sp.), walnut (Juglans), elm (Ulmus), alder (Alnus), willow (Salix), mulberry (Morus), oak (Quercus), sweetgum (Liquidambar), and poplar (Populus) are the plants they are most frequently found on. 

While it is not a harmful species within its own native range (America)  they are very succesful and destructive as an introduced invasive species because of their great polyphagy – for it is their polyphagy allows them to survive in a great variety of alien environments and more importantly  allows them to defoliate a great variety of native plants, outcompeting native species that are dependent on these plants in the process.

Young larvae show social behaviour. Using their silk glands, the caterpillars cover their host plants with silken webs. In these webs they form nests and often feed, travel and rest together in a group. The webs offer them a certain degree of protection from the elements and various predators, and allow them to detect eachothers movements, enabling social behaviours. In the wild these webs can cover entire trees and despite not being very harmful to the host plants themselves some people consider them to be harmful for they can be unsightly in gardens and parks.

Hyphantria cunea in a silk web

Northern populations have only one or two generations, whereas Southern populations have three to four.  This is likely due to natural key factors such as daylength and temperature regulating the induction of diapause in pupae (overwintering) – the local climate regulates the diapause. In most localities they are found from late summer to early fall – with moths hatching in summer – giving them the common name ‘fall webworm’. The cocoons overwinter, although this species can be continuously brooded in warmer climates where overwintering is not needed.

  • Difficulty rating:  Extremely easy
  • Rearing difficulty: 1/10 (From egg to pupa)
  • Pairing difficulty: 1/10 (Archieving copulations)
  • Host plants:  Nearly every type of shrub and tree, in the wild over 600 food plants have been recorded. Despite being extremely generalist the caterpillars do show some degree of favoritism; hickory/pecan (Carya sp.), walnut (Juglans), elm (Ulmus), alder (Alnus), willow (Salix), mulberry (Morus), oak (Quercus), sweetgum (Liquidambar), and poplar (Populus) are the plants they are most frequently found on. 
  •  Natural range: North America, Canada and Mexico by origin; but has been introduced by humans in Europe and Asia, where it has established itself as a destructive invasive species. 
  • Polyphagous:   Extremely polyphagous – over 600 recorded host plants(!) in the wild from plant families including but not limited to Aceraceae, Anacardiaceae, Annonaceae, Aquifoliaceae, Berberidaceae, Betulaceae, Bignoniaceae, Buxaceae, Cannaceae, Caprifoliaceae, Celastraceae, Chenopodiaceae, Comaceae, Compositae, Cupressaceae, Ebenaceae, Ericaceae, Ericaceae, Euphorbiaceae, Fagaceae, Geraniaceae, Gramineae, Hamamelidaceae, Hippocastanaceae, Juglandaceae, Lauraceae, Leguminaceae, Leguminosae/Fabaceae, Malvaceae, Magnoliaceae, Moraceae, Naucleaceae, Nyssaceae, Oleaceae, Platanaceae, Pinaceae, Portulacaceae, RanunculaceaeRhamnaceae, Rosaceae, Rutaceae, Salicaceae, Saxifragaceae, Scrophulariaceae, Simaroubaceae, Staphyleaceae, Taxaceae, Tiliaceae, Ulmaceae, Vitaceae – in reality likely feeds on thousands of plant species
  • Generations: Multivoltine – has one to four annual broods per year, depending on the geographic origin. Northern populations have only one or two generations, whereas Southern populations have three to four.  This is likely due to natural key factors such as daylength and temperature regulating the induction of diapause in pupae (overwintering) – the local climate regulates the diapause. 
  • Family: Erebidae (Arctiidae)
  • Pupation:  Spins a cocoon
  • Prefered climate: Nearctic; from temperate to subtropical
  • Special notes: Extremely easy to raise in all regards
  • Wingspan:  20mm-50mm
  • Binomial name:  Hyphantria cunea (Drury, 1773)

Adult Hyphantria cunea moths, in most localities, appear in late summer to early fall, despite being continuously brooded in localities with warmer subtropical climates. The larvae that spin webs on the host plants give them the name ‘fall webworm’. Adults are pale white in most cases but can have thousands of different phenotypes, as they appear to have variable black pigmented spots. The degree of pigmentation varies a lot per individual and may range from a few hardly noticable dots to heavy black spots to adults that appear to be almost completely black. The adults have no functional mouthparts and do not feed, and only live for about a week. After hatching from their cocoons, the males seek a female and pair. After pairing the females disperse and lay up to a few hundred pale, translucent green eggs upon a rough surface (usually a leaf or twig, but it can be random). Eggs hatch quickly in two to three weeks time, depending on the temperature.

Hyphantria cunea, adult

Hyphantria cunea female laying eggs

In captivity the moths are very eager to pair. Males will quickly rush to any females they may encounter – even in small, airtight plastic boxes it is easy to archieve multiple copulations. After pairing, the male and female stay connected from the tips of their abdomen for about 12 to 24 hours.

Hyphantria cunea copulating

Hyphantria cunea copulating

After hatching, the larvae spin silken nests to protect themselves. Inside these nests they hide, feed, travel and also shed their skins together. These silk nests are mainly spun over the host plants they are feeding upon. The feeding habits of Hypantria cunea  caterpillars vary with their age. Young larvae appear pale, naked and ‘maggot’ like, and only seem to eat the top layer (epidermis) of the leaf, leaving the leaves mostly intact. From the second instar and beyond, larvae become increasingly hairy and seem to consume all the leaf membrane, leaving only the veins intact, effectively skeletonizing the leaves. From the third instar and beyond they start consuming the leaves entirely, defoliating the plants. At these stages they still spin silken webs and show social behaviours.

Hyphantria cunea first instar

Hyphantria cunea first and second instar

Hyphantria cunea second instar

Hyphantria cunea second instar

The third, fourth and fifth instar have more pronounced tufts and setae. From the fourth instar and beyond, larvae become solitary instead of social, and will prefer to feed alone, although they tolerate eachother in high densities. Interestingly, larvae show geographical variation and can be divided in several races, mainly distunguished by the colour of their head capsules – an orange, red and black one are reported. On a smaller level scale larvae may still show morphological variations on a geographical level.

Larval morphology and species status of Hyphantria cunea
Many experts doubt the status if Hyphantria cunea as a single species. Most interesting are the different larval phenotypes. For example, a “black headed” and “red headed” race of caterpillars can be distinguished.  Some quotes: “Fall webworms, Hyphantria cunea (Drury), found in Louisiana differ in appearance and morphology. Those designated as an “orange race construct compact nests. Nests of the black race are diffuse and generally subdivided into several small nests. The “black race”infests 34 hosts, of which sweetgum, persimmon, and willow seem to be preferred, and the “orange race” infests 7 hosts, of which pecan and persimmon are preferred. Adults of the two forms will cross in the laboratory and produce viable Fi and F2 progeny. There are considerable clifferences in manner of egg deposition and larval behavior.  – A Behavioral Study of Two Races of the Fall Webworm, Hyphantria cunea, (Lepidoptera: Arctiidae) in Louisiana.  – Another example:  “There are three races of fall webworms in the US,
characterized by head capsule color: black, red, or mosaic.
Black and red head capsule populations overlap in the eastern
US (Oliver 1964), but populations remain reproductively
isolated because their mating period is separated in time
(Takeda 2005). The mosaic (black-red head capsule) race
exists only in the Pacific Northwest (Tufail et al. 2014). Fall
webworm colonies produce large and visible silk tents, but
IPM managers must realize that other poplar pests may
produce silk refuge for protection against vertebrate and
invertebrate predator” – FALL WEBWORM Insect Pest Management in Hybrid Poplars Series Fall Webworm.  Thank you for your research, John J. Brown and A. D. Oliver.

Looking at these different geographical races that seem to have slightly different lifestyles, morphologies and ecology, it may be true that Hyphantria cunea is actually a complex of multiple species that interbreed in some instances. More research is needed to confirm if they are simply races or subspecies of the H. cunea or perhaps seperate species.

Hyphantria cunea fourth and fifth instar

Larvae have an interesting defense mechanism; if startled they contort their bodies and quickly swipe their thoraxes over the leaf from left to right in a rapid swinging motion. This seems to make a rattling sound. Even more interestingly, it seems that this behaviour stimulates larvae in the vincinity to show the same behaviour. At this moment the true purpose  of it becomes clear; the sound is greatly amplified if multiple larvae show this behaviour at the same time, rattling and shaking the leaves of the host plant collectively in great numbers. At one point, I was raising hundreds of individuals of this species, and the sound of hundreds of caterpillars collectively shaking the branches of the food plant, made an impressive noise, similar to a larger animal or human shaking the plant!

The hairs of the caterpillars are generally not known to be irritating or urticating, although they may still make it more difficult to breathe for the most sensitive of people if they are present in great numbers.

Hyphantria cunea fourth and fifth instar

Hyphantria cunea fourth and fifth instar on Prunus laurocerasus

When the larvae are mature and fully fed, they descend to the floor from their respective host plants looking for a sheltered spot to spin a cocoon. This is usually done in leaf litter or against tree bark.  A small percentage of larvae does not bother to build a cocoon at all, and pupates on the floor, usually within leaf litter or small crevices. When spinning cocoons, the larvae of Hyphantria cunea incorporate their body hair into the silk, forming a hairy cocoon. This protects them from the elements and allows them to overwinter, but also shelters them from certain kinds of predators. Pupae can remain dormant for an extended period of time, potentially hatching after a year. In captivity, if kept warm, they will likely produce continuous generations. The development from egg to cocoon takes about 1.5 to 2 months on room temperature, although warmer temperatures speed up the process.

This species is considered to be ‘not (economically/ecologically) harmful’ in its native range for they mainly defoliate host plants around autumn, when the plants are ready to drop their leaves anyways.
However, in places where it was introduced, such as Europe and Asia, it is a very destructive invasive species, that can defoliate or destroy native forests if not controlled.

Hyphantria cunea spinning a cocoon from silk and body hair

Hyphantria cunea cocoon mass including prepupal larvae and pupae produced in captivity; in the wild it is unlikely that larvae will pupate in such a high density – but for these were raised in small breeding cages, larvae tend to pick the same spots to spin their cocoons, forming an unnatural mass – so please note that is this captive behaviour only, in the wild cocoons are single and usually solitary.

 Hyphantria cunea overwintering pupae

Hyphantria cunea fresh pupae

Hyphantria cunea pupae harvest

During winter, cocoons can be stored cold in a well isolated box, garden shed or basement. If kept warm they will probably produce consecutive generations.

Hyphantria cunea egg clusters and adults

Hyphantria cunea, pigmented variation

Hyphantria cunea freshly hatched from the cocoon; the bright yellow femora are visible

Ethical breeding – While Hypantria cunea is a fascinating and very easy species to reproduce in captivity, hobbyists should keep a few ethical questions in mind when breeding them. Hyphantria cunea has proven to be a destructive invasive species outside of its native range. Strict quarantaine protocols should be followed, and while breeding them they should never be exposed to the environment in any way. Eggs, pupae, larvae and adults can escape from captivity in multiple ways. In some cases they are thrown away with the old discarded leaf litter or frass.

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Citations: Coppens, B. (2019); Written by Bart Coppens; based on a real life breeding experience [for citations in literature and publications] 

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