The fall webworm is a moth in the family Erebidae known principally for its larval stage, which creates the characteristic webbed nests on the tree limbs of a wide variety of hardwoods in the late summer and fall. It is considered a pest but although unsightly, does not harm otherwise healthy trees. It is well known to commercial tree services and arboriculturists.
Description
The adult fall webworm has a wingspan of approximately 30 mm and is generally white-colored, although some individuals may have dark-colored marks.
Distribution
The moth is native to North America, ranging from Canada to Mexico. It is one of the few insect pests introduced from North America into other continents. Introduced to what was formerly Yugoslavia in the 1940s, it now has occupied probably its entire range in Europe from France to the Caspian Sea in the east as well as penetrated into Central Asia: Turkmenistan, Uzbekistan, Kyrgyzstan, and southeastern Kazakhstan. It was also introduced into Japan in 1945 and has adjusted its number of generations per year since its arrival. It spread into China, southern Mongolia, Korea and southern Primorsky Krai of Russia so that now it is considered holarctic in distribution. The fall webworm originated in North America but spread very quickly across the world due to factors such as trade and rapid transit.
Life cycle
One generation per year emerges in the northern part of North America, with larvae appearing in late summer through early fall. South of an approximate latitude of 40°N there are two or more generations annually, with webs appearing progressively earlier further south.
Eggs
The adult moth lays her eggs on the underside of leaves in 'hair'-covered clusters of a few hundred. Eggs hatch in about a week.
Larva
The caterpillars are highly variable in coloration, ranging from a pale yellow to dark grey, with yellow spots and long and short bristles. There are two cream stripes along the sides. The two races—one more common in the north, the other in the south—differ in head capsule coloration. The maximum length of larvae is 35 mm. Webs are progressively enlarged and much messier looking than those of tent caterpillars ; also, webs from the fall webworm are concentrated to the tips of the branches, whereas the tent caterpillar webs are largely found in the unions. Larvae feed inside the tents until the late instars. Very young larvae feed only on the upper surfaces of leaves; later, they consume whole leaves. The larval stage lasts about four to six weeks. Larvae are known to wiggle vigorously at periodic intervals in synchrony. How they synchronize these movements especially when distributed over a wide area has not been established.
Pupa
The pupa stage overwinters in the bark and leaf litter at the base of the trees. It is dark brown and about 10 mm long. The thin brown cocoon is made of silk with bits of detritus interwoven.
Adult
The adult is mostly white in the North America regions, but in the south it may be marked with black or brown spots on the forewings. It is quite 'hairy' and the front legs have bright yellow or orange patches. The underwings will have less marking than the forewings, and the abdomen often has a sprinkling of brown hairs. It has a wingspan with a range of 35–42 mm.
Food plants
The fall webworm feeds on just about any type of deciduous tree, where leaves are chewed; branches or the entire tree may become defoliated. Worldwide, it has been recorded from 636 species, and it is considered to be among the most polyphagous of insects. In the eastern U.S., pecan trees, black walnut, American elm, hickory, fruit trees, and some maples are preferred hosts; in some areas persimmon and sweetgum are also readily eaten. In the west, alder, willow, cottonwood and fruit trees are commonly used. Additional host plant examples include madrone, mulberry, ailanthus, American sycamore, and Asian white birch. Host plant selection is dependent on factors like the plant's degree of sun exposure, age, environmental stress undergone, toughness, and nutritional quality. For example, for an insect that needs energy for processes like dispersal or diapause, consuming plants that provide a lot of carbohydrates could be beneficial; for a female insect that is producing eggs, consuming plants that provide a lot of protein could be beneficial.
Behavior
The fall webworm is a gregarious insect. Groups of larvae live in self-created large webs that are found on tree branches. These webs allow for the finding of mates, temperature regulation, increased growth rate, and protection from predators, but also result in higher rates of infections and predation. The caterpillars of the fall webworm have several strategies to defend against threats. Some examples of protective behavior or defense are shaking and jerking together, repellant scent, and irritants on hairs or spines. The fall webworm exhibits a form of parental care, where the female will try to protect the eggs after oviposition by covering her newly laid eggs with her abdomen hairs.
Reproduction
Mating limitations
Male fall webworms are typically only available for mating for about 30–60 minutes per day.
Sex pheromone
The sex pheromone consists of the components - 9,12-octadecadienal, -9,12,15-octadecatrienal, cis-9,10-epoxy--3,6-henicosa- diene, and cis-9,10-epoxy--1,3,6-henicosatriene.
Physiology
Thermoregulation
Fall webworms experience behavioral thermoregulation. The fall webworms' self-created web is able to trap heat. Due to this, the fall webworm is able to maintain a warm temperature of about 40-50 °C, which allows the larvae to grow and develop faster. Inside the web, there is a temperature difference because the central part tends to have a higher temperature, while the back part tends to have a lower temperature. However, a web's heat trapping process is not always stable; wind can disturb the heat trapping.
Intestinal tract
There are several components to the fall webworm gut, such as the foregut and the midgut. The foregut and midgut of the fall webworm are alkaline, and Johnson and Felon found that the midgut pH ranged from 8.7 to 11.4.
Natural enemies
Predators
There are many predators of the fall webworm, such as Monomorium minimum, Vespula spp., Coleomegilla maculata, and Rogas hyphantriae.
Parasites
The fall webworm is parasitized by around 50 species, such as Musca domestica L, Muscina stabulans, and Chouioia cunea. There are also several species of microsporida that can be harmful, such as Nosema necatrix Kramer, Pleistophora schubergi hyphantriae Weiser, Nosema sp., and Nosema bombycis . Nosema bombycis can affect many body tissues, such as the midgut, fat body, gonads, nervous tissue, and Malpighian tubules, and the length of its polar filament is 100 µm. Spores from Nosema of fall webworms are typically egg-shaped and can sometimes be attached to each other, and the length of its polar filament is 57.5 µm ± 8.7 µm.
Diseases
Several types of pathogens can infect this species, such as granulovirus and nucleopolyhedrosis virus. Granulovirus has a rod-like shape. Also, an individual will be more likely to be infected at a younger age.
Interaction with humans
The fall webworm’s interaction with humans is important because of how adaptable this species is, which is one of the main reasons that it spread so widely across the world. Fall webworms are able to forage on large amounts of leaves inside their web, they are able to create webs on many different types of host plants, they are able to create webs at many different elevations and of many different shapes, and they spread very quickly. In addition, the fall webworm tends to create lots of webs or defoliate plants in places that humans spend time, such as recreational areas and parks; fall webworms tend to create their webs on branches and therefore defoliate parts of the tree that are very visible and draw attention. There have been methods with trying to control this organism, such as physical removal or chemical methods; however, there are some issues with these options because physical removal can be challenging with certain types of trees, and the side effects of chemicals. New, effective methods are needed to control this organism; Wang showed the effectiveness of community-based system to monitor webworms, which involved using mobile devices.
