Introduction to Invasive Terrestrial Isopods

Introduction

Terrestrial isopods, often called pillbugs, roly pollies, or doodlebugs, are some of the most common organisms in Pennsylvania's forests. While these little creatures are often called bugs and confused with insects, they are, in fact, crustaceans and closely related to crabs, lobsters, and shrimp.

Terrestrial isopods live in soil and under debris such as leaves, rocks, and logs. These small, pill-shaped creatures are detritivores—they break down and consume rotting material, similar to fungi, wood-boring beetles, and earthworms. There are many species of terrestrial isopods. Most of them are non-native, and many are invasive.

What do they look like?

Terrestrial isopods are typically no bigger than the size of the average human's thumbnail, and are easily identified by their hard, segmented outer shells. They possess seven pairs of thin legs, along with two antennae and an oval (pill-shaped) body. The color varies by species, with some often brown or muddy and others grey or nearly black. While pillbugs are famous for rolling up, only Armadillidiidae are able to conglobate—roll up into a ball (Figure 1). Other isopod species, such as those within the Trachelipodidae family (which includes Rathke’s woodlouse), cannot conglobate, and when threatened, will either remain still to hide or will run as fast as possible.

Habitat

Terrestrial isopods are habitat generalists, but they need to remain moist and prefer areas that are cool, damp, and shaded. Isopods are most active in the late spring and summer, where they can commonly be found under decaying logs, rocks, and mulch. In the fall and winter, isopods tend to burrow into the soil to avoid the cold temperatures. They may move into basements or other damp, dark areas as they seek to avoid the cold.

Native Isopods

Due to the increasing number of invasive species of isopods, many of the native isopods like eastern rockslater (Ligidium elrodii) and the Alabama sowbug (Miktoniscus medcofi) are not as common as they once were (Shultz, 2018). The eastern rockslater is dark-brown and has a stout body. At a glance, rockslaters look somewhat like small cockroaches, with longer antennae, but they have multi-segmented bodies (cockroaches, being insects, have only three segments) and a pair of cerci, sharp-looking appendages at the rear of their abdomen. Alabama sowbugs are light brown and longer than the eastern rockslater. Both species are harmless to humans, but at high densities, they can damage young plants. 

Invasive Isopods in Pennsylvania

Some of the most common species of terrestrial isopods in Pennsylvania are common pill-bug (Armadillidum vulgare), Rhathke's woodlouse (Trachelipus rathkii), and Mediterranean isopod (Chaetophiloscia sicula). All of these are isopods are invasive and native to Europe. They have spread widely and rapidly through human-mediated transmission. They are accidentally moved in mulch and garden material (e.g., plants and compost), as well as intentionally through the pet trade.

Impact of Invasive Isopods

Terrestrial isopods are detritivores that primarily feed on dead plant material. However, invasive isopods compete with and displace native detritivores, which reduces biodiversity, makes it more difficult for forests and other ecosystems to recover from disturbances, and disturbs relationships between native organisms. As detritivores, they increase rates of decomposition, which increases the rate at which carbon is released from forests, reduces forest litter that is used by native flora and fauna, and replaces native detritivores.     

Non-native invasive isopods, in general, compete with and replace native isopods. At high densities, non-native isopods feed on the roots, leaves, and shoots of live plants, which can stunt plant growth and potentially kill them. Young herbaceous plants are especially vulnerable to isopod damage in the spring, as all parts of them are soft and easy for isopods to feed on. Not only can invasive isopods harm ecosystems, but they can also become a nuisance to people. Gardeners, landscapers, and foresters can lose plants to invasive isopod damage.

Control

Area-wide control. Unfortunately, due to the large scale of the invasion, non-native isopods cannot be controlled in the forest.

Local control. Individual invasive isopods can be squashed. When present in large numbers, they can be sprayed with a herbicide like neem oil. This is a common organic pesticide derived from the neem tree (Azadirachta indica). It can be applied directly to isopods or to plants with high concentrations of isopods. Unfortunately, due to the tendency for terrestrial isopods to dig, each plant needs to be treated, and isopods may burrow under the neem oil to escape it. When using insecticides, be sure to read the label, wear all required personal protective equipment, and use only as directed on the label.

Key Takeaways:

• Terrestrial isopods are very common in Pennsylvania's forests, though many species are non-native invasive and can cause problems for local flora and fauna.

• Isopods are usually detritivores, but when resources are scarce or plants are vulnerable, they can feed on living plants.

• Invasive isopods can be managed on a local level, but they are too numerous and widespread to be managed on a landscape scale.

References

Hornung E, Szlavecz, K Dombos M (2015) Demography of some non-native isopods (Crustacea, Isopoda, Oniscidea) in a Mid-Atlantic forest, USA. In: Taiti S, Hornung E, Štrus J, Bouchon D (Eds) "Trends in Terrestrial Isopod Biology." ZooKeys 515: 127–143.

Szlavecz, K., Jones, N. T., Noël, F., Boeraeve, P., & De Smedt, P. (2025). Chaetophiloscia sicula Verhoeff, 1908 (Isopoda, Oniscidea), an invasive isopod currently spreading in North America. ZooKeys, 1225, 65–75.

Shultz, J. W. (2018). A guide to the identification of the terrestrial Isopoda of Maryland, U.S.A. (Crustacea). ZooKeys (801, 207–228).

Tuf, I. H., & Ďurajková, B. (2022). Antipredatory strategies of terrestrial isopods. ZooKeys, 1101, 109–129.