In previous articles on soil builders, we’ve explored earthworms, soil-dwelling microorganisms, and, most recently, the strange world of microarthropods. Many larger, more visible and familiar arthropods are found in soil and leaf litter. Many native bees and wasps build nests in the ground. In warmer weather, predaceous beetle larvae are abundant and active in the soil. All of these underground residents, including spiders and some fantastically alien larvae, have served as inspiration for movie monsters; Antlion larvae are reputed to be the model for the terrifying sand creature in the Star Wars movies.
Many insects pupate in soil, often spending the winter safely tucked away below the ground. Some arthropods spend their entire life in topsoil and leaf litter, while others have both underground and aboveground life stages. Some arthropods including some spiders, earwigs, and beetles, spend the day hiding in soil and leaf litter, emerging in the dark of night.
Macroarthropods help build soil structure as they tunnel, creating channels for smaller organisms, water, air, and roots to travel through. While some soil macroarthropods are herbivorous and feed on roots, in a healthy garden with a diversity of plants, most of these will not have a significant impact. Some macroarthropods, such as soldier flies, are not strictly soil dwellers, but are found below the surface in decomposing organic matter. In fact, soldier fly larvae are so effective at munching through organic material they are now used in some composting systems.
Roly-polys (pillbugs and sowbugs) are the only fully terrestrial crustacean (a subphylum of Arthropoda); most common species found in western North America are introduced. Crustaceans are distinguished by having gills and need moisture to survive. Pillbugs roll into a ball as a defense, to the everlasting delight of children. Sowbugs are flatter, have two short “tails,” and are not able to roll up. Roly-polys, like other arthropods, must molt their exoskeleton to grow, but they are unique in molting only one end at a time, with a few days elapsing before their second half is shed. The result can be pillbugs with different-colored halves! Pillbugs are primarily detritivores (literally, detritus eaters), although gardeners sometimes complain that they have a taste for strawberries and violas. A judicious sprinkle of Sluggo® Plus (iron phosphate—for slugs—plus spinosad, an organic pesticide) where damage is found will eliminate roly-polys along with slugs, snails, and earwigs, but is relatively harmless to other animals and the environment.
Earwigs in the west are largely exotic species. When present in large numbers, they can occasionally become significant pests in the garden. Surprisingly most earwig species are omnivores, detritivores, and even predators in their natural environment. They are known to dine voraciously on aphids and, especially in trees, may be beneficial insects. In Europe, “earwig homes” are sometimes suspended in trees, a practice that may seem questionable to gardeners in the western United States. Where they are pests, earwigs can be controlled easily by trapping. Earwigs rarely fly, although many species have large membranous wings tucked under their short elytra (hardened forewings). Besides defense, they use their pincers to carefully fold the flying wings under the elytra.
The poor potato bug, or Jerusalem cricket, suffers from an identity crisis. It only incidentally eats potatoes, preferring to dine on decomposing vegetation and an occasional insect. The formidable mandibles reveal that it is not a true bug, which has piercing, sucking mouth parts. It isn’t from Jerusalem, but from the western United States and Mexico, where it is known as “nino de la tierra” or “child of the earth.” Potato bugs are large and harmless, but can deliver a painful bite when handled. In the garden, we sometimes unearth them in the compost or find them under pots and when digging in rich soil.
To encourage beneficial soil macroarthropods, let natural mulch accumulate under shrubs and trees, and take care to disturb it as little as possible. Many beneficial insects—for instance soldier flies and ground beetles—pupate in leaf litter and undisturbed soil. For plants such as roses and fruit trees that are prone to fungal diseases, spread a thick layer of mulch on top of the leaf litter instead of raking it up and throwing it away. As the autumn leaves fall in the shrub borders, I take pleasure in knowing that I am encouraging the next generation of beneficial insects that will greet me come spring.
In a Nutshell
Popular Name: Roly-polys, crickets, earwigs, ants, termites, soldier flies, antlions, spiders, millipedes, centipedes, bees, wasps, ants, aphids and other true bugs, moths, and many other macroarthropods.
Scientific Name: Phylum: Arthropoda Subphyla: Chelicerata – spiders, scorpions, sea spiders, horseshoe crabs. Myriapoda – centipedes and millipedes. Crustacea – brine shrimp, barnacles, crabs, seed shrimp and roly-polys. Hexapoda – insects and a few other small groups. Many taxonomic groups, especially among the chelicerata and crustacea, are not found in gardens.
Common Garden Species: Roly-polys and earwigs may be the most commonly spotted soil dwelling arthropods. Ants, Jerusalem crickets and other orthoptera, ground beetles, and spiders are also easily found. Some arthropod species spend only a part of their life in the soil, and are more easily found in above ground life stages, especially among beetles and flies. Soldier beetles, soldier flies, and sphinx moths all have ground-inhabiting life stages. Many species of bees and wasps nest in the ground, and termites and antlion larvae may sometimes be found in gardens.
Distribution: Many groups are widely distributed. Soils have characteristic arthropod fauna; unique soils such as serpentinite soils have a unique associated fauna.
Life Cycle: Varies. Insects with complete metamorphosis often occupy different habitats in different stages of life.
Appearance: Macroarthropods include those visible to the unaided eye.
Life Span: Varies. Many soil arthropods live one year or less. However, larval and pupal stages may take years in some species. For instance, cicada nymphs live in the soil for up to 17 years, depending on species.
Favorite plants: Many soil macro arthropods function as detritivores, and eat organic matter in various stages of decomposition. Often, once the soil microbial community has initiated the process of decomposition, macroarthropods are not choosy. They may eat where they live, and be found in leaf litter, manure, and compost piles, or sometimes deep in the soil. Many insect larvae eat roots; these are often specialists on particular plant species.
Benefits: Aid in creating pores for movement of air, water, nutrients and roots. In sufficient number, arthropods can change soil structure and even chemistry, and may contribute to soil nutrients.
Problems: Some soil dwellers are garden pests, for instance some larvae (crane fly and June bugs) eat roots, and cicadas and other homopteran species may attack plants.
Interesting facts: To attract mates, Jerusalem crickets play the ‘drum’ by beating their abdomen on the ground. Each species produces a characteristic ‘song’.
Sources: One way to encourage soil dwellers such as soldier beetles is to provide undisturbed mulch in places with nectar resources; natural leaf litter is best.
Life in the Soil: A Guide for Naturalists and Gardeners, by James Nardi. This book is a must-have for any gardener interested in soil dwellers (find Pacific Horticulture review here)
Soil food web: https://www.landfood.ubc.ca/soil200/index.htm