Classifying Arthropods

By: Jeff Lowenfels Wayne Lewis

Jeff Lowenfels is a garden writer and attorney in Alaska, and a leading proponent of gardening using the concepts of…

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Dust mites (Tyrophagus putrescentiae), 100x. Photograph by Eric Erbe

Dust mites (Tyrophagus putrescentiae), 100x. Photograph by Eric Erbe

Besides being food for other members of the soil food web, soil arthropods are important to the community as shredders, predators, and soil aerators. The presence or absence of certain of these key players can tell a gardener much about the health of soils and the plants growing in them.
Jeff Lowenfels and Wayne Lewis
Teaming with Microbes: A Gardener’s Guide to the Soil Food Web

Without more than a passing interest in them, most gardeners lump all the arthropods together as simply “insects” or “bugs.” Any given gardener may know a few of the popular and unpopular ones that inhabit local gardens but, for the most part, not many more. Part of the problem is that there are too many arthropods. The phylum Arthropoda is by far the largest in the animal kingdom—so large that it presents a real challenge for us: how can we, the authors, show you, the readers, how to use the soil food web without overwhelming you with information? There are just too many kinds of soildwelling arthropods to describe them all, or even come close to doing so, and, frankly, there is too much scientific nomenclature as well. Bear with us for the little that we do use.

Gardeners are agreed that using scientific names, usually derived from Latin or Greek, is truly the only way to accurately identify a plant; but most have not learned the alphabet soup of words scientists use to classify members of the phylum Arthropoda, whose members have the greatest impact on the soil food web. Here, we list the classes, as a start:

Class Arachnida: spiders, scorpions, mites, ticks, and daddy longlegs

Class Chilopoda: centipedes

Class Diplopoda: millipedes

Class Insecta: springtails, silverfish, termites, mayflies, dragonflies, damselflies, stoneflies, earwigs, mantids, cockroaches, walking sticks, grasshoppers, katydids, crickets, rock crawlers, web spinners, zorapterans, psocids, book lice, bark lice, chewing lice, sucking lice, scorpion flies, fleas, thrips, lacewings, ant lions, true bugs, moths, butterflies, flies, beetles, sawflies, bees, wasps, and ants

Class Malacostraca: sow bugs and pill bugs

You are already familiar with many members of the class Insecta. Tens of thousands of different kinds of insects live in and on the soil and plants, as few gardeners need to be reminded. Surely you have seen representatives of one order of this one class, the order Coleoptera (beetles), as you go about your gardening chores; with approximately 290,000 species described, it would be hard to miss them.

Millipede foraging on soil. Photograph by Frank Peairs, Gillette Entomology Club

Millipede foraging on soil. Photograph by Frank Peairs, Gillette Entomology Club

Soil Food Web Functions

Most soil arthropods, particularly those that reside on the soil surface, are shredders. They chew up organic matter in their constant quest for food, creating smaller pieces. As a result, fungal and bacterial activity is increased because shredding exposes surfaces on organic litter that give bacteria and fungi an easier avenue of attack.

As they shred and move about, arthropods also taxi microbial life attached to their bodies or in the debris they push or carry about. Since most arthropods are food for still larger animals, the total distances microbes can be moved (consider a bacteria colony eaten by a grub that is then ingested by a robin) can be truly great. Microbial activity is increased if the taxi takes its fare to a good food source. Still, it is the shredding that is most important. Two common arthropods, mites and springtails, are alone responsible for recycling up to thirty percent of the leaves and woody debris deposited on a temperate zone forest floor. In the face of insufficient dead organic matter, arthropods often attack living sources of organic nutrients. And even if the supply of available organic matter is abundant enough to satisfy any reason-able arthropod, some (mole crickets, root maggots, cicadas) subsist on roots anyhow. Fungus gnat larvae, for example, hatch and immediately start eating root hairs, eventually eating their way into the roots and stem to the great detriment of the invaded plant. Still other arthropods eat other members of the soil food web in order to survive; by removing their fellows, these predator arthropods make room for other arthropods to fill the emptied niche, helping to create complete digestion of soil matter. Finally, in much the same way protozoa and nematodes do, some arthropods eat fungi, others bacteria, but this time releasing nutrients on a larger scale, befitting their greater numbers and size.

Female Mormon cricket. Photograph by Michael Thompson, USDA-ARS

Female Mormon cricket. Photograph by Michael Thompson, USDA-ARS

Many arthropods carry out their daily routines only on the surface of the soil. A surprising number, however, live at least part-time below the soil surface. As these arthropods go about their business, they mix and aerate soil; their waste products also add organic matter.

Excerpted from Teaming with Microbes: A Gardener’s Guide to the Soil Food Web, an important new book from Timber Press (www.timberpress.com) reviewed by Frédérique Lavoipierre on page 14. Frédérique will soon begin a series of articles for Pacific Horticulture on insects in the garden.