Soil Arthropods

The soil under your feet and in your garden is [or should be] teeming with life. Worms, roots, microorganisms, and insects call the soil home. The insects are called arthropods and they play a major role in soil health and plant vitality.

200 microscopic mite species from 1 sq. ft. of the top 2” of forest litter and soil (USDA, Public Domain)

In a single square yard of topsoil, there may be 500 to 200,000 individual arthropods.

What are arthropods?

Arthropods get their name because they have paired, jointed (arthros) legs (podos). Arthropods are invertebrates, which means they do not have a backbone. Instead, they have a hard outer covering, known as an exoskeleton or cuticle, made from chitlin. Arthropods range in size from microscopic to a few inches long. As they outgrow their exoskeleton, it is shed by molting.

Sowbug

Soil arthropod species

There are four types of arthropods with many familiar members:

• arachnids - mites, spiders
• crustaceans - sowbugs
• insects - ants, beetles, springtails
• myriapods - centipedes, millipedes, scorpions

Arthropods are commonly grouped according to their feeding habits. There are fungal-feeders, herbivores, predators, and shredders.

Fungal feeders

Arthropods that feed on fungi and bacteria include silverfish and springtails, and a few mite species. As they feed, they scrape the fungi and bacteria from the surface of plant roots. As these microbes graze and poop, they make many mineralized nutrients available to plants. Fungal feeding arthropods and the fungi they feed on tend to keep each others' populations in check. 

Silverfish (Barb Sendelback, Public Domain)

Herbivores

Cicadas, mole crickets, root maggots (anthomyiid flies), rootworms, and symphylans (garden centipedes), feed on plant roots and can become major pests.

Garden centipede, a pseudocentipede (Soniamartinez) CC BY-SA 4.0

Predatory arthropods

Predatory arthropods can be generalists or specialists, eating many types of prey, or only one. Ants, centipedes, ground beetles, pseudoscorpions, rove beetles, scorpions, skunk spiders, spiders, and some mites can be predators, feeding on nematodes, springtails, other mites, and insect larvae.

Rove beetle (Daniel Ullrich) CC BY-SA 3.0

Shredders
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Shredders tend to be larger and may be seen on the soil surface. They feed on decomposing plant material and the fungi and bacteria growing on those dead plants. As they feed, they shred the plant material, increasing its surface area and speeding its decomposition. This group includes millipedes, roaches, sowbugs, termites, and some mite species. When dead plant material is not available, shredders can become pesky root-eaters.

Millipede (Gillian Floyd, Public Domain)

Beneficial arthropods

As arthropods feed and burrow, they provide many benefits to soil health. Moving through the soil, they aerate and gently churn it, improving porosity, water infiltration rates, and bulk density. As they feed, they shred organic matter, speeding decomposition. And when they excrete waste products, they release mineralized plant nutrients and enhance soil aggregation because their waste is coated with mucus. Their feeding also curbs the populations of other soil organisms and opens the way for a wider variety of other, smaller decomposers. 

Arthropods often carry around beneficial microbes, in a method known as phoresy, on their exoskeletons and in their gut. These microbes end up helping decompose far more organic matter than they might have, left to their only very tiny devices. 

You can help beneficial soil arthropods in your garden by avoiding the use of broad-spectrum pesticides, employing no-dig gardening methods, and installing a wide variety of plant species. Since most soil arthropods live in the top 3” of the soil, the use of stepping stones, stumperies, rain gardens, and water features will all help provide the food, shelter, moisture, and biodiversity needed for healthy arthropod populations.

Which arthropods are in your soil?

You can create two different types of traps to learn more about the arthropods in your soil. Pitfall traps can be used to collect larger arthropods. You can make a pitfall trap with a large yogurt container sunk into the soil so that the rim is at soil level. Add 1/2” to 1” of rubbing alcohol or non-hazardous antifreeze to the container to act as a preservative. If rain is expected, or to keep out curious pets, you can create a makeshift roof over your pit trap. Leave it in place for one week.

The other trap is called a Tullgren or Burlese funnel. Burlese funnels use heat to dry a sample, forcing populations through a screen and into a waiting pool of alcohol. You can make your own using a jug, a funnel, a piece of screen, and a light bulb. Add 1/2” to 1” of alcohol to the jug before using your finger to push the screen material part way into the jug neck. Then insert the funnel. Place a soil sample in the funnel. Hang a lit 40 watt lightbulb or headlamp 4” to 6” above the sample for 4 or 5 days.

​Let’s us know what you find in the Comments!​

Burlese funnel (Wikipedia) CC BY-SA 4.0