Garden Symphylans

Garden symphylans (also called garden centipedes) are not insects; they are in their own arthropod class called the Symphyla. When full grown, they are not more than 0.33 inch long and have 15 body segments and 11 to 12 pairs of legs. They are slender, elongated, and white with prominent antennae.

Symphylans are long lived; some adults may live several years. The adult females lay eggs in the soil that hatch into small versions of adult symphylans, but with fewer segments and legs. These early instar symphylans can easily be confused with white springtails, which can also damage seedlings. They move long distances in the soil, yet cannot tunnel through soil and must rely on existing soil pores. The primary food of symphylans is decaying organic matter, but they will actively feed on root hairs when available. Ideal soils for symphylans are those with good soil structure and high organic matter content—the same soil characteristics most growers strive for in their fields.

There is a normal cyclic nature to symphylan populations during the year, with active feeding phases and less active phases when numbers appear low and apparently are deeper in the soil profile. There may be a difference in the amount of damage seen in affected areas of the field from crop to crop because of these population changes, but once a problem is seen in an area of a field, it tends to reoccur over many years.

Symphylans may damage sprouting seeds, seedlings before or after emergence, or older plants. They feed primarily on root hairs and rootlets and their ability to injure the crop decreases as plants get larger, however, their pitting of older roots may provide entryways for pathogens. Transplants may be stunted by their feeding as they "prune" the new roots attempting to grow out of the transplant plug.

Management of symphylans has been difficult at best and largely depended on the use of soil insecticides. As the available soil insecticide registrations are diminishing, symphylans are becoming an increasing problem. Insecticides can be applied to infested soil, but their effect is limited because of the symphylan's ability to move deep into the soil. Soil insecticides may help in giving the plants a chance to establish in a protected zone. Careful soil tillage and moisture management may help reduce damage.

Numerous organisms prey and parasite on symphylans in the field including true centipedes, predatory mites, predaceous ground beetles, and various fungi; however, little is known about their ability to reduce symphylan numbers.

Symphylan damage is generally associated with heavier soils that are high in clay content and have good soil structure. Symphylans do not thrive in either compact soil or sandy soils because these soils do not provide them with adequate tunnels for their movement (symphylans cannot make their own burrows). Therefore, intensive tillage that breaks the soil aggregates and seals the soil pores is also suggested.

There is some evidence that packing down the soil surface after planting may reduce injury.

Flooding the soil has been used to control symphylans in some situations but has been unsuccessful in others. Flooding requires at least 2 to 3 weeks and is more effective in late spring or summer than in winter. Symphylans may be found more than 3 feet below the soil surface and flooding to this level in many soils is difficult to achieve. Even in the best circumstances, flooding will only reduce numbers that can be expected to increase when conditions are again favorable. Effectiveness of rotations with nonhost crops has not been studied in California.

Use biological and cultural controls in an organically certified crop.

Sampling for symphylans is difficult and visible detection of any symphylans often indicates a population large enough to cause economic damage. A sampling plan modified from one developed by researchers at Oregon State has proven very efficient and relatively easy.

1. Place thick slices of raw potato on the soil surface at the level at which moisture is clearly visible in the soil. Be careful when removing dry soil from the surface not to disturb the pores in the moist soil to prevent symphylans from reaching the bait. This can be done by raking the dry soil away with a lettuce knife, rather than slicing into the soil with a knife or spade.
2. Then cover the bait with a solid plastic dome to protect it from drying out. This plastic dome or cap must be large enough not to cause excessive heating of the area or to accumulate excess condensation. A 6 X 6 inch round white plastic pot with no drainage holes or a plastic cup is adequate. Cover with stone or soil to prevent its removal by wind.
3. Leave the bait in place for 24 to 36 hours
4. Remove the cover to count the symphylans, both on the potato slice and on the soil surface underneath. Count the soil surface first as the symphylans there will quickly hide.

If symphylan counts approach 75 per potato slice, complete stand loss may occur. Significant stand loss will occur even at lower symphylan numbers.

To control symphylans apply insecticide just before planting. Spot treatments may be adequate.