Garden Symphylans

Garden symphylans (also called garden centipedes) are not insects; they are in their own arthropod class Symphyla. When full grown they are not more than 0.5 inch long and have 15 body segments and 11 to 12 pairs of legs. They are slender, elongated, and white with prominent antennae.

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.

Symphylan damage is generally associated with soils that are high in organic matter 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). There is some evidence that packing down the soil surface after planting may reduce injury.

Flooding has been used to control symphylans in some situations but has been unsuccessful in others. Flooding requires at least 2 to 3 weeks, is more likely to be effective in late spring or summer than in winter, and is probably most effective where there is a high water table. Symphylans may be found more than 3 feet below the soil surface and flooding to this level in many soils is difficult. Even in the best circumstances, flooding will only reduce populations; they can be expected to increase when conditions are again favorable. Effectiveness of rotations with nonhost crops has not been studied. Soil fumigation can kill populations in the upper soil levels; eventually, however, the soil will be reinfested by populations deeper in the soil.

Numerous organisms prey on symphylans in the field including true centipedes, predatory mites, ground beetles, and various fungi; however, little is known about their effect on symphylan populations.

Cultural practices such as packing the soil surface after planting and flooding are suitable for organic crops.

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. 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; disturbing the pores in the moist soil will prevent symphylans from reaching the bait. To avoid such disturbance, rake the dry soil away with a lettuce knife, rather than slice into the soil with a knife or spade. Then cover the bait with a solid plastic dome to protect the bait from drying out while it is allowed to attract symphylans. 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 styrofoam cup is adequate. Leave the bait in place for 24 to 36 hours and then 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 at lower symphylan populations.

Infested soil can be treated with insecticides, but their effect is limited because of the symphylan's ability to migrate deep into the soil. Insecticides may help in giving the plants a chance to establish in a protected zone. Treat for symphylans just before planting. Spot treatments may be adequate.