The garden symphylan is an occasional but very destructive pest of most field crops. Symphylans are not insects. They are more closely related to centipedes and millipedes. Garden centipede is another name commonly used for the garden symphylan.
Garden symphylan infestations are widely scattered throughout the Commonwealth. Most problems occur in the eastern half of the state. Very few infestations have been found in the northern tier and northwestern counties. Infestations seldom encompass an entire field, but rather comprise one or more small areas of 1/4 acre to several acres.
Usually, the first indication of a symphylan infestation is a relatively small area of stunted, unhealthy plants. In some areas, the plant stand may be only a fraction of what it should be. In the infested areas, crops may be completely destroyed or severely damaged. Crop losses continue in the same general areas of the field year after year, with the affected area increasing in size by about 10 to 20 feet each year.
The total number of infested acres in the Commonwealth probably does not exceed 5,000 acres. However, total production loss can be expected on the infested areas unless the symphylans are treated properly.
Garden symphylan damage on corn roots. Dennis Calvin collection.
Garden symphylan damage on soybean roots. Dennis Calvin collection.
Mature symphylans are white, slightly less than 1/4-inch in length, with 12 pairs of legs and a pair of long-beaded antennae. Their entire life is spent in the soil. Their life span is probably 1 to 2 years.
The tiny, round, white eggs are covered with a network of small ridges. They are laid in clusters of 10 to 12 several inches below the soil surface. The egg color becomes a light tan shortly before hatching.
The nymphs have 6 to 11 pairs of legs, depending upon age. They are white and resemble the adults except for size and number of legs.
Garden symphylan Dennis Calvin collection.
Symphylans overwinter in the soil as adults. In the spring, they move up into the top 6 inches when the soil temperature rises above 45 degrees F.
Eggs are deposited in soil crevices and tunnels in late April, May, and June. The eggs hatch 2 to 3 weeks later into tiny, white nymphs that resemble the adults in appearance except they have only 6 pairs of legs. As the nymphs develop, they increase in size and add a pair of legs at each molt until they have 12 pairs of legs. About 3 months are required to complete development from egg to adult. The adults remain in the upper 6 inches of soil until extreme dryness or cold weather drives them deeper into the soil.
Garden symphylans feed on sprouting seeds and underground stems of seedlings. Depending upon the extent of feeding, plants are weakened or killed. On alfalfa and corn, they chew off the hair roots and prevent development of a healthy root system. Infested corn plants often will produce a mass of numerous, fine roots in the areas that have been chewed.
On crops such as potatoes, beets, and carrots, symphylans will render the tubers or roots unfit for use by chewing out numerous shallow pits. Severe damage usually is confined to relatively small areas in a field. Uniform infestations throughout an entire field are not likely to occur.
Control of garden symphylans involves preventive treatment before or at time of planting. No rescue treatment can be used effectively while the crop is growing.
When poor crop growth occurs, the cause should be determined immediately. To check for symphylans, turn over at least 10 shovels of soil. Sift the soil while looking for active symphylans. An average of one symphylan per shovel of soil is a signal that it will be profitable to treat the infested area before planting the next crop. Mark off the infested area; the entire field need not be treated.
Conventional tillage permits the selection of either broadcast treatment and incorporation before planting, or band application over the row at planting. The broadcast treatment of the target areas is the preferred treatment, especially for non-row crops.
For reduced tillage and no-till methods, the only practical control is to use a granular insecticide banded over the row at planting.