Introduction
Garden symphylans—Scutigerella immaculata (Newport) (GS)—are centipede-like soil arthropods which infest many home gardens and agricultural soils in western Oregon and Washington. Symphylans are omnivores, feeding on germinating seeds, seedlings, roots, plant parts in contact with the soil, and other organic material including decaying plants and fungal hyphae. Poor stand, seedling death, poor growth, stunted plants, reduced vigor, and yield reduction result. Recognizable damage is typically a series of holes chewed in the host plant. Chronic feeding on the roots of both annual and perennial plants reduces a plant’s ability to acquire water and nutrients. This results in a poor root system that manifests as general stunting and distortion of plants as well as increased susceptibility to plant pathogens. Sampling and control of GS is complicated by daily and seasonal vertical movement in the soil which is influenced by soil structure, soil moisture, temperature, time of day, season, crop stage, and their feeding cycles.
Conventional growers, organic growers, and small-scale gardeners often approach symphylan management from different perspectives, primarily due to economic and scale-dependent factors. Selection of appropriate tactics to manage GS is largely determined by the cropping system (no-till versus tillage), and use of soil applied insecticides. However, in all systems, effective management results from accurate identification of GS and the damage they cause, a general knowledge of their ecology, as well as appropriate sampling methods and control strategies. Correct diagnosis of a GS problem is sometimes tricky, since damage may be atypical and GS are not always easy to find when damage is observed.
Identification
Symphylans are soil-dwelling myriapods, not insects. They are found worldwide but are poorly described, with only about 160 total species. The class Symphyla contains two families: Scutigerellidae and Scolopendrellidae. Garden symphylans (GS) belong to the family Scutigerellidae, which is characterized by large dorsal tergites that have rounded or slightly lobed posterior margins. Several species occur in Oregon, but the S. immaculata is the primary species causing crop damage in the U.S. GS are by far the most common symphyla species found in PNW agricultural systems.
Garden symphylans are white and “centipede-like”, measuring about 0.25 inch long when mature. They have 6 to 12 pairs of legs (depending on age), which make them easy to differentiate from common soil insects which only have 3 pairs of legs. Due to their subterranean habitat, symphylans are blind but are able to move quickly though the soil, especially when disturbed. Though their color may vary depending on what they have eaten, they are generally paler and smaller than true centipedes, which are also soil arthropods with many pairs of legs (one pair per body segment). Another similar group of soil arthropods are millipedes, but these myriapods are generally slower moving and possess two pairs of legs on each body segment.
Garden symphylan biology
Symphylans exhibit incomplete metamorphosis. Stages include eggs, immatures, and adults, which can be found together throughout most of the year. Males deposit sperm packets on stalks and on the soil where the female will pick them up and place them in special glands in her mouth. Eggs are found in groups of four to 25 and are pearly white to tan, spherical with hexagonal shaped ridges. Immediately after each egg is laid, she places them in her mouth where they are fertilized before being laid in a cluster, approximately 12 inches deep. Temperature plays a key role in regulating oviposition, and the greatest numbers of eggs are most commonly deposited in the spring and fall. First instars emerge from the egg with only six pairs of legs and fewer body segments than adults. Newly hatched GS may resemble subterranean springtails, but differ in that a pair of legs are attached to most segments, rather than just the thorax, as in the springtails. Symphylans also possess bead-like antennae with numerous segments, compared with the usually 4-segmented antennae of springtails. The GS has an exoskeleton and, like an insect, sheds it (molts) periodically to grow and enlarge body size. Each of the six subsequent molts results in the addition of a body segment with a pair of legs and more antennal segments. Total time from egg to sexually mature adult (seventh instar) is about 2 to 3 months during typical spring soil temperatures in western Oregon. Two complete generations per year can occur.
Occurrence and movement
Garden symphylans are generally a problem in irrigated crops grown on alluvial soils with very good soil structure. Within these soils, GS tend to occur in circular “hotspots” encompassing a few square feet to several acres. Hotspots often remain consistent from year to year with little change in populations and only minor lateral spread.
Within a favorable soil habitat GS can migrate from the soil surface to a depth of over 3 feet. Symphylans are not capable of making their own burrows but instead their movement depends on cracks and crevices in soil and runways created by roots. The soil profile, structure, composition, water table and water holding capacity, determines the depth to which GS migrate. Vertical migration is primarily related to interactions among moisture, temperature, crop stage and endogenous feeding cycles. A general understanding of these interactions is important both for timing and interpreting sampling efforts, and for selecting management tactics.
Garden symphylans tend to aggregate in the top 6 inches of soil when the soil is moist and warm in the spring and fall. They move to deeper soil strata during July and August, though can stay at the surface if sufficient moisture is present and no plants are growing. Garden symphylans migrate to the root zone to feed, then return to the deeper strata to molt, evidenced by the large number of molted skins that may be observed in these strata. Since migration is not entirely synchronized within a population, GS are usually present throughout the habitable portion of the soil profile. Presence of GS in the surface soil may also be influenced by other variables that impede movement, such as tillage and compaction from heavy objects (such as tractor tires).
Sampling
Many of the difficulties in effectively managing GS result from unknowns concerning the density and location of populations in a field. Sampling, although often time-consuming, can provide information critical to managing populations effectively. For annual crops, sampling is commonly conducted in April, May, or June, prior to planting. GS will generally be visible in upper six inches of the soil when air temperatures are above 45°F. In general, the later in the spring that sampling occurs, the more GS will be found in the soil. Samples that include crop or weed roots generally contain more root-feeding GS than those taken in bare soil. The type and extent of sampling may vary depending on the site conditions (e.g., vegetation, size of area, cropping history), and whether populations have been historically problematic in certain areas of a site.
Three main sampling methods are used: baiting methods, soil sampling methods, and indirect sampling methods. Each method has benefits and drawbacks, and the selection of a sampling method will vary depending on the objectives of the sampling (e.g., detection vs. precise population density estimation), time of year, and site conditions.
Part of the difficulty in sampling is a result of the patchy spatial distribution of GS populations. It is important to be aware that an individual sample unit count provides information about a local region within which that sample unit was taken. Counts will often vary from zero to more than 50 GS per sample unit (i.e., soil core or bait). To obtain information about the spatial patterns of the population, sample units are often taken in a grid pattern. Areas with different cropping histories are generally sampled independently.
In most cases, sampling only measures the density of GS in the surface soil. Therefore, sampling should only be conducted when GS are within this region. The best sampling conditions are, generally, when the soil is warm and moist. Sampling within 3 weeks after major tillage, such as disking, plowing, or spading may not reflect the true population because GS often have not had ample time to reestablish in the surface soil.
To detect or identify a GS problem in a crop, bait for GS in suspected areas within 3 weeks of planting. To sample seedlings or established plants, dig them up in the early morning when GS are close to the soil surface. Inspect their roots, along with those of grassy weeds in the area, for evidence of nibbling. Check for presence of neatly chewed round holes in crowns of grasses, as well as the soil around the roots.
Soil sampling is the standard/historic method for estimating how many GS are in a field (i.e., approximate number of GS per unit of soil, or population density estimate). Sample unit sizes vary; the most common soil sample units are 6 x 6 x 12 inches (length, width, depth) or cores of 2.5 inches in diameter by 6- to 12-inch depth. When soil samples are taken, the soil from each sample unit is usually placed on a dark piece of plastic or cloth where the aggregates are broken apart and the GS are counted. Sampling is usually conducted when GS are present in the top 6 to 12 inches of the root zone.
Bait samples are generally much faster to take than soil samples, but they are also more variable and more sensitive to factors such as soil moisture, temperature, and presence of vegetation. To bait a sample, dig into the soil until moist soil is exposed and place one-half of a sliced potato on the soil surface with exposed cut area of potato against the moist soil surface. Shelter with a protective cover (e.g., white pot or 4-inch PVC cap). Baits are generally checked one to three days after placement. Lift the bait, counting the GS on the soil first, then the GS on the potato bait. During warm and/or dry conditions, baits are generally checked one to two days after placement as counts decrease if baits are left to dry out. In cooler conditions, baits are commonly left out for three to five days. Bait sampling works very well for some applications, though it cannot be used during heavy rains, as the bait stations will fill with water. Baiting works best at least two to three weeks after tillage, when the soil has stabilized but before plants are well established. Therefore, both soil sampling and the bait method can be used to determine the presence/absence of GS.
Plant growth can sometimes be a useful, yet indirect measure of GS populations and is often a good starting point for assessing GS populations. Indirect measures, however, should never be used without some direct sampling to confirm the presence of GS.
Determining the number of samples
Sampling requirements will often vary by site, depending on factors such as cropping history and time of year. Sampling involves establishing a balance between the need to be confident about estimates of the number of GS present (implying a large number of samples) and not investing excessive time and energy into the sampling endeavor (implying a small number of samples).
Follow these guidelines for determining the sample size:
- Sampling for low population densities (e.g., early in the spring or of highly susceptible crops) requires a greater number of sample units (e.g., 100+) than sampling for high population density (e.g., 30 GS/foot) as smaller population clusters are more difficult to detect. Ten samples may be enough to confirm that a high population density exists.
- As the variability of the sampling method increases, so does the number of sample units required. Since the baiting method is more variable than the soil sampling method, two to three times more bait than soil sample units are required.
- For estimation of “economic” population densities in moderately susceptible crops, at least 35 soil sample units, or at least 50 bait units, are commonly used. Depending on the size of the field, and the time of year, considerably more sample units are sometimes used.
Action thresholds
Management decisions, such as those regarding pesticide applications and the intensity of tillage, are sometimes made based on pre-plant GS population density estimates. Owing largely to the difficulty in sampling and the numerous crops to which GS are pests, action thresholds for individual crops are not well developed. The relationship between GS population density (estimated by sampling methods) and crop health is often influenced by several factors, including tillage intensity, crop species, planting date, and crop stage.
In the field, noticeable damage has often been observed if populations exceed an average of five to ten GS per cubic foot (or 1 to 2 GS per 6 x 6 x 12-inch sample) in moderately to highly susceptible crops, such as broccoli, squash, spinach, and cabbage. In conventional cropping systems, pesticides are often applied to susceptible crops if populations exceed three GS per cubic foot. In more tolerant crops, such as potato and small grains, GS feeding may not lead to significant damage, even at considerably higher population densities.
Management and control
For management purposes it is important to make a distinction between tactics that may decrease GS population and those that may reduce crop damage but not necessarily reduce pest populations. In most cases, effective GS management involves establishing a balance between these two tactics. It is important to note that in most cases little can be done without replanting after damage is observed. Sampling is, therefore, important in determining the proper course of action.
Tactics for population reduction
No simple, inexpensive, and completely reliable method of controlling GS has been developed. No method will eradicate GS from a site, and the effect of most tactics will not last longer than one to three years. Very little is known about symphylan population dynamics in agroecosystems due to the complexity of their movements up and down in the soil profile. Many control tactics have been successful in some cases but unsuccessful in very similar situations.
Tillage is probably the oldest control tactic used and is still one of the most effective. Tillage can physically crush GS, thus reducing populations. Tillage may also decrease populations of key GS predators such as centipedes and predaceous mites. However, in annual crops, benefits of increased predator populations in reduced tillage systems have not been shown to be as effective as tillage in decreasing GS populations. In general, for most effective control, till when the GS are in the surface soil, and when soil moisture allows preparation of a fine seed bed. Since only a portion of the population is in the surface horizon, tillage never provides complete control; however, surface populations are commonly significantly lower for at least two to three weeks after tillage. Research suggests that symphylans are more often associated with unbroken down organic matter with good soil structure rather than in compact or sandy soils. There is some evidence of reducing populations and injury by packing down the soil surface after planting; and flooding areas for 2 to 3 weeks.
In conjunction with tillage, pesticides are used to manage GS. Plant protection is probably achieved by direct mortality as well as by repelling GS from the root zone. The use of pesticides has been effective to some degree in conventional systems, but many growers still have perennial problems with symphylans. Pesticides are most effective if applied before planting as broadcast and incorporated applications. Banded/incorporated applications may provide acceptable protection for some crops. In some perennial crops, such as hops, post-plant pesticide applications can reduce GS sufficiently to promote plant vigor. Fumigants, organophosphate, and carbamate pesticides have historically been the most effective, but many are no longer registered for GS in many crops. Pyrethroid pesticides generally do not provide as high a level of control. Soil-applied organophosphate insecticides (e.g., Mocap, Lorsban Advanced) have been effective at protecting crops sufficiently from GS, although chemistries containing chlorpyrifos will be banned on food, forage and seed crops after December 31, 2023. Soil fumigation, when properly performed, can reduce symphylan populations enough to allow 3 years or more of crop production with no additional control efforts during that period. Refer to individual crop sections for current registrations.
Insecticide registration is continually changing: always check specific insecticide labels for current registered uses. The following may have registered insecticides for symphylan control: asparagus, snap bean, table beet, blueberry, blackberry, broccoli, Brussels sprout, cabbage, cantaloupe, cauliflower, carrots, celery, chickpea, sweet corn, cucumber, orchard floors, garlic, lettuce, pepper, potato, pumpkin, rhubarb, spinach, sugar beet, hop, mint, strawberry, silage and feed corn, clover, grass seed, wheat, barley, radish seed, sugar beet seed, home garden vegetables, home garden strawberries, and home landscape plants.
Crop rotation may partially explain seemingly sudden shifts in GS populations. While GS feed on a wide range of plants, and can even persist in fallow soil, plants vary greatly in their suitability for GS population development. Populations have been shown to decrease significantly in potato crops, even allowing subsequent cultivation in rotation of susceptible crops. Though at this point no other crops have shown to be nearly as effective as potato, numbers have also been found to be lower after a spring oat (‘Monida’) winter cover crop than after a mustard (‘Martiginia’), barley (‘Micah’), or rye (‘Wheeler’) winter cover crop. Mustard and spinach crops have been shown to be very good hosts and may lead to increasing populations in some cases.
Little information is available on the effect of natural enemies on symphylan populations, which include fungal pathogens, predaceous mites, ground beetles, centipedes, and spiders. Large numbers of small brown centipedes, similar in size to symphylans, have been observed aggressively attacking symphylans and their presence could indicate presence of symphylans.
Tactics for crop damage reduction
Most plants can tolerate some level of GS feeding during all or part of the growing season, and numerous tactics can be used to grow healthy crops successfully in GS-infested soil. These tactics can be classified as those aimed at 1) reducing crop damage under high GS populations and 2) reducing the number of GS on crop roots during establishment, when plants are often most susceptible.
Susceptibility to GS feeding can vary dramatically among different soil types, plant species and crop cultivars. Generally, smaller seeded crops tend to be more susceptible than larger seeded crops. Commonly damaged crops include broccoli and other brassica crops, spinach, beets, onions, carrots, corn, and squash. For some crops (e.g., squash), damage can be reduced by increasing the plant density. This can dilute the number of GS per plant and increase survival of young seedlings during highly sensitive stages. The stand can be thinned after establishment, if needed. Beans and potatoes are rarely damaged even under high GS populations. Perennial crops, such as strawberries, raspberries, blueberries, hops, and bare root trees can also be damaged, particularly during establishment. Within a crop, susceptibility is often related to the stage of the crop planted. For example, direct-seeded tomatoes are generally more susceptible than transplants. Broccoli transplants, conversely, often fail to establish under high GS populations.
Garden symphylans are quite active and surprisingly mobile for their size, moving vertically for soil moisture and temperature fluctuations, and horizontally through the soil profile. They rely on soil pores and channels made by roots and other soil organisms in order to move through the soil. Therefore, access to roots is strongly correlated with soil structure, bulk density, or “fluffiness” of the soil and pore connectivity. Some tactics focus on temporarily reducing the number of GS in the surface soil, then planting, thus allowing these plants to establish while GS densities are low. Sandier soils are less likely to contain harmful quantities of symphylans
Tillage is an important tactic for decreasing populations in the surface soil. Along with directly killing garden symphylans, tillage breaks apart soil aggregates, modifying soil pores and pore connectivity. The effects of tillage may vary with the type of implements used. In general, the more disruptive the tillage the greater effect it will have on GS movement and feeding. Plowing or disking, followed by thorough preparation of a fine seedbed with a rototiller or roterra, often reduces surface feeding GS populations for two to three weeks. Light rolling, with a landscaping roller or similar implement, is used under some conditions to reduce the size and/or number of macropores, thereby restricting GS movement.