Symphylans are not insects. They are more closely related to centipedes and millipedes. Garden centipede is another name commonly used for the garden symphylan. (DREVES, 2017) They feed on sprouting seeds, roots and other subterranean plant parts. Economic damage occurs from direct feeding on roots, rhizomes and tubers from establishment through plant maturity. Seedling death, poor growth, stunted plants, reduced vigor and yield reduction result. 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.
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.
Occurrence and Movement
Within a favorable soil habitat GS can migrate from the soil surface to a depth of over 3 feet. The soil profile, structure, composition 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 and tend to form a circular pattern covering a few feet to a number of acres. 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). (DREVES, 2017)
Factors regulating symphylan populations in agricultural soil systems are poorly understood, particularly the effects of farming practices such as cover cropping and reduced-tillage. Eliminating spring tillage may have increased symphylan populations but the effect of reduced tillage on symphylan populations was less important than cover cropping. Predaceous mites were more abundant in soil under large amounts of cover crop residue but these predators were not correlated with lower populations of symphylans. (PEACHEY, 2002)
Many Brassicaceae are eaten by symphylans and economic injury to crops such as broccoli is common. Using mustard crops and other brassica as trap plants may encourage symphylans to feed on those roots as opposed to the cannabis roots. Typically, the economic injury level of this arthropod is reached when ten or more symphylans are found per shovel full of soil, but this level is somewhat arbitrary and lacks a sound scientific basis. Alternative control measures to chemicals have been slow to develop, but some of the possibilities include, cultural control methods, host plant resistance, crop rotation, entomopathogens, including fungi and nematodes, and cover cropping and tillage practices.
Pergamasus quisquiliarum is a soil dwelling predator mite that has been shown to control symphylans as well. However, there is not a commercial variety available for purchase.
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.
- 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.
- 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.
- Leave the bait in place for 24 to 36 hours
- 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. (UC ANR Publication 3450)
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. (DREVES, 2017)
Recommended controls for symphylans
Each control will have its own set of parameters that will be best suited for individual environments. Certain controls may only be available for commercial application.
Commercial farmers are required to reference their own state laws to ascertain if the recommended controls fall within compliance of their states regulatory guidelines.
Mechanical and physical Controls
- Frequent monitoring if suspected as described above.
- 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.
- Cover crop treatments had a greater impact on symphylans than tillage system, but the effect of cover crop on symphylan populations was not correlated to biomass accumulation. The cause of cover crop treatment effects on symphylans might be explained by two hypotheses: (1) changes in the food resource provided to symphylans by the cover crop, or (2) indirect changes in symphylan density caused by predator/prey alteration of the entire foodweb. Cover crops have been shown to increase beneficial predator species such as Pergamasus quisquiliarum. Specialized OMRI cover crop blend for winter cover in soils that incur high disease pressure is:
- 30% Bracco White Mustard
- 30% Nemfix Black Mustard
- 10% Canola
- 30% Oriental Mustard
- 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. Beans and potatoes are rarely damaged even under high GS populations(Dreves, 2017).
- We have first hand evidence from AlpenGlow Farms who has planted potatoes in around the base of cannabis plants in large soil beds. They noticed a distinct reduction in symphylans activity and also were able to harvest potatoes along with the cannabis flowers, polyculture at its finest.