Description of the Pest
Garden symphylans are slender, white arthropods, closely related to insects, about 0.33 inch (8 mm) long, with 10 to 12 prolegs and distinct antennae. These fast-moving arthropods live in soil and move up and down in the soil profile with the moisture. They run when exposed to light. They occur mainly in soil with high organic matter and can cause considerable damage in asparagus plantings.
Damage
Garden symphylans cause injury by chewing large numbers of small, round holes in storage roots, crowns, and on the belowground portion of the spears. They also predispose the asparagus plants to additional damage from disease organisms (Fusarium, Phytophthora, etc.) that invade the wounds they create. The insects are a particular problem during periods of extended wet weather in northern California production areas, primarily the Delta, or on water-saturated soils. A good indication that these insects are present is circular areas in the field or along edges of the field in which there is little or no asparagus or weed growth.
The practice of mounding soil against the spears to produce white asparagus increases the damage potential of this pest. Since the demise of the white asparagus industry in California, crop loss has been reduced.
Monitoring with bait traps and examining harvested spears can help detect the presence of symphylans, although no treatment thresholds have been developed. Cultivation and flooding may provide control.
Cultural Control
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 (when fields are fallow), 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.
Cultivation to dry out the surface soil of the beds has reduced injury by driving the insects deeper into the soil.
Organically Acceptable Methods
Flooding fields before planting or in winter, and cultivation, are both organically acceptable control strategies.
Monitoring and Treatment Decisions
Confirm symphylan presence by digging a few feet below the surface with a shovel and examining moist soil. Research from other areas of the country and in other California crops such as lettuce and tomato indicate that symphylans can be detected with bait traps, but their success may depend on location, weather, and soil conditions. Symphylans are most easily detected in moist, warm soil. Baits placed on dry soil or on very hot or cold soils are unlikely to attract symphylans even when present, as symphylans migrate deeper in dry or hot soils and are less active in cold soil. Scrape off the top layer of dry soil to expose moist, firm soil. Cut a potato in half longitudinally and scratch the cut surface just before placing it on the soil to ensure that the surface is moist. Use half of a potato, not a sliver or a chip. Cover the bait with an opaque pot or cup to block out light and air, and mark the spot with a flag. Symphylans have a spotty (aggregated) distribution pattern, so use at least 30 bait traps per field. After 1 to 5 days, examine the cut potato surface and the soil it was resting on for evidence of symphylans.
Treatment thresholds are not well defined for symphylans in asparagus. If symphylans are present, fumigate or plant the field with a different crop. Because symphylans are likely to occur in the same areas over many years, these pests are very difficult to manage in permanent crops like asparagus.
Symphylans may be detected in established fields by examining the base of the harvested spear for small punctures. Also, if weak areas appear in fields and weeds are not present in that portion of the field but are doing well in other areas, suspect garden symphylans.
Pesticides and Natural Enemy Releases
Not all registered pesticides are listed. The following are ranked by their IPM value, with the most effective and least harmful to natural enemies, honey bees, and the environment listed at the top of the table. When choosing a pesticide, consider information related to water and air quality, resistance management, and the pesticide's properties and application timing. Use PestManage to compare summarized management options for different pests in the same crop. Always carefully read the label of the product being used and take all necessary precautions when handling pesticides.
| Rank | Active ingredient | Example trade name | Group | Group Order | MoA 1 | Amount per acre | REI (hours) ‡ | PHI (days) ‡ | Comments | Selectivity 2 | Bees 3 | Predatory mites 4 | Predators 5 | Parasitoids 5 | Residue duration 6 | Leaching(fish) 7 | Adsorbed runoff(fish) 8 | Solution runoff(human) 9 | Leaching(human) 10 | Solution runoff(human) 11 | Last updated 12 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Telone II | Preplant | 1 |
8A |
Label rates | See label | NA | Fumigants such as 1,3-dichloropropene ar... | — | — | — | — | — | — | low | low | low | intermediate | high | 08/2019 |
Legend
- ‡ a b Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without personal protective equipment. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases, the REI exceeds the PHI. The longer of the two intervals is the minimum time that must elapse before harvest.
- * a Permit required from county agricultural commissioner for purchase or use.
- § ↩ Do not exceed the maximum rates allowed under the California Code of Regulations Restricted Materials Use Requirements, which may be lower than maximum label rates.
- 1 ↩ Group numbers for insecticides and miticides are assigned by the Insecticide Resistance Action Committee (IRAC). Insecticides with unknown modes of action are assigned mode-of-action group numbers (MoAs) that begin with UN. Rotate pesticides with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B.
- 2 ↩ Range of insect and mite groups affected by a pesticide. Broad means the pesticide affects most groups of insects and mites; narrow means the pesticide affects only a few specific groups.
- 3 ↩ Risk of harm to honey bees. For more information, see Bee Precaution Pesticide Ratings.
- 4 ↩ Risk of harm to predatory mites. Toxicities are generally to western predatory mite, Galendromus occidentalis. Where differences have been measured in toxicity of the pesticide-resistant strain versus the native strain, these are listed as pesticide-resistant strain or native strain.
- 5 a b Risk of harm to parasitoids and general predators. Toxicities are averages of reported effects and should be used only as a general guide. Actual toxicity of a specific insecticide depends on factors including the application rate, environmental conditions, and the life stage and species of a parasitoid or predator.
- 6 ↩ Length of time residue affects natural enemies. Short means hours to days; moderate means days to 2 weeks; and long means many weeks or months.
- 7 ↩ Risk of harm to fish from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 8 ↩ Risk of harm to fish from adsorbed runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 9 ↩ Risk of harm to fish from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 10 ↩ Risk of harm to humans from leaching, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 11 ↩ Risk of harm to humans from solution runoff, based on USDA Natural Resources Conservation Service Windows Pesticide Screening Tool (WIN-PST).
- 12 ↩ Date information was last updated in the UC IPM Pest Management Guidelines.
UC IPM Pest Management Guidelines: Asparagus
UC ANR Publication 3435
E.T. Natwick (emeritus), UC Cooperative Extension Imperial County
B.J. Aegerter, UC Cooperative Extension San Joaquin County (European Asparagus Aphid)
Acknowledgement for Contributions to Invertebrates
R.J. Mullen, UC Cooperative Extension San Joaquin County
W.E. Chaney (emeritus), UC Cooperative Extension Monterey County
C.F. Fouche, UC Cooperative Extension San Joaquin County
