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.
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.
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.
|Common name||Amount per acre**||REI‡||PHI‡|
|(Example trade name)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Telone II)||Label rates||See label||NA|
|COMMENTS: Fumigants such as 1,3-dichloropropene are a source of volatile organic compounds (VOCs) but are minimally reactive with other air contaminants that form ozone. Fumigate only as a last resort when other management strategies have not been successful or are not available.|
|(Lorsban 15G)||10 lb||24||180|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: Lorsban 15G used as a postharvest treatment. Limited to ground application. Avoid drift and tailwater runoff into surface waters. Additional application restrictions may apply; for more information on current California permit restrictions, see the Department of Pesticide Regulation's Chlorpyrifos Interim Recommended Permit Conditions.|
|**||See label for dilution rates.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|1||Rotate insecticides 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. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|