Franklin Dlott, UC Cooperative Extension research assistant for Monterey County, has been refining sampling techniques for the pest as he looks for answers.
Slender, whitish in color, and no longer than a half-inch when mature, a symphylan in a single day can move 10 feet in loose soil and consume three times its weight in roots.
Not a true centipede, the species is in an arthropod class by itself and has a dozen pairs of legs. Other characteristics are its erratic, "nervous" movements and the ability to survive five years in a laboratory.
It prefers root hairs and fine roots and the feeding stunts young plants. It may also feed on older roots, creating entry sites for soil pathogens. It avoids light and moves at night to feed on leaves above ground. It goes to lettuce, cabbage, peppers, and other Salinas Valley crops.
Dlott told growers and PCAs at a recent meeting in Salinas he is trying to find a management practice to fend off symphylans until young plants start growing rapidly and producing enough roots to overwhelm the pests’ feeding damage.
Growers and PCAs say, regardless of location, attempts to control it have inconsistent results. "One method may work for one grower or location," Dlott noted, "but not for another. On the basis of my research, I have to agree."
He began the project using a sampling method, developed by researchers at Oregon State University, that uses slices of potato as bait. The slices are gently placed in a field on a porous soil surface free of compaction that might stop movement of the symphylans. The pests are captured in plastic pots placed over the bait and counted.
"Placing these bait stations is more art than science, but we found that we can correlate counts of symphylans to damage. Twenty per bait station is enough to stunt plants at 35 days after planting, and 68 per station can destroy plants," Dlott said.
The baiting, he said, revealed circular patterns of damage within a test plot. He also noticed that where an irrigator’s footprint compressed the soil and prevented symphylan movement, plants apparently emerged unharmed.
Once the crop is planted in an infested field, Dlott said, little can be done other than chemigation with pyrethroids through sprinklers or accepting losses.
But several preplant options might be considered. Oregon growers have had some success with avoiding planting during June and July when the pests there are more active. Dlott said the time of their peak activity in the Salinas Valley, however, is not yet known.
Soil fumigation is an option but is clouded by the phase-out of methyl bromide and inconsistent results.
Incorporation of diazanon has been a typical practice for symphylans and other soilborne pests in the Salinas Valley, but according to Bill Chaney, Monterey County entomology farm advisor, the future availability of diazanon is in question. Its granular form has been discontinued and a phase-out of its sprayable formulation has been proposed.
Dlott said his anecdotal observations suggest tillage might be an answer where the pest population is small.
Use of treated seed or pesticide-drenched transplants are also possibilities.
Regardless of which method is attempted, Dlott reiterated, inconsistent results can be expected. "To be successful, the treatment has to be in the right place at the right time for the particular field."
His trials near San Juan Bautista in 2003 with several insecticides, banded along the seedline, were inconclusive. He said he plans this year to follow the methods used by Benny Fouche, farm advisor for San Joaquin County, where symphylan has been a problem in transplanted tomatoes.
Fouche, who recorded some success with the unregistered product Force in trials with a group of materials, sprayed bands on the plant line, incorporated them into the soil, and set out tomato seedlings.
Performance of materials, optimum rates, band widths, and timing of applications remain questions for Dlott. "Part of the problem," he added, "is that the materials we will be using degrade relatively rapidly, so we have only a small window of protection."
Another characteristic of symphylans is their tendency to appear year after year in the same field, but, he said, they are also quite mobile.
"Don’t think a farm roadway will stop them. The soil under it may be compacted but not deeply enough to prevent their movement. Earthworms can make the soil porous enough for symphylans to slip through."
Practical experience in the Sacramento-San Joaquin Delta demonstrated that flooding of fields halts these movements, but such use of water is not an option for Salinas Valley growers.
Symphylans have several natural enemies, including true centipede, predatory mites, predaceous ground beetles, and certain fungi. Little is known about their effect, although they are not considered significant controls.