Many PCAs and growers are refining use of potato baits as an aid to management of tiny, root-feeding pests on lettuce and other vegetable crops along California's Central Coast.
Bill Chaney, Monterey County farm advisor, says symphylans, also known by the scientific name Scutigerella immaculata, have been a lingering concern for coastal growers since the 2002 season.
For the 2005 season he will continue evaluating several chemical controls, but for the moment the only defense is to avoid planting in fields known to be infested or to change the crop or timing of planting.
That's where the potato-bait technique can be useful to show the presence of the white arthropods, which are about a third of an inch long and have a dozen pairs of legs. They are highly mobile in soil several feet below the surface and are therefore hard to reach with materials.
Damage from their feeding on root hairs shows as loss of vigor in new fields at about the first true-leaf stage. From that point the infested area of the field continues to decline. Stands on the coast are affected from March into August, so growers are avoiding planting susceptible crops in infested areas during the late spring.
“We see this not only with lettuce, but cole crops, spinach, and broccoli. It can happen with transplants, but we see it most on direct-seeded fields,” Chaney said. A curious trait of an infestation is that both crop and weed species are destroyed.
Chaney pointed out that the 2004 season saw less damage than the previous season. “Symphylans like to eat a lot of things, not just plant roots, and there may have been other things available to them, so that they did less damage to crops.”
A complication is that damage from another pest, seed springtail, is at least as damaging as symphylans. Potato baiting will attract both species. However, springtails tend to be transitory from one season to the next, whereas symphylans remain.
Chaney and research assistant Franklin Dlott have been using a sampling method developed by researchers at Oregon State University to identify the pest in grasses for seed. It can help distinguish a symphylan infestation from poor irrigation, nematodes, faulty planting, or some other cultural problem.
Basically, it involves placing a fresh slice of potato on surface of smooth, moist soil and covering it with an inverted plastic cup or other container to hold in moisture and block out light.
The tests are done preferably prior to planting, perhaps in February or March, but they can be done at any other time. Five or six bait stations, in the seed line, can be distributed and mapped throughout a field.
“After 24 hours you come back and examine the piece of potato to see what's running around on it,” Chaney said. Without a magnification, symphylans may appear quite similar to springtails. The quickest way to tell is that symphylans run about, while springtails jump.”
No thresholds have been developed, but Chaney said just their presence alone is sufficient indication of a potential problem.
Chaney and Dlott have used the technique mainly to test some management steps they have tried. “Of course,” Chaney said, “the bottom line is to find out if what we do works. Some of the things we will be trying out don't kill symphylans but will force them to move away.”
In their research, they have noticed extreme sensitivity of the pest to soil compaction. Even in an infested field, compacted areas, even around footsteps of an irrigator or PCA, plants free of damage can be found.
“Growers have told us they once typically rolled beds before planting and that is practiced less often these days. Of course, you don't want soil packed so that it interferes with seed emergence. This is one of the things we'll be looking into.”
Symphylans are difficult to reach, and research literature from the 1920s and 1930s says they range as deep as 10 feet in the soil, although they prefer higher organic matter of shallower depths.
Organic matter may be important in getting to know their habits. Chaney said organic growers tell him they have fewer symphylans problems than conventional growers. It may be, he suggested, that when they have enough other organic matter to feed on they ignore crop roots.
Move through soil
They move readily through the soil via through loose particles and channels made by earthworms. They have a life expectancy of 1 to 3 years in the laboratory, and they tend their egg masses in nesting sites to prevent fungi.
Symphylans have long been known as a pest, including in asparagus fields in the Sacramento-San Joaquin Delta, so why have they only recently reappeared along the coast?
“It's only speculation and we can't prove it,” Chaney replied, “but it may be connected with the cancellation of use of many of the old soil pesticides, particularly the organochlorines, including DDT, heptachlor, and others that came before organophosphates. They were once widely used in agriculture and some of their residues have persisted until recently.
“Perhaps it has taken 30 to 40 years for those residues to get down to where they no longer impact these very sensitive little animals.”
Chaney will be evaluating chemical options this coming season. “We have some indications that once we learn how to apply them correctly there may be some pyrethroids that can provide protection. We're still struggling with what is the best way to do that.”
Trials with chemical applications so far, he added, have shown reductions in some cases and none in others. Some of the products, however, are already in use and would require little modification in labeling.
Although some runoff issues exist for those products that are prone to binding to soil particles, he said he suspects the optimum timing of them would not be prior to rains when drainage would be of concern.
The research effort is being funded by a cross-section of crop industries, including research boards for lettuce, celery and peppers, as well as the IR-4 program and various other contributions.