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Management program to be developed: Insecticide lettuce trials have impact on symphylans

Trials with the pyrethroid insecticide Warrior had impact on symphylans in lettuce in California coastal counties during the spring and summer of 2005, although a management program for the root-feeding pest remains to be developed.

The whitish arthropods, about a third of an inch in length and having a dozen pairs of legs, have long been a recurrent problem in asparagus in the Sacramento-San Joaquin Delta.

They have a history on the Central Coast too but were of little concern there for many years before reappearing in 2002 in lettuce and other crops in the Salinas Valley.

Their ability to be highly mobile in soil at depths of several feet makes them particularly difficult to control.

Bill Chaney, Monterey County farm advisor, has been investigating symphylans for the California Lettuce Research Board and gave a progress report at the board's recent gathering in Seaside. One of the primary objectives was to find a replacement for diazinon in the event of its becoming unavailable.

In his trials at Soledad this spring, Chaney, using experimental methods with diazinon and other insecticides, dipped lettuce plugs in solutions of them before transplanting them into the soil. Then 24 days after treatment, when the plugs were sending out roots, he measured plant weights to learn what effect symphylans had. Direct counts of the pest are impossible because of their mobility.

Heavier plants

He said the results for pyrethroids were encouraging. The Warrior treatment had plants with the greatest weights, about 65 grams, contrasted with less than 30 grams for plants in soil treated with the other materials and 10 grams for the untreated check.

He also found some improvement over untreated checks with insecticide treatments made to the soil before transplanting, but he said the plug treatments were more concentrated with active ingredient and therefore had greater effect.

“Pyrethroids,” he said, “do show effectiveness against symphylans, but the method of application has to be refined. These materials adhere tightly to the soil, so incorporation of some type will be necessary unless they can be placed below the seed.”

Chemicals of this class and others, he said, appear to not actually destroy symphylans but may repel them from the plant's root zone.

He said the critical point for plant injury is the interface between the plug and the soil where symphylans congregate. Numbers of them could be found where new roots emerged from the plugs.

As a tool for estimating harmful symphylan populations, coastal PCAs and growers are using a potato baiting technique developed at Oregon State University. It can help to distinguish between an infestation and other problems, such as improper irrigation, nematodes, or faulty planting.

Results inconsistent

Chaney stressed that one thing he has learned with trials so far is that results are inconsistent. “One time something will work; the next time it will not. Another thing we learned was that diazinon has not worked that well. So we want to try to find something better.”

His separate research with dipping celery plugs in pyrethroids also showed some impact on the pest but the plant weights recorded were not as dramatic as those of the lettuce trials.

He has also been working on western flower thrips for the lettuce board, and he found that in grower fields with high pressure from the thrips, several products gave significant control.

The standout performer was Dow Agroscience's experimental GF 1587, which he said shows promise. Others were Success, Entrust, and Valent S-1832, applied individually, and Lannate plus Mustang Max, which was better than either material used separately.

Thrips control “is a constant battle, no matter how good the material is,” he told the board, because of the continual migration of adults into treated fields from nearby hosts such as artichokes or weeds.

Biological control

Chaney is conducting a biological control project on lettuce aphid in lettuce with board funding and a grant from EPA's Biologically Integrated Farming Systems Program.

“We are concentrating on this because so much is being done by organic growers with practices like insectaries and interplanting of alyssum or other flowering plants. We wanted to get some data to see if these are useful techniques,” he said.

The reason for these practices is to attract and support the key natural enemies of lettuce aphid, a number of syrphid fly species. To thrive, syrphids adults need pollen and nectar that flowering plants provide, and the larval forms are predators of the lettuce aphid.

“We are also looking at permanent hedgerows and windbreaks to see what role they have in supporting syrphids, which are very sensitive to winds common in the Salinas Valley.”

He is also evaluating crop diversity and sequential planting of crops to support syrphids for what applications they might offer conventional farming.

Western flower thrips

Eric Natwick, Imperial County farm advisor, has been researching western flower thrips management in romaine grown in the low desert with his trials at Holtville.

“We have some pretty good materials for control of aphids and lepidopterous pests in the desert,” he said, “but thrips is a different story. They've been a particularly difficult marketing problem for us, largely because of the damage and contamination they cause.”

During the 2004-2005 season, thrips counts were low, and that prevented sharp separations between efficacies of the several insecticides, most of them available for lettuce, that he tested. Consequently, there was little difference between treatments in amounts of heads and pounds for domestic markets.

However, Natwick reported, “there were significant differences for percentages of insect-free heads that could go to the export market.” The best treatment for amount of insect-free heads was from Admire followed by a combination of Success with Warrior. This treatment yielded nearly 79.8 percent in insect-free heads, versus 46.5 percent in the untreated check.

Due to the scarcity of thrips, he explained, most contaminating insects found were likely aphids.

He noted that all of the materials gave thrips control to some extent, but the timing and number of applications and the level of control must be acceptable to buyers. “So I'm not saying we are there yet in solving our problems with thrips in lettuce.”

He said for his next round of trials in lettuce, he hopes to include experimental materials showing promise for thrips control in onions.

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