Control of lettuce aphid in the Salinas Valley is largely a matter of coverage, since several available materials work well against it, according to Bill Chaney, entomology farm advisor for Monterey County.
At a recent seminar in Salinas attended by some 125 growers and PCAs, Chaney explained his comment did not relate to methods of application as much as to timing of treatments early before lettuce heads grow closed and conceal the aphid.
Although coastal growers have developed strategies and confidence after dealing with Nasonovia ribis-nigri for several years, the species continues to invade other new areas with considerable impact.
He said the aphid recently moved into fields in Australia and New Zealand, prompting a flood of telephone calls to his office from anxious growers there asking for guidance in stopping it.
“They are really struggling down there, like we were the first year or so after it came here, to control this pest. It will take them some time to develop their controls just like we have done here.”
One reason lettuce aphid can be so difficult to control, he said, is growers attempt at first to treat it like the more accessible green peach aphid, which stays mostly on outer leaves.
Lettuce aphid has a history of development of resistance to insecticides, as seen in Europe, Chaney said. Realizing that concern, the California Lettuce Advisory Board funded research to develop ways to monitor for any resistance to the two popular imidacloprid products, Provado (foliar applied) and Admire (soil applied).
Chaney and research assistant Franklin Dlott developed tests to establish a baseline for imidacloprid dosage to signal if and when resistance occurs. These tests have revealed no resistance.
They grew individual lettuce plants and collected adult aphids from the field. They segregated similarly aged insects and placed them on the young plants. Minute droplet dosages of the material were then applied precisely by micropipette directly to each insect. The plants were capped to prevent escape of the aphids, and then mortality was recorded. The tests are used on aphids collected from a field where resistance is suspected.
Chaney said a dosage of one-tenth of the field rate provides a complete kill. In fact, aphid mortality does not start to drop until one-hundredth of the field rate is applied.
“That gives an idea of how toxic these materials are to the aphids. It also drives home our point that coverage is the key, and that's how you control lettuce aphid.”
He added that the tests have shown no evidence of a lettuce aphid population having resistance to imidacloprid. “We've even tested fields where PCAs said they saw aphids after Admire was in the soil and Provado was applied on top.”
Still, he added, the fact remains that when any single material is used against an insect pest for an extended period, resistance will occur. That's why rotation of imiacloprid products with other classes of chemistry is important.
Several pyrethroids perform well against the pest, provided they are applied early when the crop canopy is still open.
In describing natural enemies of lettuce aphid, Dlott said the most important are surfid fly (hover fly) and entomopathic fungi.
Since the Salinas Valley climate does not support effective populations of the fungi, the chief lettuce aphid predator there is surfid fly in the larval form.
The several species of surfid fly are susceptible to insecticides used for aphid pests, so their application pertains mostly to organic farming.
While adults, identified by their hovering behavior, feed on pollens and nectars of plants such as alyssum and reside in windbreaks, the larvae occur on lettuce plants, where they each consume 30 to 40 lettuce aphids per day.
“They are very thorough and get all the aphids on a plant,” Dlott said.
The larvae, more efficacious than insecticides because they move deeper into heads than sprays can reach, leave no trace of their prey.
In a given field, multiple species of surfid flies in varying proportions can be found, but they may not be present in sufficient numbers when lettuce aphids are on the crop.
Chaney and Dlott did trials with white alyssum and other insectary plantings, which they say impacted surfid fly populations and the amount of aphids they consume.
The researchers reported that fewer aphids were recovered nearer to the insectary plantings. The distance varies in the range of eight to 12 rows. They added that many factors come into play in this practice and more research would be needed before any generalized recommendations could be made.