Biological control is the cheapest and easiest means of controlling pests, but it is not always successful, according to Bill Chaney, Monterey County farm advisor.
Nevertheless, Chaney and others have been evaluating syrphid flies as a control for lettuce aphid, a persistent pest in coastal lettuce, especially romaine. The lettuce aphid is tough to control with insecticides because it colonizes deep in the lettuce head.
The use of various flowering plants, some of them common ornamentals, to support a number of syrphid species, natural enemies of the aphid, originated as a control for organically grown lettuce, but Chaney sees a potential application of it in conventionally farmed lettuce fields.
At a recent meeting of growers and PCAs in Salinas, Chaney said the lettuce aphid appeared in Salinas Valley lettuce in 1989 and has since become a serious pest.
Classical biological control, or the use of any natural enemy to reduce a pest population — while not always a smash success — does have noteworthy accomplishments to its credit, Chaney said.
The prime example in California was the release of the Vedalia beetle more than 100 years ago to control cottony cushion scale which, at the time was devastating citrus groves. The Vedalia, a lady-beetle species introduced from Australia, was credited with saving the California citrus industry from the scale.
More recently, about 10 years ago, Chaney said bluegum psyllids threatened the $30 million, floral eucalyptus industry in Monterey and Santa Cruz counties. The psyllids' feeding deformed branches of the eucalyptus and left spots were the branches and foliage would not accept dyes. Multiple sprays required for control of the psyllid damage were not economical, but a parasitic wasp from Australia was introduced and cleaned up the pest in two years.
Chaney said the campaign against lettuce aphid, however, mostly employs what is termed conservation biological control, or conserving the natural enemies that exist locally.
In 2001, he said, lettuce aphid populations in lettuce in the Hollister area were observed to reach a peak and then suddenly disappear without a trace. Investigation showed the presence of syrphid fly nymphs, which had devoured the aphids deep in the lettuce heads.
“Organic growers recognized what was going on here with biological control and began setting out strips of insectary plants with their lettuce and other crops,” Chaney said.
Insectary plants are annuals, such as alyssum, clovers, or mixes of species, placed either across a field at intervals or around the edges.
The flowering plants supplied syrphids, also known as hover flies or flower flies, with nectar and pollen to support adult and younger stages, which in turn overcame the aphids.
“We call this habitat manipulation, or supplying these resources in the field in a way to reduce lettuce aphid populations before the harvest date of the crop. We do not yet have the answers, but we are trying to learn more about it.” One question is which of the many species of syrphid flies are impacting the pest.
There are additional beneficial insects that might be potential “players” against lettuce aphid, he said. Among them are green lacewings, minute pirate bugs, bigeyed bugs and others.
However, the downside with biological control, Chaney cautioned, is the possibility of small-seed-feeding lygus or other pests that could also be drawn to the lettuce by flowering insectary strips.
Chaney said the early data was taken to the EPA, which funded the current project of further investigations of ways of suppressing lettuce aphid begun in February 2005.
Hugh Smith, a University of California Cooperative Extension entomologist assigned to the Monterey County office, has been conducting the research.
Smith said the Salinas Valley has abundant floral resources of mustard, radish, and other wild plants that support syrphids. Many other weeds, including poison hemlock, are also favored habitat for the insects.
He sampled syrphid species most common in romaine throughout the valley. He found about a dozen are present, and he is studying the egg-laying and other behaviors of the main seven species.
The key to the conservation biological control approach, Smith said, is that several species prey on other aphid species and lay eggs in lettuce before the lettuce aphid appears. Other syrphids, however, appear only when lettuce aphid infestation is high.
“We learned is that conservation biological control is probably successful here because we are not relying on one or two species. There is a mixture of syrphid species and they are taking advantage of different situations,” Smith said.
This year, he continued observations of the role of syrphids. By excluding them from lettuce plants with covers or spray treatments to kill only syrphid larvae, he wanted to learn if other predators also go to the aphids. He is also monitoring use of a fungus and a lady-beetle predator that attack lettuce aphid.
Smith said much is to be learned, since individual organic growers use several different patterns and species of insectary strips to sustain syrphids.
“We can't just tweak a certain system, such as changing insectary strips from one every eight rows to every 12 rows. We found that growers are doing some very different things with very similar levels of success.
“That's why it is important for us to think about all the large, naturally-occurring areas of insectaries we have. Syrphids are very mobile and can come from far away as well as next door.”
Smith's work has been funded by EPA's Region 9 for another year to research similar ways to control insect pests of celery and cole crops.