Research generated by the USDA’s Agricultural Research Service in the heart of America’s Salad Bowl is showing how lettuce growers can control pests without the use of insecticides, by allowing a few flowering plants to grow among the salad greens.
Organic farmers have long known that planting sweet alyssum throughout a lettuce field effectively controls aphids, a major pest of salad crops. Sweet alyssum attracts beneficial insects including hoverflies, whose larvae each chomp down as many as 150 aphids per day.
The trade-off for this type of chemical-free pest control used to be the loss of up to 5 percent of yield, due to lettuce being displaced by the alyssum plants.
But thanks to a technique of “additive intercropping,” developed by USDA researcher Dr. Eric Brennan, growers can now plant enough alyssum for pest control without displacing any lettuce, or reducing crop yield. A well-respected crop researcher, Dr. Brennan is a former OFRF grant recipient.
“I see additive intercropping as a sustainable, win-win approach for dealing with major insect pests,” Brennan said. “Farmers can provide the beneficial insects with the resources they need to control aphids, without giving up any valuable lettuce-growing area.”
Brennan’s planting guidelines were developed over a period of nine years, on certified organic research plots, while producing romaine lettuce on a commercial scale. Additive intercropping involves planting one or two alyssum plants per every 50 lettuce plants. The alyssum are planted between regularly-spaced lettuce starts, where they thrive without crowding the lettuce, or reducing its size.
Hoverfly adults feed on alyssum’s nectar and pollen, while the larvae feed on aphids and other insects.
Lettuce is the most economically important crop grown in the Salinas Valley, with annual production values of $1.2 billion in 2013. Aphids are a constant threat to the crop, and are difficult to control, because they often hide in the cozy, protected interior of the lettuce head.
Conventional lettuce farms typically fight aphids with systemic pesticides, which are taken up by the plant and kill the insect when it sucks the plant’s juices. Systemics can reach sucking pests in areas of the plant where sprays do not reach. Growers can also plant lettuce varieties that resist certain aphid species.
Sweet alyssum, a common garden plant of Mediterranean origin, is particularly useful as an insectary plant in the Salinas area because it flowers year-round in California’s mild Central Coast region. Lettuces also grow year-round here, with prime farmland producing multiple plantings, and commanding some of the highest rents in agriculture.
Brennan’s research results are free and available online. He has also produced a lively, interactive 18-minute video that describes his research, shows various experimental planting patterns, and includes graphic footage of a hoverfly larva sucking down a fat aphid.
See the video at https://www.youtube.com/watch?v=KVLgt2_J1Wk
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