Efficient Intercropping for Biological Control of Aphids in Organic Lettuce

Organic lettuce growers in California typically use insectary strips of alyssum [Lobularia maritima (L.) Desv.] to attract hoverflies (Syrphidae) that provide biological control of aphids.  A two-year study with transplanted organic romaine lettuce in Salinas, CA, investigated agronomic aspects of lettuce monoculture and lettuce-alyssum strip intercropping on beds in replacement intercropping treatments where alyssum transplants replaced 2% to 8% of the lettuce transplants, and in additive intercropping treatments where alyssum transplants were added to the standard lettuce density without displacing lettuce transplants.  Alyssum and lettuce dry matter were determined at lettuce maturity.   Alyssum transplants produced less shoot dry matter in the additive than in the replacement intercropping treatments.  The number of open inflorescences of alyssum increased with alyssum DM, and among treatments ranged from 2 to 15 inflorescences per lettuce head.  Compared to monoculture lettuce, lettuce heads on intercropped beds were slightly smaller and had with lower nitrogen concentration in the both additive treatments and in some replacement treatments. This research provides the first information on a novel additive intercropping approach to provide alyssum floral resources for biological control of lettuce aphids, and suggests that this approach may be more land-efficient, particularly for producing slightly smaller lettuce heads for romaine hearts or for local markets with less strict size requirements.  Additional research is needed to determine if the increased competition between alyssum and lettuce in additive intercropping would reduce lettuce yields for wholesale markets with larger head size requirements.  Practical aspects of implementing the various intercropping arrangements and alternatives will be discussed.