When you cut into a head of romaine lettuce, you get a nice view of the densely packed interior leaves. Unfortunately, some of the most challenging aphid species are prone to infest this interior area.
For almost a decade, we here at USDA-ARS tested how well aphids were controlled in romaine lettuce when the insectary plant alyssum was interplanted. One of the main aphid predators we wanted to support were hoverflies.
Once we realized it was effective, we turned our attention to testing how to have our cake and eat it, too — or how to use insectary plants in the quantity needed to be effective, but not take away valuable space needed for crops you can sell.
First, allow me to explain how the system works. Intercropping lettuce with insectary plants is a common and effective strategy that organic farmers often use to control aphids.
Insectary plants can provide an energy source and protein to naturally occurring beneficial insects (i.e., hoverflies), which help growers by attacking the pests that are attacking their crops. These plants flower quickly, giving the beneficial predators a food source throughout the season.
We conducted our study on our USDA-ARS farm, a 23-acre site within an ongoing, long-term, organic systems experiment where we’ve grown romaine, broccoli, and strawberries on a commercial scale in rotation with various cover crops for the past 10 years.
Now, back to what you want to know. Here’s what we learned.
Alyssum Helps Aphid Control
When we started our long-term study, alyssum insectary plants were concentrated in two lines on eight of the 48 total beds in the test field. Single alyssum beds were on the edge of the field. Otherwise, the pattern was 10 beds of lettuce, then two beds of alyssum, followed by another 10 beds of lettuce, and so on.
We used this ratio the first four years and found it to be highly effective in controlling aphids. But I wondered if I could reduce the amount of space allocated to alyssum and still control aphids.
The eight beds that were devoted to alyssum during the first four years reduced the area for lettuce by 17%. This displacement of lettuce for insectary plants is a major concern for farmers in Salinas, because the land rent is extremely high here (often more than $2000 per acre per year!).
Our research is partially funded by the wholesale of marketable vegetables from the experiment. So I was highly motivated to continue the research by maximizing the marketable yields and the efficiency of the lettuce production.
Video Explaining How Brennan’s System Works
How the Research Field Was Set Up
- Tractor: GPS-guided tractor
- Lettuce bed width: 40-inches wide
- Bed prep: Organic fertilizers injected
- Spacing: The lettuce was transplanted in 2 lines, 12 inches apart with 11 to 12 inches between plants.
- Transplant age: Approximately 30 to 35 days old at transplanting
- Transplant timing: During the first 10 days of May, except for year three, when the rains delayed it until late May.
- Irrigation: Sprinkle irrigation was used to establish the transplants, but drip irrigation was used for most of the season.
- Feeds: Liquid organic fertilizers were injected through the drip tape approximately 30 days after transplanting
- Weed control: Tractor cultivation and hand weeding once during each lettuce crop.
- Harvest timing: At maturity, about 39 to 49 days after transplanting.
Attempt 1: Reducing the Rows of Alyssum
I trialed a few approaches that increased the amount of area for lettuce to see if I could continue to get the aphid control I wanted along with increased crop levels.
We started off by reducing the overall area devoted to alyssum by half. Where before there was a double row of insectary plants on eight beds, we now had just one row (or line) of alyssum and one line of lettuce.
This still provided excellent aphid control, and it boosted the potential lettuce yields by about 8% because there were about 8% more lettuce plants.
But I wanted even more space for lettuce, and the final part of the study was really the most radical.
Attempt 2: Additive Intercropping
This change was inspired by a competition experiment between alyssum and lettuce that I conducted during years five and six.
As you can see in the photo below, I tried all kinds of crazy combinations of these two plants. If you’d like to read all the details from that competition experiment, they’re described in a recent publication at: https://is.gd/1gxmW1. The most exciting result from the experiment was from what we call ‘additive intercropping’ where we kept the lettuce’s normal spacing on the bed but added alyssum transplants in between the lettuce transplants within a row. As a result, we got an intercropping pattern that has twice the normal transplant density. (We call this additive intercropping because we added the two densities together.)
Now there’s obviously more competition for nutrients between the romaine and alyssum in the additive pattern because it’s more crowded.
But the amazing thing about this additive pattern is that the increased competition reduced lettuce biomass or lettuce size by about only 25% and the alyssum biomass by about half compared to when they were growing separately and in their own beds.
Attempt 3: Low-Ratio Additive Planting
Here’s how I used the information from the additive planting experiment to improve the efficiency of intercropping lettuce and alyssum during years eight and nine. Take a look at the photo 3-A. This is the field 20 days after transplanting during year eight.
Now you might be wondering, “What’s happened? Where’s the alyssum?”
Now here’s the field 44 days after transplanting and about a week before harvest during year eight, photo 3-B. There’s lots of alyssum flowers out there, but they’re just not as obvious as in the previous years where alyssum displaced lettuce.
I want to point out two things:
First, notice that most of the alyssum is still concentrated in a few beds. These are the same eight insectary beds used during the previous years. The close-up shot photo 3-C shows the additive pattern that we used in the insectary beds during year eight.
Notice that there’s only one alyssum transplant between every three lettuce transplants in one line of the bed. We used a similar pattern in year nine, except there was only one alyssum transplant between every five lettuce transplants in one line of each bed.
The intercropping patterns used during these last two years were designed to reduce the potential for competition between alyssum and lettuce.
The second thing to note is that we also planted lines of alyssum that ran perpendicular to the bed direction.
We added these perpendicular lines to create this grid pattern because I was concerned that the relatively low-intensity additive pattern on the eight insectary beds might not provide quite enough to encourage hoverfly movement through the whole field. But I really don’t know if this concern was justified.
How We Added the Low-Ratio Additive Alyssum
We first transplanted lettuce across all 48 beds using a tractor-drawn transplanter. We then inserted (by hand) one alyssum transplant between every three or five lettuce plants during years eight and nine, respectively.
For each of the nine perpendicular lines, we walked across all the beds and inserted one alyssum transplant, again by hand, between two lettuce transplants in one line for each of the beds.
That final pattern of one alyssum to five lettuce in the insectary rows got us to our goal.
In a subsequent study, I found there was no difference in the marketable weight of a box of lettuce from beds with the additive pattern compared to the weight of a box of lettuce from beds without any alyssum.
Now this is a very, very important point. Because it means that with these less intense additive intercropping patterns, we can produce lots of alyssum flowers for the beneficial insects without losing any lettuce yields. It’s really a win-win situation.
This experience leads me to conclude that during the first seven years we were providing far more alyssum flowers for the hoverflies than were necessary. I estimate that additive intercropping with about 500 to 1,000 alyssum transplants per acre distributed throughout the field should provide sufficient pollen and nectar for the hoverflies to control aphids in transplanted romaine lettuce.
Based on my experience, I highly recommend the additive intercropping approach for transplanted lettuce because it’s much more land efficient. It doesn’t reduce marketable head weight of lettuce, yet it still provides beneficial insects, like hoverflies, with the food that they need to survive and control aphids. Hopefully additive intercropping is an approach to intercropping in lettuce that more growers (organic and conventional) are interested in using to improve aphid control.