Efficient Intercropping for Biological Control of Aphids in Transplanted Organic Lettuce

Watch this video at https://www.youtube.com/watch?v=KVLgt2_J1Wk

How do organic farmers control aphids and produce high quality lettuce without pesticides? With naturally occurring beneficial insects like hoverflies that eat aphids live! Farmers attract these good bugs into the field by intercropping lettuce with flowers like alyssum. This video shows how this system works, and a more efficient and novel way to achieve biological control of aphids with less land area and fewer weed problems. It is based on research by Eric Brennan at the USDA Agricultural Research Service during 9 years of commercial scale organic lettuce production in the Salinas Valley, California.

This video was modified from a video presentation at the American Society of Horticulture annual conference in July, 2013.

Click following link to download a pdf of a related publication (Agronomic aspects of strip intercropping lettuce with alyssum for biological control of aphids)

Agronomic Impacts of Strip Intercropping Lettuce with Alyssum for Biological Control of Aphids

Video Transcript

Hi everybody. My name is Eric Brennan. I’m a scientist at the USDA Agriculture Research Service based in Salinas, California. Salinas valley opens to Monterey Bay, which acts as a natural air conditioner for much of the area. This climate is ideal for lettuce and the gross production value for lettuce here was nearly $1.3 billion in 2012. I’ve worked here since 2001, and my research over the past 12 years has focused on high value, organic production systems. Today, I’ll share some of the lessons I’ve learned over the past 10 years on how to use intercropping to biologically control aphids in transplanted organic romaine lettuce.

When you cut into a head of romaine lettuce, you can get a nice view of the densely packed interior leaves. Unfortunately, the most important insect pest of lettuce in California is a nasty aphid species that likes to infest this interior area and is easy to see. Intercropping or interplanting lettuce with plants that flower quickly like alyssum is a common and effective strategy that organic farmers in this region often use to control aphids. Alyssum is referred to as an ‘insectary plant’, because when it's intercropped with lettuce it attracts naturally occurring beneficial insects, like hoverflies, into the field. Hovering in midair requires lots of energy which the adult hoverflies get from the sugary nectar of the alyssum flowers. The pollen provides the adults with the protein that they need to reproduce. After feeding on the flowers, the females fly through the field searching for lettuce plants where they will lay their eggs. The females prefer to lay eggs on lettuce plants with aphids because the larvae that hatch from the eggs in a few days, feed on the aphids. I like to think of aphids as walking milkshakes for hoverfly larvae. Infact, the larvae of some hoverfly species can eat up to 150 aphids per day before they mature into flying adults.

In highly disturbed agricultural landscapes such as those used for vegetable production in Salinas, the presence of hedgerows around the farm and the frequent use of annual cover crops help to protect and maintain populations of beneficial insects year round. These habitats and the use of insectary intercrops, like alyssum, enhance the ability of beneficial insects to control economically important pests like aphids. We refer to this pest management strategy as Conservation Biological Control.

Let’s now move to the USDA organic research farm, where I’ll share how my approach to intercropping alyssum and lettuce has become much more efficient over the past 10 years. This 23 acre site includes an ongoing, long-term, organic systems experiment where we have grown 2 acres of romaine lettuce, broccoli, and strawberries on a commercial scale in rotation with various cover crops and compost rates over the past 10 years. Today, I’m going to focus on the intercropping practices we used to maximize the potential marketable yields during 9 years of lettuce production. This research is partially funded by the wholesale of marketable vegetables from the experiment. Therefore, in order to continue the research, I highly was motivated to maximize the marketable yield and the efficiency of the lettuce production.

Here are some details about the management of the lettuce that were consistent across years. A gps guided tractor was used to form beds that were 40 inches wide, into which we injected preplant organic fertilizer. After bed shaping, the lettuce was transplanted in two lines 12 inches apart with 11 to 12 inches between plants within each line. The transplants were approximately 30 to 35 days old at transplanting. Transplanting was usually during the first 10 days of May except during the year 3 when rains delayed it until late May. Sprinkle irrigation was used as needed to establish the transplants, but drip irrigation was used for most of the season. Liquid organic fertilizers were injected through the drip tape approximately 30 days after transplanting. Weeds were controlled by tractor cultivation and by hand weeding once during each crop. And the lettuce was harvested at maturity 39 to 49 days after transplanting.

The alyssum insectary plants were concentrated in 8 of the 48 total beds in the field. Notice that alyssum beds 1 and 8 on the edges of the field were single alyssum beds, followed by 10 beds of lettuce, then 2 beds of alyssum and 10 more beds of lettuce, etc. This picture shows 4 different alyssum varieties including the sweet variety that is the typical insectary variety in California. The alyssum and lettuce in the background were all transplanted 46 days ago and it's clear that sweet alyssum is much more vigorous and bushy than the three ornamental alyssum varieties shown here.

I’ll now highlight 3 major changes in the way that lettuce was intercropped with alyssum during the 9 years and explain my rationale for making each change. The first change occurred after year 2 and involved switching from using alyssum seed to using transplants to establish the insectary beds. Alyssum seed is extremely small and the seed of the sweet variety used for insectaries is also inexpensive. During the first two years, I thought that direct seeding alyssum would be more cost-effective than using alyssum transplants. However, direct seeding alyssum in dense lines in the field had two major problems. I’ll use a few drawings to illustrate the first problem that involved weed management.

This drawing shows a single bed with the two transplant lines of lettuce approximately 2 ½ to 3 weeks after transplanting. The field is ready to hand weed at this stage and the red dots represent emerged weeds. Note that the weeds had already been removed from the bed center and furrow by tractor cultivation. Hand weeding in a situation like this is relatively easy, because the weeds are small and easy to distinguish from the larger and evenly spaced transplants. This drawing shows weeds interspersed with 2 lines of direct seeded alyssum plants. The green dots are the densely seeded alyssum plants and the red dots are the weeds. Note that the density and location of the weeds here is the same as in the previous drawing with lettuce transplants. However, in this case, the weeds and alyssum emerged together and if had not colored the weeds red they would be very difficult to distinguish from the alyssum plants. As you can imagine this was extremely difficult to hand weed and the situation only got worse as the weeds and alyssum plants got bigger and tangled together. Furthermore, many weeds in direct seeded alyssum lines escaped control and produced seed that added to the weed seed bank.

The second major problem with direct seeding alyssum in transplanted lettuce is that even in the summer, alyssum seedlings often need to grow for about a month before they begin flowering. In fact, this alyssum seedling didn’t flower until it was 36 days old. In contrast, alyssum transplants are usually flowering at transplanting. Early flowering of the insectary plants is important for transplanted crops like lettuce that may be harvested at 39 to 49 days after transplanting.

The fact that lettuce from the first two years was not infested with aphids suggests that flowering from direct seeded alyssum was adequate for biological control. However, the cost of alyssum transplants seemed worthwhile for both weed control reasons and the likely benefits of earlier flowering for biological control of aphids. After 4 years of successful lettuce production without any major aphid problems, I wondered if I could reduce the amount of space allocated to alyssum and still control aphids. The 8 beds devoted to alyssum during the first 4 years were obviously effective, but they were also reducing the area for lettuce by 17%. This displacement of lettuce for insectary plants is a major concern for farmers in Salinas where the land rents are high.

The last two intercropping changes I’ll discuss are 2 approaches I used to reduce the field area that was displaced by insectary plantings. This photo shows the intercropping pattern during years 5 to 7. Notice, that rather than 8 solid beds of alyssum that were used during the first 4 years, the insectary beds now included 1 line of alyssum and 1 line of lettuce. This still provided excellent aphid control, and boosted lettuce yields by 8% because there were 8% more lettuce plants in the field.
Let’s now move onto the last intercropping change that was the most radical. This change was inspired by a competition experiment with alyssum and lettuce that I conducted during years 5 and 6. As you can see I tried all kinds of crazy combinations.

All the details are described in this recent publication. However, I’ll describe the most exciting results from this experiment with a simple addition equation. If we add the transplants from one bed of lettuce to the transplants from one bed of alyssum we get an intercropping pattern that has twice the normal transplant density. We call this additive intercropping because we added the two densities together. There's obviously more competition in the additive pattern because it’s more crowded. The amazing thing about this additive pattern is that the increased competition only reduced lettuce biomass by about 25% and alyssum biomass by about half compared with when they were growing separately on beds of their own. I’ll now show how the information from this competition experiment was used to improve the efficiency of intercropping lettuce and alyssum during years 8 and 9.

Here’s what the field looked like 20 days after transplanting during year 8. You might be wondering what’s happened? Where’s the alyssum? That question “Where’s the alyssum?” reminds me of a well-known and beautiful song by Pete Seeger. Sing along if you like as I play a line or two on my guitar.

[Music]. Where have all the flowers gone?
[Music]. Long time passing.
[Music]. Where have all the flowers gone?
[Music]. Long time ago.

That’s a great song, but let me answer the question: Where are the alyssum flowers? Here’s the field 44 days after transplanting and about a week before harvest during year 8. There are lots alyssum flowers out there but they’re just not as obvious as in the previous years where alyssum displaced lettuce. Here’s another shot the next day when the lighting made it easier to see the alyssum. I want to point out two things in this picture. First, notice that most of the alyssum is still concentrated in a few beds. These are the same 8 insectary beds that were used during the previous years. This close up shot shows the additive pattern that we used on the insectary beds during year 8. Notice that there's only 1 alyssum transplant every 3 lettuce transplants in 1 line of the bed. A similar additive pattern was used during year 9 except that there was only 1 alyssum transplant between every 5 lettuce transplants in 1 line of each bed.

This figure with white symbols to represent alyssum, illustrates the difference in the extremely intense additive intercropping pattern that was used in the competition experiment described earlier, compared with the additive patterns that were used during years 8 and 9. The intercropping patterns used during these last 2 years were designed to reduce the potential for competition between alyssum and lettuce. In fact, in a subsequent study, I found that there was no difference in the marketable weight of a box of a lettuce from beds with the additive pattern used during year 8, compared to the weight of a box of lettuce from beds without any alyssum. This is a very important point because it means that with these less intense additive intercropping patterns, we can a produce alyssum flowers for beneficial insects without losing any of lettuce yield. The second thing I want to highlight about the additive intercropping patterns used during years 8 and 9 are these lines of alyssum that ran perpendicular to the bed direction. If you looked at the field from the top it would be a grid like this with the insectary and lettuce beds running from the bottom to the top of the figure and the perpendicular lines running from the left to right. You might wonder why we added the perpendicular lines during years 8 and 9 to create this grid pattern. This was done because I was concerned that the relatively low intensity additive pattern on the 8 insectary beds alone might not provide quite enough alyssum flowers to encourage hover fly movement through the whole field. However, I really don’t know if this concern was justified.

You might be wondering how we created this additive pattern through the field. First, we transplanted lettuce across all 48 beds using a tractor drawn transplanter. And then in one line on the 8 insectary beds, by hand we inserted one alyssum transplant between every 3 or 5 lettuce plants, during years 8 and 9, respectively. For each of the 9 perpendicular lines, we walked across the beds and inserted one alyssum transplant by hand between two lettuce plants in one line for each bed. Our lettuce yields were highest these last 2 years when we used the additive intercropping approach because alyssum didn’t displace any lettuce.

I’ll summarize my experience with intercropping lettuce with alyssum over the 9 years with 2 figures. This first figure shows the dramatic change in the amount of lettuce that was displaced by alyssum over the years. Based on my experience, I highly recommend this additive intercropping approach for transplanted lettuce because it is much more land efficient, it didn’t reduce marketable head weight, and yet it still provided beneficial insect like hoverflies with the food they needed to survive and control aphids.

This last figure illustrates how the density of alyssum transplants changed over time. It is interesting to note that we achieved excellent aphid control all year despite the drastic reduction in the number of alyssum transplants per acre. This experience leads me to conclude that during the first 7 years we were providing far more alyssum flowers for the hoverflies than was necessary. I estimate that additive intercropping with about 500 to 1000 alyssum transplants per acre, distributed throughout the field should provide sufficient pollen and nectar for hoverflies to control aphids in transplanted romaine lettuce.

I hope this presentation has helped you to understand the value and complexity of intercropping lettuce with insectary plants like alyssum for biological control of aphids. Thanks for watching, and stay tuned for more exciting sustainable ag research. And when you eat your next organic lettuce, think of all the flowers, and hardworking people, and hoverflies, that it took to produce it!