EFFICIENT Intercropping for Biological CONTROL of APHIDS in Transplanted Organic Lettuce


Hi everybody. My name is Eric Brennan. I’m a scientist at the USDA Agricultural Research
Service based in Salinas California. Salinas Valley open to Monterey Bay which acts as
a natural air conditioner for much of this area. This climate is ideal for lettuce and over a billion dollars worth of lettuce is
produced here annually. I’ve worked here since 2001 and my research over the past 12 years has
focused on high-value organic production systems. In this video I’ll share some of the lessons I’ve
learned over the past 10 years on how to use intercropping to biologically control aphids on 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 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 an effective strategy that organic
farmers here 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 mid air 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’ll lay their eggs. The females prefer to lay eggs on lettuce plants with aphids, because the larvae that hatch from these eggs in a
few days, eat aphids. I like to think of aphids as ‘walking milkshakes’ for
hoverfly larvae. In fact, 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 like
those used for vegetable production in Salinas, the presence of hedgerows around the farm and the frequent use of 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 call this pest management strategy
‘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’ve grown two acres of romaine lettuce, broccoli, and strawberries on a commercial scale in rotation with various cover crops over the past
10 years. Today I’m going to focus on the intercropping
practices that 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 to continue the research I was highly motivated to maximize the marketable yields and the efficiency of the
lettuce production. Here are a few details about the lettuce
management. A GPS guided tractor was used to form beds that were 40 inches wide and into which were injected preplant, organic
fertilizers. After shaping the beds, the lettuce was
transplanted in 2 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. And transplant usually occurred during the first 10
days of May except for during year 3 when the rains delayed it
until late May. Sprinkle irrigation was used 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, and weeds were controlled by a tractor cultivation and by hand weeding once during each lettuce
crop. The lettuce was harvested at maturity, about 39 to
49 days after transplanting. The alyssum insectary beds were concentrated on
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’s the typical insectary variety that’s used in
California. The alyssum and lettuce in the background were
all transplanted 46 days ago and it’s really clear that the sweet alyssum variety is much more vigorous and bushy than the 3
ornamental alyssum varieties that are shown. I’ll now highlight 3 major changes in how lettuce
was intercropped with alyssum during the 9 years and then I’ll explain my rationale for making each
change. So the first change occurred after year 2 and basically it 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 is also pretty inexpensive. Now during the first 2 years I thought that direct-
seeding alyssum would be for more cost-effective than using
alyssum transplants. I was really wrong though because 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. So this drawing shows a single bed with 2 transplant lines of lettuce approximately 2
1/2 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 the furrow by a tractor cultivation. Now 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 lettuce transplants. This drawing shows weeds interspersed with 2
lines of direct-seeded alyssum plants. The green dots here represent densely seed
alyssum plants and the red dots represent weeds. Now note that the density and the location of the
weeds here is the same as in the previous drawing where lettuce transplants were shown. However, in this case the weeds and the alyssum
emerged together. And if I hadn’t colored the weeds red they’d be very difficult to distinguish from the
alyssum. As you can imagine this was extremely difficult to
hand weed. And the situation only got worse as the weeds and the alyssum plants got bigger and tangled
together. Furthermore, many of the weeds in these direct-
seeded alyssum lines escaped control and went on to produce seed that added to the
weed seed bank. The second major problem with direct-seeding
alyssum in transplanted lettuce is that even the
summer alyssum seedlings often need to grow for about a
month before they can begin flowering. In fact this seedling didn’t flower until it was 36
days old. Now in contrast, alyssum transplants are usually
flowering at transplanting. Early flowering of the insectary plants is really
important for transplanted crops like lettuce that may be harvested at 39 to 49 days after
transplanting. The fact that lettuce during the first 2 years of
production was not infested with aphids does suggest a flowering from direct-seeded
alyssum was adequate for biological control of aphids. 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. So after 4 years of successful lettuce production
without any major aphid problems, I wondered if I could reduce the amount of space
that was allocated to alyssum and still control aphids. The 8 beds that were devoted 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
like alyssum is a major concern for farmers in Salinas because the land rent is quite high here. The last 2 intercropping changes that I’ll discuss are 2 approaches that I used to reduce the field
area that was displaced by insectary plantings. So 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 alyssum
and 1 line of lettuce. This still provided excellent aphid control and it boosted lettuce yields by 8% because there
were 8% more lettuce plants in the field. Let’s now move on to last intercropping change
that was really the most radical. This last change was inspired by a competition
experiment between alyssum and lettuce that I conducted
during years 5 and 6. As you can see I tried all kinds of crazy
combinations of these 2 plants. All the details from that competition experiment are
described in this recent publication. But I’ll just describe the most exciting results from
the experiment with the simple addition equation. So if we add the transplants from 1 bed of lettuce to the transplants from 1 bed of alyssum we get it intercropping pattern that has twice the
normal transplant density. We call this additive intercropping because we added the 2 densities together. Now 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 or lettuce size by about 25% alyssum biomass by about half compared to when they were growing separately
and beds of their own. I’ll now show you how I used the information from
this experiment to improve the efficiency of intercropping lettuce
and alyssum during years 8 and 9. So here’s what the field looked like 20 days after
transplanting during year 8. Now 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 a player line or 2 of that
song 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? Now here’s the field 44 days after transplanting
and about a week before harvest during year 8. There’s lots of alyssum flowers out there but there 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 2 things in this picture. First, I want you to notice that most of the alyssum
is still concentrated on 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 in 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 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 was white symbols to represent
alyssum illustrates the difference in the extremely intense
additive intercropping pattern that was used in that competition experiment that I
described earlier compared with the additive patterns that were used
during years 8 and 9 on the insectary beds. 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 lettuce from beds with the additive pattern that we used during year
8 compared to the weight of a box of lettuce from
beds without any alyssum. Now this is a very very important important point
because what it means is that with these less intense additive intercropping
patterns we can produce alyssum flowers for the beneficial
insects without losing any lettuce yields. It’s really a win-win situation. The second thing that I want to highlight about the
additive intercropping patterns that we use 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 left to the
right. So why did we add these perpendicular lines to
create this grid pattern? Well, basically 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 hoverfly movement through the whole
field. But you know I really don’t know if his concern was
justified. You might be wondering how we created this
additive intercropping pattern through the field. First, we transplanted lettuce across all 48 beds
using a tractor-drawn transplanter. And then in 1 line on the 8 insectary beds, by hand, we inserted 1 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 all the beds and inserted 1 alyssum transplant, by hand, between 2 lettuce transplants in 1 line for each of the beds. 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. So 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 the
additive intercropping approach for transplanted
lettuce because it’s much more land-efficient it didn’t reduce marketable head weight of lettuce and yet it still provided beneficial insects, like
hoverflies with the food that they needed to survive and control aphids. This last figure illustrates how the density of
alyssum transplants changed over time. It’s interesting to note that we got excellent aphid
control all years 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 really necessary. I estimate additive intercropping with about 500 to 1000 alyssum transplants per acre distributed throughout the field should provide sufficient pollen and nectar for the
hoverflies to control aphids in transplanted
romaine lettuce. I hope this video has helped you to understand
the value and the 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.

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