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.