Dealing with pests is a significant part of managing a vegetable garden, and there are
several different strategies that growers can use. Within the context of this RED Gardens
project I tend to rely on specific approaches as the primary way of dealing with particular pests.
I use a lot of netting as barriers to prevent caterpillars and birds eating my brassica plants,
and a finer mesh to prevent carrot fly damage. I provide places for slugs to hide around the
gardens and then I kill them, and I use a similar approach with rats. I have found trap crops useful
for distracting the flea beetles away from the brassicas, and more recently these have evolved
into trap and burn crops. But for aphids I have generally relied on the healthy population of
ladybirds or ladybugs as great natural predators for this potentially problematic pest. But the
ladybird population didn’t seem to be as abundant in the past few years, and they were initially non
existent when there was a real problem with aphid in one of the polytunnels earlier this season.
But a short while later there were lots of these beneficial insects doing what I needed them to do,
and it seems that this sudden change was because of a barrier I put up to keep out another pest.
For the most part I don’t have a lot of issues with aphids in the gardens,
I think mainly because I have gotten a lot better at growing healthy plants. And aphids seem to be
more attracted to plants that are struggling or stressed, and if I am able to provide the
vegetable plants with everything they need then I seem to be able to avoid significant infestation.
In the polytunnel I generally end up finding these sap sucking aphids
on the lettuce plants that I grow as the spring turns into the heat of the summer.
Lettuce plants are less tolerant of the heat, which I think is why the aphids appear. I
don’t mind this so much as by then I usually have lettuce available from the outside gardens,
and I figure that this gives the adult ladybird enough food to reproduce,
so that these natural predators are available later in the season for other crops. I
also tend to have aphids on my tomato plants in one of my polytunnels in the summer,
though not so much in my other polytunnel. I'm not sure why this is the case,
but it seems the plants are struggling with some nutrient issues or some other factor, but
the plants still produce quite well, especially if there are enough ladybirds around to help.
But in the last few years I've noticed that there are fewer of these beneficial insects around,
even though there is good habitat for them to live in and plenty of food.
I generally haven’t worried so much about this, but then a large batch of eggplant or aubergine
plants that I transplanted into this polytunnel were really badly affected by aphids for the first
part of the season this year. I'm not sure why this was, but the plants likely suffered
from transplant shock as they were a bit root bound in the pots and needing to get into the
soil earlier. There were also signs of nutrient deficiencies for a while after transplanting,
which may have been exacerbated by not having enough soil moisture. And
we also had weather that was cooler than would be needed by these heat loving plants. But
whatever the reason, a huge infestation of aphids appeared, enough to seriously impact the plants.
I didn’t really deal with the issue early enough, mainly because I was hoping the extra care we were
giving the plants would help them grow out of the issue they were having. And I was also hoping that
the ladybirds would arrive to help deal with the pests. But they didn’t seem to be around and we
started to resort to other methods of reducing the population of the fast breeding aphids.
I have been wondering if the increase in number and diversity of birds in and
around the gardens have been impacting on the population of ladybirds. And
there are 4 different types of birds that regularly come into the polytunnels in search
of food. I often see robins, sparrows and wrens in the polytunnel when the doors are open, either
in amongst the plants, pecking at the soil, or picking insects from around the polytunnel frame.
And there are regularly blackbirds digging at the compost on the surface of the soil
in search of worms, and later in the summer they also have a habit of eating the ripe
tomatoes. The blackbirds have made so much of a mess this year, including pulling up seedlings,
that I decided to use netting to cover the doorways at both ends of the polytunnel,
in an attempt to keep them out. I stapled two lengths of butterfly netting to the door frame
and weighed down the base with some stones, which produced makeshift curtains. This barrier does
make it more of a hassle for us to get in and out of the polytunnel, but once I blocked off
a few holes at the base of the plastic, we were able to keep the birds out, most of the time.
We put up the netting to keep the blackbirds out, which were the ones that I thought were
causing the most damage, and I was disappointed to also be excluding the robins and wrens who
are both lovely companions to have hanging around while I was working in there. But I also wondered
if excluding all the birds would have an impact on the insect population in this growing space.
I seems that only a few days later I noticed a ladybird on the aubergine plants and a
while after that I noticed the first of the ladybird larvae on the plants as well. And
the population of ladybird larvae seemed to rapidly increase to match the abundance of
the food source, and it got to the point where I had never seen so many ladybirds at various
stages of development. And gradually the numbers of aphids decreased on the aubergine plants,
then the ladybird adults and larvae started to migrate to other parts of the polytunnel,
searching for more aphids on the other plants in this growing space. I have seen ladybird
populations rise and fall in the past but I have never seen anything like this before,
as there were hundreds of helpful creatures when only a short while ago very few could be found.
It was fascinating to watch the lifecycle of these insects and it was great to be able to see
different stages of their development on the one plant. There were lots of the small crazy looking
larvae crawling around, and they seemed to grow so quickly. They can apparently eat a huge number
of aphids and there was an abundance of food for them. Then after a while the larger larvae
seemed to stick themselves into a leaf and after a weird cycle of shaking they gradually curled up,
and developed a shell and entered the pupae stage. A while later the young adult beetles
emerged from the shell, and initially they were pale yellow without any spots, and then
their shells gradually darkened to a red colour and their very recognisable spots appeared. And
they set off consuming enough aphids to be able to reproduce and lay eggs so that the cycle could
continue all over again. And the full life cycle seemed to take only a few weeks, which was a lot
faster than I though it would be. I guess with the warmth of the polytunnel and the abundance
of food they were able to grow and reproduce quickly, especially with no predators around.
So there is currently is a huge population of ladybirds in the polytunnel with a large
number of the second generation currently in the pupae stage. Soon there will be more of
these iconic adult ladybirds crawling through the polytunnel than I have ever seen before. And
no doubt many of them will wander outside in search of more food, but at the moment there
are very few ladybirds in the outside gardens, despite there being some aphids around. And
this dramatic change in population of these beneficial insects coincided with all of the
birds being excluded from the polytunnel. Of course correlation is not necessarily causation,
and this could be simply a coincidence, it could be just down to the delay in the
development of the aphids, and the excessive number of them. There could be other factors
involved including the changing weather, or some other issue that I'm not aware of, but I strongly
suspect that it was excluding the wrens from the polytunnel space that was the key factor here.
When the wrens were regularly in the polytunnel there were very few ladybirds around,
and when they were kept out of the polytunnel, the population of ladybirds increased dramatically. It
could also be the robins or the sparrows, but the wrens apparently eat a lot of insects, and I often
used to see them hopping and flying in amongst the leaves of the plants in search of food. I
don’t know if they would eat the larvae or the adult beetles, or even the pupae stage,
or perhaps all of them, but this seems to be the most likely cause for the absence and then
the abundance of these natural aphid predators. Of course I could open the curtains at either
end of the polytunnel space for a few days and see what happens, but I'm hesitant to do that.
I think this is a fascinating example of a 3 tier predator prey relationship, that has significant
impacts within the vegetable gardens. I really want a healthy background population of ladybirds
in order to help keep the aphid population under control. And this is why I'm generally a bit
more tolerant of having aphids on some of the vegetable plants, because I know that the adult
ladybirds and the larvae need a lot of aphids to eat. And I know that it takes a while for the
ladybird population to catch up to the occasional infestation of aphids. But this delay can be
worrying, and the last few years the beneficial predators did not show up in the numbers that I
was expecting, and that was especially the case with the crop of aubergine plants this year.
And now I wonder if this decline coincided with the wrens appearing in the area around the gardens
a few years ago. Unless I discover some other factor, it seems that I need to continue to use
the barriers to keep out the birds, so that they don’t eat the ladybirds, so that I can
rely on them as the natural predators of the aphids. And I wonder how excluding birds from
this polytunnel will impact the populations of the other insects in this space, and I would
not be surprised if I end up wanting to let the birds back in to deal with another pest issue.
This is part of the complex interactions of the webs of biodiversity, and an interesting
example of how a vegetable garden is both a highly controlled space and a natural system. My main job
of course is to try to give the plants everything that they need to grow strong and healthy,
so that any pest issues are less likely to become a real problem. And this may include not
growing some types of vegetables because they need growing conditions that I cannot reliably provide,
at least during some times of the year. But I also need to be mindful of the interactions
between predators and prey within the ecosystem in and around the gardens, and to perhaps step in
when one part of the wonderful biodiversity is eating something else that I rely on.