Scientific agroecology and ecosystem services in the vegetable garden.

Can we protect crops by relying solely on the natural regulation of ecosystems?

It is not uncommon to read in certain press articles or forums dealing with biodiversity or agroecology that in nature ecosystems eventually balance themselves. It is not necessary to intervene against bio-aggressors with chemical products and it is enough to let nature take its course. The idea of natural balance has long prevailed in scientific ecology taught at university. This is certainly true in natural forests, but in a cultivated environment it is more logical to speak of a managed ecosystem because the choice of plants, often imported when not modified by selection, responds to precise objectives imposed by the farmer or the owner of a pleasure garden to feed the population or to build a plant environment that responds to cultural trends.

However, trusting the balance of a natural environment to control the pressure of all pests is more complicated than one might think at first sight.

If we take the case of the PACA region with its hot and dry climate, it is difficult to imagine that we can trust only one set of natural factors intervening from the forest to the vegetable garden to control all bio-aggressors, because the ecosystem of a Mediterranean forest and that of a vegetable garden are very different. Mediterranean flora and fauna can withstand droughts lasting at least 3 months.

On the other hand, most cultivated fruits, vegetables and cereals need periodic watering. With some exceptions (such as lavender), cultivated plants are not able to survive a hot, dry climate lasting 3 months. Irrigation has consequences for the immediate environment. All the inhabitants of this environment are affected. Some are boosted, others cannot withstand the changes in the environment imposed by the farmer. What can we expect from the Mediterranean forest where the inhabitants are often different from those living in a hot and humid cultivation environment in constant imbalance caused by the farmer! Only the farmer can control this imbalance by acting on the ecosystem of his vegetable garden (by destroying, for example, weeds that benefit from a humid environment, by encouraging the establishment of predators of bio-aggressors, etc.).

However, some pests and their predators can thrive in different ecosystems, such as aphids and ladybirds. A natural regulation of certain pests in a vegetable garden characterised by its humid biotope is therefore sometimes possible from a dry and warm Mediterranean ecosystem located near this garden, but not always in the desired direction.

Human activities and biodiversity

European agricultural landscapes have been modified for more than 2000 years. Over time, many wild and imported species have adapted to their new environments, resulting in the development of ecosystems dependent on agricultural activities. Other wild species have become scarce or have withdrawn from areas reserved for agricultural activities. In France, 60% of the land is reserved for agriculture and only 5% for natural biodiversity. 35% of non-agricultural areas are dependent on anthropogenic factors modifying biodiversity.

When you clear a forest to introduce new plants that did not ask to grow there, you disturb the natural balance of an ecosystem. No one can claim to be acting without consequences for the environment when one intervenes in the components of that environment. Planting a single lettuce from a selected seed already interferes with the surrounding environment because we will have to work the soil it will be planted in, water it frequently to prevent it from withering away and prevent slugs from eating it instead of us.

For food crops, particularly cereals, this imbalance is all the more intense because millions of the same variety are reproduced in the same area, making them more vulnerable to pests.

By dint of selections, mutations and cross-breeding, farmers and agricultural engineers have managed to create species that are better able to live in their new environments, provided they are protected and cared for like many domestic animals. These species still require a great deal of care and have great difficulty surviving on their own in the wild. With rare exceptions, no one meets them on roadsides and paths.

Agricultural activities and changes in natural ecosystems

The environmental impact of the activities of amateur gardeners and farmers takes place in two directions:

- On the biotope (a) when amendments and fertilizers are spread.
- On the biocenosis (b) by modification of the different plant and animal species.

We may regret these consequences, but if tomorrow we decide to abandon agricultural activities to be more in line with the concerns of naturalness activists and return to spontaneous biodiversity, we will have to convince 65 million French people to return to the lifestyle of Cro-Magnon man, i.e. to find their food by gathering and hunting. The entire French territory (reforested to shelter animals that have become wild again) would still have to be sufficient to feed this entire population. Is this reasonable? Who is ready to try this step backwards?

It is clear that agriculture has an impact on biodiversity with a reduction in the population of some native species. Whether the farming system is conventional or organic, there is always a loss of biodiversity for one simple reason: the animal biodiversity of an ecosystem is itself closely linked to plant biodiversity. However, in an agricultural plot, plant biodiversity is inevitably reduced by the farmer who seeks to plant only food species that are inevitably fewer in number (for cereal farmers, it is even a single species).

In some agricultural areas, intensive practices have led to a loss of habitat for certain species, causing their decline. For example, the widespread application of highly beneficial herbicides to oilseed rape has indirectly contributed to the decline of pollinators by eliminating semi-natural habitats around agricultural fields in periods when oilseed rape is not flowering. These semi-wild habitats, where different plant species thrive, provided floral resources for pollinators before and after crop flowering (3).

According to some authors, in general, the increasing intensification of agricultural practices, which has resulted in the reduction of mixed crop-livestock farms in favour of large specialised monocultures, has led to a decline in biodiversity, particularly in populations of insects, birds, arthropods and higher animals such as hedgehogs (4). On a global scale, the loss of biomass is estimated at around 25% per decade. However, if we take the case of insects, they are still very little studied. For example, only 3% of arthropods are known. In-depth studies by Klink on biomass and insects provide a more nuanced picture than that predicted by extrapolations. The opposite is even reported, e.g. an increase in the abundance of freshwater insects of the order of ~ 11% (5)

However, there are also observations that show that in other cultivated areas, or following the return of forests, the development of waterways, the multiplication of ornamental gardens in cities or in private homes, biodiversity has been enriched by the gradual adaptation of native species to their new environments and the introduction of new species. Wherever humans have settled, new biodiversity has been created over time, and this has been the case for as long as agriculture has existed, i.e. for thousands of years.

So can we expect to benefit from the ecosystem services that this new biodiversity is still able to offer us in agriculture?

Agriculture is not the only cause of the depletion of certain animal species. Ornithologists deplore modern construction methods in our towns and villages. For example, in the PACA region, land pressure, habitat fragmentation and agricultural practices have reduced species specialising in forest environments by 29%, species specialising in agricultural environments by 25% and species specialising in built-up environments by 34% (6).

However, there are also observations that show that in other cultivated areas, or following the return of forests, the development of watercourses, or the multiplication of ornamental gardens in cities or in private homes, biodiversity has been enriched by the gradual adaptation of new species. Wherever humans have settled, new biodiversity has been created over time, and this has been the case for as long as agriculture has existed, i.e. for thousands of years.

a) Biotope: A biological environment with relatively uniform living conditions.
b) biocenosis: All the animals and plants belonging to an ecosystem.

The biotic interactions.

Forests, grasslands and all other non-cultivated areas are considered natural reservoirs of living diversity where pests and their own predators live together. For example, when a plant is invaded by aphids, the proliferation of aphids eventually attracts its own predators (ladybirds, hoverflies, midges, lacewings, etc.), which in turn reduces the proliferation of pests. A new balance is established to correct a natural disorder.

However, this theoretical model is often unstable, especially in cultivated areas. The more effective the predator of the pest, the more unstable it is, giving rise to the "biological control paradox" for the following reason:

When the pest population decreases, so does the food supply for the pest's predator, which in turn leads to a depletion of the beneficial insect, which eventually starves to death. For this reason, it is impossible to achieve both a strong and long-lasting reduction of the pest. In many cases, the extinction of the beneficiary (by natural emigration to a region or starvation) is followed more or less quickly by the return of the pest, especially if it can move over a long distance (as is the case with aphids). The beneficial insect also returns, if its new host is not located too far away and if the beneficial insect is still sufficiently abundant; a situation that depends on many changing and uncontrollable factors (such as climate, interaction with other competing species, etc.). To give a few examples, how often does a home gardener have to deal with massive aphid attacks on cucumbers, beans, carrots in the middle of summer! Such attacks are possible until the first frosts of spring in the PACA region and can cause considerable damage to crops.

In some texts dealing with biodiversity, it is often stated that the greater the diversity of animal and plant species, the more the ecosystem tends to be in balance. In a well-diversified environment, a pest would be more likely to encounter one of its predators. Conversely, anything that reduces the diversity of species in an area of biodiversity would have the consequence of impoverishing a possible response of useful auxiliaries against a pest invasion. But it is not that simple. It is now well accepted that it is not the number of species that determines the functioning of an ecosystem, but the functional diversity and the functions that these species perform in the ecosystem (7). A meta-analysis of the scientific literature by French researchers at INRAE showed that while tree diversity protects forests against attacks by herbivorous insects, it is above all the composition of the mixtures (e.g. their composition of hardwoods and softwoods) that protects the trees against pests (8).

The different host varieties capable of harbouring a pest and the more or less extensive diet of the pest's predators are important factors modulating the pest's population. Knowledge of these relationships and how they regulate each other can lead to interesting conclusions that can be applied to practices that reduce the impact of pests. To understand the interactions between hosts, pests and their predators, I cite the olive tree as an example, as it is very present in the PACA region.

It is well known that the fruit of the olive tree is attacked by the larva of the fly Bactrocera oleae, which can cause considerable damage. Fortunately, for the grower, but not for the olive fly, the latter is also a prey appreciated by various predators that are not difficult to find substitute food when the olive fly becomes scarce. This interesting feature deserves to be investigated further

Some natural enemies of the olive fly

The Pnigalio mediterraneus, which is a hymenopteran parasitoid of the olive fly, is known to parasitise the holm oak moth as well as the apion croceifemoratum beetle, which is fond of a Mediterranean plant: the fetid anagyre or stinking wood.

The Psyttalia concolor is an endoparasite known since the beginning of the 20th century to paralyse the diptera infesting the caper, the lyciet and the jujube.

Most plant pests are also prey for other organisms (insects, glass, bacteria, etc.) and the latter, like their victims, are sometimes polyphagous. This means that if these pest predators do not find their favourite prey, they can feed on other pests that parasitise other plants, which can then serve as reservoirs of useful insects. To take the case of the olive tree, some predators of olive tree pests have been known for a long time. Some of these predators are already mentioned in a book dating back to the colonisation period, where the author (R. Poutiers (9)) recommended associating other fruit trees with olive trees and keeping hedges and bushes in their vicinity to feed the parasitoids of the pests.

The albizia, an often overlooked friend of the gardener

In vegetable growing, there are also multiple interactions between pests, their predators and their hosts. The carrot fly, for example, can attack celery, parsnips and parsley. Carabid beetles and staphylines devour the eggs and larvae of the carrot fly. These predators are polyphagous and also feed on various pests, including snails and slugs.

Albizia ♦

here is Another very interesting and useful example for all forms of crops subject to aphid infestation:

Ladybirds are known to feed on aphids. When ladybirds run out of aphids, they either migrate to another area or look for a substitute prey in the neighbourhood. Ladybirds can be forced to stay in the vicinity of a vegetable garden by providing them with food and shelter by planting an albizia tree which is known to be frequently invaded by whiteflies and psyllids (a). The ladybirds will then thrive in this ecological niche (provided they never use treatments to destroy whiteflies) until they have exhausted this food supply.

When there is no more prey, ladybirds will migrate to flowers known to attract them where they will feed on nectar. It is therefore a good idea to set aside an area of the ornamental garden for them, with seeds of blueberries, dill, marigolds, coriander, tansy, geranium, cosmos, yarrow, tuberous milkweed, coreopsis....

If aphids colonise a vegetable garden, there is a good chance that adult ladybirds will migrate to this new feeding ground. If this is not enough, the gardener can also transfer them manually. Rhododendrons and azaleas (to be planted in heather and acidic soil), chelids, mint and verbena can also be invaded by whiteflies, as well as some vegetables (tomatoes, cabbages, aubergines, cucumbers, squash, beans...) which are best avoided.

The albizia psyllid is a few millimetres long and has yellowish-green roofed wings during the summer season, turning brown in the autumn. For more information click here

Biological control by conservation or augmentation.

Larva and adult ladybird on a whitefly-infested albizia leaf ♦

The reader of these lines now understands why it is astute to look for all possible associations between plants likely to keep pest predators in place, which will result in a considerable reduction in some cases (or even total) in the use of synthetic or organic plant protection products. These plant protection methods are part of the biocontrol techniques known as the fight biological by conservation.

Unfortunately, scientific studies aimed at quantifying all the biotic interactions that could be useful in agriculture are still insufficient, especially as not all species are known, particularly in the infinitely small, since microorganisms also have an important role to play in the balance of ecosystems. For example, it has been discovered that bacteria and viruses can attack bio-agressors...

It is also necessary to take into account the favourable or unfavourable abiotic interactions, which are often uncontrollable (physical factors such as climate, sunshine, water resources, type and configuration of the habitat, cultivation practices, etc.) and which also intervene in the functioning of ecosystems.

Some studies are still partial, or even involve a poor appreciation of useful beneficials and their roles in integrative biological control. For example, the parasitoid eupelmus urozonus is often cited in some writings on olive cultivation for its regulatory properties against the olive fly. This parasitoid parasitises a fly (myopites stylatus), a cousin of the olive fly, which thrives on the slime plant (Inula viscosa), a common perennial in the Mediterranean region. This parasitism takes place in winter, and after a period of overwintering in the soil, it was thought that in spring the eupelmus parasitises the olive fly. Recent studies have shown that there are several species of Eupelmus associated with the olive tree and the slimy inula that are relatively more host-specific, thus counteracting the natural control scenario of the olive fly that was imagined a few years ago.

When the mobilisation of natural regulations in a cultivated landscape is often not sufficient to regulate pest populations, it is possible to go further by importing some native pest predators into a cultivated area.

This active technique where the farmer intervenes directly in an ecosystem to reduce the pressure of a pest is called the fight biological by augmentation. This involves artificially increasing a population of predators living in the area to a density that allows satisfactory control of the pest. If regulation is sought over time by an insubstantial supply of the predator when it is deficient, this type of crop protection is also called " the fight biological by inoculation ". the fight biological by Flooding " is a variant of this technique of plant protection by a massive import of a pest predator in order to act quickly. In this case, an attempt is made to use a curative action to combat a pest outbreak in the same way as a pesticide, without the disadvantages.

the fight biological by augmentation is developed in the chapter of this website: "Importation of beneficial insects" with some specific examples to control the pests most often encountered in the vegetable garden.

The fight biological by acclimatisation

For bio-predators accidentally imported from another continent, The fight biological by acclimatisation consists of identifying a useful pest, usually from the area of origin of the pest, and introducing it into the area to be protected. This transfer is only effective if the beneficial insect can survive in its new environment and does not produce a harmful imbalance in local biodiversity. As this strategy is more delicate, parasitoid Hymenoptera are nowadays preferred in research because of their narrower host range. These important discoveries have shown their effectiveness, especially in greenhouse crops and sometimes in open environments. There have also been setbacks, especially in the open, with a very low efficiency rate (detailed here). LBiological control by acclimatisation in the vegetable garden is not discussed on this website because of its difficulties, especially since for a small family garden, biological control by conversation and augmentation with physical protection are largely sufficient to control many pests.

Ecosystem services; sometimes conflicting biotic interactions

Green Nazera Viridula bug on a ripe tomato with yellow spots caused by the insect's bites ♦

One might think that the protection systems outlined above are easy to implement for many pest predators in relation to their alternate hosts. Unfortunately, this is not always the case. Interactions between them are not always beneficial, especially when they favour other pests that are difficult to control.

For example, the hibiscus, which is a shrub that is very present in ornamental gardens in the Provence-Alpes-Côte d'Azur region, has the advantage of producing flowers throughout the summer until the first frosts in the autumn. The hibiscus is frequented by all pollinating insects and hoverflies (the adults feed on nectar while their larvae are aphid predators). This shrub is therefore very useful for the preservation of biodiversity and as a reservoir for hoverflies.

Hibiscus is a favourite prey of aphids in early spring. Although the hibiscus does not seem to be particularly affected by these sucking insects, it can serve as a vector for the spread of aphids, but also for the establishment of their predators, particularly ladybirds. For this reason, I do not try to destroy the aphid colonies that invade the hibiscus in my ornamental garden in early spring.

The hibiscus is also much appreciated with the green bug Nazera Viridula which is very common in the Provence-Alpes-Côte d’Azur region. It is assumed that this green bug originates from Ethiopia and can wreak havoc in a vegetable garden. This polyphagous bug has the particularity of invading tomato, squash and bean crops. Thus, planting hibiscus in the hope of encouraging pollinators and fixing aphid predators, also creates breeding grounds for the Nazera Viridula bug.

Fortunately, there is a parasitoid fly called Trichopoda Pennipes from the USA whose larvae feed on the internal organs of the Nazera Viridula bug. In general, this fly, which resembles a hoverfly, lays white eggs that are easily detected near the head of the pest. These eggs give rise to small larvae that migrate into the body of the bug. This fly is found in our gardens. However, the damage to crops is sometimes severe. In some seasons, this fly is unable to control the proliferation of the Nazera Viridula bug, which requires the use of a pesticide.

Another solution for controlling the Nazera Viridula bug is for the home gardener to wrap the tomato bunches with insect-proof netting held to the stem by UV and moisture resistant thread. The net should not be in contact with the fruit to prevent bugs from biting through the net.

Hibiscus is not an isolated case. The oleander, which is very common in the Provence-Alpes-Côte d’Azur region, flowers all summer long. This is why it is considered a very useful shrub for the preservation of all pollinators. However, the carrot fly Spodoptera littoralis can also be found in the flowers of this laurel and feeds on pollen and nectar while the larva develops in the roots of carrots and other root vegetables by digging galleries.

And incompatible proximities that need to be known

The planting of certain trees near a vegetable garden is problematic if the area is particularly affected by moth attacks. There are three varieties of moths in relation to the feeding habits of their larvae:

These predators are very active in the Mediterranean region and are polyphagous and can also attach themselves to certain ornamental plants in the ornamental garden (notably phlox, carnations, roses and pelargoniums) and vegetable plants. Carrots are good hosts as well as tomatoes, lettuce, radishes, celery, cabbage, artichokes, spinach and others. Thus, the choice of certain plant varieties in an ornamental garden juxtaposed with a vegetable garden can have more or less harmful consequences on vegetable plants.

So should we chase pelargoniums, roses and carnations out of the pleasure gardens? Not necessarily. These plants participate in other ecosystem services, many of which are still poorly understood. Any action that is thought to be favourable in order to modify an environment can have other effects, often in cascades, some of which could be negative. The above examples simply show the complexity of the interactions between the different actors in an ecosystem and why we cannot envisage an agriculture that would be protected solely by interactions between plants (which is advocated in permaculture). In biological pest control, ecosystem services therefore have their limits, especially as they are influenced by changing and uncontrollable factors.

This is another example of how difficult it is to rely on natural ecosystem balances to regulate the pressure of certain pests. All amateur gardeners know that open-air radish and carrot crops easily catch the maggot, which digs galleries in the roots, even if the crop plots are surrounded by grassed paths, orchards or meadows. Year after year, the losses become more and more important. The problem is such that many amateur gardeners who do not wish to use pesticides end up abandoning the outdoor cultivation of these root vegetables.

The expected results of preserving so-called "natural" biodiversity as the only control system for reducing pest pressure are therefore largely dependent on complex and random factors. Biological control by augmentation or acclimatisation also has its limitations. Many examples are described in another article on this website, which can be accessed by clicking here.

Are crop pests few in number?

In writings or websites promoting various variants of non-scientific agroecology, it is sometimes stated that pests are few and most insects are not harmful. For some crops, however, the number of pests is quite high. Alliums (onion, shallot, garlic) are more or less attacked by a dozen species of invertebrates: nematodes, mites and insects. For peas, 25 pests have been identified, 16 for cherries, 22 for raspberries and 21 for apples....(11).

Moreover, plants are not only attacked by pests. Crops are often victims of cryptogamic, bacterial or viral diseases that can cause significant damage or even the loss of an entire crop. Certain pathogens often return to crops, such as downy mildew on tomatoes and potatoes in the north of France, bean mosaic (transmitted by aphids), scab on apple trees, etc. These endemic infections cannot be controlled by pest predators.