Agroecology: Ecosystem Services in the Vegetable Garden

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.

The albizia, an often overlooked friend of the gardener

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

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 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. 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

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::

• Defoliator caterpillars feed on leaves,
• The soil caterpillars (also known as cutworms) eat the crowns and roots,
• The leafminer caterpillars dig galleries in the stems, petioles and flower stems.

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, when seeking to preserve so-called “natural” biodiversity as the sole system for reducing pest pressure, therefore remain largely dependent on complex and unpredictable factors..

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.