Integrated biological protection in agriculture

For more than 50 years, the development of the chemical industry has provided an effective response to crop-threatening pests becoming an essential modelhas and widespread model for plant protection. Fertilisers and pesticides have enabled high yields to be achieved to meet the growing world population (a). But pesticides have a negative effect on the environment and have eventually selected for bio-aggressor that are resistant to the molecules on the market. These pesticide resistances have also developed in the vegetable garden; for example, the green aphid Myzus persicae, some strains of which survive neonicotinoids and pyrethroids (and related molecules; the pyrethrins approved for organic farming) (1). This aphid, which likes peaches, is very polyphagous and is content with at least 50 botanical families, including cucurbits (cucumbers, gherkins, etc.), solanaceous plants (potatoes, etc.) and brassicas (turnips, etc.). Research was then directed towards less aggressive alternative solutions: selection of varieties more resistant to diseases, biological control by spraying predators of bio-aggressors, use of traps containing hormones, etc.

Nowadays, we no longer treat crops blindly with multipurpose combinations containing different plant protection products as soon as we discover a spot on a fruit or a leaf. A pest does not have to be dreadful, a disease can be benign and not require any treatment, and the removal of a diseased organ is sometimes enough to eradicate an emerging disease. Plant protection products have also been improved to reduce their dosage and to target pests more effectively, with less impact on the environment.

To reduce the use of pesticides, an overall strategy has been defined called Biological Integrated Protection (BIP) or, for fruit, Integrated Fruit Protection (IFP). BIPs and IFPs include biocontrol techniques that consist in optimising the interactions between the different components of an ecosystem in order to reduce pest pressure (biodiversity corridors, importation of beneficial insects, physical protection against insects, sterile insect technique (SIT), biocontrol plant protection products....). However, it is also necessary to take into account the formidable capacity of pathogens and pests to adapt to their environment, which sometimes requires the use of pesticides approved for use in organic farming, or even a return to synthetic pesticides.

In integrated biological protection, pesticides are used when all other methods have failed, but not under any conditions. Pest populations must exceed a threshold beyond which irreversible crop damage occurs, resulting in unacceptable economic losses. Biocontrol plant protection products, including those approved for organic farming, are used first. If this fails, synthetic pesticides are then used.

Biocontrol techniques cannot solve all the problems encountered. In addition, the farmer often has to deal with problems of use that are less flexible than those of plant protection products, such as the constraints of conserving living organisms (bacteria, beneficial insects, etc.), negative interference with other biological phenomena, and the specificity and formulation of applications.

As for professional farmers, the gardener is confronted with the hazards of biological control. They sometimes have to deal with highly contagious diseases causing considerable losses (rust, mildew, etc.) which require anticipation of attacks. But, before taking their sprayers, many gardeners should first inform themselves about the characteristics of the bio-aggressors that would threaten their crop. Confusion with beneficial insects, unnecessary treatment of infections accompanying natural ageing, physical stress mistaken for microbial or fungal infection are some examples of common mistakes made in the vegetable garden.

Most of the biocontrol techniques that can be used in a vegetable garden are described by clicking on one of the menu items at the top right of this page.

a) From 1960 to 2005, the world population increased from 3 billion to 6.5 billion and the utilised agricultural area (UAA) per capita decreased from 4,300 m² to 2,200 m² (source UN-FAO).

When to act against bio-aggressor ?

Home gardeners find that they are often confronted every year with invasions of certain pests (e.g. aphids, larvae that burrow into carrots). Other bio-aggressor are occasional and are more or less aggressive depending on changing factors that the gardener cannot control (such as the climate). In the same region, a cultivated plot may be commonly invaded by one or more dominant bio-aggressor that do not exist (or are less numerous) in another plot located a few kilometres away.

The existence of dominant bio-aggressors that frequently return to crops requires anticipating their attacks by starting to modify the environment (for example, plantations that favour the attachment of pest predators, etc.), installing physical protection (anti-insect nets), and providing for the importation of useful auxiliaries. Occasional pests are treated on a case-by-case basis, depending on the degree of invasion and their aggressiveness, the resistance of the crops, which varies from one species to another, and above all the cultivation method (a poorly nourished and weak crop has difficulty resisting pest attacks).

Some examples of benign diseases that do not require phytosanitary treatment:

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Apical necrosis on tomatoes (also called "apical rot" or "black ass")

This disease is characterised by black spots appearing on the fruit, but opposite the stalk. This benign disease is favoured by too much watering, or a boron deficiency revealed by laboratory analysis, or the choice of certain very sensitive varieties such as 'Roma'.

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Growth cracks in tomatoes

caused by excess water.

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Black root rot

blackening of the roots of carrots, endives, celery, melon, lettuce, tomatoes, beans, cucumbers: these are cryptogamic infections resulting from poor growing conditions that weaken the plants. To fight against these opportunistic infections, it is necessary to act on the cause and not on the consequence: excessive humidity, unbalanced or insufficient fertilisation...

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Fumagin

black crust formed by several fungi accompanying insect bites, especially aphids, leafhoppers and mealy bugs. The fungi develop on the honeydew left by the biting insects without entering the plant. The plant is not invaded. It is sufficient to cut off the diseased parts, isolate the plants with physical protection and possibly treat the aphids if their predators are not yet present, or import aphid predators.

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Opportunistic infections accompanying the natural ageing of a crop:

Some diseases considered problematic before harvesting, appear at the end of a crop cycle such as verticillium wilt and powdery mildew. It is then preferable to pull out the infested plants after harvest and before these diseases are transmitted to other more recent crops.

Limitations of biocontrol methods.

In market gardening, biocontrol methods using beneficial insects are fairly easy to implement for crops grown under greenhouses or protected by anti-insect nets. For field crops, the use of phytosanitary products is still the preferred means of control for many farmers because of the difficulties encountered in implementing alternatives to pesticides. For example, in south-eastern France, apple growers are confronted with the ravages of codling moth, which has a high dispersal capacity and a strong preference for pome fruit. Even at very low population levels, this pest can cause significant damage. The natural enemies of codling moth, notably the parasitic Hymenoptera whose females lay eggs and larvae, are proving insufficient to neutralise this formidable bio-aggressor.

Other examples of difficulties encountered in importing auxiliaries for use in open settings include :

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An introduced population from beneficial insect breeding may experience genetic drift. Depending on the number of individuals selected, the different genetic variants are more or less well transmitted. For the breeder of beneficial insects, the smaller the starting population, the lower the richness of genetic variants, the greater the risk of producing crosses between related or inbred individuals, and the greater the likelihood that an individual will carry unfavourable genes resulting in reduced fecundity or susceptibility to disease. Populations introduced for biological control are often characterised by a decrease in genetic diversity with low rates of heterozoology. However, parasitoid Hymenoptera do not experience this genetic drift because of their haplodiploid mode of reproduction.

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Some beneficial insects have a very wide diet and can even attack plants. For example the predatory bug Nesidiocoris tenuis is very polyphagous and attacks a wide range of prey, and for this reason it was used in biological control programmes in Maroc and in greenhouses in France before it was found that this bug can bite tomato apices, sometimes causing significant damage. The causes of this diversity in behavioural adaptation are still poorly understood.

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The natural control of a pest predator is always delayed by the time the pests reproduce to provide sufficient food for the predator. During this period, the farmer experiences varying degrees of crop damage. This damage is problematic when the pests carry pathogens that produce incurable diseases such as virus infections. Natural pest population control is also influenced by other factors such as weather conditions that can delay the emergence of beneficial insects.

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The effectiveness of a biological treatment depends on the mode of action of the beneficial insect. While an immediate benefit is possible by a massive importation of certain predatory insects such as ladybirds, other pest predators such as parasitoids only produce a delayed benefit while their larvae develop in the host before neutralising it, leaving the pest free to ravage a crop.

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The establishment of biological control by importing a useful insect may be accompanied by a beneficial or negative symbiotic process with bacteria or viruses. For example, the introduction of enthomopathogenic nematodes into the rhizosphere of a plant (to neutralise other root-destroying nematodes) is characterised by the emergence of bacteria that are symbiotic with the introduced nematode, and whose metabolic activity is essential to attack the pest. In addition, these bacteria deliver a cocktail of natural insecticides, fungicides and antibiotics that protect the crop. On the other hand, this introduction of useful nematodes may be accompanied by the introduction of opportunistic pathogenic organisms (fungi, bacteria, etc.), which reduce the expected effect.

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Some predators introduced for biological control purposes may interact with other natural enemies present spontaneously in the crop area, producing in return an imbalance in biodiversity. For example, abundantly introduced beetles of the genus Pterosichus can consume aphids previously parasitized by the larva of a hymenoptera, thus altering a natural factor of balance in the aphid population.

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The actionf predators of bio-aggressors can be altered by intra-guild predation. Intra-guild predation is the predation of one predator by another pest predator, resulting in the protection of the pest being controlled. For example, under certain environmental conditions, birds may feed on flying predatory insects that feed on lepidopteran pests of cabbage.

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In areas where pests have become resistant to chemical insecticides, mating disruption (which consists of diverting male moths with hormones to prevent fertilisation by females) is an alternative solution with clear advantages. This technique is, for example, effective against the fruit moth. In case of heavy rainfall, the sprayers remain active while the chemical protection is washed away. However, this technique of male confusion is considered ineffective for small amateur plantations (2). This technique has proven to be most useful in integrated agriculture for uniform areas larger than 5 ha, or even 10 ha when pest pressure is not very high.

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The introduction of beneficial insects from another continent to control an invasive pest is not easy to manage. Some beneficial insects find substitute hosts that allow them to spread to other territories, which results in new pressure on native species, leading to their rarefaction or even their disappearance. This is the case of the Asian ladybird (Harmonia axyridis) introduced into Europe and North America. This introduction soon created serious problems through damage to biodiversity (this insect feeds on the larvae of native ladybird species). In Europe, this ladybird is now considered invasive and has been classified as a pest in Great Britain.

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Another example of a failed import is the weevil Rhinocyllus conicus, introduced in the 1970s to America to reduce pressure from the leaning thistle of European origin. This weevil now thrives on several North American Cirsium thistle species with undesirable effects on the associated phytophagous community.

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Birds, bats and all insectivores do not distinguish between the usefulness of certain insect predators of pests, which may have consequences for natural pest control. Cats destroy lizards, even though lizards are insectivores. Cats also kill snakes, which are useful in reducing the proliferation of snails (3), but the cat population has increased significantly in housing estates, resulting in the depletion of some useful species.

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As with plant protection products, biocontrol methods can be subject to resistance, some examples of which are given below:

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The beet moth (Spodoptera exigua) originating from South-East Asia, which is known to cause many problems in greenhouse crops (including peppers), has become resistant to plant protection products and Bacillus thuringiensis.

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For orchards, it has been possible for some years to use the granulosis virus (Cydia pomonella granulosis virus), which produces diseases in insect pests. Resistance to this virus has been demonstrated. An INRA study published in 2009 (4) states that since 2005 "a strong resistance of codling moth to granulosis virus (CpGV) has also been demonstrated in AB orchards in some localities of south-eastern and central France... Generally speaking, the risk of acquiring resistance is high in AB orchards due to the low diversity of solutions that can be used and their lower efficacy requiring repeated applications (10 to 15 annual treatments with CpGV over more than 10 consecutive years in certain plots, for example, to avoid the appearance of resistance, it is preferable to use this product in alternation with other phytosanitary products)."

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Resistance to mating disruption has been observed in some lepidopteran species (5)

Biocontrol methods are being extensively researched around the world and it is likely that in the coming years we will see an improvement of existing solutions and the emergence of new effective techniques to control bio-aggressors.