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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Some bio-aggressors are a serious threat to the environment as soon as they appear. This is the case of xillela fastidiosa in olive trees, flavescence dorée in grapevines, sharka in stone fruit trees, and most viruses that can infect all kinds of ornamental, fruit and vegetable plants. The most threatening attacks must be declared to the Prefecture and treatment of these pests is compulsory (see the article "Compulsory control of regulated pests").

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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 (1) - (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. You can follow the evolution of this ladybird in France by clicking here.

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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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After effective treatment of a crop, an imported beneficial insect may have difficulties in maintaining itself in its new environment, especially when it cannot find a substitute host. It can then disappear, notably through the "allee" effect (reduction of the reproduction rate when the population falls below a certain threshold).

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Colorado beetle

It only takes one pest species to destroy an entire crop. This is the case, for example, with the Colorado beetle, an oligophagous insect accidentally imported from America that can quickly destroy an entire potato or aubergine crop. The fight against this pest still relies mainly on chemical pesticides. In France, for the amateur gardener who can no longer use these pesticides from 1 January 2019, rapeseed oil ester, which belongs to the group of biocontrol plant protection products, seems to be effective.

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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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Parasite pressure is increasing with the importation of new species facilitated by international trade and tourism. These new species often have no predator in their new habitat and therefore cannot be controlled naturally. It is very difficult to avoid this pitfall unless tomorrow we ban all freight transport and citizens from travelling. The number of new pests introduced each year has increased from 1.61 species between 1950 and 1999 to 6.78 species between 2005 and 2014 (4). Here are some examples:

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Drosophyla suzukii originating from South-East Asia was introduced into France in the 2000s. Unlike its cousin Melanogaster, which prefers decaying fruit, Drosophyla suzukii prefers ripe or ripening fruit, to the great displeasure of fruit growers. This devastating fruit fly is terribly invasive and has no predators in Europe capable of regulating its development. Apart from anti-insect nets, there are no really effective organic techniques against this bio-pest to date. Arboriculturists are faced with a real scourge that would jeopardise the entire European and even global economy if they decided to abandon pesticides.
In vegetable growing, the tomato, potato and aubergine leafminer (Tuta absoluta) originating from South America has recently been introduced into the Mediterranean region.

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In vegetable growing, the tomato, potato and aubergine leafminer (Tuta absoluta) originating from South America has recently been introduced into the Mediterranean region.

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The Californian thrips (Frankliniella occidentalis) occurs on a wide variety of plants including cucumber, pepper, aubergine and ornamental plants and weeds.

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Other recently introduced alien species include the Asian hornet, Japanese knotweed (an invasive plant listed by the International Union for Conservation of Nature as one of the 100 species of greatest concern), and water primrose (an invasive plant in ponds and rivers). There is a lot of talk about the disappearance of certain animal species as a result of the action of pesticides, but between 2005 and 2014, 61 new insects appeared in France (i.e. about 7 per year).

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As with plant protection products, biocontrol methods can be subject to resistance:

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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 (5) 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 (6).

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.

Limitations of biocontrol methods.