Beneficial insects against pests

A beneficial insect is a living organism that naturally regulates a pest population. There are two main groups. Predators consume their prey directly, like the ladybird that devours aphids. Parasitoids, on the other hand, lay their eggs inside a host, whose body nourishes the larva before it emerges.

This distinction is not merely academic. A predator acts quickly and is visible, which is reassuring. A parasitoid acts slowly and stays out of sight, but establishes itself durably when conditions suit it. Against an active infestation, the two are often used together: the predator to bring the population down, and the parasitoid to maintain control over time.

Most of these organisms are reared and sold by specialist European producers, packaged in vials, sachets, or hanging cards. They travel poorly beyond a few days, which explains the short delivery windows and the importance of releasing them promptly after receipt.

In conventional growing, a gardener can apply a treatment and then wait out the pre-harvest interval. In aquaponics, that logic simply doesn't hold. Water circulates in a closed loop between the fish tank, the biological filter, and the growing beds. Any molecule that washes off a leaf enters the circuit and stays there, with no dilution or renewal.

Pyrethroids, which are widely used in commercial insecticides, are particularly toxic to fish at very low concentrations. Copper-based treatments cause the same problem and also disrupt nitrification. Even black soap, often recommended as a safe option, warrants caution if the runoff finds its way back into the pond.

Sticking to biological methods is therefore not an ideological stance, it is a technical necessity. It is also what makes aquaponics both demanding and rewarding, since the health of your crops depends on an ecosystem you manage yourself, as illustrated by the most common beginner mistakes.

The aphid remains the number one pest in water-based growing systems, particularly on lettuces, chard and herbs. It is identified by clusters of soft-bodied insects, green, black or pinkish, gathered on young shoots and on the undersides of leaves, where the sap is most accessible.

Two signs betray its presence before it is even spotted. Honeydew, the sticky, shiny substance it excretes, coats the leaves and subsequently attracts a black sooty mould. Ants moving up and down the stems are a second warning signal, as they farm and protect the colonies.

Its strength lies in its speed. A female gives birth to fully formed offspring without mating, and a population can double within a few days when temperatures approach twenty-five degrees. Dealing with the first colonies takes infinitely less effort than trying to regain control three weeks later.

Everyone recognises the adult ladybird, far fewer people recognise its larva, which looks like a small slate-grey alligator spotted with orange. Yet it is the larva that does the real work. Depending on the species and conditions, a single larva typically consumes several hundred aphids during its development, whereas the adult's appetite is considerably more modest.

This difference has a practical implication. Suppliers most often sell larvae rather than adults, because adults tend to fly off shortly after release, while the larva, unable to fly, stays on whichever plant you place it on. If you spot these larvae appearing on their own in your garden, whatever you do, don't crush them.

Ladybirds give good results against concentrated, visible infestations. They are less suited to diffuse ones, where parasitoid micro-wasps have the upper hand thanks to their ability to actively seek out their hosts.

Aphidius colemani is a parasitic wasp measuring two to three millimetres, completely harmless to humans. The female seeks out aphids and lays a single egg inside each one. The larva develops within the host, whose body then hardens and takes on a characteristic golden-beige colour.

These husks are known as mummies, and they are your best indicators. A mummy with a round exit hole means a new wasp has already emerged and is hunting in your greenhouse. Never clean leaves that carry them, they represent a free, self-sustaining colony.

Commonly recommended release rates are around 0.5 individuals per square metre as a preventive measure, 0.5 to 1 for an early infestation, and up to around 2 for reinforcement, with repeated releases every eight to fifteen days. The species performs well between fifteen and twenty-five degrees, which covers the majority of European greenhouses during the main growing season.

The green lacewing, recognisable by its translucent veined wings and golden eyes, is one of the most useful beneficial insects in protected cultivation. As with ladybirds, it is the larva that does the hunting. Equipped with long pincer-like mandibles, it attacks aphids as well as young whiteflies, thrips, and spider mite eggs.

This versatility makes it a sound choice when several pests are present at once, a frequent situation in a small domestic greenhouse growing a variety of species. It avoids the need to order multiple highly specialised beneficial insects.

The larvae are generally supplied in a vermiculite or buckwheat carrier, to be distributed directly over the affected foliage. Commonly cited application rates range from around ten to fifty larvae per square metre, depending on the severity of the infestation. One important point: the larvae are cannibalistic, so they must be spread out and never grouped together in the same spot.

The whitefly, often called the greenhouse whitefly, rises in a cloud the moment you brush against a plant. It is particularly fond of tomatoes, aubergines, cucumbers and basil, and also produces a sticky honeydew that soils the harvest. Its larvae, flat and translucent, remain fixed to the undersides of leaves and often go unnoticed.

Encarsia formosa is the reference parasitoid against this pest, used in professional greenhouses for decades. It is supplied in the form of cards bearing glued pupae, to be hung in the foliage. Whitefly larvae that have been parasitised turn black, making monitoring very easy to follow with the naked eye.

Published application rates vary according to pest pressure, generally from 1.5 to 6 individuals per square metre per release, with two to three releases spaced approximately eight days apart after planting, then monthly maintenance. This beneficial insect requires warmth and becomes noticeably less active below eighteen degrees, which limits its use at the beginning and end of the season.

Red spider mite is not a spider but a tiny mite, barely visible without a magnifying glass. It thrives in warm, dry conditions, exactly the climate of a closed greenhouse in high summer. Leaves become speckled with pale dots, turn yellow, and then become covered in a fine, distinctive webbing.

Phytoseiulus persimilis is the predatory mite specifically targeted against it. Faster and slightly larger than its prey, with a bright orange colouring, it consumes both eggs and adults and can clear an infestation in two to three weeks. Recommended rates are typically between two and ten individuals per square metre, adjusted according to the severity of the attack.

The most effective lever, however, remains climatic. By raising relative humidity above sixty per cent through floor damping and controlled ventilation, you significantly worsen the reproductive conditions for red spider mite while favouring its predator.

Beneficial nematodes are microscopic worms that live in moist grow medium and penetrate insect larvae to kill them. Steinernema feltiae is the most widely used species against fungus gnat larvae, while other species target vine weevils or certain soil-dwelling larvae.

They come in powder form, to be diluted in non-chlorinated water and then applied with a watering can over the entire surface of the growing bed. One technical point is particularly important: the temperature of the grow medium must generally exceed twelve degrees, and the medium must remain moist for several days to allow the nematodes to move around.

Aquaponics offers them precisely these favourable conditions, with a bed of clay pebbles kept constantly moist. Store the sachets in the refrigerator, observe the use-by date, and apply preferably in the evening, as ultraviolet light destroys them rapidly.

Cabbage white and noctuid moth caterpillars attack mainly brassicas, chard and lettuces, leaving clean bite marks and clearly visible green droppings. On a small growing area, hand-picking in the evening remains the most straightforward and economical method.

When pressure increases, Bacillus thuringiensis is the benchmark treatment in organic growing. This bacterium produces protein crystals that destroy the gut of caterpillars after ingestion, with no effect on insects that do not feed on the treated leaves. The kurstaki strain is the one that targets lepidopteran larvae.

Its selectivity makes it more reassuring than a broad-spectrum insecticide, but caution remains essential in aquaponics. Treat on overcast days, avoiding any run-off towards the fish tank, and stop the water circulation during application if your system allows it. If in any doubt about a product, withholding treatment and picking by hand is preferable to a risky trial.

The best beneficial organism is the one you never need. Before ordering anything living, keep pests out in the first place. In a greenhouse, that means fitting all vents and doors with a fine-mesh netting, the most cost-effective physical barrier there is.

The mesh size determines what you stop. A mesh of around one millimetre blocks cabbage white butterflies and most flies, whereas you need to go significantly finer to contain whitefly and thrips. The finer the mesh, the more it restricts airflow, so you should check your ventilation after installation.

Complement this with a few simple habits. Inspect every new plant before bringing it in, as pests most often arrive with a purchase from a garden centre. Remove dead leaves lying on the ground and space plants far enough apart to allow air to circulate between them.

The first mistake is leaving yellow sticky traps in place after the release. These traps are excellent for detection and monitoring, but they also catch your winged beneficials, particularly the parasitic wasps. Use them for diagnosis, then remove them before introduction.

The second mistake is prior treatment. An insecticidal soap or oil applied a few days beforehand leaves residues that kill the beneficials on arrival. Allow a minimum of one to two weeks between any treatment and a release, and longer for persistent products.

Then there is impatience and poor weather. A parasitoid will not bring a population down in three days, it takes two to four weeks to see a clear effect, and generally requires several successive releases rather than a single large one. Finally, release at the end of the day, never in full sun on scorching leaves, and do not forget to manage the ants that actively protect aphid colonies from your beneficial insects.