# Biological control of pests in agriculture: methods, examples and planning

URL: https://leafwrit.com/ie/biological-pest-control/
Reviewed: 2026-10-29

**Biological control uses living natural enemies to suppress a pest.** Predators consume prey, parasitoids develop in or on a host, and pathogens cause disease in the target organism. Combine these approaches with prevention, monitoring and crop husbandry in integrated pest management. Biological control is established practice in Irish protected nursery production.

## Identify the target before selecting an enemy

Identify the pest, crop and growing conditions first. Conserving resident beetles and introducing specialist glasshouse predators need different evidence and management.

Note the crop name, the propagation or production stage, whether the crop is protected or outdoor, and the symptoms you see. Write down where the organism was found: growing point, flower, underside of a leaf, root zone, substrate or harvested material. Take clear photographs and ask the crop adviser to identify the species if it is unresolved.

Give the supplier the identified species and life stage, not a description such as "small flies". Ask what evidence supports the proposed agent for that target, and check whether it comes from a commercial crop, a laboratory demonstration or a trial on another host plant.

Record the pest and damage that prompted the enquiry, and mark the affected block so later scouts can return to it and tell old injury from new damage.

## Understand what each control mechanism does

Select the mechanism for the target: predation, parasitoid development or microbial infection.

| Mechanism | Question to ask |
| --- | --- |
| Predator | Which prey stages does it consume, and where does it search? |
| Parasitoid | Which host species does it attack, and how do you recognise successful development? |
| Pathogen | Which organism or strain is supplied, and what conditions support its activity? |
| Conservation of resident enemies | Which existing habitats and management practices support the relevant population? |

Decide before release what will count as establishment and as suppression. An emerging parasitoid, an established predator population and reduced fresh injury are different observations. Write down the assessment method so everyone on the staff reads the evidence the same way.

## Separate conservation from purchased introductions

In field crops, conserve existing beneficial organisms by supporting their habitat. In protected production, managed introductions can form a more intensive programme. These routes differ in the amount of environmental control available to the grower.

For conservation, map the crop and the surrounding features the adviser's recommendation refers to. Discuss where flowering resources go, access for machinery and the existing spray pattern. Find out which natural enemies are actually present before you credit a habitat change for fewer pests.

For introductions, give the supplier the structure, compartments, crop spacing and crop duration. Say whether the release is for a whole house or an isolated infestation. Pass on the history of incoming plants and of any organisms introduced earlier. If the agent or formulation is substituted, treat it as a new proposal and ask for an explanation.

## Check the conditions where the agent must work

Ask the supplier for the operating conditions of the organism. Give them measured conditions from the production zone, including the overnight pattern and where the sensors sit, and measure where the agent will actually be released.

Think about seasonal changes, supplementary lighting, ventilation and how irrigation is distributed. Note which parts of the structure are shaded, exposed or regularly wetter, and ask how those differences affect the proposed distribution and the follow-up survey.

For a substrate-dwelling agent, describe the growing medium, floor covering and drainage. For a target in flowers or foliage, describe the canopy and access. Keep those notes with the recommendation for the next crop manager.

## Ask where a beneficial insect completes its life cycle

The aphid predator Aphidoletes aphidimyza pupates in soil. Polythene over the soil or growing medium can interrupt that stage and make establishment less successful. Discuss both the foliage where larvae feed and the ground where the midge pupates.

When you talk to the supplier about this midge, describe ground coverings, exposed substrate and the position of pots. Ask how adults will emerge and whether your layout lets the population keep developing.

## Review pesticide compatibility before placing the order

Pesticides can affect parasitoids directly and indirectly. The concern therefore extends beyond whether a live organism is visible during spraying.

Give the specialist the complete recent treatment history: formulation names, application dates and treated areas, including drenches and any other applications in your system. Ask for compatibility information that names the proposed beneficial organism and the exact formulation.

Keep the compatibility advice, its date and the products it covers. Distinguish mortality observations from effects on reproduction, development or persistence. Get the recovery or introduction advice for your actual combination instead of copying a waiting period from another product.

To resolve the identity of any formulated pesticide you are considering and its Irish use, see the [Irish register guide](/ie/pesticide-register-ireland/), and check the use conditions for every product you plan to use alongside natural enemies.

## Make delivery and handling part of the programme

Name one person to receive each consignment and read the supplied handling instructions. Ask the supplier which package condition, batch details and delivery observations to report at once, and plan storage and release around that instruction.

Before you open packs, prepare the area labelling: match the agent, crop block and intended distribution to the agreed recommendation. Keep unused material identifiable and handle it as the supplier instructs. Do not carry a storage rule over from one organism to another because the packaging looks alike.

If a delivery is delayed, tell the supplier what actually happened and keep their instructions. Record where and when you released each delivery.

## Monitor the pest and the beneficial population

Agree a scouting route, observation points and the wording of reports. Name the person who inspects the crop and the person who decides whether the programme needs changing. Keep trap observations separate from plant observations.

Record localised problems so the next scout can find them: bench, row, bay, compartment or mapped sector. Photograph an affected sample with a scale and a readable location reference, and keep observations from apparently unaffected plants if the specialist wants comparison material.

Set the next review when you plan the introduction, and ask which signs deserve an immediate call rather than waiting for the routine review. Those thresholds depend on the crop and the agent, so get the specialist's criterion for the programme you are buying.

Parasitic wasps deposit eggs inside aphids. The developing larva consumes its host; the swollen aphid becomes a mummy, and the emerging adult leaves an exit hole. Temperature affects development, so absence of visible mummies does not reliably establish absence of activity.

Prepare reference photographs for staff that show an intact swollen host, an emergence hole, a living aphid colony and unrelated debris. Send unidentified aphids, mummies and beneficial insects to the diagnostic service with their crop references. Keep the variety and plant position with each image; magnification and lighting can make a useful photograph hard to read.

Look for bright orange Aphidoletes larvae and lacewing eggs on stalks. Train scouts to recognise beneficial larvae and eggs alongside adults.

During induction, have each scout describe what they see before naming it. In reports, keep abundance, distribution and feeding injury separate.

## Recognise the limits of a successful example

In strawberry propagation, preventing virus transmission changes the aphid-control objective. Tell the supplier whether the crop is for fruit or for propagation.

Check a supplier's case study against your crop, target species, starting infestation and growing conditions. Ask what improved, over what period, and which other treatments were used.

For purchased plants, find out what health assurance your customer expects. A programme meant to protect saleable fruit may answer a different question from one meant to stop infected planting material being distributed.

## Watch for effects on the crop itself

The tomato predator Macrolophus pygmaeus can feed on plants when prey becomes scarce. Monitor how the introduced organism affects both the crop and the pest.

Discuss this risk with the specialist before introduction. Ask what injury to look for, where it is likely to appear, and how to document a suspected case. Record observations neutrally rather than assuming every mark comes from the original pest.

Give the adviser a timeline linking pest observations, introductions, environmental conditions and later injury, and keep what the scout saw apart from what the specialist concluded. That makes later adjustments easier to judge.

## Types of biological control: classical, inoculative, augmentative and conservation biocontrol

Biological pest control has three broad types: conservation, classical biological control and augmentation. Classical biological control is the practice of importing and releasing natural enemies so that they establish against an introduced pest. Augmentation adds natural enemies: a few released at a critical time is an inoculative release, and millions released is an inundative release. Conservation protects the enemies already in the field.

Ask the supplier which of these your purchase is. An inoculative release needs the agent to establish and breed on the crop. An inundative release needs repeat orders and a record of every delivery.

## Biocontrol examples: entomopathogenic fungi, nematodes and bacterial pesticides

Two living-agent groups are used against insects: nematodes and fungi. Insecticidal nematodes, the entomopathogenic (insect-killing) kind such as Steinernema feltiae, were first developed as growing media drenches against sciarid fly larvae. Fungal biopesticides are built on entomopathogenic fungi, which are microbiological bioprotectants. A thrips killed by such a fungus is covered with small round balls of white fungus. Bacillus thuringiensis (Bt), a bacterial biological insecticide, does not penetrate plant tissue, so it works on young larvae grazing in the head of the plant. Btk stands for Bacillus thuringiensis subspecies kurstaki, one of the Bt strains in the Irish register of active substances.

Treat a natural or organic pesticide like any other product: find its name in the [Irish register](/ie/pesticide-register-ireland/), read its label and write it into the spray record like any other treatment.

## Biological control by pest: slugs, fungus gnats, whitefly, spider mites, mealybug, codling moth, cutworms and woolly aphid

Each pest has its own agent, so name the pest before you order.

| Pest | Biological control | Point to settle with the supplier |
| --- | --- | --- |
| Slugs and snails | The nematode Phasmarhabditis hermaphrodita, watered into the soil | It affects all slug species, including beneficial ones, so use it only where slugs damage the crop |
| Fungus gnats (sciarid fly) | Steinernema feltiae nematodes, Hypoaspis miles and Macrocheles robustus predatory mites, the rove beetle Atheta coriaria | Agents are added to the compost; keep the delivery date and batch |
| Whitefly | The parasitoid wasp Encarsia formosa, which preys on whitefly scale nymphs | Introduce it before plants are heavily affected, because it cannot give instant control |
| Spider mites (Tetranychus urticae) | The predatory mites Phytoseiulus persimilis and Amblyseius, the predatory midge Feltiella acarisuga and the rove beetle Atheta coriaria | Count pest and predator on the same leaves each visit |
| Mealybug (mealy bug) | The ladybird Cryptolaemus montrouzieri released into greenhouses | Its larvae look like large mealybugs, so train every picker to leave them alone |
| Codling moth | A mixture of pathogenic nematodes sprayed on trunk, branches and soil | Spray after the caterpillars have left the fruit; it gives no protection against moths flying in the next year |
| Cutworms | A nematode mixture watered into moist soil | Soil temperature must be 12 to 20 degrees Celsius |
| Woolly aphid | Natural enemies: ladybirds, ground beetles, hoverflies, parasitoid wasps and earwigs | Earwigs on fruit trees can reduce aphid numbers and do not cause damage there |

A ladybug, a parasitic wasp or a predatory mite is a living product, like the ladybird Cryptolaemus montrouzieri and the Encarsia formosa wasp in the table. Check the pest and the crop, confirm the agent was not killed by a recent spray (see “Review pesticide compatibility before placing the order”), and write the release date, the agent and the quantity in the field record. Leaf miner, flea beetle and downy mildew have no agent in this table: read the crop-specific guidance before you buy anything. For flea beetle in oilseed rape there is no beneficial insect to order. Cabbage stem flea beetle has developed resistance to pyrethroid insecticides to various degrees, so target sprays or avoid them as far as possible. The ragwort flea beetle is a different insect: it is a weed biocontrol agent used against ragwort, described in the [biological weed control guide](/ie/biological-weed-control/).

## Next step

For a protected crop, match the identified pest stages to the agents in the [thrips biocontrol guide](/ie/biocontrol-protected-crops/).