Parasitoid wasps
Overview
Parasitoid wasps are small beneficial insects used in biological pest control programmes against aphids, whitefly, leafminer and some caterpillar pests.
Unlike predators, parasitoids usually develop inside or on a single host. The pest is eventually killed as the parasitoid completes its development.
Parasitoid wasps are especially useful in preventative and early-curative IPM programmes where pest numbers are still low enough for natural enemy populations to keep pace.
How parasitoids work
A female parasitoid searches for a suitable host and lays an egg inside or onto it.
The developing parasitoid larva feeds on the host, eventually killing it. In aphid control, this often produces a visible aphid mummy. In whitefly control, parasitised scales may darken depending on the parasitoid species.
Main groups used in horticulture
Aphid parasitoids
Used against aphid colonies in protected and outdoor crops.
Examples include:
Whitefly parasitoids
Used in protected crops for glasshouse whitefly and tobacco whitefly programmes.
Examples include:
Leafminer parasitoids
Used against leafminer larvae within leaves.
Example:
Caterpillar egg parasitoids
Used against eggs of some moth pests.
Example:
Strengths
- Highly targeted biological control
- Good for early pest detection programmes
- Can spread through the crop searching for hosts
- Useful in residue-sensitive crops
- Strong fit with monitoring-based IPM
Limitations
- Usually slower than predators or pesticides
- Often best at low to moderate pest pressure
- Species choice is important
- Some parasitoids are host-specific
- Broad-spectrum pesticides can disrupt establishment
Environmental requirements
Temperature
Most parasitoids need suitable temperatures for flight, host searching and development.
Cool conditions slow activity and can reduce control.
Light
Good light supports searching activity in many parasitoid species.
Host availability
Parasitoids need suitable host stages. If pest pressure is absent or too high, establishment may be poor.
Practical IPM strategy
Parasitoids work best when used early.
A typical programme includes:
- Regular crop monitoring.
- Early introductions when first pests are detected.
- Repeat releases to maintain pressure.
- Predator support where pest numbers increase.
- Careful pesticide compatibility checking.
- Follow-up inspections for mummies, parasitised scales or reduced pest growth.
Common reasons for failure
Wrong parasitoid for the pest
Aphid species and whitefly species differ. Matching parasitoid to host is important.
Pest pressure too high
Parasitoids may struggle to catch up once colonies are already large.
Chemical disruption
Broad-spectrum sprays can collapse parasitoid populations.
Poor monitoring
If pests are found late, parasitoids may not work quickly enough alone.