Mail-order predators and nematodes are marketed as a chemical-free way to deal with greenhouse pests, but they are living animals that work only in narrow conditions. The Royal Horticultural Society (RHS) publishes a page on biological control in the garden that explains what to expect. This article summarises it, together with the RHS pages on aphids and on indoor-plant sap feeders where they add detail. It reflects RHS gardening advice for UK gardens and greenhouses, not a legal standard, and it is general information. It does not recommend particular suppliers or products.
What biological control is, according to the RHS
The RHS says the biological controls available to home gardeners are either predators or pathogenic nematodes, and that they can give effective management of some plant-damaging garden invertebrates. It lists advantages over pesticides, which are usually broad spectrum:
- they cause no damage to plants and leave no residues;
- once established, natural enemies can breed until the pest is reduced to an acceptable level;
- parasitoids are usually specific to certain prey and will not harm non-target creatures;
- they can be used where sprays would scorch plants or where no synthetic pesticides exist for some food plants;
- normal glasshouse jobs such as ventilating and watering are not affected, although pesticides must be avoided; and
- the target animals do not develop resistance.
It also lists cautions. Some predatory mites, bugs and beetles are generalists and should be used with care, and nematodes are usually insect-specific or mollusc-specific but can infect non-target animals within those groups, so they should be used in a targeted way.
Parasitoids and predators: why timing matters
The RHS explains that many of the “predators” sold are more accurately parasitoids, which lay eggs in a host and kill it only when their grub has finished developing. From that it draws two practical rules. Predators and parasitoids rarely give instant reductions in pest numbers because they need time to multiply, so they should be introduced before large pest populations develop. Conversely there is no point releasing them before the pest is active, since they can breed only when prey is present. Its aphid page follows the same logic: in spring, aphid populations rise before natural enemies are active in large numbers.
Temperature and season in the glasshouse
According to the RHS the effective season for predators and parasitoids in glasshouses is late March or April to September. They need warm conditions, generally daytime temperatures of at least 21 degrees C (70 degrees F) and high light intensity to stimulate breeding, although they can usually survive at 13 degrees C (55 degrees F). Glasshouse biological controls often die out over winter, so reintroduction may be needed every year. The indoor-plant page makes a related point about mealybug controls: they need relatively high temperatures, so are likely to be effective only from May to September. It adds that biological controls are most suitable for a glasshouse situation.
Pesticides and predators do not mix
The RHS says predators and parasites are very susceptible to most pesticides, which should be avoided. It gives persistence figures: pesticides can kill biological controls and persist for at least six weeks, for example ten weeks for deltamethrin-based products. It adds that natural organic pyrethrum can be used up to seven days before predator release, while pesticides based on fatty acids and plant oils or extracts can be used with care up to the day before biological controls are introduced. It also says fungicide use should be kept to a minimum. Its pages repeat that the RHS does not support pesticide use and recommends non-chemical options wherever needed.
How nematodes work and how to use them
The RHS describes the nematodes sold as controls as species that infect insects or molluscs. They enter the pest’s body and release bacteria, causing a fatal infection, and then feed and multiply on the decomposing body. The RHS says these species pose no risk to plants or vertebrates. Practical points it lists:
- Nematodes come in packs that are mixed with water and watered onto plants and soil.
- Being living organisms, they should be used as soon as possible after purchase and all manufacturers’ instructions followed.
- They generally need moist conditions and are best applied in cool, damp weather.
- Species differ in temperature needs, requiring temperatures above either 5 degrees C (41 degrees F) or 12 degrees C (54 degrees F).
- They should be used with care and only when there is a specific problem to treat.
Where they can be bought
Because they are living animals, most biological controls cannot be stocked on shop shelves and are usually supplied by mail order. The RHS says the exceptions are formulations of some nematodes used against ants, chafer grubs, leatherjackets, slugs and vine weevil larvae, which some retailers stock. Success depends on following the supplier’s release and aftercare instructions carefully, and the RHS offers a downloadable list of biological controls and their suppliers.
Matching the control to the pest
The RHS page lists controls pest by pest, and the site has separate articles on several of the pests involved: ants and aphids in the garden, glasshouse red spider mite and glasshouse whitefly and Encarsia wasps.
Questions people ask
Can predators be released straight after spraying? According to the RHS, pesticides can persist for at least six weeks, with shorter gaps only for pyrethrum and for fatty-acid, plant-oil or plant-extract products.
Will they stay all year? The RHS says glasshouse controls often die out in winter and may need reintroducing.
The bottom line
According to the RHS, biological control works when the release is timed early, the greenhouse is warm and bright enough, and pesticides have been kept away for the gap the RHS gives. Nematodes need moisture, the right temperature and prompt use, and both groups are living animals that usually come by mail order.
Sources
- Royal Horticultural Society, “Biological Control in the Garden” (rhs.org.uk)
- Royal Horticultural Society, “Indoor plants sap feeders: Identify” (rhs.org.uk)
- Royal Horticultural Society, “Aphids” (rhs.org.uk)