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Biological pest control by natural predators and parasites

By chadchaffin | Gardening

Dec 20

Biological pest control has, of course, been around for millions of years, so the concept is not new. Ladybirds have always fed on aphids, frogs have always eaten slugs and birds have always devoured juicy caterpillars – the list goes on. Sadly the proliferation of chemicals used in farms and gardens in the 20th century often upset the natural balance of the ecosystem, removing micro-organisms from the soil and disrupting the food chain.

Pesticides caused collateral damage to the friendly creatures as well as the ones targeted, which also adapted to become more resistant to everything that man could throw at them, clearly an unsustainable course. When discussing biological pest control we need to consider it in two ways. Firstly, what can we do to harness the natural predators in the garden to work for our benefit, and secondly, what can we introduce to control specific pests?

It starts with the soil

It all starts with the soil. Feeding your soil with compost and organic matter will encourage the development of a healthy population of micro-organisms and bacteria, processing nutrients for plants and forming the first link of the food chain. Strong, properly nourished plants are also more likely to recover from and survive pest attacks. Providing wild patches, a pond, habitats and nesting sites for predators will all help to sustain a balanced ecosystem, with nature keeping things in check. Ladybirds, hoverflies, lacewings, beetles, frogs and toads, hedgehogs and birds are all examples of creatures that will help police our precious plants. Some slugs will eat other slugs. 

A modern solution

Biological pest controls for gardeners have  developed over the last 25 or so years as research and technological advances have enabled the controlled breeding of predatory or parasitic organisms that are specific to control various pests. The great thing about these is that the solution never becomes the problem; they will do their job but won’t persist in the garden or become an infestation in their own right.


Nematodes are the most commonly used biological control these days to control a wide range of pests. They are threadlike microscopic worms that live in the moisture surrounding soil particles. When they come into contact with their target pest they will multiply inside it, causing it to stop feeding and die. They have a finite life of a few weeks, either because they have exhausted the supply of host pests or because they themselves have become prey for native species that exist in the soil.
Nematodes arrive in a pack containing several million, which is added to water and applied using a watering can or sprayer, when the soil temperature is warm enough and when the pest is active. They are virtually invisible to the naked eye, safe on food crops and will not harm pets, children, wildlife or bees.

  1. Nematodes for slugs

    Proven and most effective against immature slugs under the soil surface, preventing them from reaching maturity to reproduce, their presence has also been shown to deter larger slugs. Each application is active for around six weeks, after which another may be needed depending on what you are growing. Usage should be able to be reduced over time as the slug population comes down to manageable levels.
  2. Nematodes for vine weevils

    This was the first nematode to be available for garden use and was a true breakthrough in controlling the very destructive larvae of this difficult pest that lives mainly in pots and containers. It can be applied in spring or autumn, stopping the larvae from eating away whole root systems and killing them before they develop into adults. Nematodes can also combat carrot root fly, cabbage root fly, leatherjackets, cutworms, onion fly, ants, sciarid fly, caterpillars, gooseberry sawfly, thrips and codling moth.

    This cocktail of various nematode species, sold as Nemasys Natural Fruit and Veg Protection, is designed to target pests when they are active, particularly useful early in the season when plants are most vulnerable. Used as a programme of two-weekly treatments after planting out, the gardener does not have to worry about application times or thorough investigations as to what the pests actually are. Alternatively, use to treat at specific times for particular pests. Caterpillars on leaves need to be directly contacted by spray for control.
  3. Nematodes to deter ants

    If ants are a problem then this nematode species will repel them from the treated area. It does not kill them but will encourage them to seek an alternative nest site.

Other biological controls

  • Ladybirds to control aphids
    Both larvae and adult ladybirds can be introduced into the garden and will get straight to work feeding on greenfly, blackfly and whitefly. If there is enough food they will stick around, but you can further encourage them by providing a ladybird shelter to give them a place to rest and multiply, and a ladybird feeder to see them through any lean times. An established colony will continue to be of benefit for many years.
  • Encarsia for controlling greenhouse whitefly
    A tiny parasitic wasp which lays its eggs in young whitefly scales, destroying them in the process. It should be introduced on low-medium infestations, and will give protection over a long period. Supplied as pupae from which the wasps will hatch when hung in the greenhouse. The best coverage is achieved using a programme of three applications 14 days apart.
  • Phytoseiulus for controlling red spider mite
    A tiny predatory mite which feeds on the red spider mite, used mainly in greenhouses. It breeds faster than the pest, so will outnumber it relatively quickly depending on the level of infestation. When there are no red spider mites left to eat, the predators will also die out.
  • Aphidius for control of aphids under cover
    A slender black insect about 2mm long that lays single eggs into immature aphids killing them as the young aphidius develops. One female can lay 100 eggs in her lifetime, so population numbers will grow rapidly until full control is achieved
  • Predatory mites
    This phoretic mite (sold as Mighty Mite) has a voracious appetite and a rapid reproduction rate at temperatures above 10ºC (50ºF). Primarily a soil-dwelling predator, it feeds on the larvae of dipteran flies, fungus gnats, sciarid flies, eggs and young maggots of the housefly, blowfly, stable fly and flesh flies. It also feeds on vine weevil larvae, cutworms, woodlice and springtails. It can be applied to pots and containers or direct onto the soil and is also effective added to mature compost heaps. When the compost is used, it will contain a population of mites ready to go to work against soil dwelling pests. 1000 mites will treat up to 4sq m.
  • Hypoaspis for control of sciarid flies (fungus flies)
    Sciarid flies are 3-4mm long black flies that jump or hover over the soil surface in the greenhouse and around houseplants where compost is warm and moist. The small white larvae live in the top few millimetres of compost, attacking rooted cuttings and seedlings. Both adults and larvae of hypoaspis feed on these larvae. One treatment remains active for four to five months.
  • Cryptolaemus for control of mealybugs
    These ladybird beetles love eating mealybugs on ornamentals. With a life cycle of several months one treatment is sufficient for an average greenhouse or conservatory although severe infestations may need more.

From Kitchen Garden Mag

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