Managing Biological Control in Gardens, Allotments and Polytunnels.

Biological Control, Aka Biocontrol, Is The Use Of Natural Organisms To Manage Unwanted Organisms. It Works In Gardens Allotments, Greenhouses And Polytunnels. Here’s How To Use It.

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Sooner or later we get pests and diseases in gardens and allotments. It could be slugs, snails, whitefly, red spier mite … I don’t really need to remind you about what can strike the best run garden. But what I can do is to explain how we can use natural predators and organisms to fight fire with fire.

Encarsia formosa predated whitefly on tomato leaf from wiki images. — Encarsia formosa predated whitefly on tomato leaf

The beauty of biological control is that rather than use synthetic chemicals to poison pests, we use natural processes and let nature do the controlling for us.

How Does Biological Control Work?

Biological pest control starts in one of two ways. We either introduce a predator or similar or we create conditions that encourage them to visit and take control.

In nature wherever numbers of a given species rises so too will associated species such as predators. So where aphid numbers build up in a natural environment we soon start to see the build of of their predators such as ladybirds. This of course assumes we have a natural environment and gardens aren’t always that natural, so often we need to introduce biological control ourselves.

As well as ladybirds other forms of biological control can include nematodes, lacewings, beetles and parasitic wasps. Many of the organisms, for example nematodes are microscopic. And even the parasitic wasps, such as Encarsia formosa, which parasitises whitefly, is extremely small.

All the above are examples of natural control methods that already occur in nature. All we are doing is to introduce a little help to speed the process.

Introducing natural predators isn’t a new idea. It dates back hundreds of years.

Biological Control History

It’s often thought that first example of biocontrol dates back to 1868, when newly established citrus groves suffered from devastating attacks by the cottony cushion scale insect (Icerya purchasi) which caused defoliation and dieback of the branches. There were no natural predators for this pest until a natural predator from Australia, the vedalia ladybird beetle Rodolia cardinalis), was introduced to the citrus groves. It worked well but introducing an organism from another area, or continent, is a dangerous thing to do as it could become a pest itself. An Australian example of an introduced species that caused severe problems was the rabbit. More on that another day!

Cane toads are another Australian introduced species disaster. They were introduced to sort out a sugar cane pest, Dermolepida albohirtum, (greyback cane beetle). The cane toads then proceeded to eat everything from frogs, lizards, snakes and more. Cane toads have persisted despite extensive extermination efforts. they have become a bigger problem than the pest they were brought in to sort out!

To discover much earlier examples of biological control we need to go back 4000 years to Egypt. The Egyptians had a problem with rats eating the grain they grew in the fertile Nile valley. So its thought that they introduced cats, which they also worshiped.

A successful example of moving an organism from one country to another can be found in Mauritius. When, in 1762, they had a locust problem they introduced the mynah bird from India.

Science and scientists have shown a great deal of interest in biocontrol. Malthus mentioned it in his work on populations. And in 1800 Erasmus Darwin encouraging various ichneumonid wasps, aka Darwin wasps, and syrphid flies aka hoverflies because they predated cabbage-feeding caterpillars.

In 1843 Boisgiraud reported that he had used Calasoma sycophanta, a predatory carabid beetle, to control gypsy moth larvae on poplars. His interest didn’t stop there. He also reported work on earwigs where he used staphylinid beetles to predate the earwigs.

Why Use Biocontrol?

Biocontrol is very specific. It’s unlike synthetic chemical control methods that affect lots of other organisms with collateral damage. For example neonicotinoids are excellent for controlling aphid on sugar beet and other crops, but were banned throughout Europe because they also killed bees. the irony of this situation is that neonics are based on a natural product, nicotine. And nicotine is allowed in many organic systems as a botanical pesticide!

Personally I believe nicotine is far too toxic to be used in gardens. It can be absorbed by humans through the skin and is deadly in the concentrations needed to kill insects.

Other non specific, but arguably more benign control methods include things like soapy water for aphid control. But remember the soapy water can also kill beneficial insects. It’s notoriously non specific, whilst biocontrol is far more specific.

Another benefit is that whilst pests have a great track record in becoming immune to many synthetic pesticides they are not so good at developing immunity to natural predators.

The downside is that natural predators often have specific environmental requirements. For example, the nematodes that predate slugs and snails only work when it’s warm enough and the humidity is right. This often leads to a logistics problem as we need to order our biocontrol agents in sufficient time for them to arrive and be used in optimum conditions. That’s not always easy.

Types of Biological Control

The following list contains some of the more usual biocontrol agents and their uses. I’ll be adding more extensive pages on specific biocontrol agents over the next few months.

Biological control is the use of living organisms to control or suppress pests or diseases. The following are different types of biological control methods, along with examples of the control agents and pests targeted:

Classical biological control: This involves the introduction of natural enemies, such as predators or parasitoids, from the pest’s native range to control invasive pests. Example: The introduction of the parasitoid wasp, Tamarixia radiata, from Asia to control the Asian citrus psyllid, Diaphorina citri, in Florida.
Augmentative biological control: This involves the release of large numbers of natural enemies to control pests. Example: The release of the predatory mite, Phytoseiulus persimilis, to control two-spotted spider mites, Tetranychus urticae, on greenhouse crops.
Conservation biological control: This involves the conservation and enhancement of natural enemies by providing them with suitable habitats, food sources, and other resources. Example: Planting flowering plants to attract pollinators, such as bees, which can also serve as natural enemies of crop pests.
Microbial control: This involves the use of microorganisms, such as bacteria, fungi, and viruses, to control pests. Example: The use of Bacillus thuringiensis, a soil bacterium that produces a toxin lethal to certain insect larvae, to control caterpillars on crops and nematodes to control slugs and snails.
Genetic control: This involves the use of genetic methods, such as the release of sterile insects, to control pest populations. Example: The release of sterile male Mediterranean fruit flies, Ceratitis capitata, to control wild populations of this pest. Sterile male mosquitos are used to combat malaria.

It’s important to note that each type of biological control has its own set of advantages and limitations, and the choice of method depends on the target pest, the environment, and the desired outcome.