What is Biological Control?
Biological control is the use of natural enemies to control 'pests'. In polytunnels or greenhouses, this involves introducing predators or pathogenic nematodes.
· Biological controls cause no damage to plants and do not leave residues
· Once established natural enemies can breed and increase in numbers until the ‘pest’ has been reduced to an acceptable level
· The predators and parasites are usually specific to certain prey species and will not become a nuisance or cause harm to non-target creatures
· Biological controls can be used where pesticides cannot be used, for example on plants that may be scorched by sprays and there are no synthetic pesticides available for some food plants
· The presence of biological controls should not interfere with normal gardening activities such as ventilating glasshouses and watering, although it is necessary to restrict the use of pesticides
· The prey do not develop resistance
Using biological controls: predators
To get the best results it is important to know how biological controls behave. Many of the predators used for biological control are more accurately termed parasitoids as they kill the host whereas a true parasite does not; these animals lay eggs in a host, the grub grows and feeds inside its prey, only killing the host when it completes its development.
Using them correctly
Predatory and parasitoid biological controls rarely give instant reductions in prey populations; they need time to multiply. It is therefore necessary to introduce them before plants become heavily infested. Conversely, there is no point in introducing the biological control before the prey becomes active, since they can only breed when their prey is present.
The effective season for biological control with predators and parasitoids in greenhouses is late March/April to September. Careful use of pesticides may be required at other times of year. Glasshouse predators and parasites, just like the animals they control, require warm conditions, if they are to thrive and breed rapidly. They generally require daytime temperatures of at least 21ºC (70ºF) and high light intensity to stimulate breeding, although they can usually survive at temperatures of 13ºC (55ºF). Glasshouse biological controls will often die out during the winter and reintroduction may be necessary each year.
Biological controls and pesticides
Predators and parasites are very susceptible to most pesticides and these should be avoided for at least six weeks before biological controls are introduced, (e.g. ten weeks for deltamethrin based products; but natural organic pyrethrum can be used up to seven days before predator release). Pesticides based on fatty acids and plant oils/extracts, can be used with care up to the day before biological controls are introduced. Fungicides use should also be kept to a minimum.
Some Common Pests
By growing flowers, we are encouraging wildlife, especially pollinators, which is great. Unfortunately, you will also find there are some much more prolific beasts that will also be attracted to the garden. Below are a few of the most common culprits and how to handle them without chemicals.
Aphids
Often known as greenfly or blackfly, aphids' numbers get out of control very quickly. Their ability to reproduce is astounding, they are born pregnant! Aphids are sap suckers, they feed on the plants leaving them wakened, ravished looking, and sticky. They can also spread plant diseases.
Firstly do walk around the cutting patch daily and pick off infested leaves.
Alternatively, rub off aphids between thumb and forefinger without damaging the plant. Then resort to a biological control if necessary.
A secondary problem of an aphid infestation is ants, ants love aphids because they feed on the honeydew that is created when the aphids feed on plants. So you now have aphids and ants…what to do?
Firstly you’ll need to control these ants, you can do this with a species of nematode (microscopic eelworm) that you apply to their nests causing them to move on.
Then to the aphids, as aphids are one of the most common and widespread garden pests, there are lots of commercially available natural predators. The most well know and popular is the ladybird. Ladybirds and their larvae devour huge quantities of aphids. Next up are the beautiful Lacewings, gorgeous green creatures with delicate translucent wings, their larvae are ferocious eaters and can consume 35 or more aphids each!
There are other options too, tiny parasitic wasps and special aphid midge all do a wonderful job at controlling an aphid infestation.
We encourage birdlife here at Field Gate Flowers and have lots of blue tit nesting boxes. A family of blue tits can eat over 100,000 aphids a year.
Thrips
Next up is Thrips, another sap sucker causin flowers, fruit and foliage to be deformed, weakening the plant and eventually killing it.
Thrips are tiny, very difficult to spot and often gardeners don’t know they have thrips until they start to notice brown petals once buds open and deformed, stunted fruit.
Where possible, remove infested leaves, buds and stems because this will initially reduce numbers. Thrips are very difficult to reach with chemical sprays; but predatory mites are small enough to find and eat them, making biological control (the process of introducing natural predators) the best answer to this problem.
You can introduce a predatory mite to thrips – Ambleyseius cucumeris are the best to find and eat them
Vine Weevil
If the colour in your garden comes from pots or border shrubs like Rhododendron then you need to know about vine weevil.
Vine weevil are black beetles about 8mm long, narrower at the front than the back. If you see bitten edges around the leaves of your plants, then suspect vine weevil. They are rarely seen in the daytime.
The larvae are grubs which feast on plant roots, often completely severing them from the upper stems. They have a soft creamy-white body, up to 1.5 cm long and brown heads. They often lie in a sort of “C” shape. The damage caused by the adults is unsightly, but the grubs can cause the plant to die.
Vine weevil are very easy to treat, these nematodes control the grubs and stop them from feeding on plant roots and these traps get rid of the adults.
Solutions are vine weevil traps and vine weevil nematodes.
Slugs
Slugs and snails are the number one garden pest in the UK -must love all that rain.
They over winter underground then in Spring the young slugs emerge to feed on the appetizing menu of new young plants – juicy and new. Their rasping mouth parts chew great holes in the leaves or eat stems and the plants simply die.
Nematodes reproduce outside the body of the young slugs multiplying and spreading rapidly providing they have plenty of food and the soil is warm and wet. The slugs die within days and their bodies decompose underground. Nematodes are best used with the young juvenile slugs so don’t wait until you see big black monsters before you act. Once the ground temperature is over 5 degrees the nematodes become active and if it is a little warmer, they will work quicker.
Nematodes can work on snails by attacking the softer parts of the body but are less effective.
Red Spider Mite
This destructive pest is active indoors and under glass so causes problems all year round. Spider mites are best spotted by looking for plant damage. They pierce the leaf cells with their mouthparts and suck the plant juices. The leaf damage initially results in a fine speckling effect o the leaves where the cells have died. When the infestation becomes advanced, fine webbing spears on new growth. When webbing is widespread the leaves will typically turn brown and drop off.
Two biological solutions are Phytoseiulus Persimilis mites which are the best for large outbreaks and Amblyseius Andersoni for smaller outbreaks.
Here at Field Gate we purchase our biological controls from
www.ladybirdplantcare.co.uk and the lovely Tessa will help you select what is right for you.