Biological control

Biological control sounds like it has something to do with labs or school science lessons. I prefer to think of it as sending a little army into battle for you.
Put simply, biological control is when you release a natural predator of a particular plant pest into your greenhouse or garden so it can do the dirty work for you – finding and killing whatever bug is troubling your plants. It can be unnerving, if you haven’t used biological control before, to release thousands of bugs into your garden especially because the problem you’re trying to solve was caused by thousands of bugs in the first place!
But if you think about it, it happens naturally all the time: ladybirds and their larvae seeking out and eating greenfly for example. Sometimes though, the predator of a certain pest won’t just turn up so you need to introduce it. Nearly all of our most troublesome pests can be kept under control this way and best of all, it means you don’t have to resort to chemicals.
In many cases, biological control is actually more effective than a chemical spray. That’s because quite a few of these pests (aphids and whiteflies, for example) have developed resistance to the pesticides that are used on them. Biological controls can be used on both edible and ornamental plants, whereas many chemicals are restricted in their use on fruit and veg, with no fear of danger to pets or children.

Pest: Slugs
Control: A nematode called phasmarhabditis – in products such as Slugsure T or Nemaslug
How it works: Use this if you have problems with those pesky slugs that live in the soil. The microscopic nematodes swim about in the soil and seek out slugs. When they find one, they enter its body and release a bacteria that kills the slug.
When to use it: The soil temperature needs to be at least
5C (41F) so best used from spring onwards.

Pest: Vine weevil   
Control: The nematode Steinernema kraussei – look for Nemasys or Grubsure LTT
How it works: Vine weevil larvae eat the roots of plants while the weevils they grow into eat unsightly notches from leaves. Water the nematode onto containers or open ground during the larval stage. The nematodes attack the larvae, release bacteria inside them to kill them and then reproduce inside the dead host to give you a new generation of weevil-hunting nematodes.  
When to use it:  The soil temperature needs to be at least 5C (41F). March to May and August to November are when larvae are present.

Pest: Aphids (greenfly, blackfly and so on) in greenhouses or conservatories   
Control: Aphidius (right) or aphidoletes   
How it works: Aphidius is a tiny parasitic wasp. It lays its eggs into immature aphids. The egg hatches into a larva inside the aphid, killing it. Best used on a small aphid problem. Aphidoletes is a tiny midge. It’s attracted to the honeydew that aphids excrete and lays its eggs, which hatch into orange larvae. These seek out and eat their way through aphids like there’s no tomorrow, so are better if you’ve got a large infestation.   
When to use it:  Aphidius needs a minimum temperature of 10C (50F) to work. Aphidoletes needs a minimum temperature of 15C (60F) and is best used between May and September.

Pest: Aphids outdoors  
Control: Ladybirds and lacewings  
How it works: Although ladybirds and lacewings are already present in most gardens, you can give nature a hand and introduce more. Believe it or not, you can buy ladybirds, both adults and the larval stage and lacewing larvae. All love eating aphids.    
When to use it: Ladybirds and lacewings can be released between May
and September.

Pest: Leatherjackets
Control: The nematode Steinernema feltiae
– look for Nemasys Leatherjacket Killer   
How it works: Leatherjackets (the larvae of daddy-long -legs) eat the roots of the grass in your lawn, causing brown patches. The nematode seeks out the grubs in the soil and kills them.   
When to use it: Apply two weeks after you start to see daddy-long-legs emerging. This is when they lay their eggs so the nematode will be in the right place to hunt out the newly hatched larvae. The soil temperature needs to be 12C (54F).

Pest: Chafer grubs   
Control: The nematode Heterorhabditis megidis
– look for Nemasys Chafer Grub Killer   
How it works: Like leatherjackets, chafer grubs are a lawn pest, eating the roots of your lawn and causing yellow or brown patches. Apply this nematode to kill the grubs in the soil.   
When to use it:  Apply during August and early October when grubs are active and near the surface. The temperature needs to be above 12C (54F).

Pest: Whitefly in greenhouses   
Control: A parasitic wasp, Encarsia formosa   
How it works: There’s nothing worse than discovering clouds of tiny whitefly in your greenhouse and their scale-like eggs on the underside of leaves. This tiny wasp lays its eggs into the whitefly scales, putting an end to them. In about a month, depending on temperature, a new wasp will emerge from the parasitised scale to continue the battle.   
When to use it: Use encarsia at the first sign of whitefly, but temperatures mustn’t fall below 10C (50F) at night. It is most effective when daytime temperatures are at least 18C (65F).

Pest: Red spider mite in greenhouses and outdoors   
Control: A mite called Phytoseiulus persimilis   
How it works: Red spider mite is the bane of many gardeners’ lives. Teach them a lesson by setting phytoseiulus on them! It’s another mite, but bigger and meaner than the red spider mites, actively hunting them down and eating them.  
When to use it:   Introduce Phytoseiulus persimilis
(pictured) as soon as you see signs of red spider mite, but temperatures
must be at least 20C (68F) for part of the day.

Pest: Mealybugs in greenhouses   
Control: Cryptolaemus, a small brown ladybird   
How it works: Nasty mealybugs are hard to control by chemicals, because their white waxy coating stops the pesticide from penetrating. It doesn’t stop cryptolaemus from laying its eggs into the mealybug
eggs though! Its larvae and the adults eat mealybugs too. They sometimes also eat scale insects if mealybugs are scarce.   
When to use it: Use this control (pictured left) in summer when temperatures are around 20C (68F) and there’s more sunlight.

Pest: Soft scale insects in greenhouses  
Control: Another parasitic wasp, Metaphycus helvolus   
How it works: Female wasps lay their eggs into scale insects, the larvae develop inside and kill the scale before emerging as a new wasp.   When to use it:  Late spring and summer when temperatures are at least 22C (72F) for part of the day.

Pest: Caterpillars and codling moth
Control: The nematode Steinernema carpocapsae – look for Newasys Caterpillar Killer
How it works: Nematodes enter the pest through body openings then release bacteria that stops the larvae feeding and quickly killing it
When to use it: Use as soon as caterpillars are first seen and between September and October for codling moths.

Top tips on predators

• After applying nematodes to combat leatherjackets and chafer grubs on lawns, water them in so they’re washed down to the soil where they’re needed.
• Use yellow sticky traps (left) to monitor pest problems in your greenhouse – then you’ll know exactly when you need to apply your biological control. Take them down once you’ve introduced your control or your predators might get stuck to them too!
• Live predators can’t be stored and need to be released as soon as they arrive. Don’t order and then go away when the package is delivered or you’ll be responsible for mass murder! Nematodes arrive in an inactive state and can be stored if necessary – Nemaslug, for example, will be fine kept in the fridge for three weeks before use. Read the instructions to be sure.
• You can’t use chemicals while you’re using biological control as they will kill your predators. You may also need to stop using chemicals for a certain period before introducing your biological control. For example, if you’ve used derris in your greenhouse, you need to give it at least two weeks to dissipate and leave your greenhouse safe for your new predators.
• Some active or flying controls, such as aphidius and encarsia may fly out of your greenhouse, so net vents and doors to keep them inside. Use fleece or Enviromesh – it will also help to keep pests out.
• If you’ve gone to the trouble of buying ladybirds and lacewings to boost numbers in your garden, make sure they have somewhere to live by leaving twigs and plant debris around or investing in a ladybird or lacewing home (pictured above with Aphids outdoors).