At a first glance, unleashing an army of pre-packaged creatures to eliminate all those annoying pests, seems like the perfect way of wreaking revenge. But is this really the best way to keep things under control in the garden or glasshouse?
What are biological controls?
This term applies to the useful predators or parasites that will attack or consume garden pests. Many are around anyway, but introducing them artificially can swing things the right way, helping to keep pest numbers down. Biological controls are not toxic to humans and have little detrimental effect on the environment. The most common types are mites, predatory wasps and microscopic worms called nematodes.
How to use them
It is best to add a biological control just when you notice that the numbers of the pest are starting to increase (predators and parasites will struggle to control severe infestations so this method can be a waste of money if pest numbers are already very high).
Mites and wasps are most effective when used in a glasshouse, because if they’re used outside they tend to fly or crawl away from where you’ve put them! Nematodes are watered into the soil, and move much more slowly, so these can be used outside without fear of them escaping. Most biological controls will function best within a temperature range of 10-30ºC (although nematodes can tolerate soil temperatures down to 5ºC). You should avoid using any pesticides a few days before using your biocontrol (this includes soft insecticidal soap) as such chemicals can kill them off, too. A biological control will not have the instant knockdown effect of a synthetic pesticide. There is now a whole array of biological controls available to the gardener, so just a few of our favourites are mentioned here:
Aphidius colemani is a very small, unremarkable-looking black parasitic wasp that attacks aphids. The adult wasp injects eggs into the aphid, where they hatch, grow, pupate, then eat a small hole to burst out in true ‘alien’ style. You often see the remains of aphids as swollen pale brown ‘mummies’ scattered around the plant. The wasp arrives in small vials that are left open in the glasshouse. It is very good at seeking out small numbers of aphids. If you have a large pest population, the larvae of a midge (Aphidoletes aphidimyza) has a more voracious appetite. Often the two are sold as a premixed cocktail.
Sciarid flies or fungus gnats are those annoying little flies that jump around or hover above the compost in your glasshouse. Although these are an unsightly nuisance they cause more trouble below the compost surface, where the larval maggots eat away at your crop’s roots and stems. The mite Hypoaspis miles is highly effective at reducing populations of sciarid flies. It arrives in a pepperpot shaker mixed with compost, and is scattered onto the soil surface
Although these pests need no introduction, what most people don’t realize is that those you see on the surface are only a fraction of the whole population. Most live under the ground, and by burrowing deeply they have a remarkable ability to survive cold or dry conditions. The biocontrol against slugs arrives in a little tub, which contains the nematode Phasmarhabditis hermaphrodita suspended in a gooey beige sludge. This is mixed in a watering can just before use and then irrigated onto the offending area. The soil must be above 5°C and kept moist. Treatment should be effective for at least six weeks.