Biological controls
Biological controls involve the introduction or increase in numbers of organisms which prey on garden pests in some way, which should reduce the damage done by that pest. Biological controls and their introduction can take many different forms, such as:
- Increasing habitats for natural pest enemies such as hoverflies to control aphids or hedgehogs to control slugs.
- Artificially increasing numbers of a natural predator, such as the Encarsia formosa parasitic wasp which is used to control glasshouse whitefly.
- Introducing non native species to control local pests, such as the control of the cassava mealybug in Africa by the introduction of a parasitic wasp (Epidinocarsis lopezi) from South America.
- Introducing predators of weeds to control their spread, for example the psyllid Aphalara itadori which is being imported to many regions to control the spread of Japanese knotweed.
The main advantages of biological controls are that they do not have the negative environmental impacts of chemical treatments (so long as introductions of non native species are managed appropriately), pests cannot become tolerant of them and there are no toxic affects on plants or other animals.
However, they can have disadvantages. Many biological controls require application in very specific environmental conditions (on a domestic scale they are mostly available for glasshouse use), plus they need to have sufficent volumes of their prey in order to survive, which makes timing important. Their usage in combination with pesticides is limited, as the pesticides will often kill the biological control organism as well as the pest. There have also been some notable disasters caused by introducing non-native species, such as the introduction of cane toads in Australia to control pests associated with sugar cane crops. Unfortunately the cane toads went on to decimate local fauna, including amphibians and reptiles, as well as poisoning domestic and wild animals that tried to eat the toads.
