The eagle-eyed among you may have noticed a few colourful bags hanging on the stems of trees and plants in the Tropical Zone of The Glasshouse at RHS Garden Wisley. These are not decorations, but one of the methods we use to control pests.
In The Glasshouse, we have the usual motley crew of mealybugs, aphids, red spider mite, scale insects, whitefly and thrips. Chemical spraying can be very effective but is not good for the environment. In order to reduce the amount of chemical treatments we use, we combine them with cultural and biological controls – this is called integrated pest management (IPM).
Cultural controls involve adapting planting and care to minimise a problem with pests. This won’t eradicate pests, but will minimise them. In The Glasshouse, we try to put plants where they will grow best, with the right lighting and moisture levels, because a happy plant is more resistant to pest invasion. It tends to be the soft new growth that is most likely to be attacked, and since many tropical plants are quick growers it’s possible to cut out invaded stems that will regrow in a few weeks. Old flowers are also targets for pests and are removed once they are past their best.
Chemical controls do not always have to mean spraying. If only a few plants have a problem, or if the pest is hiding in underneath leaves, which is often the case, then it can be more effective to apply an approved insecticide with a paintbrush to clean the pests off. However, always follow the dilution instructions on the insecticide packaging.
The good guys
Biological controls consist of using living parasites, or predators of the pests, and this is what the colourful bags are.
For example, Encarsia formosa is a tiny parasitic wasp that likes to lay its eggs in the early stages of a whitefly. Each one is less than a millimetre long. Encarsia usually arrives in the form of pupae attached to square cards that are hung on the plants. When the whitefly pupae have been parasitized they turn black.
Cryptolaemus montrouzieri (mealybug ladybird), is a ladybird that likes to feast on mealybugs and occasionally scale insects. It is placed as an adult, next to the mealybugs, with a small piece of moistened tissue so it has something to drink. It is quite confusing to use at first, because it actually looks like a mealybug, only slightly bigger and more active. This is a case of aggressive mimicry, where one animal evolves to look like another. In this case, the Cryptolaemus wants to avoid being spotted by the mealybug, until it’s too late!
However, biological controls also have drawbacks. They can only be used once a pest is getting out of hand, otherwise the predator has nothing to feast on, and they tend to have minimum temperature requirements. Plus they need to be kept close to the plant and will fly out of windows if they get the chance.
Using a combination of chemical, biological and cultural controls, we can offset the disadvantages of each method and keep pests to a minimum.
See a full list of biological control suppliers (248kB pdf).
Extra content for RHS members
Log in to 'My RHS' to read Principal Entomologist Andrew Salisbury's in-depth article on biological control for greenhouse pests.