Two spotted mite [Tetranychus urticae], more commonly known as red spider mite, can be a damaging pest to many crops and plants.
They often go un-noticed early in their development and with increasing temperatures their population rapidly develops.
These tiny mites feed on the plant sap and tissue, stunting and killing off plant growth.
Leaves generally turn yellow as the chlorophyll is removed from the leaf, which in severe cases can kill plants totally.
The appearance of plants is also ruined with yellowing leaves and webbing appearing.
These webs also contain huge numbers of spider mites and are often used by the mites to move from plant to plant, sometimes by being blown around or even attaching to people working with the plants.
Control of spider mites with insecticides is difficult. Spider mites are very good at building up resistance to insecticides over quite a short period of time and physical acting products generally only kill the adults. This leaves eggs to hatch and develop, which means regular application is required to break the life cycle.
A more effective and environmentally friendly method is with the use of spider mite predators. Predators will eat the entire life cycle, egg to adult and the spider mites are not able to become resistant to them. They are also easy and safe to apply with no harmful chemical residues being left on the plant or in the environment.
How and which predators we use is key to their success. There are now several species available to growers and gardeners. The main candidates are;
Spider mite predator
The most effective predator in the right conditions is: Phytoseiulus persimilis, supplied in shaker bottles or vials. It breeds quickly in the crop with faster egg laying than the spider mites and starts feeding soon after application. It eats spider mite faster than all the other available predators and will happily go into dense webbing to feed, which some of the others will not.
However it does not survive long without food and cannot be introduced before spider mites are present. They also need temperatures to be above 15C to be active and are most active from 20C-30C,temperatures above this will reduce and stop their activity. Introduce after spider mite is observed and apply them close or onto the infected areas of the plant.
In high infestations repeated applications may be required to build up their numbers to a level where they out compete and control the spider mite.
Amblyseius californicus is also an effective predator of spider mites and can be introduced at lower temperatures than Phytoseiulus. They can also survive without spider mites for some time and can feed on pollen. This enables them to be introduced earlier and before spider mite appears. Their activity starts from temperatures over 10C up until about 33C.
They are available in bottles or breeder sachets that can be hung on plants, releasing predators over a period of weeks. They can be combined with Phytoseilius for an effective bio control programme. However they are not recorded as a native insect to the UK. If non-native insects are thought not to be able to overwinter in the UK, they are often still permitted for use like Phytoseiulus.
Judgement on Amblyseius californicus is still under consideration. This means they are not available to gardeners and only to growers growing under glass.
Spider Mite Killer sachet
There is another alternative; Amblyseius andersoni. This predator is active at the lowest temperatures of the three ( from 6C) and will continue working at the highest temperatures (up to 40C).
It can be introduced before spider mite is present in breeder sachets. It is most effective when there are low numbers of spider mite and when temperatures are too cold for the other predators. Andersoni does not like feeding on spider mites in high numbers where there is webbing.
It should be combined with Phytoseiulus in warm conditions when there are high numbers of spider mites.
So what is the best strategy for control of spider mites with predators?
Here is what I suggest:
Introduce Spider mite killer sachets [Amblyseius andersoni] as early as possible in the growing season. Each sachet will release hundreds of sachets over a period of about 4 weeks. If spider mite is low or not present, continue regular introduction throughout the growing season. They can also be used on outdoor crops of soft fruit and trees and shrubs.
Introduce Amblyseius californicus sachets if you are a professional grower, growing crops under glass, as early as possible and re-introduce sachets on a regular basis.
Always introduce Phytoseilius predators, if spider mite is increasing or in high numbers. Introduce only when temperatures are above 15C and when spider mite is present. Keep repeating applications until the predators are easy to observe on the plants.
The full range of predators can be sourced from our Natural Pest Control page.