Red Spider Mite
(Tetranychus urticae). Although the glasshouse spider mite is known as "Red Spider Mites" they are actually better named "Two Spotted Mites". When active they are pale greeny-brown in colour and can be recognised by the two 2 dark dots on their backs - they are only red in winter. Spider Mites are less than 1mm in size and difficult to see without a lens or microscope unless you have very sharp eyes. They often live underneath leaves, and are only active when it is warm. In winter they hibernate either as eggs, or inactive newborns which you can often spot as tiny red or orange shiny dots
These tiny, highly destructive pests are common in greenhouses and on house plants and can also be found on outdoor plants during the warmer summer months. In the greenhouse look out for them on citrus, tomatoes, cucumbers, melons, vines, carnations and chrysanthemums. Outdoors, you may find them on day lilies, gladioli, conifers and many tender plants .
Spider mites can be spotted more easily by looking for the damage than looking for the culprits. They pierce the leaf cells with their mouthparts and suck the plant juices resulting in discolouration of the leaves and sickly looking growth, and in worst cases the death of the plant. The leaf damage initially results in a fine brown speckling effect on leaves where the cells have died. The pattern of the mottling varies from plant to plant, but it typically starts on the underside of the leaves. On some plants with tougher leaves the spider mite eats young growth and flower buds and may be even more difficult to spot.
In the early summer it is a good idea to inspect vulnerable plants every week and deal with the first signs of mite infestation immediately. Where possible, remove infested leaves, buds and stems because this will initially reduce numbers. Spider mites prefer dry environments, so spraying the infested areas of a plant with water and damping the greenhouse floor areas will raise the humidity and hopefully slow the infestation rate.
When the infestation becomes advanced, fine webbing appears on new growth. Spider mites are arachnids (spiders) and can spin very extensive fine webs. The webbing acts to shield the colony like a blanket, and it also provides a series of ladders or netting which allow the mites to crawl from stem to stem. Where webbing is widespread the leaves will typically turn brown and drop off because of the extent of the cell damage.
The Number One SpiderMite Killer - Phytosieulus persimilis
These tiny predatory have been the main commercial biological defence for the last 40 years. As long as there is a warm, light environment and plenty of light these predatory mites feed on both adult and eggs of the Two Spotted mite giving excellent population control.
You can hardly spot the mites with the naked eye, but under the microscope you can see their shiny orange bodies scurrying around searching out their prey. This picture was provided by BCP Certis, who breed the mites for us.
Predator mites need minimum 15°C but work best when temperatures are regularly 18°C to 22°C. This generally means application end April until September indoors, or June and July outdoors.
They work very effectively and can achieve a complete clean up in several weeks if the conditions are right. To avoid the mites eating themselves out of food we recommend introducing a second dose of phyto two weeks after the first as an insurance.
Phytoseiulus are supplied in plastic tubes with vermiculite granules. They should be gently sprinkled with the vermiculite onto leaves half way up the affected plant; or if the leaf shape makes this difficult, the granules can be put into small cones of paper. The mites will move onto the plant in a matter of minutes and move around the plant looking for Red Spider mites to eat. On receipt of your order, please read the instructions before opening the tube! For more information about phytoseiulus click here
The Number Two SpiderMite Killer - Amblysieus californicus
This predator is often used in addition to Phytosieulus to provide additonal control. It has advantages and disadvantages. The main plus point is that these predators will not die (or eat each other) if they cannot find enough spider mite to eat - because they can also live on pollen, or on other prey. This means that they can be used as a preventative measure in March or April when you can't yet see any spider mite - or in later in the year for small outbreaks.
The other advantage is that Amblysieus is not so temperature sensitive - it will be active to some extent from 8 degrees to 35 degrees - again this is why it is used early season and late season. And whereas Phytoseiulus requires a damp atmostphere (relative humidity over 60 percent), Amblysieus will thrive under the hot dry environment typical in a summer glasshouse.
The only disadvantage of Amblyseius (apart from the price!) is that they don't eat quite as fast as the Phytoseilius so it if you have got a big infestation we would recommend Phyto.
Amblyseius californicus are supplied in plastic tubes with vermiculite granules. On receipt of your order, please read the instructions before opening the tube! For more information about Amblysieus click here
The Flying Killer - Feltiella acarisuga
These are native midges which target red spider mite, laying their eggs among their colonies. When the eggs hatch then the worm-like larvae will eat the eggs, young and adults. Their common name is sometimes given as "Predatory Gall Midge".
These killers have some advantages over phytoseiulus, especially when used early or late season. Because the adults hunt out their prey, they will reach the hot spots that the gardener may miss. (With a large crop it is very tedious and difficult to spread phytoseiulus accurately by shaking out of the tube. Phytosieulus has to crawl to its prey, while the midges can fly quite a distance)
They will devour large quantities of red spider mite, and are active in spring and autumn when light levels are lower. They will be active from 15 degrees, although they will act quicker when warmer. They prefer more humid conditions than the predatory mites.
For that reason the midge is recommended for early season control when you know from previous experience that there are overwintering red spider mite sheltering under leaves. Like Phytoseiulus, this predator won't thrive unless they have spider mites to feed on.
This control is not recommended for the hot dry conditions of mid summer. You should also take account of the fact that the cocoons and the midge larvae are visible to the naked eye, so this is not a solution for ornamental plants (flowers).
The Predatory Gall Midge is supplied it a tub of cocoons. Adult female midges emerge from the cocoons, mate inside the tub then fly off to lay their eggs amongst spider mite infestations. The treatment is more expensive than the phytoseiulus but if you have a known problem on a sizeable crop then one tub is excellent value because if you introduce the midges in the right conditions they will establish and breed just from one tub. click here