Red Spider Mite
Red spider mite is a pest that causes severe damage - mercilessly - indoors and outside. Especially when the weather is hot and dry, a population of spider mites can explode in numbers. For many years now, it has been tackled with success by the predatory mite Phytoseiulus persimilis.
The predator mite Phytoseiulus persimilis is originally from Chile, but has subsequently been spread by humans (consciously or unconsciously) all over the planet. A Phytoseiulus predator mite is about the same size as a glasshouse red spider mite, but has a red-brown colour, stands a little higher on its legs and is much more mobile. There are usually four times as many females as males in a population.
The female lays her eggs in or near to a red spider mite colony. They can be distinguished from the spider mite eggs by their oval shape, their light orange colour and by the fact that they’re around twice as big. The six-legged larvae do not eat anything. Just as in greenhouse mites, the larval stage is followed by the protonymph, which molts into the deutonymph, and finally the adult stage.
There’s no real rest between the developmental stages. After becoming adult, it takes at 20°C around 2 days for the predator mite to lay eggs. The development lasts under normal circumstances for a shorter time than the greenhouse mite, and takes about 5 days at 30°C, 9 days at 20°C and 25 days at 15°C. The female cannot lay eggs without being fertilized. At 20°C she lays over the course of 22 days around 54 eggs, but this can be as many as 75.
Under normal conditions a predator mite population therefore grows faster than a spider mite population. At higher temperatures (above 30°C) or in drier weather (when the air moisture level is below 60%) the spider mite is favoured and its control will be harder. With too low an air moisture the predator mite’s eggs crumple up.
The diet of Phytoseiulus consists almost exclusively of red spider mites. In the absence of these the predator mite will cannibalise its own species. An adult predator mite devours spider mites at any stage of their lifecycle, whilst the nymphs stick to their eggs and protonymph stages. In a single day an adult Phytoseiulus can eat around 20 spider mite eggs or larvae, 13 protonymphs, or five adult spider mites.
Thanks to their faster development and bigger appetite, the predator mite can fully eradicate a spider mite population. Although Phytoseiulus nymphs usually tend to stay in one place, the adults do go off in search of other hunting grounds. If your plants are touching each other the predator mites can quickly spread out throughout the crop.
Phytoseiulus can be used on all sorts of vegetable and decorative plants grown in greenhouses, such as peppers, cucumbers, melons, aubergines, strawberries, green beans, gerbera, roses and all kinds of pot plant. For a successful biological treatment against red spider mite it is important to catch the infestation early on to be able to get it under control. Given that a red spider mite population multiplies more quickly in the summer, when it will be harder to bring under control biologically, it is best to start your biological control immediately from the first warm days of spring in order to catch the spider mites as they emerge from hibernation.
After detecting the first spider mite colonies, release Phytoseiulus as soon as possible. Depending on what’s being grown and the environmental circumstances you should bank on releasing 4 to 6 Phytoseiulus/m² into the full field. On and around the infected plants you should release around 20 predator mites/m². In normal conditions Phytoseiulus is capable of keeping the spider mite numbers under control for the rest of the growing season. In dry, warm weather they can still create major problems though. Experience has shown that control with Phytoseiulus can be boosted by keeping the air moisture percentage level by misting from a high-pressure hose with a fine nozzle.
After taking delivery of them, the predator mites should be released as soon as possible. They can be kept for a short while; store the small bottle lying down in a cool (6-10°C), dark place.
Let the bottle with the predator mites in reach ambient / room temperature before using them. Twist and gently shake the bottle, so that the predator mites are spread evenly between the vermiculite blocks.
Avoid allowing the bottle of Phytoseiulus to become too hot (hand temperature is best).
Remove the sticker from the distribution opening. Spread the material on the leaves. Never sprinkle by removing the cap (otherwise you’ll get too many predator mites in too few places).
A single bottle can be shaken about 190 times. Per shaken out pile, there will be on average six to seven predator mites. Release Phytoseiulus from the moment that the first red spider mites rear their heads.
It’s best to aim for a minimum of 2-3 predator mites per m². If necessary you can release a second batch a couple of weeks later. In extreme cases you can release up to 20 Phytoseiulus/m².
For a good predator mite development the relative air moisture needs to be kept fairly high (65% or more) and the environmental temperature should be regularly above 20°C.