Red spider mite is the scourge of many gardeners. This species of tiny plant-feeding arachnid often makes it presence felt in greenhouses, hence its popular alternative name of “glasshouse mite”.
Plants afflicted with red spider mites take on a dusty and unhealthy appearance, and may also acquire strands of a very fine webbing over leaves and around stems. Upon closer examination, what first appears to be dust is actually the mites clustered on the leaves. Egg cases and case shells may also be noticed.
A few spider mites quickly becomes an infestation, which, in a greenhouse, can spread rapidly between plants. This makes it crucial to deal with the problem promptly. Take particular care in spring and autumn, when these pests are at their most prolific. However, if your greenhouse is heated throughout the year, you need to be vigilant at all times. Red spider mites thrive in warmer environments and so it is entirely possible for a heated greenhouse to suffer an infestation even in the middle of winter.
The best method of protection is prevention. First and foremost, this means keeping your growing area clean and in good order. Ensure that you sweep away debris surrounding the plants and, because red spider mites favour a drier environment, keep your plants well watered. This last might extend to the use of automatic sprinklers if you are going to be away for any length of time.
Many gardeners favour controlling red spider mites via the use of the arachnids’ own natural predators. Primarily, these are lacewings and ladybirds. Predatory species of mite, midge and beetle are also used. Lacewings and ladybirds can be encouraged through good horticultural practice, which obviously means eschewing insecticides. Predatory species of mite, such as Phytoseiulus persimilis and amblyseius, can be obtained from certain garden centres and internet suppliers. Phytoseiulus persimilis is the most popular species to use, and preys on red spider mites at all stages of a mite’s life cycle. Phytoseiulus amblyseius is more likely to survive colder temperatures than persimilis and so should be considered to tackle winter infestations.
A third solution involves the use of pesticides. Most common in commercial operations, it is also an option for the home gardener, although this approach should not be adopted without a proper understanding of safe use and the potential consequences for other arthropods. This means it is also not possible to combine biological control agents, such as Phytoseiulus persimilis and amblyseius with pesticides, as the pesticides are not selective and will kill the predatory species along with the red spider mites.
Finally, as some populations of red spider mite are showing increasing resistance to pesticides, it is unwise to assume that pesticide application is an automatic solution.