They are tiny, reddish-colored bugs that begin to hatch in summer, weaving a web on the underside of the leaves of garden plants. Wherever they pass, they leave the plant discolored , and if not treated in time, they can kill their hosts. They usually form real pests, especially in greenhouses. These tiny animals, often confused with insects, are colloquially called “spiders” or “red mites”.
What is the ‘red spider’?
Despite their colloquial name, these animals are not spiders, nor are they insects; sometimes they are not even red; their pigmentation depends on their age and what they have eaten. Although like many spiders and certain insect larvae, they exude a silk that they use to weave their nest . They are actually small mites , no more than half a millimeter in length.
Its scientific name is Tetranychus urticae , it is native to the Eurasian continent, but it has traveled with ornamental plants and is present in practically the entire planet.
The reproduction of this mite is produced by a type of parthenogenesis called arrhenotochy: fertilized eggs give rise to females, while those that remain unfertilized give rise to males, as happens with bees. The eggs are tiny spheres that resemble small translucent pearls. When it hatches, a first larva is born, which goes through two more nymph stages (protonymph and deuteronymph) before reaching maturity. Adults begin to appear in late spring. Almost the whole year they are green, and as winter approaches, the latest generations have red tones. They survive the winter by entering a state of dormancy called diapause , which makes them resistant to cold.
Since the life cycle can be completed several times in the course of the same year, at the same time and on the same plant there may be many individuals in different stages of their development, and many may be relatives. Remaining in a small and relatively isolated space, inbreeding is a very real problem for populations. To avoid this, the mite is able to recognize its relatives and avoid reproducing with them.
How does it attack?
This mite is an external parasite of a large number of plants, including ornamental plants found in gardens, parks and nurseries, and in many vegetables. The damage they cause is quite significant, especially if it behaves like a pest.
These damages can be very subtle at first, since this mite is so small that it does not attack organs, not even tissues, but instead attacks cell by cell , consuming cell content, until it dies. As it spreads through the plant, the leaves are filled with characteristic brown spots and gradually lose their photosynthetic capacity. Since photosynthesis is the plant’s way of obtaining its food, these types of attacks, if extended too far, can starve the victim .
How can it be controlled?
Like any other pest, there are different procedures to combat Tetranychus urticae infestation. Methods can be classified into three types: physical, chemical and biological.
Among physical methods , manual removal is not usually an option. The size of the animals and their tendency to protect themselves with silk makes them very difficult to remove. If the plant can be moved, a good option is to leave it in a cool, moist place; the mite has a preference for warm and dry environments. Spraying the plants with water can help kill some mites, but if they have already spun their silk, this option will not always be effective.
Other physical methods have to do with the hygroscopic effect of mineral powders, which can dehydrate these animals. Granite dust and diatomaceous earth have been used for this purpose, with relative success in both cases.
Chemical methods , on the other hand, are based on altering some biological process of the insect through toxicity. For the red mite, the most widely used product is abamectin , an insecticide, acaricide and anthelmintic that attacks the nervous system of these mites, causing paralysis and causing their death. However, it is a corrosive, irritating compound that can cause damage to human health, so its application must always be carried out following the maximum precautions.
Finally, there are methods of biological control , that is, other living beings that, either by predation, parasitism or pathogenicity, are capable of reducing the populations of the pest animal and even eradicating it. In the case of Tetranychus urticae , some natural enemies stand out, such as the beetles of the Staphylinidae family or the predatory mite Amblyseius californicus , which can be found for sale in stores. Amblyseius californicus feeds on Tetranychus eggs as well as larvae and adults, without causing damage to the plant.
References:
Faraone, N. et al. 2020. Soil and foliar application of rock dust as natural control agent for two-spotted spider mites on tomato plants. Scientific Reports, 10(1), 12108. DOI: 10.1038/s41598-020-69060-5
Feiertag-Koppen, C. C. M. 1976. Cytological studies of the two-spotted spider mite Tetranychus urticae Koch (Tetranychidae, trombidiformes). I: Meiosis in eggs. Genetica, 46(4), 445-456. DOI: 10.1007/BF00128090
PubChem. 2022. Abamectin (N.o 6435890).
Tien, N. S. H. et al. 2011. Mate choice promotes inbreeding avoidance in the two-spotted spider mite. Experimental and Applied Acarology, 54(2), 119-124. DOI: 10.1007/s10493-011-9431-y
Zeni, V. et al. 2021. Diatomaceous Earth for Arthropod Pest Control: Back to the Future. Molecules, 26(24), 7487. DOI: 10.3390/molecules26247487
