These phytophagous parasites, belonging to the arthropod family, can be found on most ornamental and horticultural plants; generally they settle on the upper page of the leaves. They are very small, so much so that it is not possible to observe them with the naked eye; the body is round, soft, yellow, orange, green or red; in general, females are larger than males. The mites have four pairs of legs, this characteristic makes them belong to the class of arachnids.
Generally we note the presence of mites on the plants due to the alterations they inflict on the leaves, such as swelling or galls, or from the thin webs they weave.
The plant mites they sting the surface of the leaves, feeding on the cells that make up the leaf lamina; sometimes they can also be found on young shoots.
The attack by the mites manifests itself through a general loss of vigor of the plant; you can also observe characteristic discoloration of the leaves, which tend to become bronze and therefore to fall.
Mites spread rapidly, producing even eight or ten generations during the warm seasons; the species that develop on the plants are: Tetranychus, Panonuchus, Polyphagotarsonemus, Olygonichys (present above all on conifers); to these we add eriofidi, like Aculus and Trisetacus, which often cause huge defoliation.
Fortunately, these parasites have many predators in the wild; first of all another mite, more conspicuous, visible to the naked eye: Phytoseiulus persimilis; a small spider, 4-5 millimeters long, which feeds on adult specimens of Tetranychus. Even the ladybirds often feed on mites, as well as some small wasps, and other insects.
In case of very serious infestation it may become necessary to use chemical products to eradicate the mites, of course acaricides are used, such as: Folimat, Amitraz, Dicofol, Clofentezine, Endosulfan, Fenpiroximate and Fenazaquin.
Generally mites develop preferably in hot and dry climates, sometimes maintaining the right atmospheric humidity can help us not only to better cultivate our plants, but also to protect them from pest infestations.
Red spider mite
A very common phytophagus in crops, both in open fields and in greenhouses, is the so-called "little red spider". It makes its appearance in the case of very high temperatures and low rainfall. It is one of the most feared adversities because, thanks to its very fast reproductive cycle, it is able to rapidly colonize the whole plant and debilitate it deeply, also causing its death. The rapidity in reproduction is also the cause of the frequent establishment of resistance to pesticides and the consequent need to reduce its use to the maximum (practicing prevention) and possibly vary the active ingredients often. In recent years, however, there is a new weapon: more and more often the integrated struggle is resorted to, being helped by insects, mites or other types of predators, to be preserved to the maximum.
Description and classification of plant mites
First of all we must point out that these are not spiders, but rather microscopic insects classified as mites, and they can be of different colors (red, yellow, green), belonging to different species (sometimes specific to a single crop).
In ornamental and horticultural crops the species that does the most damage is undoubtedly the Tetranychus urticae, but we must also remember theEotetranychus carpini (yellow spider of the vine) and the Panonychus ulmi (parasite of fruit trees).
They do not manage to measure a millimeter in diameter: this is why they are almost invisible to the naked eye, especially if the symptoms are not immediately recognized.
Under the right conditions (ie with high temperatures, above 25 ° C, and a dry environment) their reproduction is extremely fast and increases proportionally with increasing heat.
Up to about 12 degrees the eggs are in a state of inactivity, do not hatch and do not reach the adult stage. When the average temperatures reach 15 ° C, the development takes place, which generally takes 5 weeks. However, if there are peaks of 30 ° C, the growth may take a total of only 7 days.
A female, on its own, is able to lay up to 100 eggs in favorable conditions. These are, moreover, able to enter a sort of hibernation to overcome unfavorable periods and then return to work when conditions permit.
The reproductive cycle
The eggs (microscopic and not visible to the naked eye, also thanks to their translucent surface) are laid on a fine spider web and reach maturity when they become bright yellow. Shortly before hatching, the development of the bright red eyes is clearly seen.
The larvae they are also translucent, round in shape and with six legs. Overall they are slightly larger than the eggs. They immediately begin to feed themselves becoming orange or light green with dark spots on their backs
The nymphs there are two phases within the nymph stage. First there is a protoninfa and then a deutoninfa, both characterized by the presence of eight legs. Males are, even at this stage, smaller than females.
Adult females retain their pale green or orange color while males turn more towards dark red. However, both retain the black spots on the back which increase in size.
Females, in the autumn, enter the dormant phase. They move, already fertilized, on the herbaceous vegetation and take on a green color that favors mimicry.
In all the growth phases they live mostly on the underside of the leaves. In no case are they able to fly, but they move between a leaf or a vegetable and the other descending with a very thin spider web that also serves to deposit the eggs and protect them.
Movements are also facilitated by raindrops and wind.
How to recognize an infestation?
Diagnosing its presence with the naked eye in the very early stages is difficult, even for the most experienced farmer. With some suggestions (and frequent monitoring of our plants during periods of probable occurrence) it is possible to intervene in time and avoid the most important damage.
First of all, the back of the leaves should be observed regularly: with a little care, you will notice very small reddish or yellow dots. These are generally followed by a discoloration of the front of the leaf: it will take on a coloring between bronze and silvery. Later it will become increasingly yellow and "dry" in appearance, increasing the necrotic areas. They often fall to the slightest touch.
By steaming the leaves we will notice a very thin spider web that, with the progression of the disease, will become more and more evident enveloping leaves, flowers and branches. Arriving at this stage the condition can already be defined as very serious and the plant will be seriously damaged, if not condemned to the final deterioration.
Under what conditions is the appearance of red spider mites easier?
The red spider is very common
- in greenhouse cultivation, where it can also never enter the dormant phase and therefore becomes much more difficult to eradicate
- in potted plants, especially if kept on balconies, terraces or on a paved area. Radiation leads to a strong increase in temperatures and heat is transmitted more easily to the container and to the entire substrate
- In plants with dark colored leaves, where the heat concentrates more
- Where there is a massive use of insecticides (especially pyrethroids) that counteract the action of its natural enemies
Red spider prevention
To prevent or at least mitigate the impact of the red spider, different strategies can be implemented
- It is essential to always keep the environmental humidity high by abundantly wetting the leaves (even with rather strong jets) especially in the evening and in the morning.
- We move, if possible, the plant in a shaded area (especially in the afternoon)
- Choose a substrate containing a good quantity of clay which, retaining water longer, maintains humidity. We avoid, if possible, extremely peaty soils that, with the sun, tend to heat up.
- If possible it is good to wet the floor around our pots several times a day: in this way we will get a considerable increase in environmental humidity.
- If we live in a condominium and it is not possible to implement the previous advice we can opt for the preparation of saucers filled with expanded clay and water, which perform the same purpose. The same can also be placed under the plant, provided that the roots are not directly in contact with the liquid (to avoid the advent of rot).
- In the case of stone or tile floors, it is advisable to lift the vases from the floor with feet (so that the contact surface is minimal).
- We preferably use plastic or resin pots. Unlike terracotta, they are less breathable and keep the soil moist longer. Even heat accumulates to a lesser extent.
- We use various types of plant material (straw, leaves, grass) to mulch the surface and keep the roots cool. We avoid pine bark, which tends to dry out a lot and also accumulate heat.
- We frequently use cupric or sulfur-based products: they discourage the onset of these infestations.
- If the infestations are recurrent, year after year, it is good to begin the treatments already during the winter. A very good ally is the white oil, perhaps in association with a rameico, to be distributed on the branches, Agirа by suffocation against the wintering eggs.
Red spider mite treatments
If the infestation is already in progress, especially in the early stages, it is still possible to intervene with products with a low environmental impact and which, above all, do not facilitate the onset of resistance.
- First of all the leaves must be bathed several times a day with cold water.
- In the case of small plants, after having moistened them, put them in the fridge at about 4 ° C for no more than 24 hours. Generally the weather is not sufficient for it to suffer damage and the population of mites will be considerably damaged
- We spray with a solution of wettable sulfur
- We apply a mixture of nettle, garlic or chilli.
How to get nettle macerate?
We collect 1 kg of nettle leaves and put them in a bucket with 10 liters of water, possibly rain. Let's cover it with fabric so that it doesn't attract insects. A fermentation will be produced. The product will be ready for use when no more bubbles are produced. It will be filtered and diluted (1 liter in 10 liters of water) and then vaporized on the leaves of the infested plants.
In the greenhouse or on the balcony it is possible to carry out the integrated struggle by introducing a natural predator, the Phytoselulus persimillis, which is now very commonly found on the market. It is a small mite that, in a suitable environment, reproduces faster than our parasite, effectively containing its work.
It can be used all year round, as long as the temperatures are above 10 ° C.
Other useful insects are the common ladybirds and some types of Colorado beetle.
Always remember, in this case, not to use insecticides (in particular pyrethroids) that would damage our allies!
Mite: Fight with synthetic acaricides
It is good to adopt this strategy only in the event of major attacks.
On ornamental plants it is good to choose products that are possibly translaminar and that act on all the spider's developmental stages.
On horticulture, formulations are generally used which act only on eggs or on adult states, allowing however a shorter timeout period.
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