The high productive potential of the soybean crop is limited by a number of factors, such as water deficit, competition with weeds, nutritional deficiencies and the attack of pests and diseases that can be minimized through correct monitoring, identification and control.
In drier years and with high temperatures, as occurred in the last harvests, mites have become pests that are increasingly present in the soybean crop, deserving great attention. In studies carried out in the 2011/12 harvest, in the central region of Rio Grande do Sul, in areas with and without mite control, it was found that, depending on the cultivar used, losses caused by mites can reach 20 bags per hectare.¹
Most common mite species in culture
The most common species that attack soybeans are the spider mite (Tetranychus urticae), the green spider mite (Mononychellus planki) and the white spider mite (Polyphagotarsonemus latus) ².
> Spotted spider mite (Tetranychus urticae)
Adults are greenish in color with two lateral dark spots. They have as a characteristic the formation of colonies on the underside (abaxial) of the leaves, full of webs and silky threads, produced by the females where they lay their eggs.
>Green mite (Mononychellus planki)
It is characterized by having an intense green color and four pairs of front legs with a golden yellow color. Unlike the spider mite, the green mite lays eggs directly on the surface of the leaf next to the vein, mainly on the underside of the leaves ³.
>White spider mite (Polyphagotarsonemus latus)
It is characterized by its white to light green color and, like the green mite, does not produce webs and silky threads, being practically invisible to the naked eye. It lays eggs directly on the underside of the leaf along its length.
Favorable conditions for their development
The low relative humidity of the air and the high temperatures favor the development of mites (tetranychids), due to the stimulation of feeding and being a factor that increases the number of generations per unit of time, that is, the insects reproduce more, in addition to impair the activity of agents that provide biological control of mites ?.
According to Embrapa Soja (2012), “the occurrence of drought is the main factor that favors the attack of mites (Tetranychidae) on soybeans”. These conditions have been occurring more or less intensely in the last harvests and in the current harvest. On the other hand, “the white spider mite, unlike the others, is favored by rainy periods” ?.
In addition, in periods of drought, the incidence of fungal diseases in plants is low, improving the establishment of mites in host plants. Therefore, in regions with high relative humidity, such as lowland areas, we may not find mites, due to the creation of an unfavorable microclimate for their breeding and a climate favorable to natural enemies. However, conditions of low relative humidity in the air and soil and temperatures above 27 ºC favor its development.
Precipitation is a natural factor that helps control mites, as rain may be washing the mites and their eggs off the leaves, causing death by drowning, as well as death by the impact of raindrops ?.
damage caused
The mites are characterized by the reboleira attack. They have a scraper/sucker mouthpart equipped with styluses and when they attack the plants, they remove the exuded cellular content.
The first symptoms of a mite attack are the whitish or silvery color of the attacked leaflets, as these cause reduction and emptying of the cellular content. As the days go by, the leaves adopt a yellowish color, and later show a brown color due to the oxidation of the points that were attacked.
When the population of mites is high, we can see necrotic spots and, subsequently, leaf fall, which is easily mistaken for fall due to water stress. In periods of water deficit, mites perform a greater number of bites, damaging a greater number of cells to meet their needs.
After the leaves are affected, they lose their main function: the production of photoassimilates. This is due to the rupture of cells and the loss of cellular content, causing a reduction in the photosynthetic area of the plants, causing a smaller amount of photoassimilates to be assimilated and fixed in the grain, consequently causing a reduction in the weight of the grain and losses in quality and productivity. .
In addition, the lesions caused by the attack of mites increase the rate of transpiration by the plant, resulting in a water deficit and blockage of starch synthesis.
How to identify in the field?
The monitoring and identification of mites in the soybean crop must be carried out weekly so that we are assertive in decision making and control moment.
For identification in the field, in addition to viewing the symptoms, the underside of the leaf can be viewed with the aid of a magnifying glass, in order to identify the presence of mites and other pests that may be present, considering that these are insects that are difficult to see from a distance. naked eye.
If you don't have a magnifying glass, you can expose the underside of the leaf facing the sun, causing the mites to be sensitized by the incidence of sunlight and move over the surface of the leaf, thus facilitating visualization. With the exception of the white mite, the others can be seen with the naked eye without the need for a magnifying glass.
Precautions to be taken
The inappropriate use of insecticides and fungicides can increase the intensity of mite attacks. Certain non-selective insecticides, such as pyrethroids, induce the dispersion of mites in the crop and, consequently, increase the area attacked, whereas neonicotinoid insecticides can stimulate the reproduction of tetranychids. Therefore, the sequential use of pyrethroids for the control of other pests in the soybean crop in a dry and hot climate condition is not recommended, because this can lead to imbalance and increase in the pressure of mites.
Control
To control the pest, we must use products duly registered for soy. One of the control alternatives is to work with integrated pest management, being careful with the excessive use of insecticides such as pyrethroids and neonicotinoids, as we saw earlier.
Text written by Alfredo Henrique Suptiz and Igor Streit, academics of the Agronomy course at UFSM campus Frederico Westphalen, members of the Tutorial Education Program (PET) Agricultural Sciences, under the guidance of professor Dr. Claudio Jose Basso.
