Tiny and Offensive: Mite Attacks on Soybeans

Mites are secondary pests in soybeans in Brazil. However, there are regions and times in which the pest may require control measures, therefore requiring the farmer's attention when monitoring his crops.

Currently, six species of mites associated with soybeans are known in Brazil with the potential to cause damage, with the green mite and the two-spotted mite being the most common. The other pest mites are three species of red mite and the white mite. Associated with pest mites, biological control agents such as predatory mites (beneficial mites) and diseases such as fungus Neozygites floridana. Such biological control agents contribute to reducing the intensity of attack by pest mites on soybean crops, which is why farmers must make rational use of insecticides and fungicides on their crops in order to preserve them.

The occurrence of drought is the main conditioning factor for mite outbreaks in soybeans. However, inappropriate use of insecticides and fungicides can increase the intensity of the pest attack. Certain non-selective insecticides, such as pyrethroids, used in soybeans to control caterpillars and stink bugs can intensify the attack of mites, as they are harmful to predators and competitors, and induce the dispersion of mites in the crop, increasing the area attacked by the pest. Neonicotinoid insecticides can also favor tetranychids by stimulating their reproduction. In soybeans, commercial mixtures of pyrethroids and neonicotinoids are widely used for stink bug control. Furthermore, fungicides (triazoles, strobirulins and benzimidazoles) used to control soybean rust can harm the fungus. Neozygites floridana which naturally controls pest mites. The sooner the application of fungicidal insecticides is started on soybeans, the greater the damage to the biological control fungus of mites.

Mite outbreaks often occur during the reproductive phase of soybeans, when the plant is in the phase with the largest leaf area of ​​the plant, making it difficult to distribute acaricides, especially on the middle and lower leaves. Due to the difficulty of control and the cost of treatment, strategies for preserving natural enemies through the rational use of agrochemicals prevail for the management of mites in soybeans. In this context, the commercial release of Bt soybeans (intact) brings good prospects due to the possibility of reducing the use of insecticides. However, it is essential that Bt soybeans are managed correctly, controlling stink bugs only after the emergence of the pods, when the level of action is reached, and using fungicides only in the reproductive phase of the crop.

A study carried out by Emater/Paraná with the collaboration of Embrapa Soja, in the 2013/14 harvest in 107 farmers' fields in the state of Paraná, indicated that it is possible to reduce the number of insecticide and fungicide applications, as well as delay the date of the first application, without productivity losses for the farmer. In another study where 189 soybean crops were monitored by agronomists from Coamo, in the states of Santa Catarina, Paraná and Mato Grosso do Sul, in the 2010/11 harvest, it was found that it is possible to reduce by half the number of sprays for management of stink bugs in soybeans using integrated pest management.

Recent studies indicate that cultivar resistance can be a tool for managing mites in soybeans. But it is still a challenge to obtain knowledge about: the critical phase of soybean to attack the pest; the level of control; the definition of practical and efficient sampling strategies and the viability of applied biological control, with predators and pathogens.

There is still no consolidated level of control for mites in soybeans in Brazil. However, scientific research has shown that, depending on the intensity of the attack, there may be losses in productivity, justifying the use of control measures. In intensive crops (flowers, vegetables, fruit trees) mite control has been carried out using predatory mites (biological control), however, in soybean crops, due to the scale of production, mite control is carried out predominantly with acaricides chemicals. Currently, there are a small number of acaricides registered for the control of mites in soybeans, the active ingredients being: spiromesiphene, diafentiuron, abamectin and profenofos+lufenuron. As the attack can occur punctually in the crop, it is advisable to inspect all plots and spray only the attacked areas, following the registered dose of each product.

How they behave

Mites are tiny organisms that feed on soybean leaf cells, reducing photosynthesis and energy production by the plant, and intense attacks can reduce crop production. In the case of the white mite, the attack can cause wrinkling of the leaves and deformation of the plant.

The green mite (Mononychellus planki) is the most common mite on soybeans in Brazil, but it is less aggressive than the others. It is commonly found in low population density, however, during periods of drought during the soybean grain filling phase, population outbreaks of the pest may occur. This mite has an intense green color with four golden-yellow front legs. The eggs are deposited directly on the surface of the leaf, mainly on its underside and along the veins. It produces a small amount of web, used to attach its eggs to the leaf and in the process of dispersion by the wind. Symptoms of green mite attack are clear spots (dead cells) along the entire surface of the leaf, on both sides, but more intensely on its underside. At first, the attack is concentrated near the veins, but over time it is distributed over the entire surface of the leaf, which presents a grayish color. Leaves at different heights of the plant present similar intensity of attack. The green mite attack is well distributed across the crop, with large grayish patches occurring. As the transition from areas with little attack to others with more intense attack is gradual, the beginning of the development of reboleiras may go unnoticed by the untrained eye.

The spider mite (Tetranychus urticae) is the second most common species in soybeans in Brazil, it is usually more aggressive than the green mite, but very sensitive to rain and attack by biological control agents. It is noticed in intense attacks on soybeans, being favored by drought. This mite is green in color with two dark spots on its sides, which gave rise to its name “spotted mite”. They live in colonies sheltered under web that is produced in large quantities on the underside of the leaves. Symptoms of attack on the underside of the leaf are clear spots (dead cells) that quickly evolve into continuous grayish spots, corresponding to the region covered by the colony, delimited by its web, while yellowish spots appear on the upper side. The spider mite, unlike the green mite, attacks very concentrated in small spots on the leaf. Over time, new colonies are established, forming new attack points on the same leaf. The attack is uneven across the plant and crop, and leaves from the same plant can present very different attack intensities. In the field, small patches of heavily attacked plants can be seen, with a yellowish appearance, generating a strong contrast in relation to the surroundings, which makes them easier to recognize.

The species of red mites (Tetranychus desertum, Tetranychus gigas, Tetranychus ludeni) that occur in soybeans are similar to each other. They have a crimson-red color that turns dark red over time. They present symptoms and attack potential similar to those of the spider mite, but are uncommon in soybeans in Brazil.

The white mite (Polyphagotarsonemus latus) is also uncommon in soybeans in Brazil and commonly occurs in low population densities. The white mite, unlike the others, is favored by rainy periods and is present in the vegetative phase of soybeans. These mites have a bright cream color and are very small, difficult to see with the naked eye. It is common to observe males transporting female pupae, with which they copulate as soon as they emerge. The eggs are deposited directly on the surface of the leaf along its entire length, mainly on the underside. The white mite does not produce webs. Due to its small size, the white mite preferentially attacks new leaves (pointers), and its attack harms the leaf's expansion process, leaving it shriveled, and can be confused with a virus. The deformation in the leaflets, caused by the white mite, is always symmetrical and occurs with the same intensity in the three leaflets of the same leaf. While, in the case of viruses, asymmetric deformation may occur in the leaflet and leaflets of the same leaf may present deformations of varying intensities. The leaflets of plants with the virus may present bursts of yellowish, black, reddish or green color, whereas leaflets attacked by the white mite do not show such changes. In farming, the white mite attack occurs in reboleiras, whereas the viruses can be verified in isolated plants. The developing stem and pods can also be attacked. The affected structures have a tanned appearance. Early attacks can significantly affect the plant's architecture.

The article is in issue 191 of Cultivar Grandes Culturas.