Intense damage

A complex of stink bug species occurs in different soy producing regions, with Euschistus heros, Piezodorus guildinii e Nezara viridula are considered the most important. The predominance of one of these species varies depending on the location, the climate of the cultivars and the stage and development of the crop. Likewise, the extent of the damage depends, among other factors, on the stage of development of the plants during the infestation period, as they occur from the vegetative phase.

Among bedbugs, the species Euschistus heros stands out with higher populations in the Cerrado. This species is native to Tropical America and is well adapted to warmer climates, being abundant in the Center-West of Brazil. E. heroes It is the least polyphagous among soybean bugs. During the harvest, up to three generations can occur, which also feed on milk, cotton and castor beans, for example. After harvesting the soybeans, it looks for later plots or shelters, where it feeds on these and other host plants. It completes the fourth generation and enters dormancy (diapause) in the straw of the previous crop or nearby, where it protects itself from the action of parasitoids and predators. During this period, they do not eat and are able to survive thanks to the fat reserves that were stored before diapause. This species is one of the least migratory among those found in soybean crops.

Damage caused by bed bugs

Like other phytophagous bugs, E. heroes is, in general, responsible for reductions in seed yield and quality, as a result of the damage caused by the stings themselves, as well as the inoculation of pathogens, such as the fungus Nematospora corylii. This fungus is a yeast that can cause seed deterioration, similar to bacterial attack. In the field, the attacked grains become smaller, wrinkled, shriveled and become darker. Poor pod and grain formation causes leaf retention on soybean plants, which do not mature at harvest time. Therefore, the stink bug complex represents a high risk to soybean productivity. They can cause irreversible damage to the crop, feeding directly on the grains from the beginning of their formation in the pods.

Stink bugs suck sap from branches, stems or pods, inject toxins and/or inoculate fungi that cause stains on grains. When they suck on branches, they cause “leaf retention" (where there is no physiological maturation of the leaves and/or stems, while the pods mature). This symptom can be caused by all species of stink bugs. This leaf retention seems, possibly, to be associated to the imbalance of indoleacetic acid (IAA) in the plant, resulting from the suction caused by insects. Among the species, and with the same population density, there is a clear greater leaf retention caused by the attack of P. guildinii, While E. heroes is the species which causes the lowest retention. Plants with leaf retention have green leaves and stems of varying intensities and, normally, demonstrate a large number of pods with flat grains which, depending on the intensity of the attack, may have all of the pods flat. Due to this intense imbalance, the plant remains green, trying to replace the lost charge.

When stink bug attacks occur in pods, losses can reach values ​​exceeding 30%. When they attack pods in formation, they can cause the emergence of pods that are flat, dry and/or without the formation of grains and, as a result, the pods dry out and turn brown. If the attack occurs on pods in the granation phase, deformations, wilting and stains may appear on the grains. In this case, the grains may lose their commercial value, due to their reduced oil content. Losses in seed germination power can exceed 50%, in addition to a drop in vigor. According to Fraga & Ochoa (1972), in less developed legumes, the contents of the seeds can be completely sucked out, resulting in flat, flat seeds, reduced to a blade. Attack during the development phase results in small, wrinkled and deformed seeds, and characteristic stains can be seen in the puncture area. At the stage when soybeans have formed seeds, but still have a high water content, the spots caused by feeding are very characteristic, however, the wrinkling is less pronounced (Turner, 1967).

When stink bugs attack the grains, there is a loss in quality and also in the seeds, mainly due to the inoculation of the fungus. Nematospora corylii, which causes yeast spot, also known as yeast spot. These stains on the grains, however, are caused especially by Nezara viridula e Piezodorus guildinii, and are less common in attacks E. heroes.

The effect of feeding by seed suckers, whether economic (reduction in crop productivity in the field) or ecological (reduction in plant fitness), results in death of embryos and weakening of seeds and, consequently, decreases their viability, germination and vigor. Likewise, there are reductions in the number of seeds produced by the plant, changes in chemical composition and the introduction of pathogens.

Soybean seeds have reduced viability proportionally to the extent of the damage, as stated by Galileo & Heinrichis (1978). The tetrazolium test, in addition to evaluating the viability and vigor of seed lots, provides diagnosis of the causes of reduced quality, such as mechanical damage, moisture deterioration and stink bug damage, which are the main problems that affect physiological quality. of soybean seed. However, in addition to these, drying, water stress and frost damage can also be easily visualized by the test (Costa et al, 2007). The percentage of bed bug damage is given by class TZ1-8. The TZ 4-5 class shows viable soybean seeds and the TZ 6-8 class for non-viable seeds, that is, those that are killed by the action of the stink bug bite, viable soybean seeds.

In a study carried out comparing different soybean cultivars and maturation groups, the results referring to the tetrazolium test showed that the stink bugs present in the uncontrolled area caused total damage to the seeds (TZ 1-8); for class TZ 4-5 the difference between the treated and untreated area was smaller. For the TZ 6-8 class, the percentage of dead seeds was higher in the area without stink bug control. However, in this class an increase in the percentage of materials from the medium and late maturation groups was observed, due to the fact that they were more exposed to a higher population and for a longer time. The data from this work confirm that the intensity of damage is correlated with the insect population, the maturation group of the materials and pest management.

Of the total damage caused by stink bugs, the most harmful are those located close to or in the embryonic region, which also make the use of grain for seed unfeasible (Gazzoni 1998).

In short, stink bugs cause damage to soybeans by reducing productivity through effects on the pods and/or grains, by reducing the germination power and vigor of the seeds, by reducing the protein and oil content of the grains and by inoculating pathogens in the seeds. . Furthermore, they promote leaf retention at the end of the soybean cycle, which generally leads to greater use of desiccants.

Control Level

Control levels indicated for soybean bugs, currently, with the objective of chemical control: a) grain production fields = 2 stink bugs/meter of crop; b) seed production fields = 1 stink bug/meter of crop. Sampling is carried out using the tapping cloth in one linear meter of plant row (beating cloth 1m x 1m)

During the hottest hours of the day (between 10 am and 16 pm) these insects are not easily located in the field, and these sampling times should be avoided. Sampling should be more intense on the edges of the crop where, in general, stink bugs begin their attack. Due to the use of soybean cultivars belonging to different maturity groups, the first harvested crops serve as a source of stink bug infestation for neighboring crops and, therefore, the later cultivars, which are still developing pods and grains, must be taken in consideration. Being Therefore, close attention is recommended with these late-season crops, as the intense and rapid migration of insects can cause irreversible damage to soybeans.

Control

The implementation of up to three insecticide sprays on borders (150m to 200m wide, during the vegetative phase) has been useful to control migrating stink bugs, especially in late cultivars, before these insects establish generations capable of causing significant damage. In fact, it is a very selective technique for natural enemies, as the area is partially pulverized. Spraying borders is efficient if the pest attack is not yet widespread throughout the crop, which can be diagnosed through sampling.

Another important moment is to carry out control in the phase between R2 and R5.1, if the pest is present at considerable population levels. This has been a very effective practice to prevent economic damage and large stink bug outbreaks at the end of the harvest. Outbreaks that have often led producers to make successive applications to control the pest, sometimes three or four of them and, even so, causing damage. Another aspect to consider is the decrease in populations at the end of the harvest, where they reach their population peak and tend to migrate to other areas or serve as survivors in the region, making it very infested. Logically, end-of-harvest applications must be carefully based on sampling, taking into account restrictions regarding the withdrawal period (withdrawal period) of the agrochemical used (Figure 1).

Figure 1 - Stink bug population dynamics in soybean crops

Figure 2 - Soybean seeds, with problems of stink bug damage identified by the tetrazolium test, subjected to a pre-conditioning period of 6 hours/41ºC. Londrina, PR, 2007. (Costa et al, 2007)