Pest control in soybeans

Among the pests that affect soybean crops, stink bugs stand out as serious causers of problems. When adopting control measures, it is necessary to pay attention to several aspects, such as the phenological stage of the plant, environmental conditions, level of damage and timing of fungicide application.

Soybean crop management is increasingly improved. With each passing day there is greater knowledge about cultivars, sowing time, planting, fertilization, liming, inoculation, among other practices that surround the factors that build crop productivity. As a result, in some points there is consolidated management, however in others there are gaps that mean that the entire theoretical productivity constructed may be threatened due to the lack of interventions that protect productivity. In this case, phytosanitary management with regard to insect pests must always seek to combine integrated management that can reduce pest pressure and maintain productivity.

Each producer has a strategy for combating insect pests, however some parameters must be taken into consideration, such as, for example, which insect pest can cause or is already causing the greatest damage. It is necessary to analyze whether this pest is recognized as the main one throughout the harvest and whether it always causes the main damage. Also, do not neglect the level of economic damage so that pest insect management can be carried out. The stage at which the plant is in relation to the presence of insect pests and the efficiency of each insecticide in relation to the target pest and climate conditions are also important. Regarding climatic conditions (environment), care must be taken with the conditions favorable to the development of pests and environmental circumstances for the application of insecticides, as well as the stress conditions of the plant.

Every year there is a large set of soybean cultivars that are available on the seed market and are recommended for each location. Naturally, those that adapt best and are most productive remain on the seed market longer and the others are replaced. As a result, the supply of cultivars varies from year to year, but over time it is clear that there is a greater preference on the part of farmers for cultivars that have a shorter cycle, are productive, and have good resistance to environmental and biotic stresses. With the choice of cultivars with a shorter cycle, some characteristics come to be assumed, such as the reduction of the plant's leaf area and sometimes the area per leaf. This means that each leaf starts to play an increasingly important role within the plant, as it is responsible for supporting (being a source) a set of pods/grains (drain). In this sense, each cultivar has a limit leaf area (minimum and maximum) on which grain productivity is based, always considering the environment in which that cultivar is located.

Ideally, insect pests should not interfere with the soybean leaf area, but this management is not always possible. It appears that the plant has the ability to withstand the damage caused by insects without interfering with grain productivity. In other words, the plant has a compensatory capacity that, depending on its leaf area (which is greater than its minimum critical value in that environment), can compensate for possible damage. However, there are economic damages in agriculture. Therefore, there must be an economic balance in relation to the productivity and economic income of the crop. From certain damage values, for some pests (or a set of them), control actions must be adopted to reduce their presence and consequently their damage.

Figure 1 shows the action levels for the main soybean pests. The information is presented by Embrapa, 2011. When adopting the control levels indicated in Figure 1, items such as economic issues, changes in cultivars and cultivars' reactions to the loss of leaf area must be taken into account. This means that the management adopted may vary depending on the objectives of each soybean producer. 

In this sense, we sought to carry out a survey of insect pests in 48 soybean cultivars, sown in Santa Maria in the 2015/2016 harvest. The experimental plots contained five rows spaced 0,5m apart, 7,75m long and three replications. Cloth beatings were carried out in each plot. One of the three repetitions always preceded the insecticide applications, as shown in Figure 2. Crop management followed Embrapa's recommendations for high production ceilings. The soybean cultivars evaluated are described in Table 1.

It can be seen that when considering the minimum level established (Figure 1) in the 111 plots (37 cultivars), 40% would require immediate application to control the stink bug in the case of grain production. For the three evaluations, it was found that in the first season of sown cultivars there was a variation of 34% to 47%, in the case of grain production. For seed production, the number of plots practically doubled. The second sowing season for soybean cultivars established that for grain production the need for applications was only 2%, but for seed production this rate varied from 17% to 71%, according to the evaluations carried out. It is also clear that problems with bedbugs increase as the culture progresses to more advanced stages, due to climatic conditions favoring the development of the bedbug population. It can also be seen that the percentage of plots with problems with caterpillars in both sowing seasons was zero. With the alert made by the high populations of bedbugs, products designed to control caterpillars were added (thus avoiding an exclusive application for caterpillars) which kept these insect pests at low levels. 

Thus, under the conditions of the experiment, it can be seen that the main problem present in the 2015/2015 harvest with regard to insect pests was restricted to stink bugs that reached the upper limits for the production of grains and soybean seeds. This verification occurred only in the experimental area evaluated. Management for caterpillars allowed the insect to remain at levels below critical levels that would justify exclusive control. There is certainly a variability of conditions in the state of Rio Grande do Sul. It was found that due to the arrangement of the cultivars, it was not possible to perceive a preference of insect pests in relation to the cultivars studied.

Pests found in the experiment

Among the pests found in the soybean cultivar competition experiment in Santa Maria, Rio Grande do Sul is the black caterpillar (Spodoptera cosmioides). This insect is a polyphagous species with a defoliating habit, which also feeds on pods (reproductive phase of the crop), damaging the grains.

Another pest present is the caterpillar (Chrysodeixis (=Pseudoplusia) includens ). It can attack the soybean caterpillar simultaneously or later. It consumes the leaves leaving a lacy appearance.

Another insect found in the experiment was the green-bellied stink bug( Dichelops melacanthus e Dichelops furcatus). They preferentially attack pods and grains. Dichelops furcatus is more common in southern Brazil while Dichelops melacanthus  It occurs more in the north of Paraná and central west.

Stink bugs suck the plant's sap, attacking stems, shoots and pods (grains). During the attack, they inject toxins that can cause leaf retention (“mad soybeans”). The grains, after the attack, become stained and/or dull.

In turn, the small bedbug  (Piezodorus guildinii)  It preferentially attacks during flowering, but generally reaches its population peak before other stink bugs.

Comparatively, it is what causes the greatest damage to soybean crops.

The Idiamin or tear (Villous tears) was another insect detected in the experiment in Santa Maria, Rio Grande do Sul. Its attack occurs preferentially during vegetative development, with a defoliating habit.

The black-winged stink bug (meditative Edessa) The preferred time of attack is the beginning of the reproductive phase, when the offensive of this insect is intensified. The pest sucks the sap from branches and pods. It can also favor the appearance of “crazy soy” due to the ingestion of toxins in plants.

The china bug  (Chinavia Orian) usually attacks at the end of the vegetative period and beginning of the reproductive period. They are polyphagous and considered secondary pests.

The green kitty  (Diabrotica speciosa) has recorded population outbreaks, possibly related to ecological imbalances. Adults attack leaves, preferably younger ones, but can attack shoots, flowers and pollen. Larvae can affect roots and reduce nodulation.

Tiago Lovato Colpo, Evandro Ademir Deak, UFSM; Ana Lucia de Paula Ribeiro, IFFarroupilha - São Vicente do Sul; Thomas Newton Martin, UFSM

Article published in issue 204 of Cultivar Grandes Culturas.