Actions to control the brown stink bug in soybeans

Of the insect pests constantly found in soybeans, phytophagous stink bugs from the Pentatomidae family are considered the main causes of economic damage to the crop. This group of insects begins to multiply at the end of the soybean vegetative period, being most harmful in the initial phase of pod formation, although they can cause damage until the final phase of grain maturation. Among the species of stink bugs that attack soybean crops, the brown stink bug stands out, Euschistus heros (Hemiptera: Pentatomidae), as it is the most abundant species in the Brazilian Cerrado region. The main damage caused by the attack of the adult, as well as the large nymphs of E. heros, occurs through the insertion of its oral stylet into the reproductive structures of the soybean, causing significant losses in the production and quality of the grains or seeds produced, which can reduce in up to 30% crop productivity.

Control of stink bugs in soybeans is often carried out through the application of chemical insecticides in the reproductive phase of soybeans, with this control being recommended from the R3 stage onwards, when two insects are found per linear meter in the rows of soybeans intended for grain production. and one stink bug per meter of plant row in seed crops.

Research was conducted at Embrapa Agropecuária Oeste which aimed to evaluate the damage potential of six population levels of adults of E. heroes in soybean cultivation, in four phenological stages of the crop, this effect being evaluated considering the yield and quality of the grains or seeds produced.

Damage at different stages of soybeans

The experiments were carried out under field conditions in the experimental area of ​​Embrapa Agropecuária Oeste, municipality of Dourados, Mato Grosso do Sul (22º16'30"S, 54º49'00"W, 408m), during the 2014/2015 harvest. For this purpose, the Brasmax Potência RR soybean cultivar was used, which presents indeterminate growth, with sowing and cultivation carried out in accordance with the recommendations for soybean cultivation in the region.

When the soybean plants reached the V8 stage, consisting of eight fully developed trefoils, the experimental units (plots) were demarcated in the experimental area. Each plot was composed of two rows of soybeans each 1m long, covering 20 plants, where cages made with PVC structures (1m long by 0,90m wide and 1,2m high) were installed, covered with fabric. of the “tulle” type (Figure 1).

Inside each cage, infestations were carried out with different population densities of the adult bed bug. E. heroes (0, 2, 4, 6, 8 and 10 stink bugs/cage) in four different phenological stages of the culture (V8, R2, R4 and R5). Each treatment (bed bug population densities) had four replications conducted in a randomized block design.

The cages were inspected daily to monitor bedbugs, and when the insects eventually died, they were replaced. To facilitate monitoring and visualization of bedbugs, white sand was placed on the surface of the ground covered by the cages. The infestations were maintained for a period of 14 days and, at the end, the cages were removed and the soybean plants were sprayed with chemical insecticide to eliminate the insects present inside the cages. This operation was also repeated weekly until the complete maturation of the crop grains.

Soybeans were harvested from the area covered by the cages, the grains were threshed, weighed and the grain yield/ha was determined in each treatment. To diagnose the quality of the grains produced, the tetrazolium test was carried out on soybean seeds harvested only in treatments in which there was infestation at the R4 and R5 stages of the crop, following the protocol by França Neto et al (1998).

Grain yield data were subjected to regression analysis at a 5% significance level. The seed quality parameters obtained by the tetrazolium test were subjected to analysis of variance and, when significant treatment effects were found, the treatment means were compared using the Tukey test at 5% probability.

Results obtained

No significance was found in the regression analysis between the population densities of stink bugs infested in the cages and the grain yield/ha for the experiments installed in the V8 vegetative and R2 reproductive stages of soybeans (Figure 2). However, the increase in population density of E. heroes in soybean plants present in the cages negatively affected grain yield when the tests were conducted at the R4 and R5 stages of the crop (Figure 2). It was also found that the percentage of germination and vigor of soybean seeds was not affected by the different population densities of the stink bug when the infestation was carried out at both the R4 and R5 stages (Table 1).

However, the percentage of seeds with damage (bites) caused by the stink bug (1 to 8) was significantly influenced in all stink bug densities tested when compared to the control treatment (without infestation) for the trials conducted at phenological stages R4 and R5, without differing between the evaluated bed bug densities (Table 1). Likewise, the rates of seeds that had bites and those that were rendered unviable by the stink bug (6 to 8) were also significantly high for all stink bug population densities studied in the trials conducted in R4 and R5, when compared to the treatment without infestation (Table 1 ).

The non-significance between stink bug population density and soybean grain yield/ha is something well documented for representatives of stink bugs from the Pentatomidae family, when tests are installed in the vegetative stage of soybeans or even in the R1 and R2 stages. However, this fact can be aggravated when population densities of E. heroes are associated with the critical phenological stage of the crop, that is, at that stage in which soybean plants are most susceptible to damage caused by stink bugs, which is the beginning of the grain filling phases of the crop (R5.1). Scope et al (2016) also found that a brown stink bug density of 12 stink bugs/m2, in the vegetative phase of soybeans (V6), did not affect the grain yield of the crop.

Likewise, Corrêa-Ferreira (2005) found that populations of E. heroes quality Piezodorus guildinii (Westwood, 1837) with up to eight stink bugs/soybean plant also showed no differences in grain yield when infestations occurred between the end of the vegetative period (V9) or at the full flowering stage of soybeans (R2), as was also observed in the present research. However, a decrease in soybean grain yield is possible when stink bug infestation E. heroes occurs from stage R4 onwards. Scope et al (2016) found that infestations with just 0,5 stink bug/m2 at the beginning of the grain development stage (R5.1) are sufficient to cause a significant reduction in grain yield, similar to what was observed in this work for infestations carried out at the stages R4 and R5, although with higher bedbug densities.

The percentage of seeds that showed damage by the adults of E. heroes was significantly influenced in all the different bed bug densities used in the cages when compared to the control treatment (without infestation). The rates of seeds bitten or considered unviable by bedbugs (6 to 8) were also significantly high for all bedbug population densities tested in the trials conducted in R4 and R5, when compared to the treatment without infestation (control).

Based on the results obtained in this research, it can be inferred that bed bug control measures E. heroes in soybeans they are unnecessary in the vegetative phase and even in its full flowering (R2), even though this pest is occurring in high population densities in these stages of crop development. However, reproductive stages R4 and R5 showed that they are susceptible to attack by E. heroes and, therefore, requires attention and control actions on the part of producers, to prevent a reduction in productivity and quality of soybeans produced.

Crébio José Ávila, Embrapa Agricultural West Paulo Henrique R. Fernandes and Ivana Fernandes da Silva, Federal University of Grande Dourados