Stink bug management in soybeans

The brown stink bug, Euschistus heros, is the most abundant species in soybeans. During the harvest it has three generations, but it can complete the fourth after harvest when it feeds on host plants and enters dormancy for seven months. Several aspects must be carefully observed, but monitoring before and after the application of phytosanitary products is an essential factor in knowing which control measures need to be adopted.

Phytophagous stink bugs, present mainly in production systems that involve soybeans as the main crop, have caused great damage to Brazilian producers. And the conditions of the Brazilian Center-West region present characteristics that favor the development of these pests.

In soybean cultivation, among the stink bug species, the brown stink bug, Euschistus heros (Heteroptera: Pentatomidae) is widely distributed in crops in the Cerrado region. Other species of stink bugs are associated with agroecosystems, such as the green-bellied stink bug (Dichelops melacanthus), the Edessa bed bug (Meditating Edessa) and the small green stink bug (Piezodorus guildinii), presenting high infestations in some cases.

In the Chapadões region, soy is the main crop, with the largest area. It is normally the first crop to establish itself in the field, receiving the beginning of colonization by generations of pests. The different hosts, in addition to weeds and tigueras from the previous crop, are part of the region's crop rotation/succession system.

Among bedbugs, the brown, E.heros, It is the species with the greatest abundance in samples taken from soybean crops. It is worth highlighting that it has great adaptation to the climate.

During the soybean harvest E. heroes normally has three generations, and can feed on other crops, such as forage radish, pastures, crotalaria, sunflower, among others, and also on weed species such as trapoeraba, wild peanuts, turnip greens, horseweed, bittersweet, among others. After harvesting soybeans, feeding on these host plants, stink bugs can complete their fourth generation and enter dormancy (diapause) in the straw of the previous crop, where they protect themselves from the action of parasitoids and predators. During this period, which lasts approximately seven months, it may also not eat, managing to survive on the lipid reserves that were stored before diapause.

The infestation usually begins at points of greatest concentration in the plots, which can lead to errors in the survey. Some areas with nearby refuges can lead to entry (migration) into the plots and consequently the beginning of the infestation.

Stink bugs, in general, have a preference for pods in soybean crops and at the beginning of their formation they are concentrated in these parts, as well as nymphs and adults. In some cases, only 10% of the total population present in the plant can be found in plant leaves.

When sucking the pods and seeds, stink bugs can inject toxins that cause “leaf retention”, that is, the leaves do not fall normally and make mechanical harvesting difficult.

Another point to be analyzed in the biology of this pest is that bed bugs can, at certain times of the day, depending on the region's climate, be more exposed. In the case of the Chapadões region, this period runs from 9 am to 11 am. However, some studies from other regions, and other species of bedbugs, did not show the same behavior.

Another species that has stood out in recent harvests is the green-bellied stink bug. Dichelops (Dichelops melacanthus), due to the increase in corn tigueras plants in soybean crops, interaction with weeds, and consequently greater adaptation to the environment. Adults have lateral spines and have a uniform brown body color and a green ventral region. They are around 9mm long. The eggs are laid in plates and are green in color. This species is one of the most important pests in corn cultivation, and its management involves primarily the predecessor crop, in this case soybeans, mainly in the cultivation of corn in the second harvest (safrinha). Leftovers from previous harvests have led to huge losses in some regions of Brazil.

Other species of bed bugs in the family pentatomidae are found attacking crops. However, in lesser abundance, as is the case of the small stink bug Piezodorus guildinii, Meditating Edessa, green stink bug (Nezara viridula), Thyanta perditor and Acrosternum sp.. These stink bugs alone may not cause losses in grain productivity and quality, but it is necessary to add their damage to that of the main species. Therefore, they are responsible for a reduction in seed yield and quality as a result of stings. The attacked grains become smaller, wrinkled, shriveled and become darker.

Losses due to suction of pods and grains tend to be greater than 30%, which can also cause stains on already formed grains. High populations lead to pod drop in some cases.

In recent harvests it has become common to find some areas with the problem of green stem, leaf retention and/or also called “crazy soybean 1”. In the attacked crops, there are patches of green plants, which do not reach senescence even with the crop desiccation process. In these areas there are usually a large number of individuals, or sometimes the producer manages to control the population in the end, and the damage has already occurred. The last areas within the property or even in the region tend to suffer from this process. The producer may have controlled the stink bug well up to a certain stage, but migration from other areas changes the population dynamics within a week, leading to high rates of the pest and consequently large losses.

Integrated Pest Management involves several practices, with sampling being one of the foundations of the program. During the survey, the technician must correctly identify the species and also note the number of nymphs, which from the 2nd instar onwards can already cause damage. In this case, only through the beating cloth can the technician or producer carry out the correct identification and verify the “real” level of infestation. This practice, despite being old, is the most assertive in management. The recommendation is that as much as possible be carried out, but with practicality due to the size of the areas, etc. Training the sampler team should be a practice adopted year after year for improvement, recycling of knowledge and understanding of the pest's population dynamics.

After knowing the infestation rate, the technician can take action. At this point, the work carried out at Fundação Chapadão, with the help of the scientific technical council, has shown that even with 1 stink bug per meter of row, the producer/technician has managed to manage the pest, without causing significant damage to grain production. The management of areas for seed production requires more attention, with the level for control being half or lower than the index for grain. Some studies are being developed to verify this level, since seed quality is a key factor for commercialization. The best seeds sold do not reach 1% bed bug attack rates. Tetrazolium tests have helped in recognizing attacked lots.

Several control strategies must be analyzed and one of them is to keep the population low in the refuges, which is essential to avoid major damage to subsequent crops. It is necessary to understand the production system when sowing crops that may be attacked, such as cotton, corn, sunflower, beans, among others. Only management in these crops can reduce the initial quantity of stink bugs for the next harvest.

Among the control strategies, chemical management should also be considered, if it presents a registered insecticide or even a biological control program.

With the advent of the use of genetically modified plants resistant to glyphosate, it is common to find corn plants, tigueras, inside soybean crops. Ears that normally fall during harvest generate more than one flow, requiring two applications of graminicides in order to remove food for the pest. These tigueras have led to an increase in problems with the green-bellied stink bug, in addition to its interaction with various weeds.

It is worth remembering that many of the individuals left for the next harvest are naturally parasitized. In the Chapadões region (Chapadão do Sul, Costa Rica – MS and Chapadão do Céu-GO) some parasitoids such as Hexacladia smitt e Trichopoda nitens has occurred leading to the death of part of the bed bug population.

A necessary issue in the analysis of infestations in September and October sowings, with stink bugs, is that if low parasitism occurs, the technician must work on the population dynamics of the pest, making it necessary to take population management measures for the pest, trying to adjust to the recommended index. of control.

Another point to report is control efficiencies, which in most insecticides have been shown to vary from 60% to 80%. These control percentages are associated with factors such as application technology, failures in monitoring and entry with high rates of the pest. The technician must take into account that he will be working with remnants of the pest in the system. The times for monitoring and applications can occur at the beginning or end of the day, times with a greater frequency of bedbugs in the crop.

Therefore, if a survey shows 5 bedbugs per meter, after applying the insecticide with 80% control, 1 bedbug will remain, which is still a control index, requiring reapplication at an interval of up to 7 days. Applications, often called “carpooling”, have not achieved good results, mainly with the addition of other products to the mixture, altering the functioning characteristics of certain insecticides and when applied at long intervals (20 days commonly used for fungicides).

Analyzing this aspect, the producer and technician must understand that even after applications it is necessary to continue monitoring the population, in order to observe its efficiency, in addition to a possible resurgence and consequently the need for a new application.

Among the insecticides available on the market, it is important to highlight that in management, although there are no great possibilities for rotation, at least two chemical groups are interesting in controlling the pest. Among them, organophosphates, represented mainly by acephate, and “ready mixes” of pyrethroids + neonicotinoids stand out.

Control alternatives can be integrated. In adults, good efficiency was achieved with organophosphates and some mixtures, but in bedbug nymphs, mixtures of pyrethroids + neonicotinoids have shown superior results than organophosphates, due to the greater residual effect of their application. It is worth noting that in the case of ready-made mixtures, the use of pyrethroids separately or even neonicotinoids alone has not resulted in good bed bug control. Other commercially available insecticides can also be integrated into the management, such as the mixture of Bifenthrin + Carbosulfan, in addition to new molecules such as Dinotefuran. Other “mixtures” are in the registration phase.

If all aspects of managing the population dynamics of this pest are not observed, the tools may not perform well.

Germison Tomquelski, Josiane Oliveira, Patricia Mariano, Chapadão Foundation

Article published in issue 204 of Cultivar Grandes Culturas.