Technical Notebook September: Profit suckers

In recent years, changes and innovations have occurred in soybean cultivation in Brazil, which have been reflected in the occurrence of stink bugs, increasing their prevalence, resulting in control difficulties and consolidating their relevance as the most important group of crop pests. Among these aspects, the following can be highlighted:

- The growth of the area cultivated with soy associated with cultivation in a greater range of seasons, allowing the crop to be grown for around eight to nine months of the year.

- Intensive cultivation systems with “host green bridges”, which allow for larger and more diverse hosting alternatives for bedbugs.

- More intense use of herbicides (eliminate weeds that host natural enemies) and fungicides (which impact beneficial fungi).

- Planting indeterminate or determinate cultivars with a longer reproductive period and longer feeding structures, making them more vulnerable to soybean bugs.

- Cultivation of soybeans bTS that control caterpillars by reducing the number of insecticide applications.

- High value of soybeans, low cost of control and impact of low populations.

Added to the factors that facilitated the growth of bed bugs are the lack of biological alternatives available and the reduced number of chemical groups and active ingredients on the market for controlling suckers.

On the other hand, for a long time bed bug control was carried out late (research recommendation errors), without attention to occurrences, their random distribution and low initial populations, resident populations and migrations from adjacent areas, as well as the real impact of very low populations per unit area.

Species that have gained importance

Due to their biological, ecological and behavioral characteristics, the species that most adapted and benefited from changes in soybean cultivation in Brazil were the brown stink bug and the green-bellied stink bug.

The impact of these species is related to their population density, the period of coexistence with the vulnerable stages of soybean, but also with the cultivar. The predominance and high populations of these species in the soybean production system require knowledge regarding correct identification, biology, damage, behavior, levels of economic damage and control and population sampling, in order to adapt control appropriately. assertive for each situation.

The brown stink bug measures 11mm in length on average and has a pointed pronotum that ends in dark spines. In adulthood, it is dark brown in color and has a light “v”-shaped spot on the scutellum. Females lay eggs in groups, mainly on vegetables, leaves and stems, and are arranged in two or three parallel rows with approximately 15 to 20 yellow eggs. The newly hatched nymphs are orange in color and the larger nymphs (third-fifth instars) vary from gray to brown, with serrated edges and several “U”-shaped light-colored spots along the entire edge of the abdomen. The egg-adult period of the brown stink bug lasts approximately 28 days, comprising 13 days between the egg stages and the second instar (period with a gregarious habit and does not feed) and 15 days from the third instar to adult (the feeding and dispersal). Adults can live more than 100 days.

The green-bellied stink bug measures 9mm to 10mm in length and has a color ranging from yellowish-brown to greyish, with a green abdomen. The head ends in two pointed projections and the pronotum with toothed anterior margins and lateral expansions in spines. Females lay on leaves, vegetables and soybean stems in masses of approximately 14 light green eggs. The small nymphs of the green-bellied stink bug can be confused with those of the brown stink bug, with their grayish-brown coloring, but they can be identified by their green abdomen and the pointed projections on their head. Very similar to the brown stink bug, the egg-to-adult period of the green-bellied stink bug lasts between 25 days and 30 days. The large nymphs, which cause damage, (third to fifth instars) take around 15 days to reach the adult stage.

Occurrence in soybeans and other crops

During the soybean cultivation period, stink bugs may begin to occur even before sowing, by individuals remaining from the previous crop (covers or winter cereals, pastures or soybeans), and may feed on weeds or cultivated plants, on fallen grains. in the soil or in cotyledons of newly emerged soybean plants, and can remain and establish themselves in the crop during the vegetative period, when they can feed by sucking any plant structures. From the formation of legumes, until the end of grain filling, stink bugs feed preferentially on reproductive structures and reproduce and increase their population in legume cultivation areas.

The occurrence of stink bugs in soybean production systems is mainly associated with the immense supply of food/crops for the survival of these populations. The cultivation system with soybeans in the first crop (crop) and soybeans in the second crop (safrinha), which, in addition to the huge Brazilian area of ​​35 million hectares, covers the months of September to May and extends the production window, allowing occurrence of higher population density, especially in later soybeans. In soybeans bTS There are fewer insecticide applications to control caterpillars, which also control early stink bug populations. Thus, there is an earlier occurrence of bed bugs that, without control, reproduce and increase population density, earlier. According to a survey by the Integrated Pest Management Laboratory (LabMIP) at the Federal University of Santa Maria (UFSM), there are at least 10% more stink bugs in soybeans bTS which in crops does not Bt. The survival of these bugs depends on soybean cultivation, and after harvesting they take shelter under the straw, where they feed on grains fallen on the ground or on the edges of crops, where they enter into diapause or inhabit other crops, the so-called green bridges, available during the soybean off-season. In other crops, such as corn in the summer (mainly off-season cultivation) and pastures, cover crops and winter cereals, they change their eating habits and start to suck vegetative tissue from seedlings and plants, instead of seeds. This year-round crop rotation system (rotation with cereals and wheat, mainly) favored the occurrence of green-bellied stink bugs and currently has out-of-control populations in some regions of Brazil. 

Spatial distribution of the population

Knowing the spatial distribution of stink bugs is an additional resource to assist in monitoring and improving the control of these insect pests in soybean crops. Monitoring stink bug populations allows visualization of their occurrence, distribution and infestation in crops. Normally, the colonization of areas occurs at the end of the vegetative period and the beginning of the soybean reproductive period. From this period onwards, the density of bedbugs increases and is capable of producing significant losses.

In work carried out by the Integrated Pest Management Laboratory Team (LabMIP) at the Federal University of Santa Maria (UFSM), in the municipality of Jóia, in an area irrigated by central pivot, with 92ha, 41ha with soybeans and 51ha with corn, The spatial distribution of stink bugs in and around crops was evaluated for two years (Figure 1). The species Euschistus heros, Dichelops sp. were predominant, but there were also Piezodorus guildinii, Nezara viridula e Meditating Edessa. In the initial phase of cultivation, stink bug populations were detected in the bush, the edge of the dam and the marsh, and then migrated to the edges of the crops (non-irrigated) irrigated under a central pivot and colonized all soybean areas. From the grain filling, the stink bug population increased in a large part of the soybean area (red), especially in the earliest soybeans, in relation to the soybeans in the area under pivot irrigation. These results confirm that earlier soybeans promote stink bug populations that will later infest adjacent later crops. Applications to crop borders can help reduce infestation in soybean crops. After the soybean harvest, in assessments carried out on the straw, the largest stink bug population was found in places outside the crop. Thus demonstrating that bedbugs seek places of refuge in the off-season.

This experience in high technology and seed production areas has shown that crop monitoring can be carried out even before soybean sowing, in adjacent and border areas, from where stink bugs will migrate to the cultivation areas. Furthermore, it revealed that the detection of small populations still in the vegetative phase of soybeans may be a time to start control early, when soybeans still allow for more effective applications, due to the low leaf density.

The monitoring

Monitoring soybean bugs serves to determine the phases (small, large nymphs and adults), the infesting species and the number of stink bugs/m2 in crops. These data must be used to make control decisions, determining whether to control or not, which insecticides are most suitable and the dose that should be applied. The way to sample stink bugs depends on the stage of development of the soybeans. In the initial stages and in the off-season, it is carried out by directly counting the number of stink bugs in an area of ​​1m2, on plants and in straw. With more developed plants, the vertical flapping cloth with gutter is used, which is practical and efficient according to studies conducted by LabMIP-UFSM, since 2006, with two sockets (beats) in a row of soybeans (2m), totaling 1m2 of sampled area. This sampling must be weekly at as many points in the field as possible to ensure the spatial variation of populations. On average, a minimum number of four sampling points can be used in areas/plots of less than 20ha, ten points in areas of up to 100ha and up to 15 points in areas larger than 100ha.

In general, only a few producers carry out sampling on their crops, which determines preventive and/or random insecticide applications, often together with other applications. These practices have negative consequences, such as late entry for bed bugs and allowing their population to grow. In work developed by LabMIP-UFSM, between 2009 and 2010, in which approximately 400 soybean producers from Rio Grande do Sul and Paraná were interviewed, around 60% from Rio Grande do Sul responded that they use the beating cloth; in Paraná, these values ​​are lower and around 41% of producers use cloth (Guedes - personal information).

Crops in rotation and succession with soy influence the stink bug population. Therefore, monitoring population density must also be carried out during the off-season. Other crops and weeds around the crop can influence the population dynamics of stink bugs. During this period, stink bugs can be sampled in the straw with visual observations, on site and/or using sampling methods that allow the insect pest population to be quantified. In work carried out by LabMIP, in which the population density of stink bugs was evaluated in the off-season, the presence of adult stink bugs was found to be protected in the straw and later in the cover crop, throughout the off-season. With monitoring, it is possible to assess the population density of bedbugs in the area and carry out control if necessary, even before damage to the crop occurs.

Injuries and damage from bed bugs vary throughout their development. Small nymphs (1st and 2nd instars) are grouped together and do not feed, like large nymphs and adults. Large nymphs (3rd, 4th, and 5th instars) feed to develop and cumulatively cause most of the damage, and only the remaining 1/3 of the damage is caused by adult feeding. Therefore, when adult stink bugs are observed in soybeans, part of the profits have already been sucked up by the developing nymphs. The cotyledons of newly emerged soybean plants are attacked by stink bugs remaining from previous crops (such as corn, wheat or even soybeans), resulting in lower initial height of the plants, as the supply of energy for their initial growth ceases. From the formation of vegetables until the end of grain filling, direct damage occurs when stink bugs reproduce and increase in number in crops, sucking vegetables and grains that become smaller, shriveled, wrinkled and weigh less, reducing yield and the quality of the grains. Furthermore, stink bug attacks on newly formed vegetables (R3-R4) can cause abortion, which can be even more severe during periods of drought. The greatest sensitivity of soybeans to stink bug attacks occurs between the beginning of fruiting (R3) and the point of maximum accumulation of dry matter in the grain (R7). Within this large period, the most critical interval and vulnerable to losses is from R4 (end of pod development) to R5.5 (end of grain filling), according to results observed in several of the studies analyzed.

Economic damages are difficult to calculate, just as it is difficult to establish a fixed damage value for each bed bug species, due to the different factors involved in estimating these damages. However, the intensity of damage caused by soybean bugs depends on (1) species of stink bug; (2) developmental stage of the bed bug; (3) phenological stage of the soybean at which the attack occurs; (4) period of coexistence or duration of the attack and (5) population level of the pest.

Level of economic and control damage

The bases for deciding bed bug control are the Economic Damage Level (NDE) and Control Level (NC). The control decision is directly related to four factors:

1) CT - control cost (insecticide + application);

2) V - value of the soybean bag;

3) D - damage from bedbugs and

4) K - insecticide efficiency.

These are the factors that feed the NDE equation and that should guide the economic control decision. Based on these data, the producer must estimate the time to control bed bugs for his situation, as the NDE value (number of bed bugs/m2) estimated by the formula (NDE = [CT/V*D] * K), expressed the maximum limit of stink bugs that the producer can tolerate in his crop. In practice, the producer must apply the insecticide before the bed bugs reach the NDE value. This number is known as the Control Level (NC).

The damage that stink bugs cause to soybeans has already been studied and an estimate of more than 50 studies shows a loss of approximately 75kg of soybeans for a density of one stink bug/m², that is, ten thousand stink bugs per hectare. Using this value in the NDE formula, with a control cost of R$50,00/ha and the value of a bag of soybeans of R$70,00, the NDE is 0,71 stink bugs/m². Obviously, this calculation is an estimate and its values ​​may change, with the price of soybeans or with greater investments in pest control.

Management moments

Knowing and understanding the reproductive aspects of stink bugs is the first step towards managing them at the right time, with insecticides appropriate for the age and population present in the crop. It allows you to be assertive and effective, reducing initial populations and losses at the end, due to a reduction in grain production and its quality.

Soybean bugs, such as the brown stink bug and the green-bellied stink bug, follow a hemimetabolous development cycle, going through the egg, nymph and adult stages, which last approximately 30 days to 40 days. Colonizing females that have already mated begin the formation of populations and can be the first target of control to reduce the infestation, which is inevitable in most regions of Brazil. At this first moment, the important thing is the use of insecticides with a shock effect and direct contamination, since many females live for a short time and eventually feed, as their functions are to produce the first generation of bedbugs that will colonize and create the first scattered population centers. , generally from the edges of crops.

The particular development of bed bugs has the egg phase and N1, N2 nymphs that are not very mobile, and with little exposure to chemical control, generally allowing them to escape and advance their development. Third instar (N3) nymphs are more mobile and begin feeding, eventually becoming more exposed to insecticides, like N4 and N5 nymphs. However, these subphases together represent a period of approximately 15 days, which allows a greater chance of control, but which depends on coverage of insecticide treatment with dose and effectiveness for the different ages present in the areas. These populations are the result of colonizing females that form the first generations of stink bugs and that generally coincide with the beginning of the soybean reproductive phase. Therefore, when there is a tendency towards uniformity in the population density of bed bugs, it is desirable to use insecticides that act on large nymphs and adults, since these larger populations tend to be more difficult to control. Invariably, the overlapping of ages and the control difficulties inherent in soybean development easily allow control overruns and densities to grow to levels that are harmful to soybeans. This is the key moment for control and in which the best resources must be invested, because if the moment is missed, the crops will have stink bugs until harvest, with their effects and losses.

However, what is very clear from the economic damage is that soybeans are very sensitive to small densities of stink bugs, which in subtropical or tropical conditions in Brazil have the capacity to grow quickly and impact the crop. On the other hand, regardless of the two or three control interventions applied to each crop, given the delay in entry or the lack of knowledge regarding the biology and behavior of the pest, soybeans are invariably harvested with stink bugs still present, adding the expense of control to the loss of production, increasing the importance of this pest.

Clérison R. Perini, Lucas Cavallin, Gabriel A. Guedes, Ricardo Froehlich, Lorenzo Aita, Letícia Puntel and Jerson C. Guedes, UFSM