Stink bug control in soybeans

Stink bug management must follow the evolution of soybean cultivation, especially considering the increase in pest populations, control difficulties (insect species and insecticide efficiency), the severe damage they cause and the high value of soybeans. Together, these components determine that control must be carried out in low populations, otherwise they will become uncontrollable, draining the producer's profits.

Stink bug management must consider the important changes that have occurred in soybean cultivation in recent times, (i) the growth of the cultivated area advancing into previously unexploited regions; (ii) cultivation at different times serving as green bridges that host pests; (iii) the intense use of fungicides, insecticides and herbicides and their impact on beneficial organisms; (iv) the increase in the reproductive period with the supply of vegetables in modern cultivars and (v) the reduction in the use of insecticides in Bt crops.

In Integrated Grain Systems, the distribution of stink bugs is influenced by the succession of crops, the presence of weeds and the crops surrounding the area, as they spend most of the year on other host plants or in straw and only two to three months in soy. Cultivations surrounding soybean crops and in the off-season influence the migration and prevalence of some stink bug species, contributing to their permanence and survival in or around areas.

The species of stink bugs that occur in soybeans in Brazil vary in response to several factors such as: the growth of the cultivated area, the number and duration of host supply, operational control failures, resistance to insecticides and the lack of innovations in the chemical control. In this scenario, the brown stink bug and green-bellied stink bugs stood out and currently predominate in the vast majority of crops in Brazil.

The brown stink bug (Euschistus heros) in its adult stage has the coloration that its name suggests, measures 11 mm on average, has a pointed pronotum that ends in spines and the scutellum has a clear “v” shaped spot. Females lay in groups on leaves and vegetables, arranged in two or three parallel lines, with approximately 15 to 20 yellow eggs. After incubation, the nymphs emerge, initially yellowish-brown in color, with serrated edges, changing to greenish-brown with spots. Like the nymphs of other bed bugs, they remain grouped until the 30th instar, when they become more active, disperse and begin to feed. The nymph phase lasts an average of 33 days and adults can live for more than 100 days. 

Green-bellied bugs (Dichelops spp.) were once considered secondary pests in soybeans, however, due to changes in the crop management system (rotation with corn and wheat, mainly), they present out-of-control populations in many crops. . Adults measure 9mm to 10mm in length and are yellowish to grayish brown in color, with a green abdomen. The head ends in two pointed projections and the pronotum with toothed anterior margins and lateral expansions in spines. Layings are done on leaves or vegetables, in masses of approximately 14 light green eggs. The nymphs are brown, with a light abdomen and dark spots on the body. The nymph phase lasts an average of 18 days.

After the soybean harvest, brown stink bugs and green-bellied stink bugs take shelter under the straw, where they feed on dry grains that have fallen to the ground. These species colonize and attack off-season corn and winter wheat, and change their eating habits throughout the year, changing from grain suckers to vegetative tissue suckers.

The changes observed in soybean cultivation were reflected in the proportion of soybean bug species. The effect of chemical control, variable on species, bioecological and behavioral strategies and greater polyphagy (legumes and grasses), allowed the greater success of the brown stink bug and green-bellied stink bugs. The small green stink bug (Piezodorus guildinii) and the green stink bug (Nezara viridula) have greatly reduced their importance in Brazil and even in Rio Grande do Sul. In the 2006/07 and 2007/08 harvests, this change was already clear. In the first year, the predominant species was P. guildinii, with 76% of the total bedbugs, followed by N. viridula and E. heros, respectively.

In the following harvest, the species P. guildinii, D. furcatus and E. meditabunda predominated. Although E. heros is not among the three that presented the highest proportion, this species increased from 4% in 2006/07 to 12% of the total number of stink bugs and is currently the species that predominates in most soybean crops in Brazil (Figure 1) . The predominance and high populations of the brown stink bug and green-bellied stink bugs in the soybean production system can compensate for their low natural capacity to cause damage.

The damage caused by bed bugs varies throughout their development. Small nymphs (1^o instars and 2^o instars) are grouped together and do not feed, like large nymphs and adults. The large nymphs (of 3^o, 4^o and 5^o instars) feed to develop and cumulatively cause 2/3 of the damage. The remaining 1/3 is caused by adult food. Therefore, when adult stink bugs are observed in soybeans, part of the profits have already been sucked up by this pest in the nymph stage.

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, the attack of stink bugs on newly formed vegetables (R3-R4) causes them to abort, which can be even more severe in periods of drought.

When soybeans are grown for seeds, a stink bug bite in the embryonic axis makes germination impossible. Stings in the vascular region, through which vessels that connect the embryonic axis to the cotyledons pass, compromise the transport of nutrients from the cotyledons to the developing plant, reducing the emergence, vigor and health of the plants. Therefore, soybeans grown for seed production must be protected and free from stink bug attacks during the reproductive phase.

The damage caused by stink bugs is not noticed during the soybean cycle and its visualization is only possible with the Tetrazolium test, which allows the site of the bite and the dead region in the grain to be seen, alerting the producer about the severe damage that the stink bugs have caused in its soybean crop.

Monitoring stink bug populations in soybeans serves to determine the stages (small, large nymphs and adults), species and number of stink bugs/m² on the farm. This data is used to make control decisions, determining whether to control or not, which insecticide and the dose to be used. 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 done by directly counting the number of bedbugs in an area of ​​1 m², in plants and straw. With more developed plants, a vertical flap with gutter is used, with two outlets (beats) in a row of soybeans (2 m), totaling 1 m² of sampled area. This sampling must be weekly at as many points in the field as possible to be sure of the spatial variation of populations (Table 1).

The basis for deciding bed bug control is the Economic Damage Level (NDE) and Control Level (NC). The control decision is directly related to four factors: 1) control cost (insecticide + application); 2) value of the soybean bag; 3) bed bug damage and 4) insecticide effectiveness. These are the factors that feed the NDE equation and guide the control decision. The producer must calculate the timing of bed bug control for his situation, as the NDE value (value given in bed bugs/m²) estimated by the formula, expresses the maximum limit of stink bugs that the producer can tolerate in his crop. In practice, the producer must apply the insecticide before the bedbugs reach the NDE value. This number is known as NC.

The damage that stink bugs cause to soybeans has already been studied and an estimate of 36 results shows damage of approximately 75 kg/stink bug/m². 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. Table 2 presents data showing that the NDE varies with the cost of control and the value of the soybean bag.

The reduction in insecticide applications on Bts soybeans favored stink bugs, which already occurred in cotton in the USA between 2001 and 2007 and in Brazil in mid-2010. With the reduction in applications of broad-spectrum insecticides to control caterpillars, which they also controlled initial populations of stink bugs, these insects occur earlier in the crop, reproduce and increase their population density. In crops with Bt soybeans there are at least 10% more stink bugs than in non-Bt crops, as observed in research by the Integrated Pest Management Laboratory (LabMIP) at the Federal University of Santa Maria. 

The success of bed bug management depends on monitoring populations and optimizing management opportunities for this pest in the soybean cultivation system. This includes the control of colonizing stink bugs in predecessor crops, in straw before sowing and in sprays associated with fungicides, at the end of the vegetative period and during the reproductive period of soybeans. The lack of monitoring (survey) and correct diagnosis (species and population) leads to late applications with infestations above the control level and a reduction in producers' profits.

Jerson V. C. Guedes, Clérison R. Perini, Leonardo Burtet, Regis F. Stacke, Adriano A. Melo, Jonas A. Arnemann, Federal University of Santa Maria

Article published in issue 205 of Cultivar Grandes Culturas.