Integrated management measures for stink bug control in soybeans

Stink bugs are pests capable of causing enormous damage to soybean crops, especially in the reproductive phase of the crop. Their control requires efficient strategies, such as the use of insecticides that respond well to the presence of these insects in cultivation areas, combined with other integrated management measures.

In recent harvests, changes have been observed in the behavior of pests that occur in soybean crops. Among these insects, stink bugs that attack the aerial part of plants have enormous potential to cause damage to soybean crops, especially in the reproductive phase. Although they are present since the vegetative period of the crop, it is during the reproductive stages that the insect causes the greatest damage, when it attacks forming pods. It is a group of insects that occurs in all soybean-producing regions.

With the intensification and changes in agricultural systems, with several successive and/or concomitant crops, which, although they provide an increase in production and optimization of land use, there is also an increase in the reproduction and dispersion of pests, due to the constant supply of hosts and the movement of insects from one cultivation area to another, increasing pest populations and control difficulties. Thus, with each new harvest, growing populations of bedbugs are seen, due to the high supply of alternative hosts and also inadequate management, with spraying outside the appropriate time and not reaching the target, triggering problems such as resistance to insecticides and re-infestation of bedbugs in short space of time.

With the introduction of genetically modified soybean varieties with resistance to several species of caterpillars (Bt), the problems with bedbug attacks may worsen due to the low use of monitoring, the delay in applying insecticides which favors the population increase of the pest, the use of insecticides with low efficacy and short residual, the application technology inadequate, the migration of bedbugs from other areas and the pest's resistance to insecticides. 

Identification of the main species

To properly manage bedbugs, it is important to know the species, that is, to identify them correctly and to know the biology and behavior of the pest.  

In soybean cultivation, stink bugs from the Pentatomidae family are the most important. Of all, the species that deserves to be highlighted in crop management practices is the brown stink bug (euschistus heros), as it is considered the most abundant in soybean crops in Brazil.

The biological cycle of the brown stink bug involves placing eggs in the form of columns, usually two, or grouped. After the incubation period, nymphs emerge, which go through five instars until they become adults.  

Damage

Damage caused by stink bugs results from sucking sap from branches, stems, pods and grains. When sucking branches and stems, stink bugs inject toxins, causing “leaf retention” or “mad soybean” (where there is no physiological maturation of the leaves, while the pods mature), making harvesting the crop difficult. When the attack occurs on pods and grains, there may be a decrease in the productivity and physiological quality of the seeds, due to direct damage resulting from the insect bite and indirect damage, with the inoculation of the fungus Nematospora corylii.

Monitoring and control

Stink bug management must be done based on the principles of Integrated Pest Management, which consists of decision-making based on population levels and the crop's development stage. Such information is obtained through regular inspections of the farm. 

Regarding bed bug attacks, colonization normally begins at the end of the vegetative phase (Vn) and beginning of the reproductive phase (R1 and R2) with the migration of bed bugs from alternative hosts and neighboring areas, with a significant increase in the number of nymphs to from phase R3.

Greater care must be taken with the crop when attacking by stink bugs, in phase R4 (end of pod development) and phase R5.1 (beginning of grain filling), when there is an increase in populations. The critical phase extends until the R6 phase (green grain or full pod), when stink bugs reach their population peak, and tends to decrease from the R7 phase (beginning of maturity) and the grains are more susceptible to attack.

Bedbug sampling is carried out using the cloth beating method, which consists of a 1m long white cloth, attached to two sticks, which must be positioned in a row of plants, which are shaken vigorously, to knock the insects onto the surface. cloth, allowing counting. This procedure must be repeated at various points in the field, preferably in the cooler hours of the day, when the stink bugs move less, with greater intensity at the edges of the field, where, in general, the insects begin their attack, and repeated at weekly intervals. , from the beginning of pod formation (R3) to physiological maturation (R7).

For crops intended for grain production, the reference value indicated for decision-making for spraying is two stink bugs (larger than 0,5cm) per wipe and for crops intended for seed production, the reference value for the Chemical control is one stink bug (larger than 0,5cm) per cloth hit, considering a row of plants. Care must be taken with the different planting dates on properties and in the region, as later crops suffer greater attack from the pest, due to the population increase provided by the larger cultivation areas sown previously, increasing the potential for damage in these areas. Therefore, damage level values ​​should be used as a reference. However, according to specific information from each region and considering variables specific to each property such as damage history, cultivars, crop age, sowing time, among others, technicians from each region can anticipate the start of spraying, remembering that if You should always use registered insecticides, follow the manufacturer's recommendations and the principles that guide integrated pest management, such as the use of selective products targeting the pest's main natural enemies.

In certain situations, chemical control can be carried out only on the edges of the crop, without the need to apply insecticide to the entire area, as the attack begins in the marginal areas. Another measure that can be recommended is the application of insecticides in straw management in direct seeding systems to reduce initial populations. These measures are valid, as long as prior sampling is carried out to detect insects in the area. Another aspect to consider is the reduction of end-of-harvest populations that tend to migrate to other areas, with end-of-harvest application, which must be carefully based on sampling and taking into account restrictions regarding the withdrawal period (grace period) of the agrochemical used.

So that the problem of insecticide-resistant bedbug populations is not intensified, it is recommended that insecticides with the same mechanism of action are not used, in the same area, repeatedly.

Research

Experiments carried out by Geraldo Papa, from Unesp in Ilha Solteira, São Paulo, demonstrated the efficiency of some of the main insecticides used to control the brown stink bug (euschistus heros) in soybeans, verifying the action and form of contamination of the organophosphate group (acephate), the mixture of insecticides from the pyrethoid group (lambdacyhalothrin) and neonicotinoids (thiamethoxam) and each component of the mixture separately.

Exclusive

- Contact effect: in the laboratory, the insecticides were applied directly to the bedbugs, under the same conditions of spray volume and doses used in the field.

- Effect of contamination via tarsus: insecticides were sprayed on soybean plants in the field. The treated leaves were collected and placed on the inner walls of PVC pipes, causing the bugs to walk on the treated soybean leaves.

- Effect of ingestion: Insecticides were sprayed on soybean plants in the field. The pods were collected and placed on the mesh lid of cages containing the bedbugs. To prevent bedbugs from coming into contact with the pods, they were placed on wooden sticks.

Results 

Geraldo Papa, Fernando Juari Celoto, João Antonio Zanardi Júnior, Unesp/Ilha Solteira

Article published in issue 194 of Cultivar Grandes Culturas.