Stink bug control in soybeans and corn

While farmers remain concerned about caterpillars and other defoliating insects that cause impactful and easy-to-see symptoms, stink bugs have seriously attacked soybean and corn crops. Often little noticed by producers, these insects are among the most difficult pests to control in the production system and which gain greater importance each year.

Currently, the management of the stink bug complex that attacks the pods, causes damage to soybean grains in the summer harvest and then migrates to the off-season corn, sucking the newly emerged seedlings is the main challenge of integrated pest management (IPM) in these cultures. The limited diversity of registered insecticides with different modes of action, the proven resistance of pest populations to some insecticides, the deficiency in application technology and the mistaken use of insecticides, due to the abandonment of sampling and action levels, are among the main causes of the occurrence of high pest infestations in soybean and corn crops in recent harvests. Managing bedbugs correctly does not mean applying insecticides more frequently or earlier. On the contrary, the abusive use of chemical control only worsens the pest problem. To counter this situation, technicians and producers must be aware that the basis for making decisions about whether or not to use chemicals is the careful monitoring of the insect's population density, seeking to abolish pre-scheduled applications - the famous calendars - and “piggyback” applications, which are quite common in the desiccation of weedy plants and/or in disease control, when an insecticide is added to the herbicide or even in a mixture with the fungicide, without adopting any technical criteria. 

There is a great diversity of stink bug species that attack soybeans during the summer harvest. Many of these insects migrate to the corn crop when soybeans are being harvested. They suck on corn seedlings, which can lead to their death depending on the intensity and stage of development of the crop in which the attack occurs. Among the most common and important species of bedbugs in this production system are the brown stink bug (Euschistus heros) and the green-bellied stink bug (Dichelops melacanthus ou D. furcatus). 

The brown stink bug, a key pest of soybeans, damages this crop by sucking out the developing pods. The period of soybean development between the “knife” phase (R3) and the end of grain filling (R6) is the most critical period for damage caused by this sucking insect. However, due to the large populations that normally occur at the end of summer, these insects have also attacked corn crops at the beginning of their development. The greatest risk of damage occurs in corn sown earlier (until mid-February), when the brown stink bug is still very active. 

In corn, the brown stink bug is usually less harmful when compared to the green-bellied stink bug. It is common to observe several brown stink bug adults aggregated on the leaves of more developed corn plants. These bugs, however, are not causing damage to plants. In addition to the presence of stink bugs in corn, the behavior of these insects must also be observed. This is important because in some situations, the brown stink bug can attack the corn stalk, similar to the habit of the green-bellied stink bug, and thus cause similar damage, including killing the plant and reducing the initial stand.

Unlike the brown stink bug, the green belly is now known as a key pest of corn. This insect's population is already growing in the final phase of the development of soybean crops (preceding corn). However, as its occurrence in soybeans is almost always at the end of the crop's development (R7 or R8), it usually does not cause significant injuries to soybeans and, therefore, its control alone is not necessary in this crop. However, throughout the reproductive period of soybeans, this stink bug is monitored and must be considered together with other species of sucking bugs to make control decisions. 

Currently, high populations of stink bugs are arriving with greater intensity and earlier in soybean and corn crops. This has led farmers to increase the use of insecticides, often without obtaining satisfactory results. This abusive use of insecticides has only worsened the problem with these pests, in addition to representing an unnecessary expense for the producer. Therefore, knowing the pest and respecting the technical recommendations for its management are crucial for successful management.

The stink bug in soybeans

Although stink bugs occur since the vegetative period of soybeans, and can even suck the plant in search of water during this phase, they do not cause any reduction in productivity, as the grains are not yet formed. The great potential for stink bug damage in soybeans occurs when this pest feeds directly on the grains. Therefore, MIP-Soja considers that its control must be carried out when it reaches the action level. In the case of crops for grain production, this level is two stink bugs larger than 0,3cm, whether nymphs and/or adults of all phytophagous species, per meter of plant row (beating cloth). When the crop is for seed production, the action level becomes that of a stink bug (nymphs and/or adults of all phytophagous species). The most critical period for bed bug damage and ideal for controlling these insect pests is between the R development stages₃ and R₆. Therefore, stink bug control applications in soybeans prior to R3 are not necessary. In the period after R6, in addition to the stink bug population density, other parameters such as the harvest of surrounding crops, time for harvest, size of the area, rainy season, etc. also need to be observed, especially if the crop is for seed production, as situations of high population levels of bedbugs, which are often common during this period, may negatively affect quality.

Unfortunately, the incessant search for increased productivity, associated with the good prices paid for soybeans and the low cost of many insecticides, has led producers to question the validity of the action levels recommended by research for bed bug control. This doubt, associated with the reduction in the number of official technical assistance technicians, contributed to many producers abandoning IPM, starting to apply insecticides to soybeans to control stink bugs along with fungicides in the reproductive phase, without any sampling of the pest population. and appropriate comparison with the level of action. This has even been done in an attempt to take advantage of the agricultural operation that is being carried out. With this erroneous application of insecticides carried out alongside the fungicides and no longer at the appropriate time (which is the level of action), the control of stink bugs in soybeans has generated disastrous results. In this context, it is important to highlight that despite questions about the need for early control of stink bugs in soybeans, recent research results, with early cycle soybean cultivars and indeterminate growth, show that the recommended action levels remain reliable, indicating the best time for the soybean farmer to start applying insecticides.

The stink bug in corn

The green-bellied stink bug, considered a secondary pest in soybean crops, has been growing in importance in corn crops in recent years. Two species have been associated with corn, Dichelops furcatus, more common in southern Brazil, and D. melacanthus, predominant in other producing regions. The latter differs in that the spines on the pronotum are black, while in the other they are brownish. Both species are equally harmful and inject toxins into the seedlings' necks during the feeding process. Being an initial pest, it has significantly harmed corn during the crop's implementation, either by reducing the stand or damaging the vigor of the seedlings, causing the plants to tiller. Greater damage has been observed when a high incidence of the pest coincides with periods of drought. This pest uses straw as a place to shelter, survive and multiply. The continuous succession of soy/corn or soy/wheat favors the development of these insects. During the soybean harvest, the grains fallen to the ground, associated with the presence of weeds, particularly trapoeraba, have favored the increase in populations of this stink bug, as they constitute excellent food in the off-season. Therefore, reducing losses in the soybean harvest and keeping the area free of weedy plants are also important factors in regulating the insect population.

The current situation of high populations requires changes in management, making it necessary to consider control in the system as a whole. It is necessary to improve the control of these bugs in crops that precede the off-season corn, especially during the soybean harvest.

Treating corn seeds with products with high solubility and systemic action (neonicotinoids) can provide good results in controlling the pest. However, the best efficiency of the products is conditioned on the correct use of the recommended dose. There is currently a great diversity in the size of corn seeds on the market, so the dosage should be recommended by number of seeds (60 thousand seeds) and no longer by weight, as previously.

As for bed bug control via spraying, you need to be aware of the most appropriate time to carry it out. Delayed spraying, that is, after the plant is 10-15 days old, can make control ineffective. In this case, even if the stink bug is controlled, nothing prevents the damage from appearing, as the toxin that the insect injected previously is already in the plant, and the damage appears later. Ideally, monitoring should take place in the first few days of corn emergence and control should be carried out whenever the action level is reached. It is not recommended to spray corn after 15-20 days of age and/or when the plant reaches a stalk thickness above 1,5cm. Comparing the two control systems, seed treatment with neonicotinoids has the advantage for being more efficient and for being selective to natural enemies.

To decide on the treatment of corn seeds, one must take into account the history of the insect's occurrence on the property and the monitoring of the straw, shortly before sowing the corn. Additional spraying may only be necessary after monitoring after corn emergence.

Often, due to lack of information, producers decide to reseed corn based on the percentage of damaged plants, without taking into account the different degrees of injury that the insect causes. The information in Figure 1 seeks to support technicians and producers when making decisions. Reseeding would only be justified in situations where plants die and at least 60###/span#< – 70% of plants with grades 3 or 4, associated with unfavorable weather.

Main recommendations for management

It is true that it is still necessary to improve the management of the bedbug complex focused on the production system. However, there are some recommendations that can be made and which, when followed, allow for good pest management and acceptable coexistence, reducing production costs and maximizing the producer's profitability. The main recommendations to be followed by farmers when managing stink bugs in soybeans and corn are summarized in Table 1.  

Table 1. General recommendations for the management of stink bugs in the soybean and corn production system.

In soy: Use insecticides for stink bugs only between the R3 (beginning of the first pods) and R6 (end of grain filling) stages in grain production crops; Respect the action levels (2 stink bugs ≥ 0,3 cm per cloth, for soybeans intended for grains or 1 stink bug ≥ 0,3 cm, for soybeans intended for seed), as they are reliable, including for early cultivars and of indeterminate growth; Always rotate insecticides using products with different modes of action; Preserve the use of mixtures (neonicotinoids + pyrethroids), acephate or other product indicated for bed bug control only for use in the soybean reproductive period to control this pest. This reduces the selection pressure for bed bugs resistant to these insecticides and preserves their action to control this pest for a longer time. In areas with the presence of weeds, a desiccant must be applied 7 to 10 days before sowing; Carry out seed treatment when you find more than 1 to 2 insects per m2 of straw; Seed treatment products are highly soluble, therefore, in situations of heavy rain immediately after sowing until 10 days after emergence, attention must be doubled; After corn emergence (2-3 days), evaluate the crop and, if you find 1 live stink bug/10 plants, carry out additional spraying; Reassess 5-7 days later. Once you find 1 live stink bug/10 plants again, reapply; Use a cone or double fan nozzle spaced 50 cm apart, with a flow rate of 150 l/ha or more. The height of the bar must not exceed 50 cm from the ground; Depending on the behavior of the bed bug and the period of greatest exposure, apply from 7:00 to 13:00 and from 16:00 to 19:00 hours, avoiding high temperature times; Avoid applications on cold days and/or with low relative humidity; Areas with a history of high infestations: carry out population management in the production system; Avoid spraying at times of high temperature and/or low relative humidity.   In corn: Never wait for damage symptoms to appear to control the pest, as it may be too late; Provide good corn nutrition. Nitrogen helps plants with symptoms of damage recover.

Table 1. General recommendations for the management of stink bugs in the soybean and corn production system.

 

In soy:

• Use insecticides for stink bugs only between the R3 (beginning of the first pods) and R6 (end of grain filling) stages in grain production crops;
• Respect the action levels (2 stink bugs ≥ 0,3 cm per cloth, for soybeans intended for grains or 1 stink bug ≥ 0,3 cm, for soybeans intended for seed), as they are reliable, including for early cultivars and of indeterminate growth;
• Always rotate insecticides using products with different modes of action;
• Preserve the use of mixtures (neonicotinoids + pyrethroids), acephate or other product indicated for bed bug control only for use in the soybean reproductive period to control this pest. This reduces the selection pressure for bed bugs resistant to these insecticides and preserves their action to control this pest for a longer time.
• In areas with the presence of weeds, a desiccant must be applied 7 to 10 days before sowing;
• Carry out seed treatment when you find more than 1 to 2 insects per m² of straw;
• Seed treatment products are highly soluble, therefore, in situations of heavy rain immediately after sowing until 10 days after emergence, attention must be doubled;
• After corn emergence (2-3 days), evaluate the crop and, if you find 1 live stink bug/10 plants, carry out additional spraying;
• Reassess 5-7 days later. Once you find 1 live stink bug/10 plants again, reapply;
• Use a cone or double fan nozzle spaced 50 cm apart, with a flow rate of 150 l/ha or more. The height of the bar must not exceed 50 cm from the ground;
• Depending on the behavior of the bed bug and the period of greatest exposure, apply from 7:00 to 13:00 and from 16:00 to 19:00 hours, avoiding high temperature times;
• Avoid applications on cold days and/or with low relative humidity;
• Areas with a history of high infestations: carry out population management in the production system;

• Avoid spraying at times of high temperature and/or low relative humidity.

 

In corn:

• Never wait for damage symptoms to appear to control the pest, as it may be too late;
• Provide good corn nutrition. Nitrogen helps plants with symptoms of damage recover.

Adeney de Freitas Bueno, Embrapa Soja; Beatriz Spalding Corrêa-Ferreira, Embrapa Soja Rodolfo Bianco; Iapar, Samuel Roggia, Embrapa Soja 

Article published in issue 196 of Cultivar Grandes Culturas.