The challenge of fighting bed bugs

The increase in phytophagous stink bugs in the production system, which involves soybeans as the main crop in the Cerrado region, has led technicians to better analyze all aspects of managing this important pest, which generates great losses and has increased production costs. .

In soybean cultivation, among the species, the brown stink bug, Euschistus heros (Heteroptera: Pentatomidae), has a wide distribution in crops in the Cerrado region. Other species of stink bugs are associated with the agroecosystem, such as the green-bellied stink bug (Dichelops spp.), the edessa bed bug (Meditating Edessa) and the small green stink bug (Piezodorus guildinii), presenting high infestations in some cases. Some species have lost importance in recent years, such as, for example, the green stink bug (Nezara viridula).

Soy is the main crop, as it has the largest area, but its fundamental role in the incidence of some species is explained by being the first crop in the field and consequently the one that begins the colonization of a new generation of individuals. Different hosts, such as tigueras weeds from the previous crop and cultivated plants that are part of the crop rotation/succession system in the Cerrado region, play an important role in the management of this pest.

Biology

Among bedbugs, the brown, E.heros, is a species native to the Neotropical Region (Tropical America), adapted to the hot climate, consequently more abundant in the Cerrado region, which has a mild winter.

The adult is dark brown in color, with two lateral extensions of the pronotum, shaped like spines. The average longevity of adults can be 116 days. The eggs are deposited in small yellow masses, normally with five to eight eggs per mass, with a pink stain, close to the nymphs hatching. Eggs are laid mainly in soybean leaves or pods. Newly hatched nymphs are brown or gray in color, becoming yellowish as they develop, with serrated edges. Newly hatched nymphs measure 1mm, moving on to the next instars when they reach 5mm to 10mm.

E. heroes during the soybean harvest it normally has three generations, and can feed on other crops (cover crops such as radish, pastures, sunn hemp, sunflower, among others) and also on weed species such as wild peanuts, turnip greens, 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 does not eat, but manages 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 (Figure 1). Some areas with nearby refuges can lead to entry (migration) into the plots and consequently the beginning of the infestation.

Figure 1 – Dispersion of stink bugs in a soybean field. Red dots: bed bug rates greater than two individuals per meter, dark green dots: 0 individuals. Chapadão Foundation 2012

Stink bugs generally have a preference for pods in soybean crops. In other crops such as cotton, they prefer apples that are just starting to form or even already formed. In corn, plant growing points (Dichelops sp.) and other crops, mostly grains in formation, are the target of these pests.

In soybean cultivation, from the beginning of pod formation, stink bugs concentrate in these parts, both as nymphs and adults. In some cases, only 10% of the total population present in the plant can be found in plant leaves.

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

The bedbug Meditating Edessa é commonly called the black-winged stink bug. It is a bed bug that, as an adult, part of its wing is dark in color and the nymphs are yellowish to green in color with few red spots on the back. This species has also been observed in other neighboring countries, such as Paraguay, Bolivia and Argentina.

In the Cerrado region, this stink bug has the cotton crop as a good host, leading to an increase in its population, year after year. In cotton, when apples are formed, their control becomes essential in order to avoid falling apples and their poor formation.

The small stink bug Piezodorus guildinii It has a wide occurrence in the central region of Brazil. The adult is a bed bug measuring approximately 10mm, light green in color. It has a transverse stripe on the posterior half of the pronotum, a dark spot that may have a reddish background. The eggs are black in color, laid in double rows. The nymphs have a voluminous abdomen, with the anterior half of the body dark brown or black and the abdomen reddish-yellow, with several black spots. Studies developed by Embrapa demonstrate that the small stink bug damages seed quality more and causes greater leaf retention in soybeans than other stink bugs.

The genus bed bugs Dichelops (Dichelops melacanthus e D.furcatus) These are species that have increased in recent harvests, due to the increase in corn tiger plants in soybean crops, 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 green eggs are placed on plates. 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 2nd harvest corn (safrinha).

Other species of stink bugs from the Pentatomidae family are found attacking crops, however, in lower abundance. They are the green stink bug (Nezara viridula), which has decreased in importance due to its lower occurrence; Thyanta perditor e Acrosternum sp., the latter often confused with the green stink bug, however, with differences in the abdomen and antennae in relation to other species. 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.

Damage

In soybean cultivation, these bugs suck the sap from the branches, stems and pods. When sucking branches or stems, stink bugs can inject toxins that cause “leaf retention”, that is, the leaves do not fall normally and make mechanical harvesting difficult.

In this way, 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.

Among bedbugs, the species Piezodorus guildinii presents the greatest damage to the crop, when compared to individuals of other species. However, E.heros It constitutes a greater risk to the crop, especially in the reproductive phase, as it is most often present in greater abundance, causing irreversible damage to soybeans, as it feeds directly on the grains, from the beginning of pod formation. Losses due to the suction of pods and grains can be greater than 30%, which can also cause stains on already formed grains. The occurrence of high populations can also lead to pod drop in some cases.

Driving

Integrated Pest Management (IPM) involves several practices, with sampling being one of the foundations of the program. When surveying, the technician must correctly identify the species and also note the number of nymphs, which from the 2nd instar onwards can, in some cases, cause damage. In this situation, only with the use of a cleaning cloth will the technician/producer be able to carry out the correct identification and the “real” level of infestation. This practice, despite being old, is the most assertive in management, and it is recommended to carry out as much as possible , however, with practicality due to the size of the areas, etc. Training the sampler team must be a practice adopted year after year to improve and recycle knowledge.

After this phase, knowing the index, the technician can take action. At this point, in work carried out at Fundação Chapadão, with the help of the scientific technical council, it has been observed that with one stink bug per meter, the producer/technician has managed to manage the pest, without causing significant damage to grain production. It is also worth highlighting that control in seed production crops must take into account the qualitative aspect. The sower often tries to guarantee this aspect as much as possible. 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 in order 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 with management in these crops can the initial quantity for the next harvest be reduced.

These strategies, when necessary, can be chemical, with a registered insecticide or even the adoption of a biological control program.

It is important to remember that with the advent of the use of genetically modified plants resistant to glyphosate, it is common to find tigueras corn plants inside soybeans. Ears that normally fall during harvest generate more than one flow, requiring two applications of graminicides. These tigueras have also led to increased problems with the green-bellied stink bug.

It is worth remembering that many of the individuals left for the next harvest are naturally parasitized. Some parasitoids, such as Hexacladia smitt, It can take up to 40 days for some individuals to die. Others, like Trichopoda nitens, also until development in some species can take 30 days to cause the death of bed bugs, in the case of N. viridula. In this way, it is interesting for the technician to analyze the phenology of the crop. For example, soybeans that are in the vegetative stage, depending on this parasitism, will not require any chemical intervention. It should also be noted at this point that there is a lack of further studies on selectivity to Tachinids, which are important in the management of bedbugs and other pests.

In parasitism surveys of Euschistus heros In the Chapadões region, it was observed in the 2012/2013 and 2013/2014 harvests that the stink bugs present in the soybean crop in October showed 70% parasitism, not laying viable eggs, without completing a new cycle. However, from the month of December onwards, the vast majority of bedbugs become viable, generating new individuals.

There are several natural enemies of bed bugs, in the case of E. meditabunda, which has also shown egg parasitism by Ooencyrtus sp. compromising its population at a certain point. In some cases, this pest can make up to 50% of the population unviable.

A necessary issue in the analysis of stink bug infestations in September and October sowings is that if there is low parasitism, it can compromise subsequent sowings, generating individuals in nearby plots and consequently losing control.

Another point to report is the low control efficiencies, which are associated with factors such as application technology, monitoring failures, with high rates. The times for monitoring and applications must occur at the beginning or end of the day, times with a greater frequency of stink bugs in the crop.

The indexes determined by the search are valid. But the technician must understand that the index also takes into account that the alternative to be used presents 100% control and, however, research has shown that control, when good, reaches 80%. Therefore, if the survey indicates five bedbugs per meter, after applying an insecticide with 80% control, there will be one bedbug left, which is still a control index, requiring a shorter interval between application. In Figure 2, it is observed, in work carried out with the aim of verifying the application intervals, in infestations of four bedbugs per meter, that after the second application satisfactory control rates were achieved. Applications often called “free riders” may not lead to good results. Another issue is that the index takes into account the cost of control (if the cost of control is lower and the price of the final product is higher, this index may decrease) .

Figure 2 – Effect of some insecticides on E. heroes in soybean cultivation. Percentage of control by Betacyfluthrin + Imidacloprid and Lambdacyhalotrin seven days after the first application and two days after the second application, seven-day interval between applications. Chapadão Foundation. Chapadão do Sul/MS 2008/2009 Harvest

Analyzing this aspect, the producer and technician must understand that even after application 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 new application.

Regarding the research results, it is worth highlighting that in the work carried out at Fundação Chapadão, it has been observed that the mixtures have reduced their efficiency in adults, however, they still have good efficiency in nymphs. At the time when the mixtures of pyrethroids + neonicotinoids were put into the field, (betacyfluthrin + imidacloprid) and (lambdacyhalotrin + thiamethoxan), in the doses recorded, showed control greater than 80% (Figure 3), reaching in some cases up to 90% ( Figure 4). The departure of methamidophos, which in the 2004/2005 harvest already showed wear due to continuous use and cases of resistance confirmed in some regions by Embrapa Soja, left a gap among the strategies, and it is interesting to note that only the acephate insecticide remained on the market. This insecticide for controlling adult bed bugs is one of the alternatives. In the 2012/2013 harvest, the insecticide (bifenthrin + imidacloprid) was also sold. Another insecticide recently launched on the market was a mixture of bifenthrin + carbosulfan. New molecules such as dinotefuran and mixtures are in the registration phase.

Among the insecticides that can contribute in part to the management of bedbugs, growth regulators, inhibitors of chitin synthesis, used to control caterpillars such as the false insect (Chrysodeixis includens), have shown action causing physiological disturbances in adults and even in nymphs with some action. However, they often require constant applications or even a greater amount of insecticide when compared to caterpillars. In Figure 5 it is observed that two applications of the insecticide lambadacyhalothrin + thiamethoxan (pyrethroid + neonicotinoid), when lufenuron (growth regulator) was added, showed greater control efficiency. The same graph shows the treatment at a dose of 0,4L/ha (pyrethroid + ne + imidacloprid).

In mixtures of pyrethroids + neonicotinoids, the technician must remember that at certain times, conditions and products, an imbalance of mites may occur (this has been observed in several studies). Only understanding the system and planning insecticides can lead the technician to adopt the best alternative.

With the advent of genetically modified plants, producers cannot neglect monitoring. With soy Bt Intacta RR Pro, some areas may need an additional strategy in managing bedbug pests. This is due to the removal of some insecticides applied to caterpillars that contributed in part to bed bug control. In this way, management without the team in the field can undermine one of the foundations of IPM, which is sampling (monitoring).

There is also the problem of resistance, in the case of the brown stink bug, observed with organophosphate insecticides and recently with ready-made mixtures (neonicotinoids + pyrethroids). This is related to the continued use of the same mode of action by producers during these years.

Therefore, producers must be aware of the efficiency of the insecticides used and whether the pest is resistant to pesticides in their crops. To do this, they must seek information from research institutions, manufacturing companies, technical assistance and consult an agronomist to recommend products, using the rotation of chemical groups, thus avoiding the development of pest resistance to defensive.

In this aspect, Irac-BR has carried out studies in partnership with some institutions in order to maintain chemical groups of insecticides/acaricides and genetically modified plants as viable alternatives, and to Euschistus heros is one of the projects, analyzing molecules such as thiamethoxan, imidacloprid, acephate, betacyfluthrin and lambdacyhalotrin.

Brazil is considered by many to be a major user of agricultural pesticides. However, it can be observed that with the increase in crop productivity, growth in the exploration of the same area and the use of technologies, the country managed to increase the quantity produced year after year, even with all the obstacles faced by the producer. Chemical control, as observed, is part of an integrated set of measures that make up the MIP, such as periodic monitoring of crops, monitoring of population levels and, also, several other technical recommendations involved.

For now, the country is winning the battle against bedbugs, but the war is not over yet.

Figure 3 – Effect of some insecticides on E. heroes seven days after application to control Euschistus heros in soybean cultivation. 2004/2005 harvest. Chapadão Foundation. Chap.Sul/MS

Figure 5 – Effect of some insecticides on P. guildinii seven days after application to soybean crops. 2004/2005 harvest. Chapadão Foundation. Chap.Sul/MS

Figure 5 – Effect of some insecticides on E. heroes in soybean cultivation. Percentage of efficiency seven days after the first application and seven days after the second application. Chapadão Foundation. Chapadão do Sul/MS. 2012/2013 harvest