“Since the 2019/20 season, the occurrence of Rachiplusia nu caterpillars has been observed in first-generation Bt soybean cultivars…”
Adeney de Freitas Bueno is an entomology researcher and head of R&D at Embrapa Soybean, with a degree in agronomy from the Faculty of Agronomy Manoel Carlos Gonçalves, a M.Sc. in agronomy from Unesp and Ph.D. in entomology from University of São Paulo.
AgriBrasilis – What is the outlook for resistant pests in soybeans?
Adeney Bueno – Pest resistance to control tactics is certainly one of the main challenges faced in Integrated Soybean Pest Management (IPM-Soybean) in the farms. In practice, the adoption of any pest control tactic in the farms without a resistance management strategy ends up homogenizing the environment, favoring the survival and multiplication of insects that are best adapted to that environment.
At first, resistant insects are a minority in the population and go unnoticed. Throughout the agricultural season, resistant insects multiply and, being the only ones that survived the adopted control measures, they become the majority. This is the moment when the population is considered resistant.
To talk about the panorama of resistant pests in soybeans in Brazil, it is important to make a distinction between the main groups of pests currently present in the crops (caterpillars and Hemiptera bugs) and also between the main pest control methods adopted in soybeans on a larger scale: the adoption of Bt soybeans; and the adoption of chemical insecticides.
“It is recommended that Bt soybeans are not cultivated in 100% of the area. It is necessary for cultivation to be carried out in a maximum of 80% of the area…”
a) Pest resistance against Bt soybeans:
First-generation Bt soybeans express the insecticidal protein Cry1Ac and have been cultivated in Brazil since 2013 as a strategy to control the main caterpillars that attack the crop. Unlike an insecticide that controls the pest only for a few days after it is applied in the field, the cultivation of Bt soybeans controls the pests targeted by the technology for the entire time the crop is there (24 hours a day, 7 days a week). Therefore, it is possible to say that the selection pressure (evolutionary pressure) for insects against Bt soybeans is enormous, selecting target pest populations for resistance throughout the whole time the plants are in the field.
It is recommended that Bt soybeans are not cultivated in 100% of the area. It is necessary for cultivation to be carried out in a maximum of 80% of the area, reserving at least 20% of the area for the cultivation of non-Bt soybeans, in what is called a “refuge area”. The refuge area will produce soybeans normally, but with pest control being carried out in the traditional way, with proper monitoring of pests and the application of insecticides only when necessary (when pest populations reach the recommended action levels). The refuge will produce soybeans profitably and efficiently, as has always been done, with the use of insecticides when necessary to control caterpillars. In addition to producing soybeans, the refuge area will also produce some insects that will survive the use of insecticides, and that will be susceptible to Bt soybeans. These susceptible insects will interbreed with resistant insects from the Bt area, giving rise to a generation of heterozygous insects (Sr, according to the image below), which can be controlled by Bt soybeans. This is what will guarantee the preservation of the efficiency of Bt soybeans over time.
Illustration of the refuge area managed with the adoption of IPM-Soybean in preserving the efficiency of Bt technology. (SOURCE: Embrapa Soybean)
The adoption of Bt soybeans has brought great benefits to the crop, including mainly the reduction in the use of insecticides (previously used to control caterpillars) and economic losses caused by the attack of caterpillars controlled by this technology. Furthermore, crop management is facilitated by reducing the demand for insecticide applications. These benefits have been so attractive that unfortunately – and wrongly – most soybean farmers have been adopting Bt soybean technology in 100% of their farming area. Given this scenario of non-adoption of the refuge area, populations of caterpillars resistant to first-generation Bt soybeans (which express only the insecticidal protein Cry1Ac) have been occurring in Brazil.
Since the 2019/20 season, the occurrence of Rachiplusia nu caterpillars has been observed in first-generation Bt soybean cultivars. In addition to Rachiplusia nu, which is a defoliating caterpillar, the occurrence of another species of caterpillar, a small borer known as the bean shoot moth (Crocidosema aporema), has also been increasing in first-generation Bt soybeans. These two species are recognized as the first two species of caterpillars resistant to first-generation Bt soybeans.
More recently, especially after the 2021/22 season, two new Bt soybean technologies have been gaining ground in the market, known as second generation Bt soybean. Intacta 2 soybean, which expresses the proteins Cry1Ac, Cry1A.105 and Cry2Ab2, and Conkesta soybean which expresses Cry1Ac and Cry1F. The expression of more insecticidal proteins in the same technology has the function of increasing the spectrum of action (controlling more target pest species – more caterpillars controlled) in addition to reducing the speed of selection of resistant insects.
For an insect to survive new Bt soy technologies it needs to be resistant to all insecticidal proteins present in the new technology. This significantly reduces the initial number of resistant insects and increases the time for resistant populations to occur (selection time will be significantly longer).
b) Pest resistance against insecticides:
As with the Bt plant, the use of the same insecticide will select populations of the insect-resistant pest that will no longer work.
Representation of the process of selecting resistant insects due to the successive application of the same insecticide or insecticides with the same mode of action. (SOURCE: Embrapa Soybean)
Regarding the control of Hemiptera insects in soybeans, there is no technology for Bt soybeans or even other soybeans genetically modified/edited for Hemiptera resistance. The use of insecticides is still the main tool for controlling this pest. Currently, approximately 50% – 60% of the number of insecticide applications to soybeans occur for Hemiptera control.
Given this greater dependence on insecticides to control Hemiptera in soybeans, it is extremely important to rotate the different modes of action. To avoid or delay the resistance of Hemiptera (or any other pest) to insecticides, it is important to rotate insecticides with different modes of action between applications. This recommendation, popularly known as product rotation – in fact it should be called mode of action rotation, as it is not enough to rotate the product if the mode of action is the same – prevents (or delays) the selection of resistant insect populations, preserving the effectiveness of products over harvests and years.
Illustration of the rotation of insecticides with different modes of action and their role in preserving the effectiveness of active ingredients. (SOURCE: Embrapa Soybean)
Rotating insecticides with different modes of action is important for all soybean pests. Recent studies have demonstrated a significant reduction in the susceptibility of the caterpillar Chrysodeixis includens to insecticides that inhibit chitin biosynthesis, and of the brown stink bug (Euschistus heros) to insecticides from the pyrethroid and neonicotinoid groups.
Specifically for E. heros and other soybean bugs, registered insecticides belong to a few chemical groups. Therefore, this limited availability of modes of action favors the evolution of resistance, due to the frequent exposure of the pest to active ingredients with the same mode of action in each harvest, making the management challenge even greater.
Summarizing the panorama of resistant pests in soybeans: currently the main challenges are the adoption of the Bt soybean refuge area for caterpillar management and the rotation of active ingredients for stink bug management. In both cases, it is crucial to adopt IPM-Soybean with continuous monitoring of pests in the crop and the application of one or more control strategies only when necessary and recommended by research.
AgriBrasilis – Why did the occurrence of Hemiptera insects increase in the 2023/24 season? What can we expect for the next season?
Adeney Bueno – The occurrence of Hemiptera in crops normally varies over the years, mainly depending on climatic conditions and the management strategies adopted.
Hotter and drier climates generally favor the occurrence of pests in crops, including Hemiptera insects. When we have more favorable climates, we will have a greater occurrence of pests, which will require greater adoption of management strategies, which includes greater application of insecticides, which is the management strategy most adopted on a large scale in agriculture.
In addition to the climate, we need to consider the difficulties of managing Hemiptera insects due to populations that are resistant to the insecticides used, in addition to the challenges in the technology for applying these insecticides. Hemiptera are pests that occur on the soybean reproductive period, where they obtain greater shelter and protection within the crop, which reduces the efficiency of their control.
Given these points, we can say that the higher incidence of Hemiptera insects in the 2023/24 season is due to the association of some factors: these factors mainly include the hotter and drier climate in several regions and the difficulties in managing the pest (application technology of insecticides and occurrence of resistant populations). The low adoption of IPM-Soybeans leads to failures in monitoring pests in the crop and also favors the greater occurrence of the pest. Monitoring failures lead to control being carried out at the wrong time, which reduces its efficiency.
AgriBrasilis – What are the damages caused by this pest?
Adeney Bueno – They feed directly on the pods, damaging the soybeans, which lose weight and quality. The intensity of the damages will depend on the intensity of the attack and on the management adopted.
AgriBrasilis – What are parasitoids and how are they used to manage caterpillars? How has the adoption of this control method evolved?
Adeney Bueno – Parasitoids are small insects that lay their eggs inside hosts, which are the pests. Parasitoid larvae hatch inside their hosts. The parasitoids will feed on these host insects, causing them to die.
In agriculture, parasitoids occur naturally and their population will be preserved with the greater adoption of IPM, which reduces the use of insecticides, being applied only when they are really necessary. IPM also always prioritizes the most selective insecticides, which harm parasitoids least.
Some parasitoids can be applied to crops, multiplied on a large scale and released massively to reduce pests in the farm. In the case of soybeans, there are egg parasitoids, which are small “wasps” that can be purchased from biofactories and released into the crop.
There are two species of egg parasitoids that are bred by biofactories and commercialized and released in soybeans, one species for caterpillar control and the other for control of Hemiptera bugs.
The egg parasitoid for caterpillar control is the species Trichogramma pretiosum, and the egg parasitoid for Hemiptera control is the Telenomus podisi species. They should be released onto crops at the beginning of the target pest infestation, as they will not directly control caterpillars or Hemiptera bugs, but will control the eggs of these pests.
The availability of Trichogramma pretiosum on the market is greater, as there are more biofactories that produce and sell this parasitoid. In addition to use on soybeans, it is sold to control different species of Lepidoptera in other crops. Despite its greater availability to soybean farmers, the use of Trichogramma pretiosum to control caterpillars in soybeans is still limited, as caterpillars in soybeans are still very well controlled by Bt soybeans and most chemical insecticides. With the emergence of populations of caterpillars resistant to Bt soybean (Cry1Ac) such as Rachiplusia nu and Crocidosema aporema, the use of Trichogramma pretiosum can grow in soybean cultivation as a preferred option for managing these resistant caterpillars, preserving the main benefit of adopting soybean Bt, which is the reduction in the use of chemical insecticides on crops.
The availability of Telenomus podisi on the market is lower, as there are still few biofactories that produce and sell this parasitoid. However, as Telenomus podisi is used to manage Hemiptera bugs, which are the main soybean pests, against which there is no Bt soybean and insecticides present control difficulties (due to the occurrence of resistant populations and flaws in application technology), the search for this parasitoid has grown a lot and its use in agriculture is actually only limited by supply.
We can say that the use of Trichogramma pretiosum and Telenomus podisi has been growing in soybeans and there is greater interest among farmers in using these parasitoids.
AgriBrasilis – Can parasitoids replace chemical insecticides? Which?
Adeney Bueno – Egg parasitoids can significantly reduce the use of insecticides, but not replace them. All pest management technologies (including insecticides) are important if used correctly and have their place within integrated pest management, which provides for the harmonious association of different management strategies.
AgriBrasilis – How to prevent chemicals from killing parasitoids?
Adeney Bueno – By adopting IPM, the farmer reduces the use of insecticides, used only when really necessary, and prioritizes the use of less impactful insecticides (more selective) to parasitoids and other beneficial organisms existing in the crop. This does not completely prevent it, but greatly reduces the mortality of parasitoids with chemical products.
AgriBrasilis – What other bioinsecticides are efficient in soybean cultivation?
Adeney Bueno – It is always important to look for bioinsecticides properly registered for use in the country and apply these bioinsecticides in accordance with the registration. This is important because the efficiency of the bioinsecticide when used as recommended was duly proven and analyzed in a rigorous evaluation process carried out by the Ministry of Agriculture (MAPA), which is the entity responsible for evaluating the efficiency and agronomic practicality of the use of insecticides and similar products in the country.
In addition to egg parasitoids in soybeans, there is also the registration and use of entomopathogens (mainly viruses, bacteria or fungi) that control some soybean pests. In addition to these bioinsecticides, others, such as the use of plant extracts, predators, entomopathogenic nematodes, for example, are being studied and may be used soon, as long as they are properly registered by the responsible agencies.
AgriBrasilis – Can the Integrated Pest Management – IPM program in soybeans in Brazil be considered a success? Why?
Adeney Bueno – Yes, mainly because it allows the reduction of the use of chemical insecticides and of the losses caused by pests and, therefore, increases profits for the soybean farmer. Unfortunately, one of the major bottlenecks that prevent the adoption of IPM in soybean farms on a larger scale is the availability of qualified labor, necessary mainly for monitoring pests and making decisions about when control measures should be adopted and which kind of control measure.
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