Pest pressure in soybean crops

The characteristics of the Brazilian intensive agricultural production system, while providing numerous advantages, result in greater challenges in pest management, due to the almost uninterrupted supply of host plants that allow these insects to remain throughout the year in the field. Adhering to good integrated pest management is the first step to preventing or reducing control difficulties.

In recent decades, global production of food had an unprecedented increase, largely thanks to the of tropical agriculture via scientific and technological advances that allowed the cultivation of up to three crops in just one year. Therefore, contributing significantly to overcome the challenge of meeting food demand growing world.

The intensification of agriculture reduced the need for deforestation to increase food production, impacting positively, for example, in maintaining biodiversity and preserving water. However, intensive cultivation faces greater difficulties phytosanitary systems when compared to more traditional systems.  

Various insect pests with characteristics polyphagous plants benefit from this intensified production system due to the almost uninterrupted supply of host plants for these pests, allowing them to remain in the field throughout the year. Consequently, the number of sprayings with insecticides, when used without criteria technicians, increases significantly. This scenario intensifies the process of selection of pest populations resistant to insecticides, resulting in reductions in productivity when pest control is not satisfactory, in addition increase the production cost, as new spraying can be carried out to attempt to correct the control failure. The same analogy must be made for other control tactics, such as the use of transgenic plants with proteins insecticides, such as soybeans that express the Cry1Ac protein.

Currently in the Brazilian cerrado, most of cultivated cotton is sown after soybean cultivation and this has an influence relevant when talking about resistance management, as several pest species (Table 1) attack both cultures. Therefore, throughout the harvest, they will receive, if the control levels established by the research are not respected, an intense load of insecticides.

Considering that they are no longer pests of a culture but rather a production system, it is important to keep in mind that the plans rotation of active ingredients with different modes of action should be carried out throughout the same harvest and in subsequent harvests. 

In addition to the common use of active ingredients to these pests in soybean and cotton crops, species such as C. includens, H. armigera, H. zea e C. virescens are in continuous exposure to Bt proteins, as there are soybean and cotton cultivars that express the Cry1Ac protein (Table 1).

Complex caterpillars spodoptera are not affected because this protein at levels expressed by plants, do not suffer its toxic effect. However, the new generation of transgenic soybeans will also contain the Cry1A.105 and Cry2Ab2 proteins that are efficient in controlling the complex spodoptera, currently important pests for soybean, corn and cotton crops. Necessary It is worth mentioning that such proteins are already present in corn cultivars and In cotton, cultivars with the Cry2Ab2 protein are commercialized. Therefore, these caterpillars may be under intense selection pressure, as the cotton, much of the corn produced in the country is sown after the cultivation of soy.

Strategies to avoid/delay the evolution of resistance

Resistance is a selection process and The speed at which it is selected depends on the intensity of the selection pressure (control method) exercised over target individuals. Therefore, the use of good agricultural practices such as Integrated Pest Management (IPM) is the first step towards adequate resistance management, because when using a correct monitoring insects and respecting the control levels recommended by research, naturally the number of insecticide sprays will be less than the currently used. Consequently, the intensification of the selection process of resistant individuals will be reduced. In other words, before thinking about resistance management strategies, the implementation of IPM is essential.

In a complementary way, one should use insecticides (biological or chemical) with different mechanisms of action in each spraying, to allow efficient rotation of active ingredients. In addition Furthermore, it is desirable to use insecticides selective to natural enemies by least at the beginning of the agricultural cycle, as studies indicate that there may be contribution of natural enemies in reducing the frequency of individuals resistant in a population.

In Regarding the management of Bt plant resistance, the use of refuge is essential to allow caterpillars susceptible to Bt proteins to reach adulthood (moth) and cross with possible insects resistant to Bt proteins for maintenance of susceptible populations, thus delaying the selection of insects resistant. Based on the flight range of the moths, it is important that the maximum distance from the refuge area to the Bt soybean area does not exceed 800m, as greater distances can make it difficult for insects to randomly mate from Bt soybean and non-Bt soybean areas. Furthermore, the area designated for refuge must contain at least 40 lines and correspond to a minimum of 20% of the total area cultivated with soybeans to allow the number of crosses necessary between resistant and susceptible individuals.

OK It is worth highlighting that the exaggerated use of insecticide spraying in areas refuge makes this control tactic unfeasible, as it drastically reduces the production of susceptible insects, consequently reducing the number of matings favoring the selection process of resistant individuals. That does not mean that the producer will suffer losses in the refuge area, as the recommendation of 20% refuge area takes into account the control of Prague. Simply respecting control levels will produce susceptible individuals enough for such matings.

O The use of Bt soybeans has been shown to be efficient in controlling target species Cry1Ac protein. However, constant monitoring of the crop is necessary to prevent the proportion of resistant individuals from increasing in the population over time. point of observing pest control failures. Only one case was reported in the Chapadão do Céu region, in the 2017/18 harvest, on helicoverpa spp. attacking Bt soybeans. However, there are controversies regarding to the species found, whether it was H. armigera ou H. zea or their hybrids. Regardless of this, technology it has only a suppressive effect on both species. However, until now no There were cases of caterpillars surviving on Bt soybeans. In other words, it is a natural process of selecting individuals with lower susceptibility to the protein Bt.

Resistance monitoring

One One of the objectives of a resistance management program is to maintain as much possible, the proportion of susceptible individuals in a population to a given control strategy. To this end, monitoring the susceptibility of populations of the target species must be carried out constantly and preferably before beginning the implementation of a given control strategy. Such Monitoring must have a regional focus, as even in species with a high dispersion capacity, variations in susceptibility to certain control tactic between relatively close populations. For To give an example, data from monitoring populations of C. includens in Mato Grosso during the harvest 2016/17 (Figure 1).

They were dose-response curves (mortality) were carried out with the insecticide teflubenzuron for five populations from Mato Grosso and compared these curves with a population maintained in the laboratory for several generations without contact with insecticides. As a comparison parameter between populations, the amount of active ingredient (µg/cm²) dispensed into the diet of insects that would provide mortality for 50% of the individuals evaluated (CL50).

To the field populations with lower LC50 values ​​were Campo Verde and Alto Garças. The populations of Rondonópolis, Diamantino and Canarana were the ones that obtained higher LC50 values, especially in Rondonópolis which required a concentration 72 times greater when compared to the susceptible laboratory population.

Same being C. includens a species with high migratory capacity, pest management in each location influenced the response (mortality), because between the populations of Campo Verde and Rondonópolis the distance in line straight line is 170km. This demonstrates the need for monitoring resistance at a regional level and what inferences for the adequate management of resistance should not be extrapolated to different regions.

Final considerations

A selection of individuals resistant to any control tactic is an evolutionary process natural, but the first step to reduce/avoid control difficulties due to resistance consists of adhering to good integrated management of pests.

Rafael Major Pitta, Sandra Maria Morais Rodrigues, Ferreira and Daniel Ricardo Sosa-Gómez, Company Brazilian Agricultural Research Institute-EMBRAPA; Fátima Teresinha Rampelotti, University Federal of Mato Grosso-UFMT

Article published in issue 231 of Cultivar Grandes Culturas, August, 2018.