The increase in areas cultivated with corn, soybeans and cotton resistant to glyphosate has brought producers the benefit of easier cultural treatments and the implementation of the direct planting system. However, both this system and the high adoption of herbicide-resistant transgenic plants generated great selection pressure on weeds, which resulted in an increase in herbicide-resistant weeds in the field (Heap & Duke 2018). This fact, in addition to making management strategies very difficult, increases the production cost, as the producer has been increasing the dose applied and needs to diversify the products used.
The occurrence of weeds in the off-season may vary, depending on the weather conditions and specific climatic characteristics of each region, in addition to being influenced by factors related to soil preparation, history of sprayed herbicides and crops grown in the area. The continuous use of the herbicide glyphosate in Brazilian agriculture has favored an increase in the frequency of resistant biotypes and herbicide-tolerant species in the main soybean-producing regions of the country (Lucio et al..
Currently, in Brazil, there are 11 species of weeds that have biotypes resistant to this herbicide; Among them, horseweed stands out (Conyza spp.), bittergrass (Digitaria insularis) and crow's foot grass (Eleusine indica) (Figure 1), such as species with wide geographic distribution (Lucio et al., 2019; Heap & Duke 2018). The traffic of machinery, agricultural implements and seeds across borders with other countries, mainly Argentina, Uruguay and Paraguay, is an important route of entry for new resistant species, such as pigweed (Amaranthus hybridus), detected in the 2018/2019 harvest in the state of Rio Grande do Sul. Wild sorghum (Sorghum halepense) resistant to glyphosate is already present in Argentina (Heap, 2020), and the probability of introduction in Brazil is imminent.
All species mentioned here stand out as common plants found in the off-season in Brazilian fields (Adegas et al. 2010; Concenço et al., 2012; Heap & Duke 2018). During this period, if not properly managed, weeds can increase their seed bank, become difficult to control and can serve as important hosts for pests and diseases (Dalazen et al. 2016).
The possibility of cultivating grain crops at various times of the year in the same area is one of the great advantages of the tropical production system, which optimizes the exploitation of the area. However, it brings with it some inconveniences, such as the difficulty of controlling polyphagous pest species in cultivation areas, since host plants are maintained in the field throughout the year (Rodrigues et al. 2015).
Among these polyphagous pests present in the field, the fall armyworm stands out, Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) (Figure 2), considered one of the most important pests in Brazil (Mendes et al..
According to a survey carried out by Montezano et al. (2018), this pest species can feed on at least 353 plant species from 76 different families, the main ones being Poaceae, Asteraceae and Fabaceae. These plants include cultivated and invasive plants that occur simultaneously in different regions and seasons. This pest's ability to feed on many plants present in cultivation systems, and the continuous maintenance of host crops in the field, such as corn, soybeans, cotton, sorghum and rice (Boregas et al. 2013) in intensified production systems, provide food for S. frugiperda in the field throughout the year, creating the so-called green bridge, which makes management strategies for the species difficult.
Furthermore, the occurrence of fall armyworm acting as threadworm has increased. This habit occurs when the caterpillar, already large in the field, feeds on seedlings in crops, which reduces the stand. This fact may be caused by the maintenance of the caterpillar on different host plants. The short period between desiccation and planting does not provide a reduction in host plants, that is, a reduction in food for the caterpillars to the point of resulting in a significant decrease in their frequency. As the new crop germinates shortly thereafter, the caterpillars begin to feed on the newly emerged seedlings (Figure 2b).
Due to inadequate management, glyphosate-resistant weeds tend to remain green for longer in the cultivation area, making them potential hosts for polyphagous pests. Therefore, knowing the suitability and biology of the fall armyworm on these invasive plants contributes to improving management practices for this pest. Since in these cultivation systems this pest can remain in the area for long periods feeding on weeds, when the main crop is planted, as this species is already in the area, it starts to cause problems in the newly planted crop, reducing stand and causing feeding injuries that reduce production potential.
In this scenario, in which weeds with resistance to herbicides are becoming more frequent in agricultural areas, a study was carried out, in which the biological aspects of S. frugiperda in specific weeds common in Brazilian agroecosystems, such as those from the Poaceae family, chicken-foot grass (Eleusine indica), wild sorghum (Sorghum vertcilliflorum) and bittergrass (Digitaria insularis), and from other families, buva (Conyza sp.) (Asteraceae), green pigweed (Amaranthus hybridus) (Amaranthaceae) and trapoeraba (Commelina benghalensis) (Commelinaceae). With these results, it was possible to infer how the presence of these plants can influence the survival of this pest in agricultural systems.
In a study carried out in the laboratory, a difference was found for the biological variables studied: caterpillar (larval) survival, caterpillar development period and biomass (weight) of caterpillars and pupae (Figures 3, 5, 6 and 7). In this sense, the present study brings good news. Horseweed, very common in production systems and with the presence of resistant plants in all grain producing regions of the country, cannot be considered a host plant for S. frugiperda, as no caterpillars survived (Figure 3).
Although Dalazen et al. (2017) described horseweed species as frequent and dominant hosts for the fall armyworm in Brazil, in the present study the horseweed did not prove to be a host plant in both laboratory and greenhouse bioassays. These data are in agreement with those found by Mendes et al., 2016, showing that horseweed can be classified as a shelter plant in the field, as not only do the insects not complete the larval phase, but the development time also increases significantly. In this way, the caterpillars survive in underdevelopment for up to 40 days, but die if they are fed exclusively with horseweed (Figure 4).
In addition to horseweed, bittergrass and trapoeraba did not provide good development of the insect in the laboratory, with caterpillar survival rates of 19% and 33%, respectively. Exclusive feeding with bitter grass and trapoeraba also greatly lengthened the development period of the surviving caterpillars, which took around 25 days to complete this phase (Figure 5). It is worth remembering that the longer the insect spends in the caterpillar phase, the more susceptible it is to other forms of mortality, such as predation.
On the other hand, this study also revealed weeds that may be suitable host plants for the development of S. frugiperda, such as wild sorghum and crow's foot grass. Both weed species provided good development of the pest, as the survival of the caterpillars was similar to that observed in corn (Figure 3), and the biomass of the pupae was also compatible with that presented by the caterpillars that fed on corn (Figure 6 ). It is worth mentioning that the biomass of pupae has a direct correlation with the fertility of adults, therefore, the greater the biomass of the pupae, the more fertile the moths will be.
In the study carried out in a greenhouse, it was observed that there was a difference in the injury caused by S. frugiperda feeding on these different weeds (Figure 7). The injury observed in crow's foot grass was similar to that observed in corn, wild sorghum and pigweed. However, in bittergrass, horseweed and trapoeraba, plants that showed worse biological performance against the pest, the injury was also lower. This shows that the presence of the insect in the field can go unnoticed by the producer, since he may not see the caterpillars or the feeding injuries of these insects.
It is important for the producer to be attentive to the observation of feeding injuries also on weed plants (Figure 8), which can be a good indicator of the presence of caterpillars feeding on the plants, especially when preparing the planting, when the presence of these caterpillars on weeds can lead to possible reductions in the stand of the newly planted crop.
In general, the results showed that fall armyworm survival and development were similar to those of maize on the weeds wild sorghum and crow's foot grass, and that horseweed, bittergrass and trapoeraba were the worst hosts of this plague.
Although trapoeraba and bittergrass received the lowest ratings for injury in the greenhouse, the ability of this pest to survive on these weed species for a certain period should be noted. Furthermore, it is important to highlight that weeds in which the fall armyworm has had a long period of development, such as bittergrass and trapoeraba, can favor the presence and persistence of the pest, serving as what can be called of “shelter”.
Recently, a crow's foot grass with multiple resistance to herbicides (glyphosate and fenoxaprop-p-ethyl) was detected in the state of Mato Grosso, Brazil (Heap & Duke 2018). The lack of control of this grass has been increasing mainly in the Cerrado regions of Brazil. This fact also contributed to the increase of this weed in agricultural production systems (Takano et al..
According to the results, the management of these weeds is very important in Brazilian fields during the off-season, with the aim of reducing the seed bank in the soil, the incidence of fall armyworm and production costs. Cellars et al. (2017) estimated a cost of more than R$9 billion annually generated by the presence of resistant weeds, considering only the soybean production system in Brazil. This, without considering in this cost the survival of pests in these weeds, such as the fall armyworm, which would increase this value. Therefore, weed management strategies must be publicized and adopted by rural producers.
For adequate weed management in the off-season, integrated control measures to prevent an increase in seed production during the off-season are necessary. These measures include the possibility of using residual herbicides after harvest, the use of cover crops and mechanical control practices.
To prevent weeds in the area from being able to serve as a green bridge S. frugiperda and do not increase your seed bank, it is necessary to carry out adequate weed control during the off-season. It is important that desiccation is carried out in advance, preferably 15 days to 20 days before sowing the crop, and that pest monitoring is carried out close to the sowing date, to check whether there is a need to apply insecticides to control the pest. -target.
Natália Alves Leite, Tamara Moraes, Alexandre Ferreira and Décio Karam, Embrapa Milho e Sorgo;
Simone Mendes,*
Embrapa Corn and Sorgo* corresponding author
With each new edition, Cultivar Grandes Culturas publishes a series of technical content produced by renowned researchers from all over Brazil, which address the main difficulties and challenges encountered in the field by rural producers. Through research focused on controlling the main pests and diseases in the cultivation of large crops, the Magazine helps farmers in the search for management solutions that increase their profitability.
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