Weed control can help with fall armyworm management

Invasive plants are not only harmful due to competition for natural resources with crops, they can also be a means of keeping pests and diseases in the field. Researchers found that the persistence and high dispersion of weeds with resistance to herbicides in the cultivation area are capable of favoring the survival of pests and diseases that threaten several crops in Brazil. This has been demonstrated for the fall armyworm (Spodoptera frugiperda) in corn crops during research by Embrapa Milho e Sorgo (MG).

Published in the journal Florida Entomologist Bio One, under the title “Survival and Development off farm armyworm (Lepidoptera: Noctuidae) in weeds during the off-season” - “Survival and development of fall armyworm (Lepidoptera: Noctuidae) on weeds during the off-season” - the work sought to investigate, in laboratory and greenhouse conditions, the insect's adaptive capacity to corn. Six weeds with a history of resistance to glyphosate were analyzed, such as horseweed, bittergrass, crow's foot grass and pigweed, all commonly found in Brazilian agroecosystems. Precisely due to the difficulty of control, they can remain in the field for longer. Therefore, scientists wanted to know whether these plants were also hosts for the fall armyworm.

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The fall armyworm (Spodoptera frugiperda) is one of the main polyphagous pests that takes advantage of the direct planting system and other cultivation systems with the presence of host plants year-round in the field. This insect is a pest not only in corn crops, but also in sorghum, cotton, rice, soybeans and other crops of economic importance.

Researcher Simone Martins Mendes, from Embrapa Milho e Sorgo, explains that in these crops it can attack plants in the germination phase, removing the stand from the crops; in the vegetative stage, when it causes major defoliation in corn, sorghum and rice crops; and in reproductive structures, as in the case of corn and cotton ears, causing losses to the farmer. 

Furthermore, there are records in the literature of the occurrence of this pest feeding on more than 300 species of plants in the field. Therefore, this insect finds it very easy to establish itself in the cultivation area”, says the scientist.

“In this context, understanding how the pest behaves - feeding on weeds that are resistant to herbicides - can help the producer to better manage the pest, also paying attention to the control of these plants to facilitate their management in the field.” Another detail highlighted by the researcher is that this pest has a history of selecting resistance to the main technologies available for its control. Resistance selection has been recorded for some Bt technologies, expressed in corn, and also for some insecticide molecules, hence the great difficulty in managing the fall armyworm. 

Research has demonstrated that the survival and biomass of S. frugiperda were significantly higher in crow's foot grass, corn and wild sorghum, both in the laboratory and in the greenhouse. On the other hand, the good news is that horseweed, trapoeraba, bittergrass and pigweed were the least suitable plants for the development of S. frugiperda. In other words, they are not suitable host plants for the pest in the field.

In addition to the adaptive capacity of this caterpillar, the results suggest that its persistence in the field may be directly related to the inefficiency in weed control during the off-season, reinforcing the importance of integrated pest and weed management during this period.

“Weed management during the off-season is a practice neglected by many producers. Due to the absence of crops of agricultural interest during some months of the year, crop areas are left fallow, being dried out close to the planting date of the new harvest”, explains Embrapa researcher Alexandre Ferreira da Silva, also from Embrapa Milho e Sorgo .

The scientist warns that, during this period, weeds can develop, complete their life cycle and increase their seed bank. “This fact can cause economic losses to producers, as it implies greater expenses with herbicides and loss of effectiveness of these products in desiccation, in addition to increasing the incidence of weeds during the development of the crop, due to the increase in the seed bank”, highlights the researcher.

Glyphosate resistance

According to Silva, this scenario becomes even more complicated when dealing with species that have biotypes resistant to glyphosate, considering that this is the main herbicide molecule used in agriculture. 

“Biotypes with resistance to glyphosate are widely dispersed throughout the main grain producing regions of the country. Currently, in Brazil there are 11 species that have biotypes resistant to the molecule: horseweed (Conyza canadensis, C. bonariensis, C. sumatrensis), ryegrass (Lollium perenne ssp. multiflorum), caruru (Amaranthus palmeri, A. hybridus, A retroflexus), bittergrass (Digitaria insularis), crow's foot grass (Eleusine indica), white grass (Chloris elata) and milkman (Euphorbia heterophylla)”, says the researcher.

“For adequate weed management in the off-season, the researcher recommends integrated control measures to prevent growth and seed production during the period. “Among the measures are: the possibility of using residual herbicides after harvest, use of cover crops and mechanical control practices”, advises Silva.

“In this scenario in which weeds are becoming more common during the off-season, favoring the survival of pests, we carried out a study in which we evaluated the biological aspects of S. frugiperda in specific weed plants common in Brazilian agroecosystems, and we infer how the presence of these plants can influence the survival of this pest in agricultural systems”, adds researcher Simone Mendes.

Weeds such as green bridge

“The so-called green bridge occurs when the insect pest finds plants in the field that are hosts, that is, in which the pests can feed and even reproduce. Because of this, maintaining host plants in the field can be a source for insect pests to maintain their population in the field”, says the researcher. Simone Mendes.

“The findings of this work show that due to inadequate management, weeds resistant to glyphosate tend to remain green for longer in the cultivation area, becoming potential hosts for polyphagous pests. Therefore, knowing the aptitude and biology of the fall armyworm on these weedy plants helps us to adapt the management of the weed and insect pest”, comments the researcher

According to her, by knowing in advance which weeds are present in the area, it is possible to adjust the monitoring carried out before planting the main crop. “If these species are in the area, the pest can continue to feed and cause problems in the newly established crop, reducing the stand and causing injuries that reduce the productive potential.”

How weeds behave and how to manage them 

Researcher Alexandre Silva comments that studies carried out by members of Embrapa's herbology team estimate that the cost of weed resistance to herbicides in the soybean production system is approximately R$5 billion per year. Adding possible crop yield losses due to weed competition, this total value could reach R$9 billion annually. 

“In this context, it is important that producers adequately tackle the problem, adopting management strategies and following the guidelines of Integrated Weed Management (MIPD), which consists of the integration of different control methods, taking into account the local reality”, says the researcher. 

According to him, among the main control methods, preventive management stands out, which is care when purchasing seeds, cleaning machines and agricultural implements from areas with a history of infestation, maintaining roadsides, carriers and maintaining terraces. free from weeds; and cultural management, which involves reducing the fallow period, producing straw to cover the soil, choosing cultivars adapted to the region, using the recommended spacing and plant population and crop rotation.

In addition to these methods, the researcher also includes mechanical management, when weeding and mowing are carried out to prevent the spread of plants in the area; and chemical control, which consists of the use of herbicides with different mechanisms of action, in different control systems, such as the use of pre- and post-emergent herbicides in the same area.

“To prevent weeds from serving as a green bridge and not increasing their seed bank in the area, it is necessary to carry out adequate monitoring of the pest and promote weed control during the off-season. It is important that desiccation is carried out in advance, preferably 15 to 20 days before sowing the crop, and that monitoring is carried out close to the sowing date to check whether there is a need to apply an insecticide to control the target pest” , advises Silva.

Furthermore, he points out that among the species that present biotypes resistant to glyphosate, crow's foot grass and pigweed were the most suitable weed plants as hosts for the pest. 

“Therefore, it is important that producers pay special attention to whether these plants are present on their property. Adequate management of the weed community, integrating different control methods and carrying out an adequate rotation of herbicides with effective mechanisms for controlling target weeds, helps the producer in the management of resistant biotypes. This biotype, in turn, helps to reduce the food and green bridge for the pest, allowing it to be better managed in the field”, reports Ferreira. 

The studies were carried out by researchers from Embrapa Milho e Sorgo (MG) Alexandre Ferreira da Silva, Decio Karam and Simone Martins Mendes; by doctoral student Tâmara Moraes, from the Entomology department of the Luiz Queiroz College of Agriculture (Esalq/USP); and post-doctoral fellow Natália Alves Leite, from the Federal University of Rio Grande do Sul (UFGRS).