How to deal with the incidence of aphids in sorghum

There are four types of sorghum [Sorghum spp] in Brazil, broom sorghum, forage sorghum, sweet sorghum and, finally, grain sorghum, which is the most commonly cultivated, as its grains are used to manufacture animal feed, replacing corn. Sorghum production in Brazil is concentrated, for the most part, in the Central-West region. With an estimated production of 1,09 million tons in December 2019, the State of Goiás has consolidated itself as the largest sorghum producer in Brazil, therefore representing 40% of national sorghum production. This quantity is 20,3% higher than the production recorded in December 2018 (912,9 thousand tons), according to data released in January 2020 by the Brazilian Institute of Geography and Statistics (IBGE).

As it is a rustic crop and tolerant to water stress, without significant losses in nutritional quality, sorghum, both for grain and forage, has been used as an alternative crop to second-crop corn, as it has a longer “planting window”. extended to a biome as specific as the Cerrado, in addition, its market value can reach up to 80% of the value of corn.

Climate instability, lower production costs, high productivity rates, good straw coverage offered for summer plantings, among other advantages, are important aspects that producers from different regions take into account when adopt this culture more frequently in the off-season. Furthermore, until recently, growing sorghum also had the “advantage” of minimal or even no use of agrochemicals.

Currently, the reality is different and to produce more and more, rural producers have started to increase the cultivated areas and invest in more suitable technologies for sorghum cultivation, such as fertilization, more productive cultivars and phytosanitary treatments.

This increase in areas cultivated with sorghum and its proximity to areas cultivated with sugar cane raised concerns in addition to those related to increased productivity. This is the case with the increase in the number of chemical interventions, as they were faced with phytosanitary limitations, mainly related to pest arthropods, such as the species Helicoverpa armigera, the complex of spodoptera spp, in addition to suckers, such as aphids of different species, particularly Melanaphis sacchari (Zethner) (Hemiptera: Aphididae), pest previously restricted to sugarcane crops.

This possibility of expansion and the potential for damage caused by the species M. sacchari were alerted in 2016 by a group of researchers from the Federal University of Pernambuco, and it took three years for this to become a reality, as in 2019 this pest was the target of numerous applications of agricultural pesticides in crops in the states of Minas Gerais, São Paulo and in the Federal District.

In the current harvest, sorghum crops in the South, Southwest, Surrounding and Central regions of the state of Goiás also showed alarming rates of populations of this aphid, requiring producers to carry out up to five interventions with pesticides on the crop to control the target and, in some cases More critical, the alternative was the destruction of the area's cultural remains, considering the intensity of the infestations and the damage caused. Furthermore, there is an aggravating factor in this issue regarding the absence of agrochemical products registered with the Ministry of Agriculture, Livestock and Supply (Mapa) to control this pest in sorghum crops. Therefore, chemical interventions were based only on information obtained from other countries or using recommendations for other aphid species and/or crops.

Confirmation of the presence of M. sacchari in sorghum crops in the state of Goiás was based on samples of individuals of this species collected in some areas cultivated with sorghum in the municipality of Indiara and sent by the Integrated Pest Management Laboratory of the School of Agronomy of the Federal University of Goiás (EA/UFG ) to the Diagnostic and Biotechnology Laboratory/National Agricultural Laboratory of Goiás (LDB/Lanagro-GO) of Mapa, with identification being carried out using a molecular technique (Conserved Region Sequencing).

M. sacchari is a sugarcane pest present in America, Asia, Africa and Oceania, recognized as an invasive pest, due to the high rate of dissemination, favored by air currents. Its presence was reported in the 1970s in North America and Central America and remained until then in these regions as a common pest of sugarcane and an important vector of some diseases of viral etiology in the crop. However, in 2013, outbreaks of a biotype of M. sacchari, in sorghum, and two years later, all 17 producing states in the United States of America (USA) reported large infestations of this pest. Some crops in Texas and Louisiana had an average of 900 aphids per leaf of the attacked plants, with reductions of up to 100% in productivity, forcing producers to carry out countless pesticide applications to control the pest.

Description and biology

The sugarcane aphid (M. sacchari) was initially described as Aphis sacchari e Longiunguis sacchari before being placed in its current gender. It feeds on several plants from the Poaceae family and is currently a key pest of sugarcane (Saccharum officinarum) and sorghum (Sorghum spp). It appears as a small aphid (< 2mm), with a soft body, pyriform, generally beige in color, but exhibits significant color variation with shades of pink, brown and yellow, depending on the host plants and climatic conditions. Its individuals are recognized for having dark colored tarsi, siphuncles and part of the antennae, distinguishing them from other species that infest Poaceae.

Within an established population of M. sacchari, nymphs and adults are found, the latter appearing in winged forms, responsible for the dissemination of its population in the crop, and apterous, responsible for reproduction within the same colony. Reproduction in tropical regions like ours is by thelytotic parthenogenesis, that is, females giving rise to new females, therefore, they can accumulate large populations in a relatively short period of time.

Some reports cite population peaks of more than 30 aphids on just one plant, however, quickly, in two to three weeks of population abundance, aphid densities decrease, leaving only exuviae and sooty mold, and this decline occurs due to the dispersal of aphids. winged individuals induced by the density itself and as a result of the poor nutritional conditions of the host due to high infestation.

In Goiás, the species M. sacchari was found in sorghum crops, specifically between the months of March and May, mainly in the final phase of the vegetative cycle, affecting, in many crops, 100% of the plants, with an average of approximately one thousand aphids per leaf. Grain sorghum was the most affected, in addition to other sorghum species and nearby grasses, such as corn and millet, which showed lighter infestations.

The attack of these insects occurs on the leaves, where both nymphs and adults feed by sucking the sap, mainly on the abaxial surface, so it is common for the infestation to start from the oldest to the youngest leaves (from bottom to top). They cause direct damage by sucking the plant's sap, and indirect damage by being important vectors of viral diseases, in addition to the fact that continuous sucking causes them to excrete large quantities of sweet substances, called “honeydew”, which are a favorite food for certain ant species. These substances leave the surfaces of the leaves sticky and shiny in appearance, over time they darken due to the development of sooty mold, preventing photosynthesis from taking place and ultimately leading to yellowing, drying and leaf fall. Furthermore, there may be a total or partial impairment of grain production, due to this depletion of the plant caused by the pest in question.

It is currently known that this pest has the potential to cause the decline of sorghum cultivation in several regions of the country, mainly in the state of Goiás, where it reached numerous locations in a short space of time, with easy dispersion and rapid population growth. . It should also be noted that there is the possibility of transmitting important diseases, both for sorghum and sugarcane and corn crops. Thus, studies on the bioecology of M. sacchari they must be urgently established as input for decision-making on the best control tactics for the affected regions, and surveys must also be carried out on the occurrence of this pest in all possible host crops.

control tactics M. sacchari  to be recommended, taking as a reference what has been adopted in other countries, consist of the use of resistant materials, the destruction of crop residues, the elimination of spontaneous grasses with the potential to host the pest, the establishment of a safe distance between the areas of sorghum cultivation and those cultivated with sugar cane, in improving application technologies suitable for reaching the target and, finally, in the study of the use of chemical and biological insecticides, considering the location of the pest and, also, selectivity, since natural enemies, such as predators and parasitoids, are highly associated with the species M. sacchari in agricultural areas, it is also of fundamental importance to study the adoption of different mechanisms of action of agrochemicals, with the purpose of reducing the selection pressure of active ingredients, given the possibility of developing resistant populations of this species.

Cecilia Czepak, Jácomo D. Borges, Vinicius O. Magalhães, Anderson de S. Zandomenighi, Lucas G. Santos, Leonardo Y. S. Schmidt and Murillo A. Gomes, School of Agronomy/UFG/Goiânia/GO; Regina M. S. Coelho, Maria da Gloria Trindade and Abmael M. de Lima Junior, LDB/Lannagro/Mapa/Goiânia/GO; Marcos Vieira, Accura - Integrated Technologies in Agribusiness