How to control stem fly in soybeans

Melanagromyza sp. is an exotic pest, important for soybeans in Asia and Oceania, also occurring in Europe and Africa. The larva bores into the soybean stem, reducing plant development and compromising grain production by up to 36%. In the 2014/15 harvest, the insect was detected in Brazil, in the states of Santa Catarina and Rio Grande do Sul.

In the 2014/2015 harvest, in harvest and off-season crops, members of the Integrated Pest Management Laboratory (LabMIP) at the Federal University of Santa Maria found the stem fly Melanagromyza sp. attacking soybeans in the municipalities of Iporã do Oeste and Riqueza, in Santa Catarina, and in the municipalities of Boa Vista do Buricá, Cruz Alta and Três de Maio, in Rio Grande do Sul. These occurrences were surprising due to the high level of infestation and the wide distribution in the region. With the occurrences confirmed in the field, the material was collected and the species of Melanagromyza (Diptera - Agromyzidae) present in soybeans was identified. This identification was made using morphological and molecular techniques, with the collaboration of Australian entomologists. The Ministry of Agriculture, Livestock and Supply (Mapa) was notified of the occurrence of the species on 1/7/2015. Before that, in the 1980s, Gassen et al (1985) had already found a species of the genus Melanagromyza in Passo Fundo, Rio Grande do Sul, and Link et al (2009) also reported the occurrence of stem flies in soybean crops in the municipality of São Francisco de Assis, also in the state of Rio Grande do Sul.

Also in Paraguay, Melanagromyza sp. it was found in the last harvest and reported by Benítez-Díaz (2015). In the South of Brazil, the occurrence of larvae, pupae and adults has been observed since the late cultivation of the 2014/15 harvest, with a higher incidence in the off-season of the same year. After the soybean harvest in the months of May, June, July, August and September 2015, stem fly larvae, pupae and adults were found in spontaneous soybeans in Rio Grande do Sul and Santa Catarina.

Description: Stem fly adults are tiny insects, measuring between 2mm and 3mm, black in color and with a metallic green abdomen. The eggs are not seen. The larvae are light yellow in color and translucent. Fully developed, they measure up to 4mm. The pupae are cylindrical, yellowish in color, turning brown and measuring 2mm in length.

Bioecology

Adult females oviposit an average of 170 eggs on the underside of developing leaves, close to the veins and at the base of the soybean leaflet and even on the single-leaf leaf. It prefers to oviposit on tender leaves, but ovipositions occur on newly opened trefoils of older plants (even flowered ones). The larvae hatch between two and three days after oviposition and begin feeding, initially consuming the leaf blade. They migrate to a vein in the leaf and continue boring towards the petiole and then towards the main stem of the plant. They go through three larval instars over a period of eight to 11 days. The pupal stage lasts between six and 12 days. The adults emerge inside the galleries and leave the plant through the emergence hole made by the larvae, before pupating. The egg-adult cycle can vary between 16 and 26 days, allowing more than one generation per culture. Insects are found on the same plant in all phases of the cycle, at the same time. In newly emerged plants, the larvae make descending galleries, close to the second and first nodes, eventually boring through the neck, up to the main root. Ascending and descending galleries occur depending on the height and period of infestation. Even in poorly developed, spontaneous or tiguerous plants, several infested larvae can be found in the later stages of the vegetative phase and even in the reproductive phase, both in the main stem and in branches close to it.

Insults

In the adult stage, the fly feeds on cellular fluids from the epidermal tissues, causing no harm to the culture. The larvae bore into the petiole and stems, creating galleries in the conductive tissues, mainly in the xylem, which can impair the flow of water and nutrients. A larva can produce a gallery measuring 5,9cm to 13,4cm long. Attacked plants may present a reduction in height, number of flowers, number of pods, number of grains, accumulated dry mass and reduction in nodulation due to Rhizobium.

Damage

Losses depend on the region or place of occurrence of the pest, the degree of infestation, the time of attack of the pest in relation to the phenological stage of the crop, the nutritional status of the plants, the soybean cultivar, the sowing time, the use of control measures, among other factors. In plants, losses result from the reduction in the number and size of nitrogen-fixing nodules, smaller plant height, leaf area, reduction in dry mass, number of grains and soybean yield. The first five weeks after seedling emergence is the period of greatest vulnerability of soybeans to Melanagromyza sp.. The combination of these factors can produce losses of 2% to 36% of soybean productivity. However, any estimate of losses for Brazil would be hasty, as the occurrence of the species in the extensive soybean areas of Brazil and neighboring countries is not known.

Detection

Confirmation of the occurrence of Melanagromyza sp. It is not possible to visualize the plant's external symptoms, except for the presence of adult exit holes. Nor is it possible to check the occurrence of eggs easily. The easiest way to detect the pest is by visualizing larvae and/or pupae or by the presence of galleries in the main stem and branches of the soybean. Sampling in crops must be done by pulling out soybean plants at random. Then, sectioning the plants with a transverse cut of the stem in the neck region and separating the aerial part from the roots. With the help of a pocket knife, the plant must have the main stem and side branches open longitudinally, from bottom to top, exposing the pith and internal tissues of the stem, making it possible to check the occurrence of larvae, pupae or empty puparium. At this point it is possible to count the attacked plants, the damage to the main and lateral stems and the exit holes. In spontaneous soybeans, sampling must be done by collecting isolated plants in or around crops, on roads or close to warehouses. The examination of these plants must follow the same procedures recommended for cultivated plants. 

Real Time

Monitoring is essential for detecting, quantifying and locating the occurrence of Melagromyza sp. in soybean cultivation. Soon after the emergence of soybeans, early occurrences of the pest can be seen. These occurrences are easily proven by examining the plants, presence of galleries and larvae and/or pupae. Later it is possible due to the presence of exit holes located below the insertion region of the cotyledons. In later attacks due to the presence of adult exit holes, above the cotyledonary region.

control methods

In regions of the world where the pest is of greater economic importance for soybean cultivation, the most important control measures are cultural control, with resistant cultivars, and sowing outside the pest's population peaks. Biological control is also mentioned as one of the strategies to reduce the impact of the pest. There are many references to the evaluation and use of insecticides applied to seeds or in foliar sprays. Under Brazilian conditions, seed treatment will certainly be the most adopted strategy to control the pest, as it is already widely used and due to the occurrence of the pest in the initial phase of soybean. The combination of seed treatment with foliar spraying is referred to as the most effective form of pest control.

Pest management perspectives

In Brazil, the action of Melanagromyza sp. on national cultivars, with also the impact of natural biological control and the effect of chemical control practiced for other pests of Brazilian soybeans. It is urgent and fundamental to evaluate the reaction of cultivars to Melanagromyza sp., to be added to other strategies and integrated into other measures to control this pest. These and other technical-scientific demands are already being researched by LabMIP - UFSM, in collaboration with colleagues from Australia and Paraguay.

Jerson Vanderlei Carús Guedes, Luis Eduardo Curioletti, Manoela Beche, Jonas André Arnemann, LabMIP - UFSM

Article published in issue 197 of Cultivar Grandes Culturas.