In pods and grains: pests that attack soybeans

Soy is one of the main Brazilian export products, and this legume is cultivated from Rio Grande do Sul to the extreme North and Northeast of Brazil. It also presents prospects for expanding cultivation into new agricultural frontiers in the coming years, and is one of the main foreign exchange-generating products for the country.

After or even during the season when defoliating caterpillars occur in soybeans, phytophagous sucking arthropods or boring caterpillars begin to appear, which can damage the pods and/or grains of the crop. To feed, stink bugs insert their stylets into different plant structures, although grains, in the filling phase, are their preferred locations. In addition to stink bugs, some species of caterpillars, which feed on soybean leaves, can also bore into pods in formation or when they are already fully formed, thus reducing crop productivity.

Phytophagous sucking bugs

Phytophagous pentatomid bugs (Figure 1) are considered the main entomological problem in soybean cultivation. The brown stink bug (Euschistus heros), the small green stink bug (Piezodorus guildinii) and the green stink bug (Nezara viridula) are the three most abundant species that occur in culture, in the Central-South region of Brazil. The intensity of damage from these stink bugs varies according to the species and population density of the insect, as well as the stage of development of the plants, with the small green stink bug being P. guildinii the most harmful, while the brown stink bug E. heroes It is the one that causes the least damage to the crop, although its population has been considerably higher than that of other species.

Adult bed bugs E. heroes They are dark brown in color and have two lateral extensions in the prothorax region (one on each side) in the form of sharp spines and a white spot in the shape of a “crescent” at the end of the scutellum. The yellow to beige eggs are deposited on soybean leaves or pods, arranged in two to three parallel rows, containing five to eight eggs per clutch. The nymphs, although they begin feeding in the 2nd instar, only cause damage to soybeans from the 3rd instar onwards. Up to three generations can occur during the soybean cycle, dispersing after harvest from early cultivars to later soybean plots or to other adjacent crops, such as cotton. During the off-season, they enter into diapause in the straw from the previous crop or nearby. The adult of the small green stink bug, P. guildinii, measuring approximately 10mm in length, is light green in color, which may turn yellowish at the end of its life. It typically presents a reddish-brown transverse stripe on the dorsal region of the thorax, close to the head. The eggs are dark in color and always arranged in double rows, containing 11 to 15 units per clutch, deposited in the pods, leaves, main stem or side branches. Studies have shown that the small green stink bug is more damaging to the quality of soybean grains and seeds and causes greater leaf retention than other species of stink bugs that attack the crop. The green stink bug, on the other hand, N. viridula, is a species that traditionally occurs in the states of Paraná, Santa Catarina and Rio Grande do Sul. Adults measure 12mm to 17mm, are green in color and have reddish spots on the last segments of the antenna. The eggs, which initially have a yellowish color, are deposited on the underside of soybean leaves in a hexagonal shape, containing 80 to 100 eggs per clutch. The 1st and 2nd instar nymphs are gregarious, while the 3rd instar nymphs abandon their gregarious habit and begin damaging the pods and soybean grains.

The colonization of soybean plants by stink bugs begins in the middle or end of the crop's vegetative period, or right at the beginning of flowering. At this time, stink bugs are emerging from diapause or alternative hosts and migrating to soybeans. With the beginning of the reproductive period, from the appearance of pods, the populations of these insects, especially eggs and nymphs, increase, reaching high levels between the end of pod development and the beginning of grain filling, a period in which soybeans is more susceptible to attack. The population grows until the end of grain filling, when it reaches its population peak, normally with soybeans at physiological maturity. From then on, the population tends to decrease and, at harvest, the remaining stink bugs disperse to alternative host plants and, later, to diapause (straw) niches, in the case of the brown stink bug.

The damage caused by stink bugs to plants is caused by the introduction of their mouthparts (stylet) into the pods, which can reach the grains or developing seeds, with this damage being irreversible after certain population levels. The attacked grains become smaller, wrinkled, shriveled and darker in color than normal, and may present with diseases such as yeast spot, caused by the fungus. Nematospora corily, transmitted during feeding. Attacks at the R3 and R4 stages can favor pod abortion, while at the pod filling stages (R5) they can negatively affect both crop yield and the quality of the grains or seeds produced, and cause changes in protein and of Oil. In addition to the direct damage, a severe attack by stink bugs on soybeans can cause physiological disturbances in the plant, which, as a result, causes the appearance of leaf retention and/or green stems, a phenomenon known as “crazy soybeans”, which delays and/or makes harvesting soybeans difficult.

Other species of bed bugs, such as Dichelops melacanthus, D. furcatus, Meditating Edessa, Thyanta perditor e Neomegalotomus parvus They may eventually attack soybeans, however, they do not reach populations that harm the productivity and quality of soybean seeds, although their damage to plants is added to that of the other three species mentioned above.

Caterpillars that attack pods and grains

In addition to stink bugs, there is a complex of lepidoptera that, although they feed on leaves and other parts of soybean plants, can also damage pods and grains and, consequently, reduce crop productivity. Among the main species of caterpillars that exhibit this behavior, the following stand out: Spodoptera eridania, S. cosmioides, Heliothis virescens, Maruca vitrata e Helicoverpa armigera (Figure 2).

The separation of species of spodoptera has been very difficult due to the great specific variability and occurrence of synonyms in this genre. S. eridania (Figure 2A) is a species of increasing importance in the Cerrados region, as it attacks soybean crops, causing defoliation or destroying pods. The eggs, flat round in shape, are deposited on plants and covered with moth scales, with the female being able to oviposit 800 to two thousand eggs during her life cycle. The caterpillars, which can reach 50mm in length, are dark brown in color and have a longitudinal yellow stripe on the body, which is interrupted by a dark spot on the thorax. These caterpillars are most frequently found in the bottom of soybean plants (lower third) and are more active at night, which is the most appropriate time to control them. Already S. cosmioides (Figure 2B) is a species that attacks a large number of hosts including cotton, vegetables, legumes, cereals, fruit trees and forests. The eggs are deposited on soybean leaves, normally in overlapping layers of brown color, similar to egg laying. S. frugiperda. The caterpillars of the last instars have a yellowish-brown head, with golden spots on the back, distributed in two longitudinal orange lines. Damage to soybeans is similar to that caused by S. eridania.

H. virescens is a long-existing species in Brazil, its caterpillar popularly known as the cotton bollworm (Figure 2C) and which traditionally attacks cotton, soybean and tomato crops. The cylindrical eggs, yellowish in color and equipped with longitudinal striations, are deposited singly on soybean leaves. The caterpillars vary in color from green, pink to yellowish, have dark spots on the back and microspines at the base of the hair insertion, which gives them a rough texture when touched. They preferentially feed on soybean pods, although they may eventually cause defoliation in the crop.

The soybean pod borer, Maruca vitrata, is considered a seasonal pest in soybean crops, its occurrence being associated with climatic factors, especially in periods of drought with high temperatures. The light yellow eggs with a slightly reticulated chorion are deposited at night, especially in flowers, flower buds, petioles and pods. The caterpillars, bright yellow to light brown in color, have very evident segmentations on the body, with dark spots and hairs. They bore into soybean pods, stems and petioles, and can eventually damage inflorescences, and when they are in the most advanced stages of development, the caterpillars can penetrate the pods or stems of the soybean plant and feed on the its content. The damage caused by this pest is difficult to perceive, but can cause plant failure due to the attack on the main stem. Observation in the field can be done with longitudinal cuts in the stem of the attacked plants.

H. armigera (Figure 2D) is a species that until recently was considered an A1 quarantine pest in Brazil. Its official detection was carried out in 2013, in the states of Goiás, Bahia and Mato Grosso, associated mainly with cotton and soybean crops, this finding being the first record of the pest's occurrence in the Americas. The eggs of H. armigera They are yellowish-white in color and have a shiny appearance immediately after their deposition on the substrate, but they turn dark brown close to the moment the larvae hatch. The apical portion of the egg is smooth, however, the rest of its surface is carved in the form of longitudinal ribs. The larval period of H. armigera it is completed with the development of six distinct instars. The first larval instars feed in the most tender parts of the plants, where they can produce a type of web or even form a small cocoon. As the larvae grow, they acquire different colors, ranging from straw yellow to green, with brown stripes laterally on the thorax, abdomen and head. The caterpillars of H. armigera They can feed on leaves and stems of soybean plants, but they prefer reproductive structures, such as flower buds, pods and grains in formation and even dry ones, causing deformation or rot in these structures or even their fall. This inherent ability to H. armigera causing damage to the reproductive parts of the crop, in association with its ability to attack a large number of hosts, is a factor that increases the status of economic importance of this pest.

Management of phytophagous sucking bugs

The control of sucking bugs in soybean crops begins at the R3 stage, that is, right after the formation of the "knives", which are the beginnings of pod development. However, the management of stink bugs in the crop must begin with the strategies used to control initial pests and defoliating caterpillars. Therefore, the use of selective control tactics, which preserve natural enemies (predators, parasitoids and pathogens) in the vegetative phase of the crop, will contribute to the establishment of biological balance in the agroecosystem, providing positive impacts on the management of stink bugs in the reproductive phase.

In soybean stages that are susceptible to stink bug attack (after R3), control must be carried out based on the action levels determined by the research, which is two stink bugs per meter of plant row for grain crops and one stink bug per meter of row for crops destined for seeds. To do this, bedbugs must be monitored through sampling using the beat cloth (Figure 3). This inspection of the crop must be carried out at least once a week, from the beginning of pod development (the “knife” phase), until physiological maturity (R7) at different points of the crop, intensifying sampling on the borders , where insects typically begin colonizing soybeans. When sampling, it is important to identify the young forms of bed bugs (nymphs) which, from the third instar onwards, must be recorded together with the adults. Simple visual observation of soybean plants does not express the real population of stink bugs that may be occurring in the area. In general, early cultivars escape stink bug damage. However, when they multiply in these cultivars, they disperse to mid- and later-cycle cultivars, where they can cause the greatest damage. The sowing time influences the population dynamics of stink bugs, and very late plantings should be avoided, where the highest concentrations of these insects occur.

The scarcity of active ingredients for bed bug control and the abusive use of products in crops have led to high outbreaks of these pests and selected populations resistant to chemical insecticides. So that these problems do not intensify, it is recommended that the same insecticide is not used in the same area repeatedly or in doses greater than those recommended.

During the colonization period, when stink bug populations are concentrated on the edges of the crop, control can only be carried out in these marginal areas, avoiding the spread of insects throughout the crop. Several insecticides are recommended by the Entomology Committee of the Central Brazilian Soybean Research Meeting (RPSRCB) for stink bug control (www.cnpso.embrapa.br). In addition to efficiency, the selectivity criterion, that is, the effect of products on natural enemies, must also be considered when choosing. Occasionally, during the months of October and November, high populations of phytophagous stink bugs can be observed in the vegetative phase of soybeans. These infestations do not cause significant damage to the crop, therefore there is no need to control the stink bug.

In very dense soybean crops, such as those that currently exist, insecticides applied by spraying may not reach stink bugs due to the phenomenon known as the "umbrella effect". Under these conditions, the use of table salt (NaCl) at a concentration of 0,5% in the insecticide solution (500g for every 100L of water) can increase bed bug mortality by at least 25%, when compared to areas applied without salt. . The salt has a arresting effect on the bed bug, causing it to remain on the treated surface for longer, which intensifies its contamination. Salt is non-volatile, so it does not attract stink bugs from neighboring areas, as some growers have feared in the past.

Several species of parasitoids are normally found in soybean crops, acting on phytophagous stink bug populations. Among egg parasitoids, the following species stand out: Trisolcus basalis, which occurs in the state of Paraná, and telenomus podisi, which predominates in the Central-West region of Brazil. Already Hexaclady smithii is a parasitoid of bed bug adults, and has already been found in the states of Paraná and Mato Grosso do Sul. The sensitivity of these beneficial insects to insecticides is high, and they are often completely decimated from crops when broad-spectrum products are applied.

Management of caterpillars that attack pods and grains

Similar to what was discussed for phytophagous stink bugs, outbreaks of caterpillars that attack soybean pods and grains are closely associated with biological imbalances in the agroecosystem. These imbalances are caused by the abusive use of agrochemicals (insecticides, fungicides and herbicides), especially during the vegetative phase of the crop, which destroy the natural enemies (predators, parasitoids and pathogens) that normally keep these pests under control. An integrated management program, which seeks to prevent the emergence of caterpillars that destroy soybean pods and grains, should prioritize the use of selective products for the complex of natural enemies of pests in the vegetative phase of soybeans, to the detriment of broad-spectrum products. action as pyrethroids and organophosphates. In addition, insecticide applications to control pests, especially in the vegetative phase of soybeans, must always be carried out in accordance with the action levels determined by research. The implementation of these two actions will contribute to intensifying the natural biological control of caterpillars that attack pods and grains in the soybean agroecosystem, reducing their occurrence and abundance in the reproductive phase of the crop. However, the use of chemical insecticides may be necessary, in certain situations, to control these caterpillars, in which case selective or even biological products should be used, following the action levels. In the case of pod borer, it is recommended to carry out chemical control when approximately 5% to 10% of pods are found to be attacked by this pest, and sampling should be carried out in at least one point per hectare, counting the intact pods and with caterpillar damage in the sampling area. Grigolli et al (2013) evaluated the effectiveness of chemical insecticides in controlling caterpillars Maruca vitrata in soybean cultivation, noting that the products chlorpyrifos, chlorantraniliprole + lambdacyhalothrin and teflubenzuron were those that performed best in controlling this pest.