In seedlings and stems: soybean pests

The expansion of soybean agricultural frontiers in Brazil also ended up impacting pest dynamics. This is the case of insects that attack seedlings, stems and petioles, whose increasing numbers have been observed in recent harvests. Soybean anteater, elasmus caterpillar, fall armyworm, slugs and snails and snake lice are among the challenges that require attention and adequate management from producers

With the expansion of soybean cultivation into new agricultural regions, an increasing number of pests have been observed that attack seedlings, stems and petioles of soybean plants. According to Hoffmann-Campo et al (2012), the emergence of these new pest organisms in soybean agroecosystems was due to their adaptation to this crop, in the absence of native hosts or as a consequence of a selective action of broad-spectrum chemical products used to control defoliators and suckers in the crop. Soybean seedlings and stems can be attacked by different groups of pests in the initial phase of crop development, with these harmful organisms consisting basically of insects, mollusks and millipedes.

soy anteater

The soybean weevil or anteater, Sternechus subsignatus Boheman, as it is popularly known, is a species in which both adults and larvae can cause damage to soybeans. In order to feed, adults scrape and shred the tissues of the main stem and, occasionally, the lateral branches and petioles of the leaves, while larvae are endophytic, that is, they feed inside the main stem, more precisely in the pith. When the attack occurs early in the development of the plants, the apical bud can be reached and the damage is irreversible, resulting in the total shredding of the main stem, causing a reduction in the plant population. When the attack occurs later and the larvae lay their eggs and develop on the stem, a gall of very fragile tissue is formed that can break due to the action of wind or heavy rain, causing the death of the plant and, consequently, a reduction in the stand. The insect develops on a limited number of hosts, and its diet is restricted to just a few species of legumes.

Elasmus caterpillar

The elasm caterpillar, Elasmopalpus lignosellus Zeller is another pest that can damage young soybean plants, especially when the insect is already present in the crop or in the cover crop to be desiccated (e.g., wheat, oats) before sowing the soybean. The insect is considered polyphagous, that is, it feeds on several species of cultivated, wild and weedy plants, especially grasses and legumes. The adult lays its eggs on soybean plants, in the soil or on crop residues present in the area. After hatching, the larvae initially feed on organic matter or scrape off plant tissue and then penetrate the plant's collar, a little below ground level, where they build an ascending gallery. Near the entrance hole in the plant, the larvae weave a cocoon made of excrement, plant residues and soil particles, symptoms that characterize the presence of the pest in the area. The same caterpillar can attack up to three soybean plants during its larval stage, with the period from emergence to 30-40 days of plant development (up to stage V2-V3) being the phase of the crop most susceptible to pest attack.

As a consequence of elasm damage, soybean plants initially wilt and later dry out, due to the obstruction of water and nutrient transport from the soil to the aerial part of the plant. When the soybean plant is more developed and has a more lignified stem, the caterpillar feeds only on the external part, leaving visible external scars from the insect's injury; In this region, a callus can form with fragile tissue, which can easily break due to the action of the wind. The intensity of elasm damage in soybeans is greater and more frequent in conditions of high temperature and water deficit in the soil, especially in sandy or mixed soils carried out in conventional planting, especially in areas of first cultivation, as occasionally occurs in the Cerrado region.

fall armyworm

Damage to soybean seedlings caused by the fall armyworm, Spodoptera frugiperda, occur when this pest is already previously present in the crop used as cover and which will be desiccated for planting soybeans, adapted to the no-till production system. In general, grasses used as cover for straw production in the no-till system (e.g. millet, oats, wheat, brachiaria, etc.) can promote the development of high populations of S. frugiperda, especially in the driest periods of the year. If soybean planting is carried out immediately after the desiccation of this cover, the caterpillars of S. frugiperda present in the area, if not controlled, can cut the soybean seedlings close to the ground or feed on their foliage, causing their death and, consequently, a reduction in the crop stand. Although S. frugiperda has been the main pest found in these conditions, other species such as S. eridania, S. cosmioides and even the cutworm, Agrotis ipsilon, may occasionally occur. During the day, these caterpillars are normally sheltered under straw or clods, coming out to feed on soybean seedlings on cloudy days or at night.

Slugs and snails

Slugs and snails are mollusks of the Gastropoda class, which are most commonly found in humid and cool environments. Slugs have a naked body, but snails carry a layer or shell of calcium carbonate on their backs, which provides them with shelter and protection. These organisms are very sensitive to dehydration and during dry periods they remain inactive, buried in the soil or under the straw of crops planted with direct seeding. These pests are more abundant in soils with a high amount of straw or organic matter and are strongly associated with plants from the legume and cruciferous groups (e.g. beans, soybeans, vetch, forage radish, milkweed, etc.). Both slugs and snails scrape the tissue of the stem, cotyledons or even the leaves of soybean seedlings, causing injuries similar to those caused by insects, and can destroy the apical portion and cause its death, thus reducing the crop stand. In Mato Grosso do Sul, the species of snails and slugs found in soybean crops were identified, respectively, as Drymaeus interpunctus (Mollusca: Bulimulidae) and Sarasinula linguaeformis (Mollusca: Veronicellidae).

 

Snake lice

Snake lice are organisms belonging to the Diplopoda class and are characterized by having a cylindrical body divided into several segments (from 20 to 100 segments). They have two pairs of legs on each segment of the body, a characteristic that differs from arthropods of the Order Quilopoda, known as earwigs and centipedes, which have only one pair of legs on each segment of the body.

Snake lice normally occur in areas with an abundance of straw, dead organic matter and living plant tissue, as is prevalent in areas where direct planting is carried out. These pests are concentrated in the soybean sowing furrow line, where the soil is looser due to the action of the furrower and straw chopper, and can periodically penetrate the surface layers of the soil. When disturbed, they protect themselves by retracting and curling their body forming a flat spiral; They show greater activity at night and take shelter under straw during the hottest hours of the day, with damage being more severe when the attack occurs in the initial phase of crop development. Snake lice feed on dead organic matter and young living plant tissue, damaging soybean seeds in the germination phase or emerging in the soil, as well as newly emerged seedlings, ingesting parts of the cotyledons or young leaves, which can kill plants and cause a marked reduction in stand in crops. The best-known species of snake lice belong to the Julidae family, being Plusioporus e Julus the most abundant genera in soybean crops in the Center-South region of Brazil.

Other pests that attack seedlings and stems

Eventually, other insect pests can attack soybean seedlings and stems, depending on the growing region, such as the soybean mealybug, Myochorus armatus, and from the little clod, Aracanthus morei. The soybean bug has been frequently observed in soybean crops in Rio Grande do Sul, Paraná, São Paulo and Mato Grosso do Sul. The adult bug has the same color as the soil in which it lives, because the soil particles adhere to its body when it takes shelter under clods and dry leaves, which explains the origin of its popular name. When present in soybean plants, when touched or disturbed, the insect drops to the ground, remaining motionless as if “pretending” to be dead. In the past, the soybean bug was considered a secondary pest of soybeans, but in recent years, its population level has increased significantly in soybean crops in Paraná, causing concern among technicians and farmers. The soybean bug’s injury is characterized by small cuts on the edges of the leaves and cotyledons, giving them a serrated appearance.

Management of pests that attack seedlings and stems

To manage the soybean anteater, before planning the next crop, samples should be taken from the plots where attacks by the pest were observed in the previous crop. This sampling should preferably be done in the off-season, between the months of May and September, by digging trenches in the soil over the rows of soybeans from the previous crop. When examining the soil sample, the number of hibernating larvae should be counted. If two to six larvae/m2 of anteater soil are found, the soybeans should be replaced in the area with a non-host crop such as corn, cotton, sorghum, sunflower, millet, Crotalaria juncea or black velvet bean, where the insect does not develop, which will consequently interrupt its biological cycle. To increase pest control efficiency, this plot of non-host plants for the insect should be surrounded by a strip of preferred host plants, such as soybeans, beans, lab-lab and dwarf pigeon pea, which will act as trap crops, attracting adult anteaters when they emerge from the soil in the adjacent area. At this time, the adults should be controlled periodically with chemical insecticides to prevent them from spreading to other crop areas. As a complementary measure, the trap crop can be destroyed with a brush cutter or Triton during the flowering phase to eliminate any anteater larvae that may be developing in these plants. With this procedure, the producer “cleans” the anteater from the problem area, and can then plant soybeans in this area normally in the following harvest. When there is no insect in the area, but the neighbor does, anteater control can be carried out through insecticides applied to soybean seeds (example: fipronil, thiamethoxam), planning a strip of treated plants on the edge of the crop of 2m to 40m to contain the adults that reach the crop.

In the case of the elasm caterpillar, it has been proven that well-distributed rain, during the first 30 days of crop development, practically eliminates the insect's infestation in soybean crops. Spraying insecticides on the aerial part of soybeans has provided low efficiency in controlling the elasm caterpillar, due to the position in which the pest is lodged on the plant. Seed treatment with systemic insecticides (example: fipronil, imidacloprid + thiodicarb and chlorantraniliprole) can be used in areas where this pest has traditionally been a problem or that require reseeding.

The beginning of integrated pest management (IPM) in soybean crops occurs when the species used as cover for straw production in the direct planting system desiccates. On this occasion, it is necessary to ask a question: are there caterpillars in the covering to be dried? If not, spraying should be carried out aiming only to desiccate the coverage with herbicide and not add a “smell” of insecticide, as is normally done by the producer. If there are caterpillars on the cover, another question needs to be asked: will soybean sowing in the area be carried out immediately after desiccation or is it possible to wait to carry it out? Delaying sowing after desiccation, by around 20 days, even with caterpillars on the cover, in the absence of food, after the effect of the herbicide, the caterpillars can pupate or die. However, if there are caterpillars on the cover and sowing is carried out shortly after desiccation, it is recommended to apply an insecticidal spray product that has little or no effect on natural enemies, such as the insecticides thiodicarb, methomyl, chlorantraniliprole, flubendiamide, spinosad and insect growth regulators (physiological).

To control slugs and snails, metaldehyde-based products and prior desiccation of the covering infested with these pests are suggested. This is an auxiliary measure to reduce the survival of these pests, since such an operation reduces the humidity and water content on the soil surface, in addition to extinguishing the food source. Preliminary work carried out by the Coamo cooperative, in Campo Mourão, Paraná, showed that the mixture of abamectin + whole milk, placed in corn kernels constituted an effective bait for controlling snails in soybean crops. It is therefore suggested that the application of insecticides or baits in soybean crops to control slugs and snails be carried out at night, a period in which these pests show greater activity due to favorable humidity and temperature conditions and, therefore, more vulnerable to the action of chemicals.

Snake lice control can be carried out, with relative success, by applying insecticide to the seeds or spraying the plants. The most effective active ingredients for snake lice control belong to the carbamate and phenylpyrazol groups. When spraying on soybeans to control snake lice, it is suggested that they be sprayed at night, the period in which these pests are most active, using fan-type spray tips in a high volume of spray ( minimum of 200L/ha).

By Credit José Ávila (Embrapa Agriculture West) and Germison Vital Tonquelski (Chapadão Foundation)

Article published in issue 186 of Cultivar Grandes Culturas Magazine