Challenges of the floodplain

The area planted with soybeans has grown steadily in the south of Rio Grande do Sul over the last five years. In 2012 there was growth of approximately 100 thousand hectares in the region's rice farming areas and between 200 thousand hectares and 250 thousand hectares in pasture areas, due to factors such as the diversification of production and escape from fluctuations in the price of rice. For the 2013/2014 harvest, according to Emater, the forecast is for an increase of 6,22% in the area planted with the crop in the State. However, soybean cultivation suffers from excess water and inadequate management can ruin the entire investment.

The secret is to drain the land, so that the water is controlled, as soybeans cannot withstand 48 hours with their roots in water. Planting soybeans in areas that traditionally produce rice has the advantage of being that, if there is a drought, the soybeans can be irrigated and their productivity will increase. Furthermore, planting oilseeds cleans

pest lands such as red and black rice, leaving them suitable for new rice crops.

In the lowlands, the incorporation of soybeans into the rice production system can improve soil fertility, reduce weeds and bring, where large areas previously remained fallow, the possibility of agricultural use and obtaining financial resources for the producer.

The soybean crop is subject, throughout its cycle, to attack by different species of insects. Although these pests have their populations reduced by predators, parasitoids and diseases, at levels dependent on environmental conditions and the management practiced, when they reach high populations, capable of causing significant losses in crop yield, they need to be controlled.

The elasm caterpillar or collar borer (Elasmopalpus lignosellus) is one of the examples. This insect prefers sandy soils, the problem being greater in dry years. The caterpillars are small (± 2 cm) and penetrate the plant just below the soil. The plant dies or becomes weakened resulting in stand failures. In recent harvests, there has been a greater occurrence of elasm in soybeans, causing failures in the plant stand, especially when planting is carried out on November 15th, with low soil humidity and high temperatures.

The armpit borer (Crocidosema aporema ) is a very small caterpillar (± 1 cm) that joins the leaflets and penetrates the stem, causing abnormal development and death of plants. Late cycle cultivars are more affected than early cycle cultivars. The greatest difficulty in control lies in the fact that the insecticide does not reach the biological target.

The soybean caterpillar ( Anticarsia gemmatalis ) is found in all cultivation areas, being the most common defoliator of soybeans in Brazil. It usually attacks crops from November onwards, in the northern regions of Paraná, and from December to January in the south of the country, causing defoliation, which can reach 100%. The caterpillar has a general green color, with white longitudinal streaks on the back. In conditions of high population, or food scarcity, it becomes dark, maintaining white streaks.

The false measure (Pseudoplusia includens) moves by measuring hands. It does not consume the ribs and is difficult to control, with cases of resistance having already been recorded. High temperatures, above 30 °C, and humidity below 60%, increase the evaporation of chemicals, making control difficult.

In recent years there has been a greater occurrence of a complex of caterpillars of the genus spodoptera, which feed on pods, grains and leaves and can cut plants at ground level, in addition to the increase in the population and importance of false caterpillars and black caterpillars and a reduction in the population of soybean caterpillars (which accumulated decades of dominance in Brazilian soybeans).

In Rio Grande do Sul, in the 2007/08 and 2008/09 harvests, among the false caterpillars, the predominance of Rachiplusia naked, about Pseudoplusia includens, which was expected to be the most common (GUEDES et al., 2010). Such changes point to the survival of caterpillars that are more tolerant to the insecticides used on soybeans and, possibly, in many cases larger doses would be needed to control the false caterpillars and Spodoptera eridania than to control soybean caterpillar.

The survival of caterpillars from these two groups demands other applications that end up putting even more pressure on the species most susceptible to the doses used and, therefore, contribute to reducing the proportion of soybean caterpillars (less tolerant to insecticides) in the total number of caterpillars.

Alert for Helicoverpa armigera

More recently, the occurrence of caterpillars from the Heliothinae subfamily, which attack crops of agronomic importance such as soybeans, cotton, corn, beans and tomatoes, has caused great concern to producers, mainly in the North, Northeast and Center-South regions of Brazil (ÁVILA et al .2013). In Rio Grande do Sul, the occurrence of H. armigera. However, this is a problem that can arise at any time, considering the border areas and the insect's great mobility. Have been observed causing damage to these crops Heliothis virescens (Fabricius), Helicoverpa zea (Boddie) and Helicoverpa armigera (Hübner). H. armigera is a species that until recently was considered an A1 quarantine pest in Brazil, but has recently been detected in the states of Goiás, Bahia and Mato Grosso, associated with cotton and soybean crops, being the first record of occurrence on the American continent ( CZEPAK et al., 2013).

In relation to bedbugs, at least 15 species of the Pentatomidae family are recorded as suckers, with the species Nezara viridula (L.), Piezodorus guildinii (Westwood) and Euschistus heros (Fabricius) are the most important of the soybean stink bug complex.

N. viridula e E. heroes are also associated with corn, causing damage similar to that of Dichelops melacanthus. These stink bugs have increased their population each year, occurring at all corn planting times, often requiring treatment with insecticides on the seeds and foliar sprays to reduce damage.

There were changes in the occurrence of species compared to what was recorded in the 1970s. At that time, in some municipalities in Rio Grande do Sul, N. viridula, E. meditabunda and P. guildinii were the most frequent species, although Link (1979) found that E. heroes was adapting to soybean cultivation.

The change in the pest occurrence scenario is related to inadequate management practices, such as: lack of pest sampling (population and location); lack of knowledge and consideration of the most sensitive phases of culture; use of “cheirinho” with broad-spectrum insecticides, after the emergence of soybeans; lack of knowledge of soybean caterpillar species; errors in identifying pest species, implicating

mistakes in insecticide doses and escape of the most tolerant species; insecticide spraying together with the application of herbicides, therefore “outside” the appropriate population moment, determining the loss of product action before the caterpillar peaks; spraying insecticides together with the application of fungicides, therefore, an application that is often late to bed bugs and mites; use of high doses, with a negative effect on natural enemies; use of insufficient doses for the target pest, allowing its survival, new growth and reinfestation; errors in the choice of insecticides for the most important target pests.

This article was published in issue 175 of Cultivar Grandes Culturas magazine. Click here to read the edition.