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Management of pests that attack corn roots

Corós, stink bugs and pinworms are among the pests that affect seeds and roots in corn crops

11.03.2020 | 20:59 (UTC -3)

Corós, stink bugs and pinworms are among the pests that affect seeds and roots in corn crops. Its management, in order to achieve efficiency, mainly involves preventive tactics, with precise monitoring of this group of insects even before the installation of the crop.

Corn plants can be attacked by pests from seed germination and plant emergence until their physiological maturation phase of the grains, these harmful organisms being made up of insects, molluscs, millipedes and mites. The problems begin with the presence of caterpillars in the cover to be dried for sowing corn and soil insects, which attack the seeds and roots of this crop, followed by surface pests that especially affect seedlings. Then come the caterpillars, which feed on leaves and cobs, and, finally, suckers such as aphids, stink bugs and whiteflies, which attack the leaves or the forming grains.

Seed and root pests

The pests that attack corn seeds and roots are normally underground insects belonging to different taxonomic groups, with Coleoptera and Hemiptera being the two main orders that cover this complex of organisms. This group of pests normally has a strong association with the soil where it occurs and can destroy corn seeds or roots, negatively affecting stand establishment, plant vigor and development and, consequently, crop productivity.

Among the pests that attack corn roots in the Central-West region, the following stand out: the underground rhizophagous larvae of melolontid beetles, also called coró, bolo-bolo or chicken-bread, the pinworm larva and the brown stink bug. root damage, which, although they can occur throughout the crop cycle, cause more severe damage in the initial stages of plant development. These pests normally have polyphagous eating habits, that is, they feed on several species of plants, whether cultivated or not.

Rhizophagous choirs

Rhizophagous choirs are larvae of coleoptera from the Melolonthidae family that have a white body, three pairs of thoracic legs and are positioned in a U-shape when at rest. Corós damage to corn is caused by the consumption of roots, resulting in a reduction in the plants' ability to absorb water and nutrients, ingredients that are essential for their development. This intensity of damage is greater in young corn plants, grown in low fertility soil, with dense layers and under water deficit conditions. Plants attacked by corós initially show reduced development, followed by yellowing, wilting and death, and these symptoms may occur in large patches distributed irregularly in crops. Under conditions of high coró infestation in the soil, up to 100% crop loss can occur, especially when the presence of more developed larvae coincides with the initial development phase of corn plants.

The choirs Liogenys suturalise Phyllophaga cuyabana These are species that have one generation/year (univoltine) and that traditionally occur in corn crops in Rio Grande do Sul, Paraná, Mato Grosso do Sul, Mato Grosso and Goiás. P. cuyabana is a species that can cause damage to both corn and soybeans, while L. suturalis it is associated only with grasses such as corn, wheat and oats. Flights of adults of these species occur during the months of September to November. After mating, the eggs are placed in the soil, where the immature stages of the insect fully develop. The larvae have three stages of development (instars) and, at the end of the third stage, they enter the pre-pupal stage, when they no longer feed and have low mobility.

In the states of Goiás and Mato Grosso, the species of coró has also been found Liogenys fusca which has great potential for damage to both corn and soybeans. After completing their cycle, the adults leave the ground and fly in the months of September and October, coinciding with the first rains in the region. In late sowing or in the cultivation of “off-season” corn, the damage is greater, since in this period there is a predominance of 2-year-old larvae.^o and 3^ourges that are more voracious. The larvae of 3^o instars are mobile in the soil and, at the beginning of the dry period, they go deeper to 20cm and 30cm, where they build their pupal chamber. When they stop feeding, they clean their abdomen and transform into pupa inside the pupal chamber between the months of July and August. Adults remain in the soil for approximately 30 days, waiting for the presence of ideal humidity for their emergence. After the first rains, between the months of September and October, the flocking and laying phase begin again.

Other species of corós of lesser economic importance such as Diloboderus abderus ou Cyclocephala sp. can eventually be observed in association with corn, especially in the southern region of the country.

Brown root bug

In Brazil, there are records of the occurrence of brown stink bugs in several Brazilian states, although there is a more pronounced incidence in the Cerrados region. Attacks by these insects normally occur in large fields in corn crops, with outbreaks of infestation of up to 70 hectares being observed. The damage results from the continuous suction of sap in the corn roots, which can lead to the weakening or even death of the plants. The different species of host plants on which the brown root bug feeds have different degrees of susceptibility to its attack. The village et al (2009) found that cotton was the species most susceptible to feeding by S. castanea, followed by soybeans, corn, sorghum and rice. These bugs predominate in sandy soils, especially those with degraded pasture. Symptoms of attack on plants depend on the intensity and time of occurrence of the pest in the crop, ranging from wilting and yellowing of leaves to underdevelopment and drying of the plant, which can cause severe crop losses.

The presence of brown root bugs in crops is easily recognized by the strong smell that these insects exude when the soil is moved in infested areas. In Brazil, the main species of brown stink bug associated with corn crops are Scaptocoris castanea, S. ruai e S. buckupi. The village et al (2009) found that at least two species of brown stink bug occur in Mato Grosso do Sul: Scaptocoris castanea e S. agorai. The first species was found in soybean, cotton and corn crops and the second in pasture areas. In recent years, severe infestations of brown root bugs have also been observed, especially in direct planting systems in the Brazilian Cerrado. In Goiás, flocks of this pest begin during the rainy season during the month of November and persist until March, a period in which there is a predominance of adults in the soil.

In Mato Grosso there is an occurrence of the species S. castanea in all regions of the State. The species S. agorai It was identified in the Eastern region, in areas of soybeans, corn and cotton. The most recent information on brown root bugs was obtained by Nardi (2006), who described the occurrence of wing polymorphism in S. agorai. This study revealed the existence of individuals with short (brachypterous) and long (macropterous) wings, highlighting the importance of these morphological changes for the colonization of new areas, since flocking, carried out only by macropterous adults, is the only form dispersion of these insects over long distances.

During periods of the year with greater humidity, this insect remains in the most superficial layers of the soil, but, in drier conditions, it moves to lower layers, reaching depths beyond 1,5m.

pinworm

Like the coró, the pinworm larva (Diabrotica speciosa) feeds on corn roots. The larvae of this pest are whitish in color, but have a dark brown or black sclerotized spot on the head and anal plate. These larvae feed especially on the adventitious roots of corn. The loss of these roots reduces the plant's ability to absorb water and nutrients, making them less productive, as well as more susceptible to disease and damping-off. Fallen plants appear curved, characterizing the symptom known as “goose neck”. Although these plants, at the time of harvest, may contain developed ears of corn, these are generally not harvested by the harvester platform.

Management of pests that attack corn roots

For the control of pests that attack the underground part of corn plants to be effective, it is necessary to accurately monitor this group of pests even before installing the crop, since all control tactics to be implemented are preventive. Both for the management of corós and the brown stink bug, it is of fundamental importance to carry out soil sampling, with the aim of evaluating the species present, their population level and the predominant development stages of these insects.

Among the techniques that can be used to control corós and brown stink bugs, the following stand out: manipulating the sowing time, preparing the soil with appropriate implements and applying insecticides to the seeds or spraying them in the sowing furrow. As coró adults are normally strongly attracted to light, the use of light traps during the emergence period of soil insects serves to capture a significant number of adults at night and thus contribute to reducing their infestation in subsequent crops. The application of insecticides to corn seeds and sowing furrows is a promising alternative for managing corós in corn crops, especially in conservation systems, such as direct planting. In the case of the brown stink bug, insecticides applied to the seeds have not proven to be an efficient tactic. However, spraying the planting furrow with chemical insecticides, especially when the stink bug is located close to the soil surface, can provide good control of the pest, depending on the product and dose used. In work carried out with vegetation cover in Mato Grosso, during the off-season, a reduction in the brown stink bug population was observed in cover containing sunn hemp, Crotalaria spectabilis, in relation to areas planted with millet, sorghum and brachiaria. In successive cotton plantings, a reduction in the brown root bug population was also observed after three years, showing that this crop interferes with the development of this insect. Biological control of the brown stink bug using entomopathogenic fungi may also be a promising alternative. Xavier and Ávila (2006) identified four isolates of Metarhizium anisopliae, that provided levels of control of S. agorai greater than 80% under laboratory conditions. However, the efficiency of this fungus in controlling the brown stink bug under field conditions has not been evaluated.

Chemical control of kitty larvae must also be preventive. However, treating seeds with insecticides typically does not protect the corn root system from chigger attack. This happens because during the period in which the larvae cause damage to the corn (more than 30 days after emergence), the plants no longer show residual effects from the products applied to the seeds. Some insecticides, when applied in granulated form or as a spray in the sowing furrow, have been shown to be effective in controlling cowpea larvae. It should be noted that there are currently suitable kits for applying insecticides in the furrow, both in spray form and in the form of granules, with the insecticide mixture or granulated products being applied concomitantly during the sowing operation.