Control of pests that attack corn seedlings

With the expansion of corn cultivation to other agricultural regions in Brazil, the number of pests that attack seedlings also increased. This is the case of the elasm caterpillar, thread caterpillar, green belly bug, leafhopper, thrips, slugs and snails. Seed treatment, combined with other management strategies, is among the main tools for controlling these insects, capable of resulting in severe losses when not managed properly.

Corn plants can be attacked by pests from seed germination and plant emergence to the physiological maturation phase of the grains, these harmful organisms being insects, molluscs, millipedes and mites. These pests are classified as being of primary, regional or secondary importance, depending on their frequency of occurrence, scope and the potential damage they can cause to the crop.

The grain production systems of the Central-West region constitute a favorable environment for the establishment of pests, as corn cultivation prevails in extensive areas both in the summer and in the off-season, in addition to the cultivation of a cover crop between crops. winter and summer. With the expansion of corn cultivation to new agricultural regions, an increasing number of pests that attack seedlings have been observed. The emergence of these new pest organisms in corn agroecosystems can be explained by their adaptation to this crop, in the absence of native hosts or as a consequence of a selective action of broad-spectrum chemicals used to control defoliators and suckers in the crop.

Elasmus caterpillar

The elasm caterpillar, Elasmopalpus lignosellus Zeller, is a pest that can damage young corn plants, especially when the insect is already present in the covering to be dried (e.g. wheat, oats) for sowing the crop. The insect is polyphagous, that is, it feeds on several species of cultivated, wild and weed plants, especially grasses and legumes. The adult lays on corn plants, in the soil or in cultural remains present in the area. After hatching, the larvae initially feed on organic matter or scrape the plant tissue and then penetrate the plant neck, slightly below soil level, where they build an ascending gallery. Close to the entry hole into the plant, the larvae weave a cocoon made up of excrement, plant debris and dirt particles, symptoms that characterize the presence of the pest in the area. The same caterpillar can attack up to three corn plants during its larval phase, with the period from emergence to 30 days of plant development being the phase of the crop most susceptible to attack by the pest. As a consequence of elasm damage in corn plants, the symptom called “dead heart” appears, characterized by the wilting of the central leaves, which easily detach when pulled, causing obstruction in the transport of water and nutrients from the soil to the aerial part. of the plant. In these cases, tillering may also occur, which makes the plant unproductive. The intensity of elasm damage in corn 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, as occasionally occurs in the Cerrado region.

Screwworm

Several species of thread caterpillar occur in culture, being Agrotis ipsilon the most frequent. The name thread caterpillar comes from the habit that this insect has of curling up, taking on the appearance of a thread when touched. In general, the corn seedling is attacked by the caterpillar when it is up to 50 cm tall, manifesting, according to Cruz et al. (1987) three different symptoms: a) Initially it shows partial sectioning of the culm and, when the lesion is large, it causes the symptom known as “dead heart”; b) appearance of spots similar to those caused by mineral deficiencies, when the lesion is small or c) tillering of the plant, making a “clump” unproductive.

Green belly bug

the species Dichelops melacanthus e D. furcatus are reported as constituents of the soybean secondary pest complex in several regions of Brazil. However, in 1993 the occurrence of D. melacanthus causing damage to corn seedlings in the municipality of Rio Brilhante, MS (Ávila & Panizzi, 1995). Since then, species D. melacanthus e D. furcatus, whether occurring simultaneously or not, have been found in corn crops in Brazil. The insect has a brown dorsal part and a green ventral part, hence the name green belly. The eggs, which are bluish-green in color, are laid on corn leaves or even weeds. During feeding, these bugs normally position themselves in the longitudinal direction of the plant, with their head oriented towards the neck region. If, during the feeding process, the apical meristem is damaged, the central leaves of the seedling wither and dry, manifesting the symptom called “dead heart”, and tillering of the plant may also occur. When the apical meristem is not damaged, the first leaves that unfold from the cartridge have transverse whitish streaks, often with perforations with a yellowish halo, originating from the punctures that the insect made when it fed at the base of the still young plant. When the leaves of the cartridge are unable to unfold, they give the plant a “wrapped” appearance. The damage caused by the green-bellied stink bug to corn has been more pronounced in the off-season, as the insect multiplies in the summer, at the end of soybean cultivation.

In addition to the green-bellied stink bug, other species of stink bug such as euschistus heros  e nezara viridula They may eventually attack corn seedlings, but with less capacity to cause damage than the green belly.

Slugs and snails

Slugs and snails are molluscs from the Gastropoda class, which occur most frequently in humid and cool environments. Slugs have a naked body, but snails carry a layer or shell made of calcium carbonate on their back, which provides shelter and protection. These organisms are very sensitive to dehydration and in dry periods they remain inactive buried in the soil or under the straw of crops planted in direct seeding. These pests are most abundant in soils with a high amount of straw or organic matter and have a strong association with plants from the legume and cruciferous groups (e.g. beans, soybeans, vetch, radish, milkweed, etc.). The eggs of slugs and snails are generally laid in large numbers in cracks in the soil or under plant debris in the process of decomposition. Both slugs and snails feed on the leaves of corn seedlings, with injuries similar to those caused by insects. In Mato Grosso do Sul, the species of snails and slugs found in corn crops were identified, respectively, as Drymaeus interpunctus (Mollusca: Bulimulidae) and Sarasinula linguaeformis (Mollusca: Veronicellidae).

Gut

The thrips, Frankliniella williamsi, are small yellowish insects often found among the leaves of corn seedlings that are still curled. Damage caused by thrips is frequently seen during periods of drought and when conditions of low relative humidity and high temperature prevail after plant emergence. Due to the scraping of the leaf blade, the leaves appear yellowish, whitish or silvery and, in conditions of high infestation, this can affect crop yield.

Corn leafhopper

The corn leafhopper, dalbulus maidis, is a small insect whose adults measure around 3 mm and vary in color, from yellow to straw. As they belong to the Cicadellidae family, the tibia of the third pair of legs has a row of spines; They are very agile and show the behavior of moving laterally when disturbed. The importance of this leafhopper in corn cultivation lies in the fact that it is a vector of the disease called “stunting”, associated with two pathogens known collectively as mollicutes, whose incidence has increased in recent years, especially in off-season corn crops. Corn plants with symptoms of this disease appear reddish-brown (Fitoplasma) or light green (Spiroplasma), resulting in the production of small ears, often with little or no grains. The earlier the plant is infected by the pathogen, the greater the reduction in corn productivity.

Management of pests that attack corn seedlings

In the management of the elasm caterpillar, it has been proven that well-distributed rain, during the first 30 days of crop development, practically eliminates the insect infestation in crops. In the direct planting system, which provides better soil moisture conservation, this pest has occurred at a lower intensity when compared to the conventional system. Spraying insecticides on the aerial part of corn has provided low efficiency in controlling the elasm caterpillar (50%), due to the position in which the pest is lodged on the plant. Treating corn seeds with systemic insecticides has proven to be an efficient tactic for controlling the elasm caterpillar. For the control of the caterpillar, it is suggested to carry out high-volume insecticide applications, with the sprays directed at the neck of the plants, preferably at dusk, using the same products recommended for controlling the elasm caterpillar.

Green-bellied stink bug control can be carried out preventively, using insecticides via seeds or spraying on the crop. Work carried out in Embrapa Agricultural West showed that the damage level for controlling the green-bellied stink bug in off-season corn is less than 1,0 insect for every five corn plants in the crop. The recommended insecticides, in spray form, for the soybean phytophagous stink bug complex are normally efficient in controlling the green-bellied stink bug in corn. Before sowing corn, it is recommended to inspect the area where the crop will be planted to check for the presence of stink bug nymphs and adults, to assess whether or not it is necessary to treat the seeds or even carry out spraying with insecticide shortly after corn emergence. The period of greatest care for the stink bug is during the initial phase of plant development, when corn is most susceptible to insect attack, that is, from the emergence of the plant to the appearance of the fourth open leaf (V₄) which corresponds to an age between 15 days and 20 days after plant emergence.

For the control of slugs and snails, metaldehyde-based products are suggested, but in addition to being expensive, they are not practical for use in large areas. Previous desiccation of the cover infested with slugs and/or snails is an auxiliary measure to reduce the survival of these pests, since this 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 quirelas constituted an efficient bait in controlling snails. However, it should be noted that, to date, there is no registration of products for the control of snails and slugs in corn crops. In addition, it is suggested that insecticide or bait applications on corn crops to control slugs and snails be carried out at night, a period in which these pests are most active due to favorable humidity and temperature conditions and, thus, greater vulnerability to the action of chemicals.

To control thrips, systemic insecticides applied to seeds provide good initial protection against the attack of this pest in corn. Likewise, spraying plants using “shock” insecticides can efficiently control thrips, especially when mineral oil is added to the insecticide mixture. To control the corn leafhopper, seed treatment can protect plants in the initial phase of the crop.

Crébio José Ávila, Embrapa Agropecuária Oeste - Dourados, MS; Silvestre Bellettini, UENP/CLM - Bandeirantes, PR

Article published in issue 202 of Cultivar Grandes Culturas.