Corn aerial pest management

In addition to soil and seedling pests, corn crops can be attacked by defoliators, borers and even suckers that feed on the aerial part of the plants. Pasture leafhopper, fall armyworm, sugarcane borer, earworm, grass leafworm, stink bug and corn aphid are among the insects that require control measures to minimize damage

Several species of insects are associated with corn cultivation, however relatively few are considered key pests, have regularity of occurrence, consistency in geographic range and potential to cause significant damage to the crop. The behavior of insect pests in corn varies according to the phenological stage of the crop, soil and climate conditions, cultivation system and local biotic factors related to their survival.

Aerial pests

In addition to soil and seedling pests, the crop can be attacked by defoliators, borers and even suckers that feed on the aerial part of corn plants. At these stages of development, the control measures used are curative in nature. The control of insects that attack the aerial part of corn has been carried out basically by spraying insecticides on the plants. In the case of defoliating caterpillars or borers, the use of transgenic varieties Bt It also constitutes an excellent management alternative.

Grasshopper leafhopper

The pasture leafhopper complex that attacks the aerial part of corn is basically represented by two species, Deois flavopicta e Zulia Entreriana. The species D. flavopicta It measures around 10mm in length, is black in color, with three yellowish bands on the wings, two transverse and one longitudinal in the alar region called clavus. When the wings are at rest, the claves are close together, forming an image similar to the letter “V”. The species Z. entreriana It measures approximately 7mm in length and is shiny black in color, with a white stripe at the end of the wing.

Temperature and humidity conditions conducive to the development of these leafhoppers, combined with the increase in areas with pastures such as Brachiaria decumbens in the vicinity of corn crops, there are positive predispositions for the occurrence of high populations of the insect. Under these conditions, insects can migrate intensely to the corn crop, where they land on the leaf blade of the plants or take shelter between the stalk and the sheath and inside the cartridge. The damage is caused exclusively by adults, who, when sucking the plant, inject a toxin that blocks the flow of sap. The symptoms of the attack are characterized by leaf spots that evolve into a well-defined, generalized chlorosis, with the onset of senescence. Younger plants are more sensitive to attack by these leafhoppers.

fall armyworm

The fall armyworm, Spodoptera frugiperda, is considered the most important pest of corn crops in Brazilian conditions. Laying is done on the leaves, in masses of approximately 50 eggs to 100 eggs, for a total of 1.360 per female. After three days of laying, the little caterpillars hatch and feed on the eggshells (chorion) that gave rise to them. The larval period lasts around 23 days, with the caterpillars reaching around 40mm in length in the last of their six instars. In the second instar, the caterpillar migrates to the corn cartridge, where it completes its development. More than one newly hatched caterpillar can be found per cartridge. However, due to cannibalistic behavior, only one developed caterpillar is normally located in each cartridge. However, it is possible to find individuals of different instars in the same cartridge, but separated by leaf blades.

The attack on corn can occur from the seedling stage to stalking and stalking. The small caterpillars begin by scraping the leaf blade, preferably the youngest leaves, causing the symptom known as “scraped leaves”. From there, they attack all the central leaves in the region of the cartridge, which, under more severe damage, can be completely destroyed. In later attacks, individuals can be found between the stalk and the ear, where they destroy the straw and some grains. In conditions of high population density of the pest in the straw where the corn crop will be installed, the caterpillar can pierce the neck of young plants, similar to the attack of the screwworm, and cause the death of the leaves of the cartridge, sometimes leading to , to tillering. The pupal period lasts about ten days in summer.

Relatively prolonged periods of drought favor the establishment and resurgence of high population levels of fall armyworm in corn. In regions with continuous corn cultivation, the pest occurs in high abundance throughout the year. Up to 30 days of crop development, up to 15% reduction in production or 34% after flowering can be observed.

Sugarcane borer

The sugarcane borer, Diatraea saccharalis, is a pest that traditionally occurs in sugarcane cultivation, but which can often also attack the aerial part of corn. The caterpillar penetrates the stalk of corn plants and feeds inside, creating galleries. Apparently, direct damage is not important, as the injured plant can produce normally. However, under the action of strong winds, the plant can fall over and the cob, when it comes into contact with the soil, favors the germination or rotting of the grains, in addition to no longer being captured by the harvester.

earworm

The adult earworm, helicoverpa zea, is a moth with a wingspan of approximately 35mm. A female can oviposit around a thousand eggs, placed anywhere on the plant, although the moths prefer the hairs (stigma-style) of the ears. The eggs measure around 1mm in diameter, are white at the beginning and brown towards hatching. After the incubation period, of three to five days, the caterpillar hatches with a brown head and the rest of the body white. Initially the caterpillars feed on the new “hairs” and then migrate inland to the apex of the cob, where they consume the newly formed grains. Close to pupation, the caterpillar leaves the cob, leaving an exit hole in the straw, heading to the soil, where it transforms into a pupa, which lasts, on average, 14 days.

Capinzais Curuquerê

The caterpillars of the grass curlew, Mocis latipes, are of the palm-sized type, presenting a yellowish color with dark brown longitudinal streaks. At their maximum development, they measure around 45mm in length and have a prominent head, with yellowish streaks. This pest feeds on corn leaves, consuming only the leaf blade from the edges, leaving only the central vein. The insect should receive greater attention from 60 days to 80 days of cultivation, a phase in which corn is very sensitive to defoliation.

Corn bug or "gaucho"

Corn bug adults, Leptoglossus zonatus, measuring approximately 20mm in length, are dark brown in color, with two circular yellow spots on the pronotum. The hemihelytra (forewings) have a zigzag yellow transverse stripe and the hind tibiae have a leaf-shaped expansion. Adults and nymphs insert the stylet into the corn grains to suck them and the puncture practiced for feeding purposes becomes a gateway for the penetration of pathogenic fungi. Their action, interacting with the direct damage resulting from the pest's feeding, causes failure of the ears, wilting and rotting of the grains. Insects normally prefer ears of open straw, which have more exposed seeds.

corn aphid

Corn aphid colonies, Rhopalosiphum maydis, they are normally seen inside the cartridge and on the tassel of the plants, where they continuously suck the sap. In corn cultivation, this insect multiplies easily, but generally does not assume economic importance. Under conditions of very high infestation, in the pre-flowering period, economic losses may occur.

Corn aerial pest management

Controlling the grasshopper can be carried out by treating the seeds with insecticides, when the insect occurs in the initial stages of crop development, or by spraying plants at a later stage.

The control of caterpillars that attack the aerial part of corn is normally carried out by spraying insecticides on the plants. In addition to organophosphate insecticides, pyrethroids, physiological insecticides and diamides provide good control of this group of pests, depending on the dose of the product used.

Sugarcane borer control is not normally carried out. In areas close to sugarcane fields and subject to frequent attack, the use of low-growing varieties minimizes the problem. In situations of very high infestations, the use of the same insect growth-regulating lizardicides, as suggested for the control of fall armyworm, reduces possible damage.

Transgenic plants that exhibit insecticidal activity constitute an important alternative for the management of insect pests in corn crops. The transgenic corn plant with insecticidal activity is known worldwide as corn Bt, by expressing a toxin (insecticide) isolated from the bacteria Bacillus thuringiensis (Bt), which is specific especially to Lepidoptera larvae.

In Brazil, several events Bt were released for commercialization, expressing different toxins such as Yieldgard, Herculex and Agrisure, also combined with tolerance to herbicides. The main target pests of this technology include the fall armyworm (Spodoptera frugiperda), the earworm (helicoverpa zea) and the sugarcane borer (Diatraea saccharalis) and even inch-long caterpillars (Mocis latipes) and the elasm caterpillar (Elasmopalpus lignosellus). The control efficiency of these species is quite high, and the application of chemical insecticides for their control can often be dispensed with. However, for the toxin Bt presents insecticidal activity, it must be ingested by the caterpillar and, therefore, the producer may notice some damage to the corn leaves, such as the symptom of scraped leaves.

The cultivation of corn Bt in large areas, it could condition the selection of biotypes resistant to the toxins of these plants. Therefore, it is extremely important to implement refuge areas in order to avoid or delay the development of resistance. Under Brazilian conditions, the refuge area basically consists of sowing 5% to 10% of the area cultivated with corn Bt, with hybrids not Bt, similar in size and cycle to the first. This refuge area cannot be more than 800m away from the transgenic plants, to allow the surviving adults from the corn area to crossbreed. Bt with those emerged in the refuge area. Monitoring caterpillars on both plants Bt as well as the refuge is extremely important, as depending on the cultivar used and the intensity of the infestation, additional control measures may be necessary. In the refuge area, caterpillar management must be implemented using other control methods and avoiding the use of bioinsecticides based on Bt.

The application of chemical insecticides in spray form to control fall armyworm must be done using fan nozzles (8002, 8004, 6502, 6504), with the jet directed towards the plant's cartridge. The volume of spray to be applied will depend on the stage of development of the crop, using 200L/ha to 300L/ha for plants up to 30-40 days old. Rainy periods in the initial phase of crop development tend to minimize the problems caused by the fall armyworm in corn, either by dropping the plant's eggs or by drowning small caterpillars. Several natural enemies are cited as important agents of natural control of the fall armyworm, particularly predators of caterpillars and eggs, parasitoids of caterpillars and eggs, in addition to entomopathogenic microorganisms such as fungi and viruses.

In situations of high infestation of corn bugs and aphids, control can be carried out using systemic phosphorus insecticides or a mixture containing pyrethroid and neonicotinoids, applied by spraying.

Crébio José Ávila, Embrapa Agropecuária Oeste; Geraldo Papa, Unesp - Ilha Solteira Campus

Article published in issue 199 of Cultivar Grandes Culturas.