Damage and behavior of Helicoverpa zea and Helicoverpa armigera in corn

One of the factors that can affect corn production lies in the incidence of insect pests, capable of causing damage to the crop. In recent years, agricultural producers have faced this type of problem, with emphasis on caterpillars of the genus helicoverpa who are intensely attacking corn cultivation, regardless of whether the plants are transgenic or conventional. Two species of caterpillars in this genus have caused damage to corn: Helicoverpa zea e Helicoverpa armigera.

The biological development of these pests goes through the egg, caterpillar, pupa and adult stages. The life cycle of H. zea lasts around 40 to 45 days. The adult of this insect is a grey-green moth with a wingspan of approximately 40mm, with yellow-brown forewings, with a darker transverse stripe, and dark spots scattered over the wings. The hindwings are lighter, with a band on the outer edges. The female oviposits on any part of the plant, but her preference is for stigmata. The eggs are spherical with lateral protrusions and laid individually. After 3 to 4 days, caterpillars hatch, which have a variable color, from green to black with longitudinal stripes. As they develop, they penetrate the interior of the ear and begin to destroy the forming kernels. They develop over a period of 13 to 25 days. At the end of the larval period, the caterpillars leave the cob and go into the soil, where they transform into pupa. This period lasts approximately 10 to 15 days depending on the temperature. Afterwards, the adult emerges, this phase lasting 12 to 15 days. Each female has the capacity to oviposit an average of 1000 eggs.

The life cycle of this insect varies from 35 days to 75 days, depending on the temperature. Laying is carried out preferably on the adaxial surface of the leaves or on flowers, fruits and terminal shoots with pubescent surfaces in isolation or in small groups of eggs; normally during the night. Newly oviposited eggs have a color that varies from yellowish-white with a shiny appearance (it has the shape of longitudinal ribs) to dark brown close to hatching. Egg incubation lasts 3 days at a temperature of 25°C and up to 11 days at low temperatures. In the larval stage H. armigera passes through six distinct instars. The larval period can vary from 14 to 21 days. The caterpillars have variable coloring. Pupae are usually found in the soil, but can also be found in corn cobs. It has a mahogany brown color and a rounded surface on the terminal parts. The pupal period is approximately 10 to 14 days. However, the pupal stage can last for several months, as this insect can enter diapause due to weather conditions. The adult males of H. armigera They have greenish-gray forewings with a slightly darker band in the distal third and a small dark spot in the center of the wing. Females have yellowish front wings. On the hind wings it is possible to see a brown border at the apex, in addition to being lighter. The size of adult insects is approximately 35mm to 40mm. Adults live for approximately 9,2 to 11,7 days, with females living longer than males. A female can lay up to 3.000 eggs and oviposition occurs between 2 and 6 days after emergence.

The caterpillars of H. zea they attack the stigmas, preventing fertilization and, as a result, failures will appear. They feed on milky grains, destroying them and finally, the holes left by the caterpillars in the ears, during the pupal stage, facilitate the penetration of microorganisms that can cause rot. The caterpillars of H. armigera They feed on buds, inflorescences, fruits, pods, ears, leaves and stems, causing injuries in both the vegetative and reproductive phases of several species of plants of economic importance. H. armigera e H. zea They are extremely polyphagous insects, with the ability to develop and feed on a wide range of hosts, such as nightshades, legumes, fruit trees and vegetables. This characteristic tends to favor the success of this pest. Furthermore, H. armigera it has high local mobility of caterpillars, migration of moths and high fecundity. These biological characteristics allow this pest to survive in unstable environments and adapt to seasonal climate changes.

In the 2013 corn harvest, a large outbreak of corn caterpillars was recorded. H. armigera in the main producing regions. Perhaps the largest ever recorded since the beginning of the commercialization of genetically modified corn seeds (Bt) in Brazil. In the region of Campo Mourão, in the west of Paraná, a high incidence of caterpillars was observed. helicoverpa spp., causing damage to corn plants Bt. In some regions of the states of Mato Grosso and Mato Grosso do Sul, several safrinha corn producers also reported the occurrence of corn caterpillars. helicoverpa spp. with a certain preference of the insect being observed for materials that presented ears with clear stigmas. It is worth mentioning that the worldwide loss caused by the attack of caterpillars H. armigera, in different crops including corn, reaches 5 billion dollars annually. It is estimated that losses caused by H. zea exceed 10%  (although still unproven).

Importance of corn (box)

Corn is the third most cultivated cereal in the world. Its economic importance is due to the different ways in which it is used, ranging from animal feed to the high-tech industry. Approximately 70% of world corn production is used for animal feed, with this figure reaching 85% in developed countries. Therefore, 15% of all world production is destined for human consumption. In addition to its economic importance, corn cultivation is important for the viability of other crops through crop rotation (CIB, 2006). In Brazil, the annual area cultivated with corn is approximately 15,5 million hectares and Brazilian production reaches 80,5 million, which places the country as third in the world ranking of harvested area (FAO, 2013).

 The article appears in issue 189 of Cultivar Grandes Culturas.