Occurrence and management of stink bugs in second-crop corn

Corn production in extensive areas can favor the incidence of pests that use the plants for food, reproduction or shelter, which can cause significant losses to the crop if not controlled. Among the corn pests, phytophagous bugs stand out, which are considered the main entomological problem in second-crop cultivation (“safrinha”), especially when these pests occur during the initial stages of plant development.

The green-bellied bug, Diceraeus melacanthus, is considered a pest in several crops such as soybeans, wheat and corn. The eggs of this species are light green in color and are usually laid on the leaves of corn and weeds. The insect goes through five nymph stages until reaching the adult phase. The adult D. melacanthus It is generally light brown in color on the dorsal region and green on the abdominal region, hence the name green-bellied bug. The total development cycle of the green-bellied bug, from egg to adult, lasts about 29 days, and 3 to 4 generations can occur during the second-crop corn cultivation period.

The first occurrence of the green-bellied stink bug in corn crops in Brazil was in 1993, in the municipality of Rio Brilhante (MS). After this report, this pest was recorded in corn or wheat crops in the states of Paraná, São Paulo and, later, in other states of the Brazilian savannah where corn was grown. In the southern region, especially in the state of Rio Grande do Sul, another species of green-bellied stink bug occurs (Diceraeus furcatus), which presents small morphological differences when compared to D. melacanthus, but which has the same potential for damage to corn crops.

The green-bellied stink bug causes significant damage to corn seedlings by inserting its stylets, present in the sucking mouthparts, which penetrate through the sheath to the internal leaf primordia that are still present in the stem of the seedlings. After these leaves unroll from the plants, they present deformations such as perforations, waterlogging, tillering of the plant and even its death. This causes a reduction in the size of the plants and the stand, as well as in the productivity of the crop. This damage is greater the younger the stage of the attacked corn plants, especially in drought conditions, which makes it difficult for the plants to recover after the stink bug attack.

Similarly, the brown stink bug (Euschistus heros) and the green stink bug (Nezara viridula), which are traditional soybean pests, can attack second-crop corn similar to the green-bellied stink bug. However, with less capacity to cause damage.

The potential for damage by stink bugs at different development stages of second-crop corn has been studied at the field level, demonstrating that stages with 1 to 3 leaves are the most susceptible to attack by these pests. Corn plants with 4 to 5 leaves are considered moderately tolerant to attack by stink bugs, while plants with 6 or more open leaves are already considered tolerant to attack by both D. melacanthus how much of E. heroes e N. viridula. From these last stages of development onwards, it is not recommended to carry out further actions to control stink bugs in corn crops. Knowledge of the degree of tolerance of the different stages of corn development to stink bugs can guide the producer on the most appropriate time to carry out control of these pests in the crop.

Adult stink bugs are most active during the day, during cooler hours of the day, such as morning or evening, while during the hottest hours of the day the insects are usually sheltered and protected under straw or in the shade of weeds. The green-bellied stink bug has a close association with weeds, especially with the species Carrapicho grass (Cenchrus echinatus), rhodes grass (Chloris gayana), mombaça grass (Panicum maximum), trapoeraba (Commelina benghalensis) and bittergrass (Digitaria insularis) (Carvalho, 2007). In these weeds, the green-bellied stink bug can develop and complete its life cycle.

Sampling of the green-bellied stink bug was carried out in Mato Grosso do Sul, at around 12:50 p.m., in three environments: dry weeds, green weeds and corn plants. In all the samples taken, using a 50 cm x 179 m iron frame as a sampler, 88 insects (adults + nymphs) were found in the environment containing green weeds, while 4 and XNUMX insects were found in the environments containing corn and dry weeds, respectively. Given these facts, we can infer that good control of these weeds in the corn cultivation area represents an important alternative for managing the green-bellied stink bug in the crop.

The annual population fluctuation of the green-bellied stink bug was determined in corn crops in Mato Grosso do Sul, finding that the greatest abundance of this pest occurs between the months of March and May, a period that coincides with the beginning of the second-crop corn cultivation season in the region.

Studies of artificial infestation of green-bellied stink bugs in screened cages containing corn seedlings were conducted to determine the level of economic damage and, consequently, the level of control of these pests under field conditions. The results showed that the level of control of stink bugs in corn was 0,8 insects (adult + large nymph) per m2, while in another study it was 1 stink bug for every 10 corn plants. These control levels are only a guide for the producer to decide whether or not to apply insecticides to the crops. However, depending on the development stage of the corn plant, the expected productivity level, the climatic conditions as well as the level of moisture present in the soil, adjustments are necessary for this decision-making. Fertile soils, without chemical and physical restrictions associated with the use of nitrogen as top dressing favor the recovery of corn plants attacked by stink bugs.

For efficient management of stink bugs in corn crops, it is essential to monitor these pests before and after planting the crop. An alternative to reducing the population of stink bugs surviving in the early stages of corn development would be to apply insecticides by spraying at the end of the soybean cycle, more precisely when it is drying out for harvest. The population of the green-bellied stink bug increases significantly at the end of the soybean cycle, because these insects “know” through temporal learning that after the cultivation of this legume comes the cultivation of corn and wheat, which are more suitable hosts for their development. The insecticides used to control stink bugs in soybeans are generally effective for use in sprayings carried out at the end of the cycle of this crop. However, care must be taken to choose a product that has an adequate withdrawal period between this spraying and the soybean harvest.

Another very useful and efficient alternative for managing stink bugs in corn is seed treatment with insecticides, especially those belonging to the neonicotinoid group. This control tactic can significantly reduce the stink bug population in the early stages of corn development, especially when the population of these pests is not very high in the area. In conditions of high stink bug populations at the end of the soybean cycle, the use of seed treatment alone in corn does not guarantee an adequate level of control of these pests.

To control stink bugs in corn, producers can also use the Plant and Apply (PA) tactic. This control strategy consists of applying the insecticide by spraying it immediately after sowing the corn in the area. This control method is based on the movement of stink bugs that occurs on the straw immediately after sowing the corn, since the planting operation disturbs the environment where the stink bugs were previously housed and thus begin to move around the area. However, for this control tactic to be effective, it is necessary to spray it no more than 1 hour after planting. In longer periods after sowing the corn, the stink bugs return to the cultivation environment, generally remaining associated with and protected by the weeds, which reduces the control efficiency. Likewise, insecticide applications on the stubble of harvested soybeans, before corn planting, are also ineffective for controlling stink bugs due to the umbrella effect caused by weeds and volunteer soybeans that develop after harvest.

Another efficient control strategy is spraying with insecticides during the early stages of crop development. We must be aware of the need to protect corn plants from stage V1 (one open leaf) to V5 (five open leaves). Corn plants with seven or more open leaves are already tolerant to stink bug attacks and therefore there is no longer any need to control these pests. Based on this, it is recommended that corn be sprayed 3 and 7-8 days after plant emergence (DAE), periods that cover stages V1 to V5 of the crop. For controlling stink bugs when spraying corn seedlings, the same products used to control these pests in soybean crops are recommended.

Carvalho (2007) found natural biological control of stink bugs in corn crops through egg parasitoids, mainly by telenomus podisi (Hymenoptera: Platygastridae) and adult parasitism by Hexacladia smithii (Hymenoptera: Encyrtidae). Unfortunately, these parasitoids have not been commercially available for use in flood biological control in corn crops.

By Credit José Ávila (Embrapa Agriculture West); Luis Henrique M. Fragoso e John Vitor Klein Dapont (Federal University of Greater Dourados)

Article published in issue 287 of Cultivar Grandes Culturas Magazine