Corn leafhopper: challenges in managing corn stunt

By Crébio José Ávila (Embrapa Western Agriculture); Charles Martins de Oliveira (Embrapa Cerrados); Suélen Cristina da Silva Moreira (Chapadão Foundation); Rodolfo Bianco (Paraná Institute for Rural Development); and Marco Antonio Tamai (Bahia State University)

24.09.2024 | 16:00 (UTC -3)
(A) symptoms of red stunt; (B) symptoms of pale stunt - Photos: Charles Martins de Oliveira
(A) symptoms of red stunt; (B) symptoms of pale stunt - Photos: Charles Martins de Oliveira

The cigarette Dalbulus maidis (DeLong & Wolcott) (Hemiptera: Cicadellidae) was until recently considered a secondary problem in corn, but since 2015 it has become one of the main pests of this crop. Thus, high populations of this insect have been recorded in the last corn harvests in different producing regions, especially in second-harvest crops.

Although the nymphs and adults of D. maidis Although corn leafhoppers can suck the sap from corn plants, the greatest losses are related to the ability of this insect to efficiently transmit pathogens that cause diseases in the crop. The corn leafhopper is capable of transmitting pathogens known as phytoplasma and spiroplasma mollicutes, which cause red and pale stunt, respectively. These stunts are currently considered the main phytosanitary challenges in the corn production chain in Brazil, which can result in significant losses in yield and in the quality of the grains produced in the crop.

Taxonomic and bioecological aspects

The corn leafhopper is a sucking insect with four rows of spines on the tibiae of its hind legs. When adult, it is straw-yellow in color and has transparent wings. Adults have two easily visible circular black spots on the back of their heads, between their compound eyes, a characteristic that makes them easy to identify in the field. The insects also have the behavior of moving laterally over the plant when disturbed and are preferably found inside the corn casing.

Females, which are generally larger than males, lay endophytic eggs in the mesophyll of corn leaves, preferably near the central vein. The eggs are shaped like a grain of rice and are initially transparent, becoming milky as they develop, and project tufts of white microfilaments externally. The insect has five nymphal instars and the egg-adult cycle lasts between 15 and 27 days, depending on the temperature and humidity of the environment.

(A) Detail of adult at rest; (B) adults present in the corn plant cartridge
(A) Detail of adult Dalbulus maidis at rest; (B) adults present in the corn plant cartridge
(C) Adults in copulation: female above (larger) and male below (smaller); (D) egg removed from corn leaf
(C) Adults in copulation: female above (larger) and male below (smaller); (D) egg removed from corn leaf
(E) Microfilaments that form 48 to 72 hours after lays; F) nymph
(E) Microfilaments that form 48 to 72 hours after laying D. maidis; F) nymph of D. maidis

Causal agents, transmission and damage

Corn stunt (red and pale) are systemic diseases caused by pathogens that colonize the conducting vessels (phloem) of corn, causing physiological, hormonal and biochemical disorders, which are reflected in foliar symptoms and deformations of the vegetative and reproductive structures of the plants.

The mode of transmission of mollicutes by the corn leafhopper is persistent and propagative. Persistent, because after acquiring the pathogens from the plants, the insect remains infective for practically the rest of its life, and propagative, because the pathogens multiply and circulate in the body of the insect vector. After acquiring these pathogens (mollicutes), the leafhopper needs 3 to 4 weeks to be able to transmit them to the plants, a period of time called the latent period.

The adults and nymphs of D. maidis can cause direct damage by continually feeding on the sap present in the phloem of plants, although this type of damage is minor compared to that caused by the transmission of pathogens that cause stunting. When present in high populations, the leafhopper can suck the sap of plants and excrete a sugary substance resulting from its feeding, called “honeydew”. This substance, when expelled by the insect, favors the proliferation of fungi of the genus capnodium which cause darkening of the corn leaf blade. The occurrence of sooty mold dries out the corn leaves due to greater absorption of solar heat, which negatively interferes with the plant's photosynthesis, thus affecting the crop's development and productivity.

Symptoms caused by mollicutes in corn are related to impaired translocation of photosynthetic compounds, changes in nutrient absorption, as well as hormonal and biochemical disorders in plants. Symptoms include the presence of chlorotic or reddish leaf spots, presence of axillary shoots, basal tillering, reduction in plant size due to shortening of internodes, presence of multiple ears with reduced size, poor formation of ears, thus making plants unproductive. Another common symptom resulting from infection by mollicutes in corn is the accumulation of sucrose in the stalk, which favors the entry of pathogens, which causes proliferation and colonization of fungi of the genera Pythium e Fusarium, especially at the base of the stem, which can cause plant lodging, particularly in susceptible cultivars.

Infographic indicating the set of agricultural practices that should be adopted, at a regional level, for the management of and corn stunt - Source: Ávila . (2021)
Infographic indicating the set of agricultural practices that should be adopted, at a regional level, for the management of Dalbulus maidis and corn stunting - Source: Ávila et al. (2021)

Corn leafhopper management

Corn leafhopper management is relatively complex, requiring the use of different strategies to reduce the damage caused by the insects and the pathogens they transmit. In order for producers to cope with corn stunt, it is essential to adopt a set of good agricultural practices that must be implemented at the regional level. It is important to emphasize that no single control measure is 100% effective for the proper management of corn stunt, and therefore requires the adoption of integrated preventive control actions.

Corn is the only known host plant in Brazil in which D. maidis feeds and reproduces. However, the leafhopper can use other grass species for food or shelter, such as wheat, sorghum, millet, colonial grass, brachiaria and elephant grass, especially when these species are grown near corn crops. During the off-season, with the absence of corn plants in the field, the populations of the corn leafhopper are drastically reduced. On the other hand, the availability of tiguera plants or volunteer corn plants in the off-season is an essential condition for the maintenance and multiplication of the insect vector, the mollicutes and, consequently, for the spread of stunting. The presence of tiguera corn in soybean crops in summer cultivation greatly increases the possibility of the occurrence of D. maidis in the second corn crop and, consequently, stunting. Studies conducted at the MS Foundation in the 2022/2023 harvest showed that 70% of the tiguera corn plants present in the soybean crop had mollicutes in their tissues. Thus, the elimination of tiguera corn plants in the soybean crop is one of the most important strategies to reduce the incidence of stunting in second-crop corn.

The use of corn genotypes resistant or tolerant to mollicutes is considered another efficient strategy for managing corn stunt. However, the corn varieties or hybrids available on the market have not shown satisfactory resistance or tolerance. Some hybrids express symptoms but remain productive, while others show symptoms and have their productivity drastically reduced. Hybrids can present rates of 10% to 60% of plants with incidence of stunt, and the use of susceptible cultivars is necessary when the populations of D. maidis are high in the corn-producing region, significant losses in crop production may occur.

The control of populations of D. maidis In the initial phase of corn cultivation, it is essential to reduce the risk of corn stunt. The initial phase of the crop is considered the critical period for control, since the earlier the plants are infected by mollicutes, the greater the losses will be. Alternatives for reducing corn leafhopper populations have been evaluated using chemical, biological and cultural methods. Regarding the use of chemical insecticides, control of the insect vector should be carried out using products applied to the seeds and foliar sprays of the crop between emergence and the V8 phase (eight open leaves).

In regions with low incidence of stunt disease, only seed treatment can ensure satisfactory control of these diseases. Industrial seed treatment has been more appropriate when compared to treatment carried out by producers themselves on their properties (“on farm”), due to the greater precision and uniformity of the dose of insecticide applied to the seeds. In places where the incidence of stunt disease is high, ideally, corn planting and spraying should be carried out simultaneously by all producers in the region, which would prevent the flow of the insect from migrating from neighboring areas.

Study on the dispersion behavior of D. maidis in corn crops at three times of the day (6:12 a.m., 17:6 p.m. and 12:17 p.m.) in Chapadão do Sul/MS, it was found that in the morning (XNUMX:XNUMX a.m.) the leafhoppers remained at rest or with little movement, even when disturbed. In the hottest time of the day (XNUMX:XNUMX p.m.), the insect was more concentrated in the plant cartridge, while in the late afternoon (XNUMX:XNUMX p.m.) the leafhoppers were more agitated, probably in dispersion behavior. In view of this, it was found that sprayings carried out in the late afternoon presented better efficiency in controlling the leafhopper, when compared to the morning and midday periods, with this improvement in control probably resulting from the greater mobility of the insect in the area, which provided greater direct contact of the insect with the insecticide solution.

Biological control of corn leafhoppers occurs naturally through the action of parasitoids and egg predators, as well as through the use of entomopathogenic fungi in sprays. Commercial formulations of beauveria bassiana e Isaria fusorosea have been used in corn crops to control leafhoppers. However, unlike chemical insecticides, biological products have a slower action and require adequate relative humidity (RH) conditions to be sprayed. The combination of chemical and biological insecticides has provided better performance in the efficiency of leafhopper control, when compared to the use of these products alone.

In general, the greatest damage caused by stunting occurs in late plantings, often when sowing is staggered, when populations of the insect vector abandon older plantings to colonize newer plantings. Therefore, new sowing should never be done near older plantings, especially when the plants in the latter already show symptoms of stunting.

Loss of corn cobs and grains during harvesting and transportation to the fields should also always be avoided or mitigated. Corn grains that fall to the ground can germinate when they find adequate moisture and give rise to a tiguera plant.

Finally, coexistence with corn stunt in Brazil requires joint and coordinated actions by official bodies, the private sector and producers to adopt appropriate agricultural practices for the effective management of the insect vector and corn stunt in agricultural landscapes containing corn crops.

* By Credit José Ávila (Embrapa Agriculture West); Charles Martins de Oliveira (Embrapa Cerrados); Suélen Cristina da Silva Moreira (Chapadão Foundation); Rodolfo Bianco (Paraná Rural Development Institute); and Marco Antonio Tamai (State University of Bahia)

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