Corn leafhoppers respond differently to seed treatment

One of the challenges in managing corn stunt disease complexes in the field lies in the inability to identify whether a corn leafhopper (Dalbulus maidis) is healthy or infected by the pathogens causing stunting, nor to estimate the proportion of infected plants in the crops. This lack of knowledge can significantly affect the timing of control measures (the ideal time to implement pest management) and the final cost of production. Therefore, products need to be highly effective, ensuring protection primarily in the early stages of the crop, from vegetative stages V2 (development of two fully formed leaves) to V6 (six fully formed leaves), which is considered the most critical, as the earlier the plant is infected, the greater the impact on production.

We know that Dalbulus maidis is a vector of mollicutes (spiroplasma and phytoplasma) that cause the stunting complex and the streak virus, which causes fine streaking in corn plants. These pathogens settle in the phloem and compromise the nutrition, growth, and development of the plants. Symptoms include yellowing of the leaves, shortening of internodes, plant deformities, and multiple ear formation. In severe situations, plant death can occur, and productivity losses can reach 100%.

However, if such pathogens negatively affect the corn plant, they can also alter the "health" of the leafhopper, which would be affected in some way. Would insecticides be equally effective in controlling diseased and healthy leafhoppers?

Conducting an experiment

To understand this dynamic, an experiment was conducted at Embrapa Maize and Sorghum, in Sete Lagoas. The results of this study are published in an article in the journal Insects, 2025: “What is the relationship between efficacy of seed treatment with insecticides against Dalbulus maidis (Delong and Wolcott) (Hemiptera: Cicadellidae) healthy and infected with spiroplasm in the corn stunt control?”.

The trials were conducted under controlled conditions, using healthy leafhoppers and leafhoppers infected with spiroplasma (the pathogen that causes corn stunt disease), infesting corn seedlings treated with different insecticides indicated for seed treatment.

Results presented

As a first result, we discovered that leafhoppers infected with spiroplasma die faster than healthy ones when they come into contact with plants grown from seeds treated with insecticides.

The product that stood out was the compound made up of the imidacloprid + thiodicarb mixture. Mortality was around 85% for infective leafhoppers. Another very important result is that plants treated with this product did not show symptoms of stunting – nor was there any impairment in productivity.

The insecticides thiamethoxam, fipronil + thiamethoxam, and imidacloprid showed mortality rates of less than 68% for infective leafhoppers. For healthy leafhoppers, the average mortality rates in the first 24 hours were 0%, 2%, and 17%, respectively. Considering that efficient control requires rapid action capable of paralyzing the insect's feeding and thus reducing pathogen transmission, it was observed that these products may not be the most effective. However, in cases where feeding cessation did not occur immediately, there was partial transmission of the pathogen to the plants, resulting in mild symptoms that did not compromise productivity.

The least effective product was the lambda-cyhalothrin + thiamethoxam mixture, which showed mortality of around 30% and caused severe symptoms in all plants evaluated.

Action strategies

When planning field management, it should be considered that leafhoppers that do not die after contact with the insecticide can become infected and become sources of pathogen transmission to the corn. Ideally, therefore, the mortality of already infected insects should be increased, reducing the spread of stunting diseases. Simultaneously, reducing the population of healthy leafhoppers is also important, as it decreases the chance of new insects becoming infected and spreading the disease throughout the crop cycle.

When insecticide efficacy is lower, there is a proportional increase in plants showing stunting symptoms. In this context, it is important to highlight that the products evaluated were chosen for their systemic action. In addition to promoting high initial mortality, these insecticides reduce feeding time, helping to decrease the transmission of pathogens.

Expiration date

Seed treatment was effective in protecting plants up to the V2 stage, which is approximately 11 days after planting. After this period, the effectiveness decreases, and surviving leafhoppers can continue to transmit the disease. This result is consistent with the profile of neonicotinoids, carbamates, and pyrethroids, insecticides that generally have a residual period of up to 15 to 21 days after plant emergence, depending on the soil, climatic conditions, and application method. After V2, with the reduction of the residual effect, seed treatments proved ineffective, resulting in the manifestation of stunting symptoms and productivity losses.

Multiplication time

The pathogen needs time to multiply and reach levels capable of causing economic damage. Therefore, the earlier the infection occurs in the plants, the greater the losses in the crop tend to be.

Early exposure of plants to infective leafhoppers resulted in greater productivity losses. This is because the pathogen had more time to multiply, colonize the phloem, and compromise nutrient transport. However, more effective seed treatments reduced this impact, preserving productive potential.

Impact on production

Plants without stunting symptoms showed greater vigor and productivity than those infected. Plants protected in the early stages grew taller, with larger ears and no symptoms. Those attacked by infected leafhoppers in the early stages of development, however, remained short, with small ears or did not produce any ears at all.

Message to the producer

Seed treatment, as well as choosing the most effective insecticide, is fundamental to reducing the risk of transmission of stunting pathogens to the plant. However, since the results indicate that the effect of seed treatment is only effective up to the V2 stage, after this period it is necessary to implement other management tactics, such as spraying with chemical and/or biological insecticides, while safeguarding the strategies of Integrated Pest Management. It is advisable to avoid staggered plantings close together, as this facilitates the migration of leafhoppers from one crop to another.

Conclusions from the analyses

Seed treatment is an important ally in the initial development of the plant, and the choice of product impacts the efficiency of corn leafhopper control.

The discovery that infected leafhoppers die faster opens up new possibilities for study and management to reduce the impact of this pest that so worries corn producers.

By Ana CM Redoan, Vinicius M. Marques, Isabel R. P de Souza, Ivenio Rubens de Oliveira and Simone Mendes (Embrapa Maize and Sorghum)

* Article published in Cultivar Grandes Culturas Magazine 321.