Chemical control against corn leafhopper

One of the most serious corn pests in Latin America, the leafhopper Dalbulus maidis, In addition to losses in productivity, it has the firepower to cause indirect effects, through the transmission of pathogens such as viruses and stunting. Chemical control, via seed treatment and application of insecticides, is one of the recommended tools for combating the insect associated with a set of integrated management measures.

Technological advances in corn cultivation in Brazil have promoted significant gains in productivity. The agroecosystem used in the Center-West is an environment favorable to the multiplication of pests, as a production system prevails in which few crops are cultivated in the vast majority of areas, often without the rotation and succession necessary for the greater diversity of natural enemies. In this way, the successive cultivation of these crops causes many pests to adapt to the system. In addition to this, other factors such as favorable climatic conditions, high temperatures and mild winters become ideal for the multiplication of insects.

Corn crops are subject to attack by various insects from germination to harvest. Among the diversity of pests, the corn leafhopper (dalbulus maidis) (HEMIPTERA: CICADELLIDAE) was found to occur in Brazil several years ago and has increased in frequency in corn crops across the country, causing significant damage to plants and requiring control actions.

This hemipteran, when adult, measures 3mm to 4,5mm in length, being straw yellow in color. It is often found in corn kernels. The egg, yellowish, has an embryonic period of around nine days, being placed within the plant tissues, preferably in the central vein of the leaf. Nymphs go through five instars, for an average period of 17 days -20 days. The complete cycle varies from 23 days to 40 days. 

D.maidis It is characterized by sucking sap, which consequently results in reduced plant development. Another important factor is the indirect effect through the transmission of pathogens, such as viruses and corn stuns. Furthermore, when they suck the sap, in high populations, in some cases the presence of sooty mold can be observed on the leaves. But it is important to note that other pests also cause this symptom in the crop.

This insect is considered one of the most serious corn pests in Latin America due to its ability to persistently and propagatively transmit the corn stripe virus (Maize Rayado Fino Virus - Marafivirus – MRFV) and two mollicutes associated with stunting, Spiroplasma kunkelii (corn stunt spiroplasma – CSS, also called pale stunt) and phytoplasma (maize bushy stunt phytoplasin – MBSP, also known as red stunting) (NAUT, 1990) (OLIVEIRA et al, 2003) (MARTINS et al, 2008). The losses in general lead to average losses in some regions of around 10% to 30% of productivity. However, losses can reach 100% due to attacks by viruses and pests.

In general, the Central-West region of Brazil presents a great diversity of environments where there are areas with summer crops and 2nd harvest (“safrinha”), areas with only summer crops, and areas with up to 2,5 harvests ( irrigated areas where the producer, depending on the market, chooses the period to sow the corn crop, given the economic condition).

In regions with production systems in which corn cultivation is more intensive, a general increase in D.maidis. 

The corn leafhopper occurs at different stages of the crop. However, it can be considered as an initial pest. It is interesting to report that in the vast majority of cases, high populations have occurred in the initial phase. At this stage, it is important to point out that the corn plant has a smaller stem in diameter, and in this way the leafhopper can reach the sap-carrying vessels (phloem) with a stylet. Consequently, these vessels are partially obstructed, causing less development of the plants. Plants with stunting (the way a plant with a symptom of a disease attack is called) D.maidis) tend to present yellowish streaks on the leaves, reddening of leaves (starting at the apex and on the margins), yellowing, reduction in the size of the plant, with a reduction in the size of ears. Plants normally acquire viruses and phytoplasmas in the initial phase, but show symptoms in later phases such as bolting and even close to harvest.

Several studies show that the plant, when acquiring the disease in the initial phase, tends to show greater losses, even leading to it not producing the ear. It has already been observed in some cases in Brazil that, with this lower development, plants tend to fall due to the wind, leading to losses during harvest and difficulties in mechanized operations.  

Differences between materials on the market are also observed due to the attack of this pest, and the producer must be attentive to the option of those with better development and establishment of stalk, which can reduce losses.

In addition to the direct and indirect damage caused by this pest, it is worth noting that by promoting less plant development, the insect can cause changes in crop management, as spaces with smaller plants favor the emergence of weeds, which, in addition to being competitors for resources, reduce productivity. and also affect the quality of the grains produced.

The increasing increase in the population of D.maidis can be explained, in part, by the presence of corn tigueras. Many of them, due to the adoption of herbicide resistance technology, end up remaining after application in soybean crops, and consequently the pest increases its population due to this food supply. In addition to corn, there are several other hosts for D.maidis, such as sugar cane, sorghum and weeds: Eleusine indicates (Crow's foot grass), Digitaria horizontalis (Crabgrass) among others. Some of these crops and weeds can host phytoplasmas.

In the integrated management of this pest, all aspects must be taken into account, such as the elimination of hosts, definition of sowing times, biological control and chemical control. Among the chemical strategies, seed treatment with neonicotinoid insecticides (imidacloprid, thiamethoxan and clothianidin) has promoted control of part of the population, being very important in management as it controls the first migrant population in cultivation. This strategy has shown efficiency up to 15 days and 20 days after crop emergence. Seed treatment is an application modality that has some advantages, such as preserving part of the natural enemies, without affecting the dynamics of crops. However, despite being an effective method, it is necessary to keep in mind that even with the use of the best insecticide, a certain level of pest attack is expected, as the seedling is treated, there will be a need for consumption for the disease to occur. control.

In work carried out by Martins et al 2008, where treatment 1- Imidacloprid 45 +Thiodicarb 135 g/ ha in seed treatment – ​​(TS); 2- Thiamethoxan 42 g/ ha (TS); 3- Clotianidin 210 g/ ha (TS); 4- Acetamiprid 70 g/ ha (TS); 5- Cypermethrin 25 g/ha in desiccation management prior to sowing and 6- Thiamethoxan 28 + Lamdacyhalothrin 26,5 g/ha in application 10 days after corn emergence (stage V2) it was observed that seed treatment promoted suppression of the pest in the order of 70% to 80% control, with the different insecticides available on the market. The application of management prior to sowing with the insecticide in question presented the smallest contribution to the work and the application after the corn exit provided efficiencies similar to the seed treatment (graph 1). 

Another strategy is spraying with insecticides in order to promote control until the V8-V9 phases, where the pest reduces its losses. These sprays tend to complement the seed treatment strategy, being essential for controlling migrant populations of other crops.

If all methods are not integrated, it will be possible to win a battle, but not the war against this important pest in corn cultivation.

Josiane Oliveira, Patricia Mariano, Germison Tomquelski, Chapadão Foundation 

Article published in issue 205 of Cultivar Grandes Culturas.