Wheat has the highest population of aphids in the decade

The 2019 wheat harvest recorded the highest aphid population in the last ten years. The insects were favored by the hot and dry winter climate in the South Region, resulting in a population five times higher than the average for the period 2001 to 2018. The information is recorded in the Winter Cereal Pest Monitoring Network database, which brings together several research institutions in the main wheat producing regions in Brazil.

The combination of above-average temperatures in the months of April, May and June, with rainfall below the historical average in the months of July to September resulted in an increase in aphid populations in winter crops in the South Region. 

At the Embrapa Trigo meteorological station, in Passo Fundo, RS, the amount of rain in winter (June to September) was the lowest in the last 40 years. “For example, in 2018 the accumulated rainfall from June to September in Passo Fundo was 734 mm; in 2019, accumulated precipitation totaled just 267 mm in the same period, well below the historical average. The dry weather, the average temperature close to 14°C, with the availability of plants in the fields, formed a very favorable environment for the reproduction and dispersal of insects”, says Embrapa Trigo researcher, Douglas Lau.

According to data recorded by the Winter Cereal Pest Monitoring Network in the months of June to September, the aphid population was 5 times higher than the average for the period 2011 to 2018. Comparing only the month of August, the population was 28 times higher. higher than the average for the period. The basis of the information consists of collecting insects in traps installed in wheat production centers monitored by a group of companies and institutions that participate in the Network in the states of RS, PR, MG and MT. 

Aphids, popularly known as aphids, transmit viruses in winter cereals. When the infection occurs early in the plant's development, the virus can affect root growth. The most visible symptom is the yellowing of the leaves. Signs of reduction in ear size and apical sterility are also common. Depending on the sensitivity of the cultivar, transmission of the virus at the beginning of the wheat cycle can compromise grain yield by more than 60%.

The researchers observed that the resumption of aphid population growth occurred earlier this year in Rio Grande do Sul: “Historically, due to the decline in temperature in May or June, the aphid population reduces and becomes a problem again only around September. . This year, the hot and dry climate favored high populations of aphids in August, with incidence reaching 70% in some crops, requiring reinforcement in insecticide applications”, assesses Douglas Lau. According to him, many of the ear problems observed at the end of the harvest, such as apical sterility and black ears, known as chocolate ears, resulted from infections of the yellow dwarfism virus transmitted by aphids at the beginning of the crop.

In Paraná, the attack by aphids occurred in the rubbering and heading of wheat, as the crops suffered a strong incidence of frost during their initial development. “It was undoubtedly the biggest spike in aphid populations since 2011, when we started monitoring”, says FAPA/Agrária researcher, Alfred Stoetzer, assessing that although the aphid caused greater damage at the beginning of the harvest, the attack on the ears certainly had an impact in final crop yields. “We are still quantifying the data, but we observed that the number of parasitoids was much lower this year. Thus, without natural enemies and with a favorable climate, the aphid population grew very quickly”, explains the researcher.

Control measures

The first precaution to avoid losses due to aphids is seed treatment. However, in this harvest, population growth occurred in August, when the protection offered by the insecticide was already reduced. Therefore, it was necessary to apply insecticides to the aerial part. The combination of two control measures, seed treatment and two aerial applications, reduced the incidence of viruses in wheat by 82%.

Monitoring the crop after 20 to 30 days after plant emergence makes it possible to assess the incidence of aphids on wheat, with the application of insecticide being recommended when the level of 10% of plants with aphids is reached.

Currently, the market has wheat cultivars that are more tolerant to the yellow dwarf virus, however, the Winter Cereal Pest Monitoring Network evaluated the reaction of 34 cultivars and found that only 3 recorded damage below 20%. “The best control strategy is still to combine genetics with seed treatment and aerial application, but always accompanied by crop monitoring to avoid unnecessary costs in years that are not favorable to pest attacks”, concludes researcher Douglas Lau.