[ RNASeq transcripts ]
The borers of the Lepidoptera insect order are important agricultural pests that cause damage to sugarcane worldwide. In this context, the sugarcane giant borer Telchin licus licus (Lepidoptera: Castniidae) is considered one of the main pests of the sugarcane culture. Originating in the Amazon region, T. licus licus is widely disseminated throughout the American continent, having been observed in Bolivia, Colombia, Costa Rica, Ecuador, Guyanas, Mexico, Panama, Peru, Suriname, Trinidad and Tobago and Venezuela, besides Brazil, being perhaps the most commonly found castnid. In our country, this pest occurs in the states of Alagoas, Amapá, Amazonas, Bahia, Goiás, Maranhão, Minas Gerais, Pará, Paraíba, Pernambuco, Rio de Janeiro, Rio Grande do Norte São Paulo and Sergipe. It is believed that T. licus licus was introduced in sugarcane production areas in São Paulo through the transport of ornamental plants and that it currently inhabits the sugarcane fields of southern Minas Gerais, Paraná and Mato Grosso do Sul. Usually, it is associated with musceas, but it also attacks bromeliads, orchids and, mainly, sugarcane.
The adults of T. licus licus are moths of medium size, with dark colored wings, with a white transverse band on each of the anterior wings, in addition to a row of white spots in the apical region. Each posterior wing also has a white diagonal stripe, which widens as it approaches the inner margin, in addition to a row with 7 reddish spots on the outer margin. The damage, however, is caused by the caterpillar forms, which are white in color, with a single brown spot, irregularly shaped in the pronotum, and can measure up to 8 cm long and 1 cm wide. The eggs of the sugarcane giant borer have a polyhedral structure, similar in shape to star fruit, with five prominent edges in color ranging from green to brown. When they hatch, the caterpillars penetrate the soil and start drilling at the base of the plant, digging an ascending gallery inside the stem, destroying, during its development, the first 2 or 3 internodes of the sugarcane and, in extreme cases forming galleries up to 1 m long. Upon completing its development, the caterpillar digs a hole at the base of the stem for the future adult to exit and builds a cocoon with cane fibers, becoming pulp inside. When they emerge, adults copulate and females lay eggs, restarting the cycle.
Due to the behavior of the sugarcane giant borer to shelter and feed inside the plant, the use of pesticides in its control is ineffective and requires manual labor (due to topical applications, in the damage holes), which is time consuming and increases the cost of production. In addition, repetitive applications can contribute to environmental contamination and generate health problems. In the case of the giant borer, there is only one product currently registered, based on the active ingredient rinaxipir, whose trade name is Altacor® (DuPont from Brazil S.A.). However, this fact conditions the massive use of only one product, which can cause, over time, the emergence of insect populations resistant to this active ingredient, making it necessary to search for new molecules effective in controlling this pest. In parallel, studies with the insects pathogenic fungus, Beauveria bassiana, for the control of T. licus licus started in the 1980s. It is reported that tests carried out in the laboratory have always demonstrated the possibility of its use in the biological control of the sugarcane giant borer. However, the test results under field conditions have always been unsatisfactory. In more recent studies, it was found that the virulence of B. basiana varies according to the isolate, causing mortality between 53.3 and 83.3%, whereas, for the fungus Metarhizium anisopliae, the percentage of dead caterpillars varied between 43.3% and 80%, demonstrating its potential for controlling this insect. However, in view of the caterpillars' habit of covering the gallery immediately after cutting the sugarcane, the efficiency of the control with B. bassiana is likely to be successful as long as the application is made at the appropriate time, that is, on the newly grown clumps cut. There is also the possibility of the fungus acting on the remaining eggs, with application in irrigation water. Studies indicate that in 2023/2024, there will be a need for Brazil to reach a planted area of 10.5 million hectares and a processing of sugarcane (Saccharum officinarum) in the order of 862 million tons to meet the growth in consumption and exports of sugar and ethanol. On the other hand, after the 2008 global financial crisis, investments in the sector ceased and the expansion of sugarcane fields was compromised, especially by the abrupt reduction in credit, which was abundant until then. For this reason, there was a fall in productivity, caused, among other reasons, by the reduction in the cultural treatment with the sugarcane fields, including the phytosanitary control. Initially restricted to the North and Northeast regions, the sugarcane giant borer was recently reported in Brazilian Southeastern, in the state of São Paulo, the main sugarcane producing area in the country, raising concerns about its control. It is believed that the populations of certain pests may increase due to the harvest of raw sugarcane, mainly due to the deposition of straw in the soil, causing change in the soil microclimate and, consequently, in its biota, despite the lack of studies on this topic. Harvesting sugarcane without burning can benefit the pest, mainly by eliminating fire, which is responsible for destroying adults and eggs. In addition, the straw that remains in the field after the harvest of raw sugarcane makes pest control difficult, which in Brazilian Northeast is carried out by picking caterpillars and pupae in the underground part of the plants, immediately after the harvest. Therefore, the deposited straw would make it difficult to identify the infested stubble. Thus, the control of the sugarcane giant borer using more than one technique, in addition to pesticides, is important and must be considered in the scope of research, development and innovation, with the possibility of generating new technologies that will meet this market demand. Although there are studies on the creation of the sugarcane giant borer in the laboratory, so far, a viable method of creating this insect has not been established, which hinders advances in the knowledge of its bioecology. Studies on plant resistance, biological control, microbial control, among other control tactics recommended in integrated pest management, depend on the creation of insects in the laboratory, as well as the correct identification, both of the pest and its natural enemies, which they are often raised in the natural host. Thus, there is no doubt that, while this impasse is not resolved, advances in this area tend to be slower, as there is a constant need for searches for eggs and caterpillars in the field, which often present problems of health and injuries, in addition to the insect's seasonality due to the difficulty of collecting sugarcane in the most advanced phenological stages. Recent reports from the Plant-Pest Molecular Interaction Laboratory - LIMPP, together with the Bioinformatics Laboratory, both located at Embrapa Genetic Resources & Biotechnology (Brasilia-Brazil), presented the first genetic characteristics of caterpillar stage of this species through transcriptome data (RNA-Seq; 454 technology), as well as a tissue-specific transcriptome of T. licus licus caterpillar (RNA-Seq; Illumina Platform), and a differential expression analysis highlighting the physiological characteristics of this pest in response to different diets (artificial diet and sugarcane).