[ Genome ] [ RNASeq ]
Cotton is a crop of great importance for the world economy and is among the ten largest sources of wealth in the agricultural sector in Brazil. However, since the mid-1980s, its cultivation has been suffering from serious agricultural problems. Among the main factors that contributed to the reduction in the production of Brazilian cotton, we highlight the establishment of the cotton boll weevil (Anthonomus grandis; CBW; Coleoptera: Curculionidae), considered the main cotton pest in the Americas. This insect can be found mainly in Argentina, Brazil, Colombia, Cuba, USA, Haiti, Mexico, Paraguay and Venezuela. In addition, CBW has a high destructive power, due to its high reproductive capacity and the numerous generations that are produced in an agricultural cycle. Due to its development within the fruiting structures of host plants, CBW is protected from innumerable natural enemies, from adverse environmental conditions and from the action of insecticides. The CBW attack is linked to climatic conditions, although low temperatures do not favor its development. CBW has been well known in other countries for a long time, having been detected in Brazil in 1983, close to Campinas-SP. From then on, the Brazilian cotton crop accumulated increasing losses not only for farmers, but also for all other segments of the productive chains dependent on fiber and its by-products. The damage caused by CBW can vary from one region to another and occur from 50 to 90 days after the germination of cotton and the appearance of the first flower buds.
The CBW adult attack begins on the banks of the cotton field. This insect perforates the flower buds to feed or lay its eggs, which are traumatic damages and contribute effectively to the abscission of fruits and floral structures, preventing plant fertilization. The perforations have varying depths, depending on the purpose, whether for feeding or oviposition. After drilling, the bracts of the flower buds are completely removed and the bottom part of the set formed by the base of the bud and bracts is discolored. The anthers and eggs of new flower buds are the main feeding targets of the insect, whose process involves perforation of the tissues and later chewing of the stamens and ovary.
During its reproductive cycle, CBW has four stages of development: egg, larva, pulp and adult, and the life cycle from egg to adult is completed in approximately 19 days under field conditions. The first three main stages of development occur within the flower bud, or cotton apple. The female lays an egg through an orifice, the cavity being subsequently closed by a waxy substance, which allows the differentiation of the insect's feeding holes, in addition to playing an important role in protecting against enemies and preventing dehydration of the egg. In general, the oviposition perforations are made at the base of the floral bud, and the eggs can also be deposited in the apical part of the reproductive structure. At the end of the cotton cycle, the CBW female can place its postures on the green cotton apples. It is worth noting that the feeding holes are wider and deeper than the oviposition holes, usually containing a yellowish ring formed by pollen grains around them. The eggs are smooth, bright white, generally elliptical in shape. The CBW female does not discriminate between previously oviposited flower buds, and after 20% of infestation, the females start laying two eggs per flower bud. Above 50% of infestation, up to five oviposition holes were found. The incubation period for eggs is three to four days. Thereafter, the eggs hatch, giving rise to the larva, which starts to attack the innermost portions of the oviposited organs. The larvae are milky white, apodic and curculioniform, or C-shaped. Larval development takes place entirely within the flower buds, in a period of 7 to 12 days. After three instars, the larvae turn into pulp, which also grows on flower buds, flowers and cotton apples. The pulp is formed in chambers built in the attacked structures of cotton, in addition to having legs, face and eyes similar to adults, and being little mobile. After 3-5 days, the pulps undergo complete metamorphosis in adults, who have a longevity of 20 to 40 days. Under laboratory conditions, raised on an artificial diet, CBW adults can live up to three months. After the cotton cycle is over, part of the population migrates to natural shelters under the remains of dry plants, going into diapause or hibernation, which can last for months, for periods varying from 150 to 180 days, until the beginning of a new cotton cycle.
Several actions can be taken to effectively control CBW in the cotton fields. Associated with rapid and early harvesting, destruction of the remains of host plants, use of bait-knuckles, crop rotation and integrated pest management, the use of biological and chemical control methods for the control of populations of this insect-pest can be highlighted. Within the organisms used for the biological CBW control, the parasitoids Catolaccus grandis (Hymenoptera: Pteromalidae) and Bracon vulgaris (Hymenoptera: Braconidae) can be considered effective natural enemies of A. grandis. On the other hand, insecticides are the main weapons for the control of CBW. Several chemical groups of insecticides can be applied alternately, such as: Organophosphates (methyl parathion, malathion, fenitrotion, among others), Cyclodiene (endosulfan) and Pyrethroids (betacyflutrin, zetacypermetrin, esfenvalerate, deltametrin, among others). However, the large number of necessary applications of these insecticides to control the populations of CBW in the field, due mainly to the inherent low efficiency of these compounds, mainly due to the endophytic development characteristic of this species, make the production cost quite high. In addition, CBW populations are becoming increasingly resistant to the main classes of insecticides used, which has been causing serious problems mainly for countries like Brazil, which has indiscriminate use of insecticides and ideal climatic conditions for the development and multiplication of CBW. Thus, the Plant-Pest Molecular Interaction Laboratory - LIMPP (at Embrapa Genetic Resources and Biotechnology, Brasilia-DF, Brazil) has been developing biotechnological tools capable of controlling CBW safely and cheaply. The researchers started their studies in 2013 aiming mainly to identify target genes that could be silenced via RNA interference (RNAi). For this purpose, transcriptomes of all stages of CBW development using 454 technology were sequenced. In this pioneering study it was possible to identify several genes whose silencing has shown promise for the control of the insect, among which can be highlighted: chitin synthase, vitellogenin, laccase, among others. Progressively, the advancement in DNA sequencing technologies has enabled the sequencing of new transcripts, now using Illumina technology, which is more robust, sensitive and, after de novo assembly, it is perfectly possible to identify complete transcript sequences from A. grandis. Thus, here is presented transcriptomes (RNA-Seq, Illumina Platform) of anterior and posterior midgut of third instar larvae of A. grandis fed with artificial diet, which aim to evaluate the intestinal physiology of this insect pest for the development of new effective biotechnological assets for its control in cotton fields.