Mecanismos morfofisiológicos da resistência de raças primitivas de algodoeiro ao bicudo

Abstract

The cotton boll weevil, Anthonomusgrandis Boheman (Coleoptera: Curculionidae), is a very important pest for the Brazilian cotton due to the injuries caused on the squares, flowers and bolls. These injuries reduce the production and are responsible for causing great economic losses to the farmer. Among the alternatives for the management of this pest can highlight the control tactics based on plant resistance to insects. This research aimed to determine the morphophysiological mechanism involved in resistance of some primitive races of cotton against boll weevil. The work were conducted in the field, in an area infested by the bolls weevils, in the greenhouse and in the laboratory of Pathology and Molecular Biology of Insects at EmbrapaCotton, municipality of Campina Grande, state of Paraíba. Two experiments were conducted. The first experiment aimed to select the most promising lines for resistance to boll weevil among the lines TB 15, TB 41, TB 75, TB 80, TB 85, TB 87, TB 90, TB 91 originating fromprimitive races of cotton and BRS 187 8H (control).The experimental design was randomized blocks with nine treatments and 4 repetitions. The treatments were represented by lines and cultivar cited cotton. Data were submitted to analysis of variance a nd the means compared by Tukey test at 5% probability. Pearson correlation coefficients were estimated between the number of punctured squares to feed and oviposition by the boll weevil and the 2 average number of trichomes and gossypol glands per cm leaf, height and diameter of the cotton rods with 120 days old. The second experiment aimed at determining the preference for feeding and oviposition of boll weevil on the squares of the six most promising lines selected in the first experiment in test with and without chance of choice.In the choice test was used completely randomized design in a factorial scheme with six cotton lines (G1=TB 90, G2= TB87, G3= TB 80; G4= TB 75; G5= TB 41 and G6= Grandless) nine hours evaluation (15 min, 30 min, 1 hour, 2 hours, 4 hours, 6 hours, 8 hours, 12 hours and 24 hours) and ten replicates. In the test no choice, the experimental design was completely randomized, with five treatments, represented by boll weevil adults fed with cotton squares of the same lines of cotton used in the test of choice except to cultivate Grandless and five replications.The average number of trichomes and 2 2 gossypol glands per cm and teeth on bracts and the gossypol glands per cm on the sepals and of the nectaries in squares of cotton lines was determined to establish a correlation with the damage caused by feeding and oviposition of boll weevil on the squares. Data from tests with and without choice were subjected to analysis of variance (ANOVA) at 5% probability and the averages compared by Tukey test at 5% probability.In the field,the less preferred cotton line for ovipositionwasTB80. In the test-choice, the cottonlinewith smaller number of the boll weevil and punctured squares by feedingwasTB41. Under no chance of choice, the cultivarsTB80andTB41 wereless preferredfor food and line TB90showedsmaller number of punctured squares by oviposition.TheMorphophysiologicalmechanismsofresistanceprimitive racetested againstthe boll weevilare related tominor amounts of glands of the gossypol andnectariespresent, respectively, in the leaves and at the base of square.