Avaliação da resistência de cultivares comerciais de milho (Zea mays) ao ataque de Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae)

Abstract

The beetle Sitophilus zeamais (Curculionidae) is one of the most important pests of stored cereals in the world, being able to infest grains in the field and in storage. The control of this pest is usually done with synthetic insecticides, which are expensive and are often used inappropriately and recurrently, causing the development of insect populations resistant to the products used. Besides this, they leave residues in food and the environment, generating a risk for both human health and the agro-ecosystem. In this sense, the use of resistant varieties stands out as one of the most effective ways to combat pests in storage, because they make the grains more resistant to attack and establishment of insects. Therefore, the aim of this work was to evaluate the susceptibility/resistance of commercial maize cultivars (BRS GORUTUBA-EMBRAPA, SÃO JOSÉ-IPA, CMS 36-IPA, AG 1051 AGROCERES, AL BANDEIRANTE-CATI AND FEROZ VIP-SYNGENTA) to S. zeamais, through tests with and without choice. In the no-choice tests, the daily emergence, the instantaneous growth rate (ri) of S. zeamais, and the grain mass loss were evaluated. Ten individual arenas were used, consisting of plastic jars (100mL) containing 20g of maize of one of the cultivars and 20 unsexed adult insects, aged up to 48h. To evaluate the emergence, the insects were confined in contact with the grains for seven days to lay eggs. After this period, the insects were removed and the grains were stored again for a period of 30 days, being proceeded the count of the number of emerged insects in each cultivar and the mass loss due to the insects consumption. In the test to evaluate the population growth rate of S. zeamais, the same procedures of assembly of the arenas were used, however the insects remained stored in the jars in contact with the grains during 37 days. After this period, the number of live insects in each cultivars was evaluated. In the free-choice test, the preference of S. zeamais for different maize cultivars was evaluated. Arenas composed by a round plastic jars (3L) containing six smaller flasks (50 mL), inserted in a Styrofoam disc, equally spaced. In the smaller jars, 20g of only one of the maize cultivars were distributed, being released in the centre of the arena 100 adult insects of S. zeamais, unsexed, aged up to 48 hours, so that they could choose among the treatments. The arenas were covered with organza and, after 48h, the number of insects in each cultivar was counted. Subsequently, the smaller flasks were covered individually and stored for another 30 days. After this period the daily and total emergence of insects was counted until no new insects appeared, and the loss of grain mass due to insect consumption. All experiments were carried out in a completely randomized design, in B.O.D. climatic chambers, at 27±2 °C, 24 h of scotophase and 70±5% RH. The no-choice test showed that there was a significant effect of the tested cultivars on the emergence of S. zeamais. Low insect emergence was observed, mainly in the BRS GORUTUBA, CMS 36 and SÃO JOSÉ cultivars. After the fourth day of evaluation no new insects appeared, except for the SÃO JOSÉ cultivar in which there still emergence on the fifth day of evaluation. For grain mass loss, the lowest values were observed for the cultivars AL BANDEIRANTE and AG-1051 AGROCERES, in both tests (free and no-choice), but in general the losses were low, ranging from 0 .5% to 2%. Regarding the preference test, it was observed that BRS GORUTUBA and SÃO JOSÉ showed greater attraction for initial insect feeding and/or oviposition than the other cultivars evaluated. On the other hand, positive values of ri were observed only in the cultivars BRS GORUTUBA and CMS-36, indicating that there was a population growth of S. zeamais in these cultivars. It was concluded that the cultivars BRS GORUTUBA and SÃO JOSÉ were more susceptible to the attack by S. zeamais while AL BANDEIRANTE and AG-1051 AGROCERES were more resistant, being observed that the mechanisms involved were antixenosis and antibiosis.