Aspectos biológicos, danos e controle de Diloboderus abderus sturm, 1826 (Coleoptera: melolonthidae) em plantio direto

Abstract

Studies were carried out in the counties of Cruz Alta and Fortaleza dos Valos, Rio Grande do Sul State, in an Oxissol (Dark-Red Latosol), from January 1990 to February 1994, aiming to determine the biological aspects, damage and control of Diloboderus abderus (Sturm) and looking for integrated management altemative practices in commercial fields cultivated under no-tillage system. In the field it was determined that D. abderus had one generation during the year, with the stages occurring within the soil profile in well defined dates. Adults show up from November to April, eggs from January to April, larvae from February to November and pupae from October to December. The female flying back, which occurs in the crepusculum and beginning of the night is an important surviving mechanism of this species, where new reproduction places are searched for. Adequate amount of cultural residues on the soil surface during the reproductive period of the insect (January to March) is an important reproductive and alimentary resource used by the adult female to build its nest, protect the eggs and nourish the new-bom larvae. Thus, the workability of strategies to reduce larvae populations for subsequent winter crops can come true with winter crops succession, such as rye-grass, rape, vetch or lupine and summer crops, such as corn, which is normally sown in September or October, in substitution to black oats-soybean traditionally used in the region, since those crops allow a smaller amount of residues on the soil during the reproductive period, thus disfavouring the female oviposition. The larval phase have three instars and, in ther active period, from May to September, are characterized by the economical damage that they cause to the root system of plants. When the soil is covered with black oats, the spatial distribution of larvae occurs in different ways depending on the size of the soil sample. The predominant spatial distribution was of a negative binomial, indicating that the larvae show aggregation. The sample size units which showed to be more efficient were 25 x 25 and 25 x 50cm, of soil dug to a depth of 30cm. It is suggested that 33 soil sample units of 25 x 50cm and 67 of 25 x 25cm, for 25% precision of the average value, offer an evaluation of the larvae population with an acceptable uniformity. The control level determined is of 5.4 larvae/m2 for wheat plants and 0.14 larva/m2 for com plants. The no-tillage, so important to soil conservation, can be continued in soils infested with larvae of D. abderus when associated to seed treatment of wheat or corn with insecticides. Another clear possibility of controlling larvae of this insect is to delay the sowing of corn, starting me operation not before the beginning of October.