Utilização de casca de arroz carbonizada e terra de diatomácea na manutenção da qualidade de sementes de trigo e no controle de Sitophilus zeamais (Motschulsky, 1855)

Abstract

One of the most harmful insects to stored wheat seeds is the Sitophilus zeamais (Motschuslky, 1855). The diatomaceous earth, which contains microscopic particles of different shapes, rich in silicon dioxide, acts on the insect exoskeleton causing its abrasion and subsequent dehydration thereof. The carbonized rice husk, abundant and a free cost residue, has similar physical and chemical characteristics to diatomaceous earth, but was never tested for its insecticide efficiency. So the objective of this research was to evaluate the effects of milled carbonized rice husk and diatomaceous earth on the quality of wheat seeds hermetic stored and its effectiveness in controlling S. zeamais. One kilogram samples of wheat seeds were placed in 2 L airtight containers, sprinkled with milled carbonized rice husk (MCRH) and diatomaceous earth (DE) at concentrations of 0; 500; 1,000 and 1,500 g.t⁻¹, and were homogenized. Therefore, the containers were stored for six months. Assessments occurred in 0; 60; 120 and 180 days. The physical quality of seeds was evaluated by parameters: moisture content, weight of one thousand seed and bulk density. The physiological quality was evaluated by: germination, first count, cold test and accelerated aging. To determine the particle size distribution of the product was adopted the technique of laser granulometry in a liquid medium. The quantitative determination of chemical insecticides was performed by X-ray fluorescent method (XRF). Fifty adults of S. zeamais were placed on 100 grams aliquots of seeds, wich were extracted at 120 days of storage. The insect mortality was evaluated in 5-day exposure period. It is concluded that the milled carbonized rice husk (MCRH) and diatomaceous earth (DE) do not cause losses to the weight of one thousand, moisture content, or the physiological quality of wheat seeds during the airtight storage. However, the two products cause greater reductions than 5% in the bulk density. The best insect control responses are obtained with 1,050 g.t⁻¹ and 1,300 g.t⁻¹ of seed, for DE and MCRH, respectively. Five days of exposure are sufficient to high mortality of insects with the application of both products. DE shows 52.5% of silicon dioxide, as MCRH presents 76%. However, the average particle diameter of MCRH is 3.66 times higher than that was found for the particles of DE. Comparing physicochemical characterization of products with its effectiveness in mortality of insects, it is concluded that the most important point for the control of S. zeamais, is the size of the particles contained in the insecticide. Both CACM as TD can be used to control S. zeamais.