Impact of physico-chemical properties of diluents on the toxicity of carbaryl in dust formulations to the larvae of Spodoptera litura (Fabricius)

Impregnated dusts of carbaryl were formulated separately with nineteen diluents having a very wide range of physico-chemical properties to study their impact on the toxicity to larvae of Spodoptera litura (Fabricius) in the laboratory. On the basis of significant differences in the LC₅₀ values as also similarities in various properties, the diluents were categorised into three groups. The first and most effective group comprised three clays, viz, kaolinite (LC₅₀, 2.105), fullers earth (LC₅₀, 2.377) and China clay (LC₅₀, 2.681), there being no significant differences in the effectiveness among these clays. The second group included only two less effective diluents, viz, tripolite (LC₅₀, 7.183) and steatite (LC₅₀, 8.158), being 3.41 and 3.87 times, respectively, less effective than the most effective kaolinite. The third group contained fourteen practically ineffective diluents, viz, magnesite, silica, calcite, felspar, dolomite, selenite, bauxite, quartz, gypsum, rock phosphate, bentonite, pyrophyllite, attapulgite and talc, since 10.0 per cent concentration of carbaryl dust with each of these diluents gave less than 50.0 per cent larval mortality. A strong positive correlation between surface acidity of the three most effective diluents and a negative correlation between pH of such diluents (Kaolinite:Ho = -3.0 to -5.6, pH = 4.6; fullers earth:Ho = -5.6 (tr.), pH = 3.5; China day:Ho = -3.0 to -5.6, pH = 3.8) and toxicity of carbaryl dusts to S. fitura larvae was evident. Considering together all the properties, viz, surface acidity (Ho), pH, bulk density (gm/100 cc), hardness, specific surface (sq.cm.1 gm), percentage of particles in different size ranges (<2μ, 2–20μ, 20–75μ), per cent moisture content, per cent moisture absorbing capacity and per cent oil adsorption; the acidic diluents coupled with high surface acidity (HO = 4.0) and low bulk density (<93 gm/100 cc) may prove to be effective in carbaryl dusts. This suggests that deactivators may be dispensed with in the formulation of carbaryl dusts with the so-called inert mineral diluents.