Studies in sampling technique. I. Estimation of whitefly (Aleurolobus Barodensis) incidence in sugarcane

(i) The incidence of white-fly is highly variable and the maximum variation occurs in leaves. The variations between the clumps and the canes if and when they exist are of a much lesser magnitude. Therefore, increase in the number of leaves alone in a sample is pre-eminently effective in the reduction of error of an estimate. (ii) In a field where the infestation is either high or fairly high (puparia per square inch of the affected area more than 5.00), Nested sampling has to be adopted so as to ensure the equitable representation of clumps, canes and leaves in a sample. In such a field, 10-13 per cent, of the 3 ft. units should first be selected and then Nested sampling done in each unit in that particular form of the alternatives, which gives the larger number of leaves in the sample, namely 2 clumps x 2 canes × 5 leaves per unit or 20 affected leaves equitably representing the clumps and canes in a unit as far as possible or 18-20 per cent, of affected leaves (i.e., about one in every five) equitably representing the clumps and canes in a unit. (iii) In a plot of low or mild infestation, Nested sampling is not essential. Only the leaves as a whole may be subjected to random selection, other requirements being the same as mentioned under number (ii) above. (iv) For a required error percentage, a medium infested field (puparia, 2.00-5.51 per square inch) requires a smaller sample size than that needed in a plot of high infestation or of an infestation which is fairly high. A difference in the sample size in the number of leaves by even about 75% gave the same efficiency. This suggested inherent homogeneity in variation occurring in mild form of infestation. (v) If in each leaf, 10 random inch units (about 25% of the total inch units) be taken instead of enumerating the whole leaf, the estimate of the incidence will be fairly reliable with a very slight additional error; but for practical consideration and working convenience, the complete enumeration of a leaf may be preferred to counting puparia in 10 random inch-units