Problems in estimating the density of larval beech caterpillar, Quadricalcarifera punctatella (Motschulsky) (Lep., Notodontidae), using frass drops by modification of the Southwood-Jepson method
1994
Kamata, N. (Tohoku Research Center, Morioka, Iwate (Japan). Forestry and Forest Products Research Inst.) | Igarashi, Y.
Three problems were recognized in estimating the last instar larval density of the beech caterpillar, Quadricalcarifera punctatella, using frass drops by modification of the Southwood-Jepson method. The first was that some frass was destroyed and washed away from the trap. The last instar's frass was exposed to the elements in a trap for 2 weeks and decreased 1.9% in weight, the difference having been washed away by rainfall. 5.9% of the frass pellets by weight were destroyed and there was a 3.3% decrease in their number. The remaining pellets accounted for 97.3% of the original so that the width threshold of last instars' pellets fell to 1.78 mm from 1.8 mm. Since this decrease was minimal we concluded that no compensation was necessary for field application. The second problem was that density would be under- or overestimated when the survivorship curve of the target stage differed. We hence decided to employ another method which used a cumulative curve of frass falling during outbreak periods. The density (Y) could be estimated from the following equation: Y 0.9881 log((10))X((a)) + 0.898, where X((a)) is the maximum value of the cline combined with the data collected two successive times on the cumulative curve. The third problem was the number of traps necessary to make the estimate at a given level of precision; this number decreased with density increase. Only five traps (1 square meter for each) were required with 0.2 of the ration of tolerance limit at the 70% confidence level. This was because this method counted frass drops rather than individuals and larval mobility and winds randomized the spatial distribution of falling frass over a long period
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