Logistic regression approach to the prediction of tree defoliation caused by sawflies (Hymenoptera: Symphyta)
Holusa, J.(Vyzkumny Ustav Lesniho Hospodarstvi a Myslivosti, Jiloviste-Strnady (Czech Republic))E-mail:[email protected] | Drapela, K.(Mendelova Zemedelska a Lesnicka Univ., Brno (Czech Republic). Lesnicka a Drevarska Fakulta)
We tried to find a relationship between the population density of two sawfly species, Cephalcia lariciphila and Tomostethus nigritus, and the consequent portions of defoliated trees using logistic regressions. In both species, only male catches were efficient for statistical analyses as they largely prevailed on sticky boards. Concerning Cephalcia lariciphila, there was no significant correlation between the mean male catch on yellow sticky boards and probability of defoliation. On the contrary, there was a highly significant relationship between the density of larvae in soil and the percentage of defoliated trees. In Tomostethus nigritus, the defoliation depended on the year. The first year three degrees of defoliation were observed and a multinomial distribution was calculated by the generalised logistic model, whereas a statistically significant binomial model using only two stages of defoliation was calculated for the second year. Mean catch of 300 to 350 males per yellow sticky board causes the defoliation of 90% of ashes. Our results confirm the suitability of logistic regressions for this type of studies.
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