Preliminary results on the effectiveness of an enumerative sampling plan for gypsy moth in Sardinian cork oak forests


Abstract: The gypsy moth, Lymantria dispar, is the main threat to cork oak forests, as it causes complete defoliation during its periodic outbreaks. In Sardinia (Italy), gypsy moth density has been annually estimated in a network of more than 600 monitoring sites, by counting the total number of egg masses on 40 cork oak trees per site following a sampling protocol developed in Moroccan forests. This sampling method is adequate for scientific studies, but it is too much time-consuming for practical purposes such as the delimitation of infested areas for control programs. For this reason, a sequential sampling plans was developed using data collected from 1999 to 2011 in Sardinian pure cork oak forests during all different phases of gypsy moth population dynamics. The Taylor’s power law (TPL) was used to evaluate spatial distribution and the Green’s method to calculate an enumerative sampling plan at a precision level of 0.10 and 0.25. Gypsy moth population density was extremely variable, ranging from 0.05 to 59.4 egg masses per tree. TPL showed an aggregate distribution of egg masses with regression slopes significantly greater than 1 during progradation, culmination and retrogradation phases. Moreover, Green’s method indicated that at a precision level of 0.25 only 14 cork oak trees can be monitored when a damage threshold of 3 egg masses tree occurs. In conclusion, sequential sampling plans can be used to reduce sampling effort mainly when high population density of gypsy moth occurs.

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