Abstract
Thisstudy was carried out in Scale Insects and Mealybugs Department, Plant Protection Research Institute, Sharkia Branch, ARC, Egypt from May to September 2017 on navel orange leaves infested with the Parlatoria black scale, Parlatoria ziziphi (Lucas) to calculate economic threshold and economic injury levels depending on the loss of leaves chlorophyll content. Simple correlation and partial regression were calculated between numbers of insect and chlorophyll loss percentage in infested and uninfested units (inch2). Positive highly significant correlation was recorded between numbers of insect and percentage of chlorophyll loss. Chi-square (X2) was used to calculate economic threshold and economic injury levels of P. ziziphi that causes economic damage in disk chlorophyll content which were 9 and 14 individuals per inch2, respectively.
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