NEW CALCULATION OF CRITICAL NUMBER OF GYPSY MOTH (LYMANTRIA DISPAR L.) EGG MASSES FOR BETTER POPULATION DENSITY PROGNOSIS

Gypsy moth, Lymantria dispar was the dominat oak forest pest in Croatia untill 1960s. After this period population particularly in the continental part decreased with cyclical outbreaks every 10-11 years. Last two outbreaks occurred in 2003-2005, and in 2013-2014. In this study a new calculation of critical number of gypsy moth egg masses has been presented in order to estimate the risk of defoliation which is crucial for the decision about application of necessary control measures. The calculation shows critical numbers as number of egg mases per ha which differ from the previous calculations based on number of trees which carry at least 1 egg mass shown in percentages. Hitherto 5 Classes were known out of which the 5th was critical. This Class presented 50% of trees which carry at least 1 egg mas and it was assessed as critical. In such calculation the age of the trees was not taken in account although the amount of leaves in the canopy strongly depend on that. Furthermore the number of eggs in the egg masses was also ignored. New calculation is based on numbers of caterpillars which have the potential to defoliate an oak tree of different ages, obtained from literature. The average number of eggs in one egg mass was calculated out of 50 sampled and analysed egg masses. This data together with the normal distribution of oak trees per ha build the basis for the formula. Based on that 3 Categories were formulated which show the potential for defoliation and help to make the decision for applying control measures: Category I - control measures needed; Category II - control measures needed only if there is a special reason; Category III - no control measures needed. Critical numbers of old and new calculation have been compared in years of outbreaks. Results between new and old calculation show 25% difference which directly means 25% less area that needs treatment with insecticides. Although the new method has advantages there is still some limitation in the calculation considering that important variables are ignored. The real number of trees per ha and the actual number of intact caterpillars should be used for more accurate calculation and further reduction of forest area that needs protection. Future research should concentrate on precise calculations of leaf weight consumed by average caterpillar in the relationship to crown volume both for oak and common beech. These values in combination with the number of leaves in the particular forest would make the decisions on use of insecticides more accurate which would bring additional substantial financial savings.