عنوان مقاله [English]
Introduction: Land use change, deforestation, grazing, intentional and unintentional fire and invasive pests and diseases are all the major cause of damage to the Zagros forest ecosystem. The green oak leaf roller (Tortrix viridana L.) is one of the important pests of Zagros forests. Larvae of the pest significantly damage the oak forests with feeding on leaves and buds of different species of oaks each year. Larvae enter 2'd instar after eating the internal contents of oak buds, then the third and 4th instars larvae consume whole buds and even oak leaves. After development to the 5th larval instar, they roll the leaves and enter the pupal stage. The attacked trees revitalize by creating new leaves from early May onwards. New leaves, pale green and are smaller than normal leaves. Continuation of the pest activity on Quercus species causes gradual weakness the trees and readiness to accept a variety of secondary pests and diseases in later years. Based on the status of Zagros forests and their importance in the conservation of soil and water sources, the nutritional indices of Tortrix viridana larvae of two species of Oak i.e. Quercus infectoria and Q. libani were determined to get a better understanding of this pest.
Materials and Methods: Since early April, with regular visits, last instar larvae (5th instar) of T. viridana in oak forests in Perdanan areas (around the village Ghabr Hossein) were collected and transferred to the laboratory chamber. The larvae were fed on the two oak species. The leaves of these species were collected and transported to the laboratory in a room at a temperature of 25°C and suitable photoperiod conditions. The larvae were fed individually or in a group. In a grouping method, 10 larvae in two replicates and in individually method 40 larvae (one larva per replicate) for each host were considered. Larval weight, amount of consumed food and weight of feces were estimated by using a sensitive digital Carriage scale (0.001 gr). Relative consumption rate (RCR), relative growth rate (RGR), the efficiency of conversion of ingested food (ECI), the efficiency of conversion of digested food (ECD) and approximate digestibility (AD) were also calculated. For statistical analysis t-test and SPSS17 software were used to compare the mean of the data.
Results and Discussion: Results of t-test (α=5%) indicated that the larval biomass, RGR, ECI, and AD were significantly different among the hosts in both methods of grouping and individually rearing of the larvae. In this study, Q. libani showed the lowest rate of digestibility. The larvae fed on Q. infectoria have the higher rate of AD than Q. libani because of high RCR, RGR, ECD and ECI. Also, the results related to the comparison of two methods (grouping and individually rearing of the larvae), on nutritional indices were significantly different and in both host trees, the rate of RGR, ECI and ECD in grouping method was more than individually rearing method. RCR is the indication of the insect's exploitation speed of food. In the other words, it shows feeding rate regarding insect's weight at the specific point of time depending on the amount of water and other nutrition physicochemical characteristics in insects. The result showed that this index rate in group rearing method on two host species is equal, but in individual rearing method on Q. infectoria, it is more than Q. libani. The possible reasons are more feeding of larvae in individual rearing method due to the lack of competition stress, the more likelihood of the desirability of Q. infectoria compared to Q. libani, the probability of the existence of antixenosis in Q. libani. The similar study investigating individual and group rearing methods in one insect species is not available. This result confirms that T. viridana larvae’s has a tendency toward social life and in the group situation, they eat effectively. In fact, although larvae in individual form feed more because of lack of competition stress, but in the group they make use of that little food in the best way.
Conclusion: In this study, Q. libani had low food quality. As a result, its relative consumption rate, relative growth rate, the efficiency of conversion of ingested food and efficiency of conversion of digested food was less than Q. infectoria. According to the results, Q. infectoria is an appropriate host for this pest. Also, it shows that the larvae were more considering the rate of larval biomass, RCR, RGR, ECI and ECD in grouping method than the single method.