| Summary: | This study was conducted to determine the larval instars of Diprion pini L. (Hymenoptera: Diprionidae) that damage Scots pine (Pinus sylvestris L.) at an altitude of 1204 m at Çankırı-Kurşunlu in 2004. In the same study, the determination of the most appropriate period if chemical control is necessary regarding the relationship between D.pini and its egg parasitoid Neochryocharis formosa (Westwood) (Hymenoptera: Eulophidae) was considered. After detection of D.pini larvae Scots pine shoots containing at least 60 D.pini larvae from randomly selected Scots pine trees were sampled at each sampling date (once a week). The larvae were brought to the laboratory in a cooler, and the head capsule widths and lengths were measured under a stereomicroscope and recorded. After the data were evaluated according to Dyar’s law and larval instars of D.pini were determined, larvae instars and proportions of instars in the population (%) were determined for each sampling date. Regression analyses were used to determine the relationships between the head capsule width, length, and the instars of D.pini larvae. Results show that the increment ratios of head capsule width increase for the 1st to 6th D.pini larvae were 1.353, 1.339, 1.400, 1.287, and 1.1249, respectively. The increment ratios of the head capsule widths are close to the value of 1.4 of Dyar’s law. Therefore, it is understood that the occurrence ratios (%) for D.pini instars can be determined by sampling larvae at different dates according to Dyar’s law. A significant relationship has been determined between the head capsule widths and lengths of D.pini larvae. Larval length can be used in practice for instar determination in field studies. There was no record on the head capsule increments of D.pini instars in the literature. Additionally in this study, the egg parasitoid N.formosa adults, which had developed in the field-collected and D.pini eggs in the laboratory culture began emerging on September 3rd, 2004 when the D.pini larvae were in the 6th instar. Therefore, it is determined that the most appropriate chemical control period without damaging the egg parasitoid may be the period when the 2nd instar D.pini larva proportion in the population reaches 50%. These results are important for the protection of egg parasitoids as well as the effectiveness of chemical control in large-scale applications.
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