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ŠUMARSKI LIST 3-4/2014 str. 73     <-- 73 -->        PDF

on SDAY. The isolates were cultured for 15 days on slopes of SDAY in tubes and on Petri dishes at 22°C, and obtained conidia were washed down with sterilized water. The concentrations of conidia were determined by enumeration in a Thoma chamber. Insects were inoculated with conidia using several methods (Table 1). Larvae of L. dispar were either dipped directly into the conidia suspension (1x108 conidia/ml), or indirect methods were applied: a) surface con­tact of larvae with 1 ml of conidial suspensions (1x108, 1x109, 3x107, 3x108, or 4x108 conidia/ml) placed on filter paper discs (90 mm in diameter) in Petri dishes (Draganova and Staneva 1990) or b) contact with oak leaves or larch needles dipped in conidia suspension. Larvae in control variants were treated with water. Second, third and fourth instar larvae of L. monacha and L. dispar were used in the experiments. In total, 11 variants were performed with 20 larvae per repetition, with 4 repetitions in each variant. The acronyms for variants are presented in Table 1. All larvae were kept under laboratory conditions at 25 ± 2 °C, 60 ± 5 % R.H and 12:12 h L: D in a climate chamber (Percival Inc.). Lymantria monacha larvae were fed with larch needles. Larvae of Lymantria dispar were reared on artficial diet of Bell et al. (1981) or fed with oak leaves in case of treatment of oak leaves. Mortality of larvae was checked daily for 20 days and the efficacy of the fungus was corrected with mortality in the control treatments and calculated according to Schneider-Orelli’s formula (Püntener 1981).
Results and Discussion
Rezultati i rasprava
The results of the conducted studies are shown in Fig. 1, 2, 3 and 4.
It is evident that L. dispar and L. monacha larvae are within the host range of the used Isaria fumosorosea isolate from H. cunea and the re-isolates obtained from infected larvae of D. pini, L. monacha and L. dispar, but their susceptibility is rather low. The mortality due to mycoses showed a slow increase in all variants and was below 20 % with the exception of the mortality in the variant Ld-v-3 (Fig.1). The cu­mulative daily mortality due to mycosis in this variant in­creased to 41.25 % ± 12.26 on the 4th day and to 60.00 % ± 12.26 on the 11th day after the inoculation with 1x109 conidia/ml of the Georgian isolate of Isaria fumosorosea.
Although insects were inoculated with conidial suspensions with very high concentrations (1x 109 conidia/ml), the mortality 20 dpi didn’t exceed 60 % (Fig. 1, 2). In experiments with third instar L. dispar larvae an increase of the concentration of the conidial suspensions of the same isolate (Georgian isolate of Isaria fumosorosea isolated from H. cunea) from 1x108 to 1x109 conidia/ml resulted in higher efficacy – from 12.37 % ± 4.40 in the variant Ld-v-2 to 60.00 % ± 12.26 in the variant Ld-v-3 (Fig. 2).
According to Keller and Zimmermann (1989) the concentration of infective material necessary to initiate infection depends largely on the host and the pathogen. In our study we observed that larvae still living after 20 days post inoculation in all variants were not infected. They successfully completed their metamorphosis and turned into pupae. This is an evidence of low susceptibility of both Lymantria species to the Georgian isolate and three re-isolates of Isaria fumosorosea.