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ŠUMARSKI LIST 3-4/2014 str. 74     <-- 74 -->        PDF

Like the other entomopathogenic fungi, Isaria fumosorosea infects its host mainly through the external cuticle penetrating by mechanic pressure of germ tubes and releasing hydrolytic enzymes (St. Leger et al. 1986; Draganova 1988; Hajek and St. Leger 1994; Ali et al. 2010; Augustyniuk-Kram 2011; Augustyniuk-Kram and Kram 2012).
As the aim of this part of the study was to investigate how different modes of exposure of host larvae to fungal conidia affected the mortality caused by the mycosis, larvae were dipped into the suspension, or they were made to walk on filter paper discs in Petri dishes soaking with conidial suspension, or they were made to walk on oak leaves or larch needles sprayed with suspension. The dipping of L.dispar larvae into the suspension (variant Ld-v-4) resulted in the lowest efficacy of 3.75 % ± 1.76 in comparison to the variants with the other modes of inoculation where efficacy was 12.37 % ± 4.40 in the variant Ld-v-2 (indirect treatment by contact with conidia on filter paper discs) and 13.04 % ± 4.93 in the variant Ld-v-6 (indirect treatment by contact with conidia on oak leaves), respectively (Fig. 2). We conclude that indirect exposure of larvae through surface contact of host with conidia caused higher efficacy of mycosis.
According to Dunlap et al. (2007) susceptible insects exposed to blastospores and conidia of Isaria fumosorosea showed declined growth and high levels of mortality. We found that the two lepidopteran hosts species were not susceptible to the examined Isaria fumosorosea isolates. Furthermore, although the larvae were inoculated with highly concentrated suspensions placed on filter papers, the efficacy was rather low.
Bioassays with Beauveria bassiana (Bals. – Criv.) Vuill. and larvae of different insect pests (Ostrinia nubilalis Hb, Lepidoptera; Leptinotarsa decemlineata Say, Coleoptera) showed that the age of the inoculated larvae is of importance for the efficacy (Feng et al. 1985; Draganova 2000). Contrary to the expectation that the susceptibility of larvae will decrease with age as described by Keller and Zimmermann (1989), in our experiments younger instars of L. dispar were more tolerant to mycosis caused by Isaria fumosorosea isolates (variants Ld-v-1 vs Ld-v-2 and Ld-v-5 vs Ld-v-6). Similar observations were made by Ferron (1967) who showed that larvae of Melolontha melolontha F. were more sensitive to Beauveria brongniartii (Saccardo) Petch with increased age.
The bioassays with L. monacha show that 3rd instar larvae have a low susceptibility to mycosis caused by Isaria fumosorosea isolated from H. cunea and to two other re-isolates of the fungus (from L. monacha and from L. dispar (Fig. 3)). The development of the infection was slow in the variant Lm-v-1 with a small increase in efficacy on dpi 7. In contrast, the mortality in the variants Lm-v-2 and Lm-v-3 increased from 14.12 % ± 7.16 and 13.92 % ± 10.04 on dpi 5 to 21.80 % ± 7.16 and 22.78 % ± 10.04 on dpi 7, respectively.
According to Goettel et al. (1990) and Lecheva and Draganova (1998), fungal isolates are more virulent to their initial hosts. In our experiments, discrepant results concerning the initial host and the virulence of the isolates to L. dispar and L. monacha larvae were obtained (Fig. 2, 4). A comparison of the variants Lm-v-2 (inoculation with Isaria fumosorosea isolated from L. monacha larvae) vs Lm-v-1 (inoculation with Isaria fumosorosea isolated from H. cunea) (Fig. 4) and the variants Ld-v-5 (inoculation with Isaria fumosorosea isolated from L. dispar larvae) vs Ld-v-1 (inoculation with Isaria fumosorosea isolated from H. cunea) (Fig. 2) confirms the findings of the cited authors. The efficacy in the variants reached values of 25.65 % ± 7.16 vs 12.66 % ± 5.52 and 3.75 % ± 1.41 vs 1.25 % ± 0.63, respectively. However, when comparing the efficacy in the