Fungal Interactions and Host Tree Preferences in the Spruce Bark Beetle Ips typographus

Abstract

The spruce bark beetle Ips typographus is the most damaging pest in European spruce forests and has caused great ecological and economic disturbances in recent years. Although native to Eurasia, I. typographus has been intercepted more than 200 times in North America and could establish there as an exotic pest if it can find suitable host trees. Using in vitro bioassays, we compared the preference of I. typographus for its coevolved historical host Norway spruce (Picea abies) and two non-coevolved (naïve) North American hosts: black spruce (Picea mariana) and white spruce (Picea glauca). Additionally, we tested how I. typographus responded to its own fungal associates (conspecific fungi) and to fungi vectored by the North American spruce beetle Dendroctonus rufipennis (allospecific fungi). All tested fungi were grown on both historical and naïve host bark media. In a four-choice Petri dish bioassay, I. typographus readily tunneled into bark medium from each of the three spruce species and showed no preference for the historical host over the naïve hosts. Additionally, the beetles showed a clear preference for bark media colonized by fungi and made longer tunnels in fungus-colonized media compared to fungus-free media. The preference for fungus-colonized media did not depend on whether the medium was colonized by conspecific or allospecific fungi. Furthermore, olfactometer bioassays demonstrated that beetles were strongly attracted toward volatiles emitted by both con- and allospecific fungi. Collectively, these results suggest that I. typographus could thrive in evolutionary naïve spruce hosts if it becomes established in North America. Also, I. typographus could probably form and maintain new associations with local allospecific fungi that might increase beetle fitness in naïve host trees.

Keywords: Endoconidiophora polonica; Endoconidiophora rufipennis; Grosmannia penicillata; Leptographium abietinum; bioassays; host choice.

Figure 1

Testing adult Ips typographus host preferences in multi-species choice arenas with a quadrant of water agar medium (control) and quadrants of spruce bark agar medium made from Norway spruce, white spruce, and black spruce. (A) Four beetles were released in each arena. (B,C) The number of beetles entering different media was registered after 18 h. (D,E) Beetle tunneling length in different media was measured after 5 days. Bars show meanvalues with 95% CIs. Dots represent individual replicates (n = 9 individual trees per spruce species). Treatments with different letters (a, b) differed significantly (ANOVA with Tukey’s post hoc test).

Figure 2

Host preferences of adult I. typographus in the presence of bark beetle-associated fungi. (A) Single-species choice arena containing semi-circles of bark agar medium made from Norway spruce, black spruce, or white spruce. The medium was either colonized by one of four fungal species 15 days earlier (right) or was left un-colonized (medium control, left). (B) Three beetles were released into each arena. Most of beetles entered fungus-colonized medium just beside the fungal inoculation plug (arrow). (C) The percentage of beetles that entered un-colonized medium (Medium control), fungus-colonized medium (Fungus) or remained on the surface (No choice) 48 h after introduction of beetles to the arenas (numbers inside the bars show total number of beetles that made their choice). p values show the outcome of Wilcoxon signed-rank tests comparing fungus and medium control for each combination of spruce species and fungal species (n = 9 individual trees per spruce species). E, Endoconidiophora; G, Grosmannia; and L, Leptographium.

Figure 3

(A) Trap bioassays to test attraction of I. typographus toward traps baited with fungus-colonized spruce bark medium (Fungus) or un-colonized medium (Medium control). Empty traps served as a negative control. Two beetles were released into each test arena. (B) The percentage of beetles that entered the different traps or remained in the test arena (numbers inside the bars show actual beetle numbers). p values indicate the outcome of Wilcoxon signed-rank tests comparing fungus and medium control for each combination of spruce species and fungal species (n = 18; two independent tests using nine individual trees per spruce species). E, Endoconidiophora; G, Grosmannia; and L, Leptographium.

Figure 4

Lengths of necrotic lesions in the inner bark of cut bolts of Norway spruce, white spruce, and black spruce 3 weeks after inoculation with four bark beetle-associated fungi. EP, Endoconidiophora polonica; GP, Grosmannia penicillata; ER, Endoconidiophora rufipennis; and LA, Leptographium abietinum. For each fungus, two isolates were inoculated (e.g., EP1 and EP2; see Table 1 for details). Agar alone (without fungus) was inoculated as a control. Bars show meanvalues with 95% CIs. Dots show individual replicates (n = 9). Treatments with different letters (a–e) differed significantly (ANOVA and Tukey’s HSD test with p < 0.05).

Figure 5

(A) Bark beetle tunneling in single-species choice arenas containing semi-circles of bark agar medium made from Norway spruce, black spruce, or white spruce. The medium was either colonized by one of four fungal species 15 days earlier (right) or was un-colonized (left). (B) Beetle tunneling length in un-colonized bark agar (Control) and in bark agar colonized by different bluestain fungi 5 days after the beetles were introduced into the arenas. Bars show meanvalues with 95% CIs. Dots show individual replicates (n = 9 individual trees per spruce species). Treatments with different letters (a, b) differed significantly (ANOVA and Tukey’s HSD test with p < 0.05). E, Endoconidiophora; G, Grosmannia; and L, Leptographium.