Components of Antagonism and Mutualism in Ips pini–Fungal Interactions: Relationship to a Life History of Colonizing Highly Stressed and Dead Trees
Efforts to describe the complex relationships between bark beetles and the ophiostomatoid (stain) fungi they transport have largely resulted in a dichotomous classification. These symbioses have been viewed as either mutualistic (i.e., fungi help bark beetles colonize living trees by overcoming tree defenses or by providing nutrients after colonization in return for transport to a host) or antagonistic (i.e., fungi compete for a limited resource and reduce brood development with no apparent benefit to the beetle). We investigated several components of one beetle-fungus interaction. Specifically, we addressed whether beetle entry into, and development within, a host tree vary with the degree of colonization by ophiostomatoid fungi. Ips pini (Say) transports several species ofophiostomatoid fungi, the most common being Ophiostoma ips Rumbold) Nannfeldt, in the process of colonizing its host, Pinus resinosa Aitman. We introduced this fungus 0, 3, 7, and 10 d before beetle entry to characterize its effects on I. pini colonization and development. This sequence allowed quantification of temporal effects and comparison of results with other systems. Fungal growth was greatest when inoculated before beetle colonization. Fungal colonization reduced beetle entry into logs, but increased brood production. Mate capture was not significantly affected by fungal growth. The benefits imparted by O. ips to its beetle vector during brood development are compared with results from other systems. This difference may in part be related to the exploitation of highly stressed and dead trees, rather than vigorous hosts, by I. pini.
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