Document: MAT-3-52-13

Community interactions and the population ecology of Dendroctonus frontalis.

AYRES, M.P.* ¹, F.LOMBARDERO ¹, R.HOFSTETTER ¹, K.KLEPZIG ² and J.MOSER ²

Dartmouth College, Hanover, NH 03755 ¹
Southern Research Station, USDA Forest Service, Pineville, LA 71360 ²

Abstract:
Population cycles in the southern pine beetle, D. frontalis, could be the result of endogenous feedback from community interactions involving mutualistic mycangial fungi, a bluestain fungus (Ophiostoma minus) that competes with the mutualistic fungi, and Tarsonemus mites that are phoretic on the beetles and transport the bluestain fungus. Mycangial fungi benefit the nutrition of D. frontalis larvae by concentrating nitrogen, which is otherwise a severely limiting resource. This dependency makes D. frontalis sensitive to abundance of the bluestain fungus, which outcompetes mutualistic fungi and thereby interferes with the nutrition of D. frontalis. Survival of D. frontalis larvae was only 0-20% within regions of phoem infested by bluestain fungus, but 60-100% within phloem that lacked bluestain fungus. Our model of community interactions postulated a mutualism between bluestain fungus and Tarsonemus mites. This was supported by: (1) laboratory studies indicating that Tarsonemus populations feeding upon bluestain fungus can increase by ~200 fold during the ~40 d from when they reach a tree via colonizing beetles and have to depart the tree with emerging beetle progeny; (2) manipulative experiments showing that Tarsonemus mites dramatically increase the proliferation of bluestain fungus within D. frontalis oviposition galleries; and (3) field surveys that reveal a strong positive correlation among infested trees between the abundance of bluestain fungus and Tarsonemus. Interactions between D. frontalis, Tarsonemus mites, bluestain fungus, and mutualistic fungi creates a negative feedback loop that could regulate the abundance of D. frontalis. Sampling of forests that were in the 1st and 3rd years of a beetle outbreak cycle indicated that mites and bluestain fungus were relatively less abundant in the early phase of the outbreak, suggesting that there is a delay in this feedback loop, which would contribute to cyclical population dynamics. This is the first example that we know of delayed density-dependence resulting from a system of mutualisms, competition, and commensalism.

Keywords: Dendroctonus frontalis, bark beetles, fungi, mites, mutualism, population dynamics, delayed density dependence

This abstract is being presented at: 3:30 PM in session:
ANIMAL ECOLOGY