Tree proximity and ethanol dose as factors in trap tree strategy for exotic ambrosia beetles

Many non-indigenous ambrosia beetles that are pestiferous in nurseries are attracted to ethanol.  Several recent studies have demonstrated that trunk injections of ethanol can induce ambrosia beetle tree attacks.  Consequently, ethanol trunk injections may be a potential tool to create trap trees and protect other desirable crop trees.  To understand how ethanol trunk injections may protect other trees, two experiments were performed.  In both tests, total ambrosia beetle attacks were compared between trees that received a 75 ml volume of either 1% or 50% ethanol.  Two tree spacing treatments were compared including 1% and 50% injected trees with growing containers contiguous and 1% and 50% injected trees with growing containers separated by 10 m.  The objective was to determine if trees with low stress levels (i.e., simulated by 1% injection dose) received protection from ambrosia beetle attacks in the vicinity of trees with high stress levels (i.e., trap trees simulated by 50% injection), but whether low dose ethanol trees lost protection when isolated by distance from high dose trap trees.  For experiment 1, zelkova trees were used and treatments were placed parallel in relation to a woodlot edge (~ 1 m from the woods edge).  For experiment 2, sugar maple trees were used and treatments were placed perpendicular in relation to the woods edge.  All experiments were done in a randomized complete block design with six and four replicates in the parallel or perpendicular tests, respectively.  Replicates were 20 m apart.  New ambrosia beetle gallery entrances were counted three times a week for two weeks and then treatments were re-randomized for an additional two weeks of counting.  New galleries were circled with a wax pencil on counting dates to avoid re-counting galleries.  As expected, high ethanol dose trees received much higher ambrosia beetle attack rates than low ethanol dose trees in both tests.  In the perpendicular test, the distance of the tree from the woodlot edge had minimal effect on beetle attacks within an ethanol dose range.  In both tests, low ethanol dose trees had less attacks than high ethanol dose trees when adjacent to the high ethanol dose trees.  However, total attacks on low dose trees in the perpendicular test were very low (n=2) making conclusions uncertain.  The results generally supported our hypothesis that trees with high ethanol dosages can provide some protection to trees with lower ethanol dosages when placed in close proximity, but the protection afforded by high ethanol dose trap trees appears to decline when trees are spatially separated.