Tree resin flow dynamics during an experimentally induced attack by Ips avulsus, I. calligraphus, and I. grandicollis

The success of tree colonization by bark beetles depends on their ability to overcome host tree defenses, including resin exudation and toxic chemicals, which deter bark beetle colonization. Resin defenses during insect outbreaks are challenging to study in situ, as outbreaks are stochastic events that progress quickly and thus preclude the establishment of baseline observations of non-infested controls. We use synthetic aggregation pheromones to demonstrate that confined Ips bark beetle herbivory can be successfully initiated to provide opportunities for studying interactions between bark beetles and their hosts, including the dynamics of constitutive and induced resin exudation. In Pinus taeda L. plantations between 12 and 19 years old in North and South Carolina, U.S., trees were affixed with pheromone lures, monitored for evidence of bark beetle attacks, and resin samples were collected throughout the growing season. Baiting increased beetle herbivory to an extent sufficient to produce an induced resin response. Attacked trees exuded about three times more resin at some time than control trees. This supports previous work that demonstrated that information on constitutive resin dynamics alone provides an incomplete view of a host tree’s resistance to bark beetle attack.

Attack rates ofSirex noctilioand patterns of pine tree defenses and mortality in northern Patagonia

Accidental and intentional global movement of species has increased the frequency of novel plant–insect interactions. In Patagonia, the European woodwasp,Sirex noctilio, has invaded commercial plantations of North American pines. We compared the patterns of resin defenses andS. noctilio-caused mortality at two mixed-species forests near San Carlos de Bariloche, Argentina. We observed lower levels of resin flow and higher levels of mortality inPinus contortacompared withPinus ponderosa. In general,S. noctilioattacked trees with lower resin compared with neighboring trees. Resin production inP. ponderosawas not related to growth rates, but forP. contorta, slower growing trees produced less resin than faster growing conspecifics. For all infested trees, attack density and number of drills (ovipositor probes) per attack did not vary with resin production. Most attacks resulted in one or two drills. Attack rates and drills/attack were basically uniform across the bole of the tree except for a decrease in both drills/attack and attack density in the upper portion of the crown, and an increase in the attack density for the bottom 10% of the tree. Planted pines in Patagonia grow faster than their counterparts in North America, and produce less resin, consistent with the growth-differentiation balance hypothesis. Limited resin defenses may help to explain the high susceptibility ofP. contortato woodwasps in Patagonia.

Seasonal fire effects on mixed-conifer forest structure and ponderosa pine resin properties

This study examined the effects of spring and fall restoration burning in an old-growth mixed-conifer – ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) forest in southern Oregon. Variables measured include fuel loads, forest structure indices, mortality of large ponderosa pines, and pine resin defenses. One year after treatment, reductions in surface fuel loads and changes to forest structure parameters suggested that burning treatments could meet restoration objectives, with fall burns being somewhat more effective than spring burns. However, mortality of pre settlement pines was significantly higher in fall burns than in spring burns, and both were higher than in unburned controls. Bark beetles (Coleoptera: Scolytidae) were important mortality agents within 2 years after burning. Resin defenses (pressure and flow) were variable over the 2 years of postburn study but showed no evidence of decrease in burned trees; rather, resin defenses were significantly higher in burned trees than in controls at several measurement dates. While increased beetle attacks have previously been documented following burning, there has been much less research on resin responses to fire. These findings suggest that current models of beetle–host interactions do not properly explain the effects of prescribed fire in ponderosa pine forests.

Composition of loblolly pine resin defenses: comparison of monoterpenes from induced lesion and sapwood resin

Even-aged loblolly pines in three crown classes chosen to represent varying levels of vigor were inoculated with the blue-stain fungus Ceratocystisminor to induce hypersensitive tissue formation. There were no differences in monoterpene composition of the induced tissue among crown classes growing on the same site. However, there were significant differences in percent composition of individual monoterpenes between trees growing on a poorer wet site and those growing on a better dry site. There were also quantitative and qualitative differences in monoterpene composition of preformed xylem resin compared with the monoterpenes extracted from the hypersensitive tissue. The differences in composition between the preformed resin and resin found in the hypersensitive tissue may reflect, in part, the energy requirements for initiation of the inducible system.