False ring formation in response to hemlock woolly adelgid feeding in eastern hemlocks

Background/Question/Methods

Herbivores can alter plant physiology through the induction of abnormal wood formation. Feeding by some insects induces the formation of false rings, a band of thick-walled latewood cells within the earlywood portion of the tree ring that reduces water transport. These false rings may be caused by the hypersensitive response, a plant defense against sessile herbivores and pathogens. Hemlock woolly adelgid (Adelges tsugae), and elongate hemlock scale (Fiorinia externa) are invasive insects that both feed on eastern hemlock (Tsuga canadensis). Adelges tsugae has a greater effect on tree health than F. externa, but the mechanism underlying their differential effect is unknown. We explored the effects of these herbivores by assessing growth ring formation in branches of trees that had been experimentally infested for four years with A. tsugae, F. externa, or neither insect. We measured false ring density, ring growth, and earlywood: latewood ratios in the two most recently deposited growth rings. 

Results/Conclusions

Branches from A. tsugae-infested trees had 30% more false rings than branches from F. externa-infested trees and 50% more than branches from uninfested trees. In contrast, branches from F. externa-infested trees and control trees did not differ in false ring formation. Radial growth and earlywood: latewood ratios did not differ among treatments. Our results show that two invasive herbivores with similar feeding modes have differing effects on false ring formation in eastern hemlock. If false rings are responsible for or symptomatic of water stress, this may provide a potential explanation for the relatively large effect of A. tsugae infestations on tree health.