Impact of light availability and water stress on eastern hemlock trees and hemlock woolly adelgid settlement

Background/Question/Methods: Eastern hemlock (Tsuga canadensis) is a dominant shade-tolerant tree in northeastern United States, but it has been declining since 1951 when hemlock woolly adelgid (Adelges tsugae; ‘HWA’) first arrived in Richmond, Virginia. Determining where HWA settles on hemlocks under different abiotic conditions is important in understanding the insect’s expansion. Resource availability such as light and water can affect herbivore selectivity and damage. Using a 3*3 factorial design, we examined how HWA settlement and survival were affected by differences in light and water availability for the host trees.  We held 160 two-year-old hemlock saplings in a greenhouse at the University of Rhode Island. All 160 trees were covered with shade cloth: 80 trees with low-light shade cloth (10% light availability) and 80 trees with high-light shade cloth (90% light availability). Half of the trees in each light treatment received ample water, and the other half were water stressed. The trees were grown under these conditions for six weeks prior to inoculating with HWA. Half of the trees (=20) in each treatment received HWA, while the other half remained as insect-free controls (=20). There were twenty trees in each of the eight treatments; watered/full-sun/infested, watered/full-sun/insect-free, watered/shaded/infested, watered/shaded/insect-free, water-stressed/full-sun/infested, water-stressed/full-sun/insect-free, water-stressed/shaded/infested, and water-stressed/shaded/insect-free.

Results/Conclusions: HWA settlement was 50% higher in the shade treatment than in the light treatment, and 37% higher on water-stressed versus well-watered plants; the treatments did not affect settlement on new-growth foliage. Once settled, neither the light nor water treatments affected HWA survival on new- or old-growth foliage. The fact that light and water availability impacts HWA settlement, but not survival, may help explain why some trees become infested with HWA more rapidly than others, a fact that may help develop more effective management strategies for hemlock forests invaded by this pest.