Infestation by elongate hemlock scale makes eastern hemlock less attractive to the hemlock woolly adelgid

Eastern hemlock (Tsuga canadensis), a native conifer from eastern North America is under severe threat by the hemlock woolly adelgid (HWA; Adelges tsugae), an exotic hemipteran introduced from Japan. Trees succumb in as little as four years as this insect continues to spread over the eastern hemlock’s range.  Interestingly, some studies show that when HWA shares its host with another exotic hemipteran, the elongate hemlock scale (EHS; Fiorinia externa) the negative impact on tree health is lower than the impact of HWA alone. This is likely due to the fact that EHS-infested trees support 40% lower HWA densities compared to previously uninfested trees. Lower HWA densities are possibly linked to a removal of nitrogen by EHS and subsequent suboptimal nitrogen content for HWA to thrive. In the present study we investigated whether HWA can discriminate between uninfested foliage over EHS-infested foliage. To test this we performed dual choice tests in petri dishes. Each choice test consisted of EHS-infested and uninfested foliage from the same tree and 3 HWA egg masses. After hatching, HWA crawlers were allowed to settled and the numbers of both settled and not-settled crawlers in each choice were counted at the end of the experiment. Our results showed that adelgids prefer uninfested hemlock foliage to EHS-infested foliage from the same tree. Being able to discriminate between these two choices is a biologically relevant decision as crawlers can actively move within a tree despite their passive dispersal between trees. Being able to avoid lower nutritional content can potentially minimize competition effects with other insects feeding in the same host.  Future studies will look at this avoidance pattern in the field.