▁around ▁ 1 9 0 0 . ▁ ▁Because ▁this ▁species ▁is ▁not ▁native ▁to ▁the ▁United ▁States , ▁the ▁F ras er ▁fir ▁has ▁not ▁evol ved ▁any ▁type ▁of ▁defense ▁against ▁it . ▁ ▁These ▁insect s ▁typically ▁lay ▁about ▁one ▁hundred ▁eggs ▁and ▁have ▁three ▁gener ations ▁per ▁year . ▁The ▁ad el g id ▁attacks ▁the ▁tree ▁by ▁feed ing ▁in ▁f iss ures ▁within ▁the ▁b ark ▁of ▁trees ▁larger ▁than ▁about ▁four ▁cent im eters ▁in ▁diameter ▁at ▁breast ▁height . ▁As ▁it ▁fe eds , ▁it ▁releases ▁to x ins ▁contained ▁within ▁its ▁sal iva . ▁These ▁to x ins ▁reduce ▁the ▁conduct ance ▁of ▁sap wood ▁being ▁built , ▁which ▁causes ▁water ▁stress ▁and ▁k ills ▁the ▁trees . ▁ ▁The ▁Great ▁Sm ok y ▁Mountains ▁National ▁Park , ▁in ▁North ▁Carolina ▁and ▁Tennessee ▁contains ▁about ▁ 7 5 % ▁of ▁all ▁southern ▁spr uce - fir ▁e cos ystem s . ▁These ▁e cos ystem s ▁covered ▁vast ▁port ions ▁of ▁the ▁S out heast ▁during ▁the ▁last ▁ice ▁age , ▁when ▁the ▁climate ▁was ▁cool ▁and ▁mo ist . ▁Since ▁the ▁retre at ▁of ▁the ▁gla ci ers , ▁this ▁e cos ystem ▁has ▁been ▁conf ined ▁to ▁the ▁higher ▁elev ations ▁in ▁the ▁mountains ▁of ▁the ▁S out he astern ▁United ▁States . ▁The ▁spr uce - fir ▁forest ▁has ▁evol ved ▁as ▁an ▁island , ▁away ▁from ▁any ▁other ▁e cos ystem s ▁of ▁its ▁type . ▁This ▁has ▁presented ▁valuable