Varying densities of the spring generation of the hemlock woolly adelgid were manipulated on 16 previously uninfested eastern hemlocks in an open-field plantation in Massachusetts. In contrast to experimentally created hemlock woolly adelgid populations in a forest, as reported previously, there was no evidence of density-dependent survival on a tree-wide basis in the plantation in the spring (progrediens) generation. There was, however, comparable density-dependent survival of settled crawlers and sexupara production when samples of the population were examined from branches with high density. Plantation hemlocks had 9.3 times more foliage and 10 times lower adelgid densities per cm than the forest hemlocks. These results show that density-dependent processes in the progrediens generation may only be evident when hemlock woolly adelgid density reaches a critical threshold. In the sistens generation that begins in midsummer, we counted a mean of 126 settled crawlers on marked branch on all 16 trees, but none of these adelgids survived the mid-summer aestivation phase, perhaps due to higher temperatures that were recorded in the plantation compared with a nearby hemlock forest, where 16% of the adelgids survived the aestivation phase. Whole tree counts of overwintering adelgids revealed that the adelgid populations had gone extinct on 13 out of the 16 trees. Mortality in the midsummer aestivation phase often exceeds overwintering mortality that has been widely thought to be the main factor that limits adelgid population growth and spread, particularly in northern states.
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Vol. 44 • No. 1