Sudden Infestation of a Blue Spruce

(Question)

Hello, we have two dwarf blue spruce plants in our front yard and our neighbour has a very similar plant that she says is a “tree” rather than a dwarf – I’m not sure of the differences, but hers when it was just planted had a longer main trunk with no branches (18″ or so) and ours didn’t (they had branches at almost ground level). So, to the question: a few days ago the neighbour’s plant got infested with what appears to be small back caterpillars in great numbers and they are destroying (eating) the needles. Do you know what these are and if there is a way to stop them? Also, since we have a similar plant about 15′ away can we prevent these from spreading to our plants?
Many thanks, Vladimir

 

(Answer)

There are a few insect pests of spruce trees which may be the culprit here: the spruce budworm, the yellow-headed spruce sawfly or the spruce bagworm, which has a distinctive pupa-like case.  In all three cases, larvae are present from May into June. Unfortunately, it isn’t possible to tell from your photograph, but you may be able to identify which has infested your neighbour’s tree from photos on these websites, and then you will have a better idea of your next steps:  http://ento.psu.edu/extension/factsheets/bagworm

http://files.dnr.state.mn.us/assistance/backyard/treecare/forest_health/whitesprucediagnosis.pdf

You may wish to consult a qualified arborist to give you a diagnosis, and also on the question of what you can do to prevent these insect pests from spreading.  Visit either Landscape Ontario at http://landscapeontario.com  or the Ontario chapter of the International Society of Arboriculture for a listing of arborists:  https://www.isaontario.com/