If they are aphids - need photo to confirm - don't panic! White pine aphids seldom do any permanent damage and I seriously doubt aphids were the cause of the death of your previous pines. There are other issues which can be much more serious to the health of these trees but it is only speculation at this point exactly what caused the problem....unreasonable to blame the aphids however.

While ants do hang out with - an even 'farm' - aphids for their sweet honeydew, they are also attracted to the sweetish sap or resin the pines can emit for any number of reasons, some, like pine borers - quite serious. Try hosing off - usually a strong stream of water will dislodge/remove the bulk of the problem. If stronger stuff needed, try a horticultural oil.

and fwiw, your other conifers are safe. White pine aphids feed only on white pines. No probs with the leylands or green giants.

thanks
please see the attached pictures.

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I have the same problem, and I'm worried it's going to kill the tree. Do white pines have any kind of defense mechanism against these aphids?

Can't identify for sure from the photos, but these are almost certainly not aphids. Most serious white pine pests are beetles or aldegids.Try Googling these to identify what you have.

They do look like aphids, probably Cinara strobi. Pretty common, and occasionally heavy outbreaks occur, but they rarely cause serious damage and are eventually brought under control by natural enemies. Gardengal48's recommendations above are good. Sometimes controlling ant nests in the area will make it easier for predatory insects to get to the aphids.

Here is a link that might be useful: White pine aphid