Should I be concerned about twisted galls on my lombardy poplar trees?

Question

We have lots of large twisted galls on our lombardy poplar trees' leaf stems. I've heard that twisted galls on poplar trees are not a threat to the tree. So, I'm not worried about them killing the trees or anything.

Do the galls in the fallen leaves contain insects or their eggs that will promote more galls in future generations? Will picking up the leaves help to reduce the problem the next year?

I looked inside a gall, and found that it appeared to be full of some powdery fungus or something. Is that normal? Was it really fungus? Could the insects causing the galls be attracting other pests or diseases to my yard? I hear the things that cause twisted galls are related to aphids, and we have a big aphid problem. It could be a coincidence, but I thought I'd ask, just in case.

Last year, our poplar trees were covered in what looked like house flies (and this was rather disturbing), but after some research, I think they were a certain kind of fly that eats whatever causes the galls. There aren't as many flies this year, but there are even more galls (and there were a lot last year). I've heard that they are supposed to be less frequent after a severe year. This does not seem to have been the case, so far. I'm kind of concerned that the contents of the galls could be bad for the yard, or my health. I've heard about tree fungi causing issues in the past.

Answer 1

You have Poplar Petiole Gall Aphids, Pemphigus populitransversus. The galls have slits in them, in which the immature aphids can go in and out. From here:

The aphids secrete a white, waxy material which coats their body and the inside of the gall⁽¹⁾. They feed for about two weeks and the females bear live young that mature into winged aphids. These winged females will leave the gall through the slit that is apparent on the side of the gall and fly to an alternate host.

And

The alternate hosts are in the Brassica family so typically mustard or canola⁽²⁾. They spend the remainder of the summer on the alternate host before flying back to the trees in the fall to lay eggs that spend the winter.

^{(1) This would be the 'fungus' you were seeing.}

^{(2) Other crops such as cabbage, broccoli, turnips, etc will be affected, and they can be damaging to root crops in the family, as it is the roots they feed on.}

There are two forms, the sexually reproductive form and the parthenogenetic form. From here:

In the spring, the overwintering eggs hatch producing wingless females known as stem mothers. Stem mothers are parthenogenetic (can reproduce without mating), and give birth to a generation of wingless, parthenogenetic females inside the galls they create. Several generations are produced, until late summer when the galls become overcrowded. At that time, the females produce a generation of winged, parthenogenetic females which migrate to a secondary host⁽³⁾. Several generations of winged and wingless parthenogenetic females are produced. The winged females return to the poplars where they give birth to sexually reproductive male and female aphids. After mating, the females lay overwintering eggs in cracks and crevices, completing the life cycle.

^{(3) The alternative host is various species from the genus Brassica}

This is not a major pest, and you don't have to worry about it. Some fallen leaves will contain aphids, in either form, but most of the aphids will have flown by the time most of the leaf drop has occurred. If you want to get rid of the aphids, picking leaves as they form galls will be the most effective means. Spraying with insecticides is largely ineffective and can be counterproductive by killing potentially beneficial insects.