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I'm trying to identify this bug on Tecoma stans (yellow elder) kept in a cool house in wintry Ontario.

Microscope image

Sorry for the quality but this is about as good as my microscope can manage. Bug is tiny, less than 1 mm., so very hard to see detail with no assistance. The bug is squat and bulky with no neck between head and thorax, with a flat head, 6 legs, 2 antennae about as long as the front legs. Mouth parts indistinguishable. It behaves like an aphid, sucking plant juice from softer growth and forming masses of bugs which become visible when numerous. Colour is black at the head and tail and lightish brown on the back of the thorax. The distinguishing feature is the two horn like appendages at the rear end.

About the closest I can come is to a flea beetle, possibly a larval instar. I did also consider a tarnished plant bug but I am much less inclined to that choice.

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The two horns at the back (siphunculi or cornicles) are common in aphids, and useful features for identification.

But less than 1mm is very small for aphids.

It may be Aphis fabae, as a search indicates it can be present on your tree, and is 1-2 mm long.

https://en.wikipedia.org/wiki/Black_bean_aphid

But not sure at all.

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    Very useful answer, particularly the terms siphunculi and cornicles. It appears they are unique to aphids, so the genus is right and narrows the field and defines the proper gardening treatment; we do require concrete identification before applying treatment. I'm very happy to give you accepted answer status on this for the major contribution. My microscope is a recent acquisition and I am just getting used to the access to the micro world - it is very helpful in distinguishing clues. Jan 12 at 12:31

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