Could be Mealy Bugs, maybe soft scale, that can be flushed off the tree with a sharp water stream. The ants are there helping you control these pests.

Here is a link that might be useful: Mealy Bugs

There's something about the image that 'bugs' me. Those appendages look hairy to me, rather than the coarser filaments I associate with mealybugs. What about another image?

Jean, what do you think?

Also, if mealybugs, the sweet excretions would be the attraction for most ants, unless they are carpenter ants (which are carnivorous).

Perhaps cottony cushion scale (the youngsters).

Look at these sites:

http://www.ipm.ucdavis.edu/PMG/r107301611.html

http://creatures.ifas.ufl.edu/fruit/cottony_cushion_scale.htm

Here is a link that might be useful: cottony cushion scale at UC

Not 'perhaps' a cottony cushion scale nymph, but I'd say without a doubt! Good one, Jean!

This scale can be a real pest of citrus. Not a big issue if you just have the one plant, unless it's very large. Read the article that Jean attached for you to familiarize yourself with the many beneficial insects that help control this scale insect. If you spot some of them, you may not need to do anything at all.

Indeed, the use of some pesticides can often result in an INCREASE of your pest.

Here is a link that might be useful: Click here for the image

Thanks Jean and rhizo! From the links, it looks like I should prune the inner side of the tree. I didn't see any beetle or parasitic fly on my tree, unfortunately, so I will try to prune and see if that helps. About half a year ago I sprayed the tree with neem oil because of whiteflies, and I hope I didn't kill the beneficial ones then! I LOVE this productive meyer lemon tree and I will try all that I can to save it :)

If the tree is of a reasonable size, take action.

The adults are quite obvious. Squish 'em. (Active vengeance feels good!) Do the same for the nymphs.

Neem oil is effective for insects that actually feed on plant tissues and juices. Once is dries, it has no effect on beneficials at all. Just so you know that your application of Neem didn't cause problems for the good guys, lol!

There are many people that seem to believe that, because of its low toxicity, Neem Oil will only affect insect pests and will not harm beneficial insects which is simply not true. If a product is toxic to any insect it will be toxic to all insects, depending on how contact with that poison is made. Once dried Neem Oio has no affect on any insect, good or bad, just as insecticidal soaps are not effective except when wet. If beneficial insects are sprayed with a Neem Oil solution it can prevent respiration and that beneficial insect will suffocate, just as it will with many insect pests. Since no one has looked closely that effect Neem Oil has on the insect DNA may will also follow with beneficial insects, simply because anything that has an adverse affect on one species DNA will also have an adverse affect on everything elses DNA, even ours.

Today I went and squashed the nymphs and cut out some crossing branches. I also wrap the tree trunk with duck tape to catch ants. The tree is about 7 feet tall and I am hoping that it'll survive! Thanks all for your reply, and I'll check again next weekend to see if they come back and let you know!

Confused, you could spray the plant with neem or other horticultural oil to take care of tiny eggs, too.

thanks rhizo_1! it's going to rain tomorrow and i wonder if it'll take care of the tiny eggs... probably not huh, when they are hiding in the canopy of the tree!

Rain will do nothing to these critters, sorry. :-(

The oils act as suffocants. Neem has the added property of causing problems for PLANT FEEDING insects long after it has been applied.

ah i see. so should i spray sorta from the ground up since the lymphs are mostly in the underside of the leaves / branches? that's tough... :)

Scale "bugs" feeds on the plant and then secrete a waste product called honeydew. You can google this and read about it. The ants come around to feed on the sweet, sticky honeydew. If you have a big infestation of scale and you walk under a large tree with it, it will "rain" down on you. Yuck. I generally physically remove as many creatures as I can. I try to prune off heavily infested branches, and keep the plant open and airy. I try to keep the plant as healthy as possible with compost and organic mulch, and pesticide and herbicide-free so beneficials including birds will come and help. I would put the scale that I removed and the pruned peices into a bag and discard. I probably would not compost this. Most plants can survive a moderate infestation, most plants are never entirely pest free. I agree horticultural oil acts by smothering the scale. Most folks feel these oils are fairly low toxicity. If you practice IPM you wouldn't really use them. Some folks make a homemade oil and water mixture. Not sure if this is less toxic, use olive oil? If you do use an oil, yes you should spray the undersides of the leaves where the scale hides. I think you would have to repeat after a time as the eggs hatch. I vote for enticing beneficials into the garden with organic practices and plant diversity. I think your Meyer will be OK.
Laurie