There are many factors that can contribute to leaf drop (and it's possible more than one is overlapping), but the pest pictured looks like a thrips ("thrips" is both plural and singular). Thrips are difficult insects to control indoors, and use of a pesticide is often needed to suppress outbreaks. Options include insecticidal soap and horticultural oil (choose one, don't combine them), both of which need to contact the pest directly in order to work (dried residues have no impact). Since thrips can wedge themselves into crevices and other areas of the plant that cannot easily be coated, it will require several applications to reach most of them. Some species of thrips pupate in the soil, and eggs can be inserted into the surface of plant tissue, so to catch these generational stages that sprays will miss, that's another reason why repeat sprays are needed. (Check the product label for details on how long to wait between sprays.)

Another option is the active ingredient spinosad, which is absorbed into the plant tissues on which it's sprayed. (It cannot move from a treated leaf into an untreated leaf, so repeat applications will still be needed as the plant produces new leaves, but at least each treatment lasts longer.) As the thrips feed (they suck plant juices), they ingest the toxin and die off.

A last-resort pesticide option is to treat the plants with systemic insecticide (often the active ingredient imidacloprid), which for indoor plants is usually applied to the potting mix as granules that gradually dissolve, so the plant absorbs the toxin through the roots and moves it into the foliage and stems. Products with this active ingredient are not legal for Marylanders that don't have a pesticide applicator's license to buy and apply, but it's permitted for use indoors. Each application may last a couple of months, but it will take time (perhaps several days or over a week) for the chemical to make its way from roots into foliage so it can start killing the thrips. There is always a chance that the insects are already resistant to this class of pesticide; it's not uncommon. This approach would be riskier to try if there are pets or children in the home that might try to eat the leaves of treated plants, but since many houseplants are harmful to ingest anyway, we're guessing this is less of a barrier.

Leaves can't heal from damage, and any leaves in the middle of the shedding process will continue to yellow and fall off, even if the thrips population is brought under control. However, it should at least slow and stop (beyond a normal, minor amount of periodic leaf drop) once the thrips are managed successfully.

There are some beneficial predators of thrips that some biological control companies sell, but they are very perishable and not always well-suited to indoor conditions (insufficient humidity levels, often). They may include predatory mites, minute pirate bug, and possibly lacewing larvae, though the latter aren't the best option for thrips outbreaks since they tend to go after larger prey.

Other causes of leaf yellowing and premature shedding include over-watering, under-watering, a need for repotting (if too many mineral residues build up in the potting mix over time), insufficient humidity, temperature swings (like exposure to air conditioning drafts or, in winter, cold drafts), a need for fertilizer (often nitrogen), and exposure to ethylene gas (such as from a incomplete combustion in a gas stove or near a garage where a car may be idled for a while). Ripe fruits also give off ethylene, but to have that great an impact, it's probably not a factor here.

Miri