The following is a list of possible suspects and some ways to curb their influence. Aphids are 2-3 mm long, soft bodied insects that feed on plant sap and often infect stressed plants. They can be winged or wingless and can be translucent green or black in colour. They congregate in bunches or colonies on the underside of leaves and new shoots. Infested plants become yellow and twisted with curled leaves that eventually fall off.
Growth is stunted and, in bad cases, new buds are poorly formed.
3-6 mm flies. The adults are harmless but the young maggots live in the soil and feed on organic matter. They thrive in moist soil and affected plants will be stunted and foliage may fall off. The best control is to remove plants from the soil, wash the roots and repot in sterilized soil. I wouldn't worry about these guys too much, for in all my years dealing with plants I have yet to see damage caused by fungus gnat maggots.