Hello Lyonsden77 , I agree, they are probably aphids, but you need someone to answer from your part of the world to be sure.
If aphids, buy pytherin based insecticides and mix yourself. Aerosols are not good as propellant burns plant. White oil mixed yourself will kill aphids as well. Similiar is make your own by mixing equal volumes dishwashing liquid with cooking oil and mix well til emulsified, then use this to add 10ml per 1L of water. Shake often during use.

Yellow leaves? Too much water (drowing plant and it is dying or leaching away food) or not enough (thirsty and is dying)or hot weather (natural result is that plant drops older leaves to assist surviving)or natural drop of old leaves or too fresh a mulch around base (microbes in soil focus on breaking down mulch and forget plant needs for nitrogen) or lack of fertilizer (new growth sucks older leaves of nitrogen if not enough coming up through roots) or too much (burned roots and plant can't suck up food as roots and plant dying ), trace element deficiency (use trace element food), fungal problems such as collar rot or phythopera (symptons will vary, drooping with collar rot and dropping leaves with phythopera are the starting considerations), old age and needs a prune (or remove and put a new one in)..as you can see, a few reasons exist and theses are the obvious ones, so unless you are able to define symptons, we are only guessing. Diagnose over words is difficult, so good luck.

The same applies for the "Blue" hibiscus. Does it have a name? Is it a Tropical/Rosa Sinesis? Could be an inability to process phorphorous as you see with Andersonaii and is nothing to worry about. Maybe too dry and too much sun (sun is good, but need water to keep plant well). May well be too much water and leaching of food is happening.

Sparse leaves? Lack of food or natural plant habit (they all vary in habits, some greatly).

Best food varies. Use food high in Nitrogen in early spring, food high in potassium in early summer and food high in phosphorous late fall.

Watering needs vary throughout year, with soil type, position of plant, vigour of plant, proximity and closeness to other plants and type of other plants close by, so again this is a difficult question to qualify.

Good luck,
Regards, Brian Kerr.

I also had little black bugs on my Hibiscus and read up that they were aphids. I used a mixture of 1 cup vegetable oil, 2 cups water and 2 teaspoons of dawn liquid dish soap. I sprayed the plant well and moved it out of direct sun 5 days ago. The bugs have died and I have sprayed the leaves with water to remove the oil and bugs. Now, the leaves are turning yellow. I hope I didn't kill my Hibiscus!! Is this normal for the leaves to turn yellow and what should I do now? Move it back into sun where it was before?

hAVE THE SAME PROBLEM - WHAT SHOULD i USE?

not sure if this thread is still open but i wanted to find out what is eating my hibiscus. i just bought it from home depot on saturday - today is wednesday. it had 25 blooms on it the day i bought it. rained for 2 days & now it has none. the bloom has been eaten off (?) at the base of the bloom - rather i should say the stalk has been eaten at the base of the bloom. blooms are on the ground. looks like someone came & cut them all off & left them laying there ? do aphids do this ? leaves are good - still on the plant but turning yellow at the base of the plant ? Help ?

You can try using Bayer 3-n-1 for trees & shrubs. I use it with great success on all my plants. You can purchase at any big box store.

Lonnie