The black specs
on the leaves could be evidence of insect infestation. A very tiny
insect known as
(the term is plural and singular) is a common hibiscus pest that can
cause flower buds to drop before opening. They feed inside the
flower buds as well as on the leaves, and plants with active thrips
populations often appear dirty and unkempt with dark spots of fecal
matter visible on the leaves and buds.
If flowers do open, the
flowers may be deformed or spoiled by irregular white streaks. To
check for thrips, shake suspect-looking buds or leaves over a sheet
of white paper. If thrips are present, you will notice small
(one-sixteenth to one-eighth inch), slender insects that are yellow,
brown or black depending on their species and stage of development.
A 10X magnifying glass or hand lens will help you to see this pest
While the product you applied is labeled to control
thrips, it probably is not the most effective material to control
this pest and it would take repeated applications at intervals
recommended by the label. Other products for controlling thrips
include insecticidal soap (Safer’s Insecticidal Soap), spinosad
(Captain Jack’s Deadbug Brew), imidacloprid (Bayer Advanced Tree &
Shrub Insect Control) and lamba-cyhalthrin (Spectracide Triazicide
Once & Done Insect Killer).
'Might be Mites'
Hibiscus is also
including two-spotted spider mites that are common pests on many
plants during hot, dry weather. Symptoms include bleached out
(“stippled”) areas on the foliage where these very small spiders
suck the chlorophyll out of the leaf. You can also see very fine
webbing when two-spotted spider mite populations are high. The main
miticides available to home gardeners are horticultural oil and
insecticidal soap. Like thrips, spider mites are not clearly visible
to the naked eye. You can check for spider mites the same way you
check for thrips. Two-spotted spider mite adults are pale yellow to
light red with a dark spot on either side of their abdomen.