Here is what Herbs2000.com has to say about "Pests and Diseases of Hibiscus"

Pests and Diseases of Hibiscus


The organic philosophy
The aim of all organic practices -permaculture, biodynamics, natural farming is not just to reduce and ideally to eliminate the use of toxic chemicals that are harmful to humans, animals, plants and the planet, but to understand the processes of nature in order to participate, rather than to interfere.

This means a change in thinking. Instead of thinking monoculture on a massive scale, think biodiversity over a smaller area. Instead of thinking of insects as pests, think of them as part of the natural order, with the plant-damaging insects kept in check by predators. Instead of thinking of perfection in ornamental and edible plants as blemish-free, know that nature is not driven by glossy advertising. Think always of the way plants live without interference from humans.

Food for resistance
If you feed your hibiscus with compost-good-quality, balanced compost -you can never give them too much. Conversely, if you feed any plant with chemical fertilizers, you can easily give them too much. Too much can kill off all micro life. With edible plants, too much can contribute to cancer promoting nitrites, and the imbalance that can result from overfeeding with artificial fertilizers can create pest and disease problems. Nitrogenous fertilizers in particular can be harmful because they promote rapid soft growth that attracts sucking insects.
Just as mammalian predators pick off the runt of the herd, so insects will overcome the weakest of the crop. If you doubt that compost is resistance food, try an experiment using both organic and inorganic fertilizers by growing carrots or cabbage in two lots. Give one lot the best compost you can and the other lot a nitrogenous fertilizer, such as sulfate of ammonia. The latter will quickly be eaten by pests.
Another analogy is stress: stress in humans results in vulnerability to colds, flu and infectious diseases. Stress in plants makes them similarly susceptible.
A soil rich in organic matter not only provides nutrients but inhibits the growth of fungal and bacterial diseases, such as phytophthora, by encouraging the natural antibiotic activities of mycorrhizal fungi. And of course, the texture of such a medium provides hibiscus roots with that high air filled porosity they need. When they are healthy, well-fed hibiscus shrug off or outgrow pest damage.

Stability in the garden
Aim to provide all plants with the conditions they need. Aim for both lateral and vertical diversity, by matching plants with the conditions that suit them at every level, from ground-covering creepers through mid-high shrubs to the top of the canopy.
A stable garden does not contain impossibly difficult plants from quite different climates and conditions. Nor does it grow a huge number of similar plants in isolation from all others. And apart from the procedure of planting and transplanting, a stable garden is not routinely dug or hoed. Rather, layers are allowed to build up in the way a rainforest floor accumulates. Mulching and layering duplicate the processes of nature.
A stable garden in which natural pest control is practiced will always have a constant, but manageable, supply of pests providing food for a constant number of predators. Both are accepted as part of the ecosystem. Not all insects are bad -some are predators, some never cause plant damage, and others never build up their numbers.
Companion planting for insect control
Companion planting is a component of the stable garden, but it is not a cure-all. On its own it is ineffectual: it doesn't rid the garden of pests, but it does aid diversity and stability. Through releasing different scents, through utilizing different levels of soil, and by occupying different garden spaces in certain combinations, companion planting can attract, or repel, specific insects. For example, plants which attract aphids and are good companions for hibiscus include the annuals feverfew, coriander and nasturtium, and Hyssopus officinalis.

Hibiscus pests
Some hibiscus pests are worse than others. The larval grubs of garden butterflies can chomp their way through many a hibiscus leaf, but their harm is mostly visual. There are grubs which live off the foliage of shrubs and soft-wooded trees and other grubs which usually hatch out simultaneously. This means that if you find one, you'll find more. Watch closely for them, especially in the early morning, and if you discover them in the early stage, you may be able to pick off and destroy the whole batch.

Chewing insects
Beetles too can be picked off. Apart from the flea-like bronze beetle (Eucolaspis brunnea), which leaps too quickly to be caught, most beetles can be hand removed if the bush is not first disturbed.
Snails and slugs, though voracious chewers, are a threat only to fresh new leaves, mainly on young plants. The best natural control is plentiful bird life. Other chewing pests include grasshoppers and katydids, those bright-green, well-camouflaged, flying bugs. Stick insects are also chewers, but fortunately don't often bother hibiscus. Excelling at camouflage on the bush, the females lay eggs which drop to the ground and emerge as nymphs in spring, climbing up the stem to feed on new leaves. A simple way to prevent them getting there is to band the base of the trunk or stem with a ring of grease -try Vaseline -that stops them in their tracks.

Sucking insects
Sucking insects may be less conspicuous. Although the green vegetable bug, Nezara viridula, and its immature form with the colorful mosaic back are often easy to spot, most sucking insects are a lot smaller. Scales are only 1/16 th in (1 mm) long; spider mites, pinprick-sized pests, can cover the undersides of leaves; powdery or fluffy mealy bugs are a menace, and there's always the common aphid. Be alert for these. Evidence of their presence is white spot or chlorosis of the leaves (spider mites), misshapen buds (aphids), or sooty mould (by-product of mealy bugs and aphids).
To avoid spraying other than a last resort, try first squashing these pests by hand or removing them with a strong-pressured hose. Squashing or hose-blasting mealy bugs will probably not work though, because they coat themselves and the surrounding surface with a waxy substance that resists dislodgment. For mealy bugs, try a paintbrush application of water mixed half-and-half with methylated spirits. Spot applications of this sort (i.e. using a paintbrush) are always environmentally safer than wide-coverage spraying.
Thrips can be difficult to deter as they are fast movers. They are worst in dry conditions, and are not usually a problem in moist areas. Frequent hosing and misting can eliminate them. Whitefly infestation most often occurs in heated glasshouses. It shouldn't be a problem for the home garden where hibiscus have good air circulation.

Other pests
A big problem is gall midge. It lays its eggs in the flower buds, which the larvae then damage. The buds eventually drop, and this bud-drop is the most obvious sign of the pest. To rid plants of this pest, remove all dropped buds or buds that begin to mold. Regularly apply orthene to the foliage and buds, and diazinon to the soil.
Another problem is Japanese beetle, Popillia japonica, fond of buds and flowers. Adult beetles can be controlled by spraying malathion, rotenone, sevin or methoxychlor.
There are other pests of course, most notably, perhaps, the dreaded hibiscus beetle (Macroura concolor) -scourge of hibiscus gardens in Queensland, Australia, to which it is fortunately confined.

Natural controls and non-toxic pesticides
For most of the above-mentioned insects, natural controls should be tried. Among natural predators are hoverflies, which can consume huge numbers of aphids and also eat mites and scale insects; praying mantis, which are fully carnivorous and eat aphids, leaf hoppers, caterpillars, beetles and sometimes moths; spiders, which besides consuming flies feed on the larvae of moths and whiteflies; and, best of all, the ever-friendly ladybug. Ladybugs should be encouraged everywhere. Upon hatching, their larvae start immediately to eat aphids, mealy bugs, woolly aphids, thrips and other sap suckers, and they continue into adulthood to clean up these menaces.
Non-toxic sprays should be used with caution, not because of harm to humans but because indiscriminate use can kill both the targeted insect and its predators. The non-toxic preparations described here are short-lived and break down quickly, so they should be used up quickly. Pyrethrum (make your own from Tanacetum cinerariifolium, or buy the ready-made produce from garden centers) is a broad-spectrum insecticide. Garlic spray (soak 3 oz (85 g) of whole unpeeled garlic in mineral oil for 24 hours and add 1 pint (600 ml) of water in which a bar of soap has been dissolved; strain, dilute by five times, and spray) will work if it is eaten -it is not a contact spray -and can kill stink, bugs and the mosaic-backed vegetable bug. Chile spray (1 cup of fresh chilli peppers pulped with 1/2 cup of dried chilli and 1 cup of water) controls caterpillars and loopers, which are also deterred by sprinkled cayenne pepper. Buttermilk spray (1/4 cup of buttermilk and 2 cups of flour mixed with 1 gallon (5 liters) of water) will control mites when applied to the undersides of leaves every two days. Onion spray can be good for scale, thrips, aphids and mites (pour 3/4 pint (500 ml) boiling water over 2 1/4 lbs (1 kg) of chopped unpeeled onions and strain; dilute 20 times with water for spraying). Applications of ash from the fireplace, provided it is wood ash and dry, can control leaf-eating earwigs, grasshoppers and katydids. Some gardeners recommend diluted antiseptic solution as a good general insecticide.
You can also try traps. Cut a piece of white or yellow cardboard the size of your hand, and coat it with Vaseline. Nail it to a stake or tie to a hibiscus branch. Insect pests, especially night-flying ones, will be lured to it, as they are to all light, and will stick fast. Such a simple device is good for assessing what is in your local insect population. A "trap" with a different purpose, that of catching aphids, can be made from a liquid containing sugar and yeast.
Another non-toxic suggestion is a diluted yeast- based spread laid around the garden in small quantities to attract predators such as hoverflies and lacewings, both of which feed on aphids.

Hibiscus diseases

Phytophthora
The soil-borne wilt disease associated with soils that dry out in summer and are wet in winter, can kill many ornamental shrubs, including hibiscus. It pays to avoid the kind of conditions favorable to phytophthora because affected plants usually do not recover. The problem rarely occurs where hibiscus are given good drainage and ample compost.

"Collar rot"
Visible as a browning discoloration, often slimy, at the root join, is prevalent where winters are cool and damp, and drainage poor. You can try a treatment of removing all affected stems and bark and painting the discolored area with Bordeaux paste. Clear the surrounding ground completely and spray with a strong garlic solution. Sclerotina rot affects stems at ground level.

Repeated applications of strong garlic or chamomile tea can help.

If your hibiscus has succumbed to tot and you want to replant in the same spot -inadvisable but sometimes unavoidable -you must first improve the site. Start by removing the affected plant and burning it, then dig up the whole area beyond root depth and spread, distributing the infected soil in an unimportant area, such as plugging a hole in the driveway. Some gardeners advocate lighting a fire and spreading it over the entire site before refilling, as fire will kill off any remaining pathogens. The next step is to infill for drainage by lining the cavity with a layer of pumice mixed with fine gravel or pumice over coarser stones, adding sand, then filling the cavity with new topsoil. To be extra-cautious, use sterilized potting mix.

Besides diseases of the soil, there are fungal diseases that can affect foliage. Leaf spot is the main one to watch for. Containered plants may be slightly more susceptible to it than in-ground plants, especially when positioned in places with less air circulation. Dark brown or black spots on leaves indicate the presence of pathogens which are most active in wet weather. Heavily infected leaves, where the individual spots have merged to discolor the entire leaf, will result in shedding, and shrubs so depleted can probably only be salvaged by the application of a fungicidal spray, such as Bordeaux. Liquid Bordeaux can be bought from garden retailers or made at home to an exacting recipe of copper sulfate and calcium hydroxide. Bordeaux paste uses the same two ingredients less diluted, with skim-milk powder for viscosity.

In general, compost and mulch are the best root-rot preventatives. Dolomite sprinkled on the area can also help. An all-purpose spray for fungal prevention can be made from a "weak tea" of nettles, comfrey, yarrow or horseradish. Even better, a weak liquid poured off from seaweed soaked in fresh water has a four-fold benefit: it is a foliar fertilizer, it promotes frost-resistance, it is an insect repellent and a fungicide, and the residual seaweed can be composted or used as a mulch.

Solutions to other problems

Sparse foliage
Apart from insect infestation or the general poor conditions that result from undernourishment, the most frequent cause of sparse foliage is insufficient sun. Site hibiscus in maximum sun.

Crinkled leaves
Check for aphids or leaf hoppers on the undersides of leaves. Large colonies of aphids can suck the life out of leaves before they are noticed. Remove the affected leaves along with the offending insects. If the infestation is severe, try spraying.

Deformed and sickly foliage
If it is possible or likely that your hibiscus has been exposed to systemic insecticides, it will quickly sicken. Hibiscus do not tolerate systemic chemicals, such as maldison, which are absorbed by the plant. Good feeding and watering may help it recover.

Yellow leaves
Yellowing of leaves at the base of branches and stems occurs naturally as the hibiscus sheds old foliage. If yellowing occurs higher up the stem, especially at the growing tips, it could indicate magnesium or iron deficiency -trace elements easily added to the soil. Alternatively, the discoloration could be a sign of over-watering or over-feeding with chemical fertilizers, in which case suspend both. Yellowing is another sign of exposure to systemic insecticides.

Yellow leaves with green veins
Yellow leaves with green veins are a clear indication of iron deficiency. Apply a good feeding of compost and manure, and scratch in kelp meal or seaweed. Iron chelate is often recommended specifically to target the deficiency. Dig this in lightly -never pack down tightly around the stem. Mulch, kept clear from the base of the stem, can also help.

Other leaf discoloration
Brown margins might be windburn, in which case watering and light feeding can restore health. Persistent and prolonged windburn, indicative of a windy climate to which hybrids may not be suited, is a problem that might be best solved by replacing hybrids with wind-tolerant coastal species.
Salt toxicity also shows up as brown leaf margins. Treat with generous watering. Leaves that turn brown at the tips, purple at the edges, and become brittle could indicate a lack of potassium.
Frost damage
Once frost has damaged your hibiscus, there's nothing you can do. If severe, the plant may not recover at all; if light, it will struggle back of its own accord to produce new growth in spring. Do not prune affected parts until the weather has warmed and new growth is established. If you live where frosts can be expected every winter, you may consider pot culture to enable you to move the plant to shelter each year.

Weeds
Any weeds growing around a hibiscus are competing with it for nutrients. Remove them. Remember that the feeding roots of hibiscus are very close to the surface, depending on the same top few inches of soil occupied by weeds. The easiest way to keep the area weed-free is to thoroughly clear it before covering with a layer of mulch; the mulch will keep the weeds at bay.

Lawn encroachment
When hibiscus are sited in a lawn, an area of about 3 square feet (1 square meter) should be kept clear of competing growth. Mulching will deter the grass from encroaching again.

Lichen
Gray, green or silver lichens occasionally establish themselves on old mature trees. They won't bother the hibiscus but may bother the gardener. It would have to cover and smother the whole shrub before harming it. However, if you want to remove the growth, scrub it off with a scrubbing brush or apply Bordeaux paste in winter.

Ants
Many ant species are valuable predators. They only invade hibiscus if there's something there for them to feed on, and although the odd ant may be attracted to nectar, ants in large numbers are usually after the honeydew excreted by aphids or mealy bugs. Deal with the aphids and the ants will go too. Moisture is an ant deterrent -they increase in dry places -as is pyrethrum spray. Banding the base of the hibiscus stem with Vaseline will prevent ants from traveling further up.

Poor flowering
Few flowers in an otherwise healthy plant may result from too much shade, or, if sun is adequate, from excessive nitrogenous feeding which promotes leaf growth at the expense of flower production. Bud damage may also be a cause.

Buds but no flowers
If your hibiscus are under attack from borers or beetles penetrating the buds before they open, the ground will show the evidence. The treatment of removing all buds, both from the bush (including the tips of branches) and from the ground below, and burning them, followed by insecticidal spraying, should reduce the problem, but may not eliminate it.
Bud drop can also result from lack of water during warm sunny weather; it is important not to let hibiscus dry out.

Flowers appear late in the season
The most likely cause of late blooming is sun and temperature -not enough sun and not warm- enough temperatures.

Blooms change color
The hybrids sold today are the result of years of intensive hybridizing, of crossing one with another, and often backcrossing again to achieve desirable attributes. As with many such "manmade" garden plants, dominant features of parent plants or earlier ancestry occasionally appear after they have been bred out. Sometimes hibiscus revert to colors of an earlier form; sometimes they'll revert from double to single, from ruffled to unruffled, or from overlapped or windmill single to plain single. This happens most often out of season, while true-to-type blooms can be expected in summer and fall.