Hibiscus Pests and Diseases
by Wally Morgan and Lee Bettridge

The prevalent Pests and Diseases for hibiscus growers vary from place to place around the world. This justifies the various quarantine laws in the different countries. Treatment varies too as different governments authorize chemicals for use in their areas. I try to rotate the types of chemical used in the garden to prevent the insects building up a resistance.

I will attempt to list as many problems as possible with suggested solutions, treatments you will need to check with your appropriate authority. Some will be the safer organic alternatives where possible. Always remember to read and follow the directions supplied with chemicals, and particularly the advice on protective clothing, goggles, etc.

THE GOOD GUYS
Beware: there are many beneficial insects, etc. in your garden. I have lots of little green frogs and the work of the Assassin Bug, the Praying Mantis, the Spider and the Lady Beetle amaze me as they (and their progeny in many cases) consume the pests.

Unfortunately, the build-up of bad guys just forces me to use chemicals from time to time and I know I deplete the good guys when I do that.

The CHEWERS
Caterpillars - Grasshoppers - Hibiscus Pollen Beetle - Monolepta Beetle -Harlequin Beetle – Bugs and Others.

Carbaryl was always a good spray for caterpillars and hoppers and most beetles.

BIFENTHRIN and IMIDACLOPRID used together is now my spray of choice, mainly because, for the time being, it gets those pesky pollen beetles which now seem to be resistant to most other sprays. The advantage in using these two chemicals together is that one works on contact, whereas the second is stored within the plant for some time.

Another good multipurpose spray to use is Baythroid which uses BETA-CYFLUTHRIN as the active ingredient.

The Monolepta beetles swarm in spring here attacking Tipuana and Cassia trees, shredding the flowers. They then drop to the hibiscus and turn flowers into lace and shred buds and young leaves. CHLORPYRIFOS or CARBARYL works wonders and creates a carpet of dead beetles under the bushes very quickly. Take care when using CHLORPYRIFOS as it will cause leaf drop.

For those with a manageable number of hibiscus, it is better to pick off your old flowers and dispose of them in a bin bag. This breaks the beetle breeding cycle where the young feed in the mushy old flower, then bore into the ground to pupate and later emerge as beetles to repeat the process. White containers, (I use ice-cream containers), with just a little water and a drop of detergent to break the surface tension also attract beetle to the trap where they sink and drown.

The SUCKERS
These include Aphids, Mealy Bug, Scale, White Fly and various Bugs.

Aphids and Mealy Bug: A sharp blast with a hose will remove most. The chemical IMIDACLOPRID which is a systemic is the spray of choice for good control. It may be necessary to treat the surrounds for ants as they manage the "herd" of suckers often moving them from place to place.

Another systemic spray currently available is Trifend, (TAU-FLUVINATE with MYCLOBUTANIL) as the active ingredients. This spray is good for Aphids, Thrips, Whitefly and acts a Miticide for Two-spotted mite.

You can also use Baythroid as a contact spray.

WHITE FLY 

I had a nasty experience the first time I found this pest, brought home on some abutilons. Months later when I noticed the swarms of tiny insects, I bagged some and took to the local Department of Primary Industries office - and they were sent to their Brisbane based entomologist who just happened to be away for six weeks. On his return and recovering my sample from his freezer, he identified the white fly, but did not know which one! He referred me to an Experimental Station supervisor who gave these instructions which worked - I have never had them since. I also used this to clear up huge infestations on other hibiscus properties.

The chemical is Confidor, (IMIDACLOPRID) used 3 times in seven days, every 3 days - to break the breeding cycle. You need a very weak white oil solution as well - and this acts as a spreader and also sticks the pests to the leaf - which must be sprayed fully, front and back, over the whole plant.
 

Another chemical that can be used effectively on white fly is Trifend, (TAU-FLUVINATE with MYCLOBUTANIL). 

White oil is available from garden nurseries, but you also mix your own at home. The oil mix I used effectively was 250 mL of Canola Oil and 150 mL of concentrated washing up detergent. This I put in a 2 litre milk bottle, adding water, little by little, shaking in between to emulsify (aerate) the mixture. After the froth settled, I made this up to onr litre and use 100 mL per 100 L of my spray tank.

For small quantities, that's about 6 mL ( a little over a teaspoon) of oil mixed with half a teaspoon of detergent in a bucket with IMIDACLOPRID according to label directions. Of course, put the oil mix in a small shakeable container with a little water first. The white oil mix must always be mixed slowly with a little water and shaken, the rest of the water then being added - otherwise, the oil will just float on top. 

SCALE 

(In particular White louse scale or Citrus snow scale): This needs a Confidor and oil mix, but with a stronger oil concentration. I have also found that Malathion in a normal mix with oil can be sprayed on the stems of the hibiscus and up into the foliage avoiding the leaves (it can burn the foliage). Unfortunately, the dead scale insects stay on the plant and need to be hosed off). Oil alone works for many scales. Spraying the stems with cooking oil (often Canola) aerosols is effective but must not be sprayed on the foliage. It will not kill the plant but will cause the leaves to die and recovery with new growth will take a while.
There can be a problem with white oil on hibiscus leaves - that is why only one part in 100 is recommended as the leaves may otherwise damage in sunlight. Spraying later in the day after 3 pm allows the oil effect to dissipate before the following days sun can cause damage.

The main reason for using oil is to dissolve waxy secretions that insects like scale use for protection so the insecticide can then do its work Homemade white oil is now used much more. Canola oil, mixed with a little washing up detergent and shaken vigorously to emulsify it, is suitable. 

Another chemical I have success with is ACEPHATE 970g/kg, currently sold as Lancer 970.

MITES: Red spider or two-spotted mite sometimes becomes a real pest after insecticide sprays in spring wipe out the good guys who are their predators. Those yellow leaf blotches with dark webbing patches at the back followed by a big leaf drop leaving many bare stems is a sure sign you are late getting to the problem. Me included.

As spring advances, there is a need for a spray for mite - the foliage must be covered front and back for an effective kill.  Vertimec (ABAMECTIN) is the spray I use. Even that is supposed to be somewhat ineffective now as the resistant mites are believed to be breeding up in large numbers. Another chemical which is newer to the nursery industry is Trifend, (TAU-FLUVINATE with MYCLOBUTANIL).

FUNGAL Diseases include black spot, mildew, bottrytis and the like.

A spray with copper oxychloride, compatible with most insecticides, is an excellent preventative. Remember that a black spot appearance is common on many varieties in autumn as new growth starts and the spotted leaves are soon shed. It is not worth a panic. Bottrytis is most evident when buds get that greyed dead area on the side and then abort. Copper is usually effective but chemicals like Rovral (iprodione) are specifically recommended. Another chemical which is good to rotate with is Trifend, (TAU-FLUVINATE with MYCLOBUTANIL).

In damp wintry weather, the old flowers hang on and get mildew which spreads to the developing buds. Here's a case for removing old blooms - or starting a copper spray routine. I usually do a copper spray combined with my insecticide and fertilizer, during autumn. I generally ignore black spot as the plants grow out of it in spring. I don't believe that it is a true black spot disease but just a symptom of old leaves. 

ERINOSE MITE

The comments from the DPI include:

Colony characteristics
As active plant growth is needed for the mites to establish, hardened older growth will not develop symptoms when exposed to the mite. Colonies of mites move onto new growth as the galls age. In severe cases, whole branches and plants are affected. Generally, pruning off affected tissue does not provide control without an appropriate spray programme. Growers, who have cut back their infested plants and allowed them to regrow, have reported that the new leaves were still infested.

 Control

Affected plants should be pruned to improve their appearance prior to spraying. Existing galls will persist on the plant until the affected tissue dies, making it difficult to judge the success of sprays. The aim of the spray program is to protect new growth.

The Department of Primary Industries has obtained a special (board) approval for the use of chlorpyrifos to control this pest in cropping situations. Applied as a spray at four weekly intervals throughout the year, chlorpyrifos appears to protect hibiscus in high risk localities. In particular, frequent sprays are needed during the main growth period to renew protection. There is no chemical currently registered for use in the home garden against this pest.

To avoid spreading this problem, enthusiasts are reminded not to move cuttings from infested areas into other districts. Badly effected shrubs and prunings should be removed and either burnt, buried, or taken to the dump in an enclosed plastic bag.

My personal view!

(Click an image for a larger picture)

The best treatment is to prune as much of the affected material as possible and dispose of it in sealed bin bags.  Then treat the branches remaining with Rogor, a well-known miticide which, unfortunately, affects Hibiscus foliage (which you will have pruned).

Now feed the plant with some good fertiliser like Nitrophoska and water well.

It is known that hibiscus plants that are well fed and watered cope better. Then it is just a matter of removing any affected leaves as they appear. Spraying with Lorsban (chlorpyrifos) is beneficial but at about 3 mL per L of water which will also damage the foliage.  It also keeps the beetle away (but at 1 mL/L).

Unfortunately, the pest is microscopic and easily moves back in from the neighbourhood.