As I mentioned in my last column, many pests can feed on hibiscus making this a very high maintenance choice for your landscape. Chewing insects include caterpillars, grasshoppers, snails and slugs, beetles, cut worms and leaf miners. Piercing-sucking insects include scale, mealybugs, spider mites, aphids, whiteflies and thrips. These pests are more of a problem in areas of poor circulation. The grenade scale can be hard to see because it blends with the bark. This soft scale will cause branches to die back.
Premature, flower bud-drop has become a more serious problem with hibiscus. Some varieties, especially some doubles, exhibit a chronic problem with premature dropping of buds. Some varieties bloom well during one period of the year and consistently drop their buds at other times. This is why variety selection can be important.
Nematodes, nutritional deficiencies, over fertilization, and environmental factors such as poor drainage and excessive water, drought, or salt spray can all cause flower buds to drop. Bud drop can be caused by insects such as thrips, caterpillars or a new insect pest called the gall midge. This flying insect larva lives in the base of the flower bud. If you pull apart the dying flower bud you will find these tiny, yellow maggots wiggling around. To combat these pests good housekeeping is a must. Try to rake the fallen flower buds as much as possible. Then spray the plant, especially the buds, and the soil beneath it with a mixture of a systemic insecticide like Merit and an insect growth regulator like Intercept or Neem. Or a soil application of Bayer Advanced Garden Tree and Shrub Insect Control. This will give you extended protection for a few months.
Hibiscus is sensitive to many pesticides so be sure to read the label before applying. The safest time to spray is early morning rather than in the middle of a hot sunny day.
Another pest, the pink hibiscus mealybug was killing the hibiscus on Marco Island in a big way. Many people arrived for the season finding damaged or dead hibiscus in their yards. This pest is reminiscent of the sago scale which caused many to remove sagos from the landscape as they became to expensive to maintain.
The Pink Hibiscus Mealybug came to us via the East Coast from Broward and Miami-Dade counties a few years ago and is multiplying and spreading very quickly throughout the island. This insect is originally from other tropical areas of the world such as Africa, Asia and Australia. It was found closer to home in the Caribbean in 1994 and made its way to the East Coast of Florida in Broward and Dade counties in mid 2002. This pest is expected to colonize the entire state of Florida and into Georgia.
The adult mealybugs are very small (3 mm long) with a pink body covered in short, waxy filaments which cover the entire body. When crushed their body fluids are also pink. The adult males are even smaller, reddish brown with one pair of wings. They also have two long, waxy tails. The freshly laid eggs are orange, turning pink before hatching. The eggs are encased in a cottony egg sac. The nymphs can and do crawl significant distances after hatching to find suitable host plants. They can also be spread by wind. This very prolific insect deposits up to 600 eggs at a time with 15 generations a year. The life cycle is approximately one month long.
The mealybug is a member of the piercing-sucking insects. As with all sucking insects it feeds on the sap of the plant releasing substances which injure or kill the plant. Sooty mold and ants soon follow. Other symptoms of an infestation include deformed leaves and shoots, unopened, bunched leaves, white cottony mass on buds, stems, fruit and roots, unopened or shriveled flowers or deformed fruit.
While it is called the Pink Hibiscus Mealybug it can be found on more than 100 different plants. It can attack fruits such as papaya, carambola, passion fruit, avocado, mango, citrus, guava and bananas. Many vegetables including tomato, pumpkin, cucumber, lettuce, cabbage, okra, peppers, beans and squash. Ornamentals affected include hibiscus, croton, allamanda, anthurium, heliconia, seagrape, schefflera, bougainvillea, oleander, ixora, ficus, ginger and lantana.
While chemical control has not been very effective there has been some improvement. The waxy covering protects the mealybugs and eggs from contact with the chemicals. But biological control does work. Greg Hodges at the Florida Department of Agriculture, Division of Plant Industry released a beneficial parasitic wasp all over Marco Island a few years ago. These beneficial wasps became harder to get and so the Department of Agriculture now reluctantly suggests spraying if you have extensive or special hibiscus plants infested with the mealybug. Spraying chemicals to try to control this insect will only kill the beneficial insects trying to kill the mealybugs. So before you reach for the spray take a closer look at the plant to see if you might have these beneficial at work in your yard. You will see the bunched growth with new healthy growth above it. If you squash the bug infested area and it is dry and powdery rather than wet and bloody the mealybugs have been successfully controlled by the beneficial insects. I see the results of these wasps everywhere and have attached a picture of what to look for on your plants.
There are two species of this wasp that will do the trick. Mealybugs parasitized by these wasps will leave behind “mummies” which will have an exit hole where the adult wasp emerges. The adult ladybeetle is also a voracious consumer of mealybugs as is the immature ladybeetle, also known as the “mealybug destroyer”. This insect looks like a colorful mealybug so don’t confuse them.
The insect can spread by wind very easily and the action of cutting infested branches will dislodge the insects for easier movement through the air. If you have cut the plants double bag the clippings in a zip lock bag if possible and put them in the regular household garbage rather than the yard trash. A lot of people gave up the fight and removed hibiscus from their yards. Hopefully enough of the beneficial insects will survive and multiply to make the pink hibiscus mealybug a thing of the past. The program certainly reduced the numbers drastically.
Diseases of hibiscus include leaf spot, canker and mushroom root rot. Canker is a fungus disease which causes twigs and branches to die back and sometimes the entire plant is killed. Reddish-orange fruiting bodies can be found on diseased bark. The best control is to prune off and destroy all diseased wood.
Mushroom root rot will cause hibiscus to wilt suddenly and die a short time later. Poorly drained soils and buried tree stumps or roots encourage this disease. Dead or dying plants should be removed with as much root system as possible and the soil should be replaced or sterilized before replanting.
Leaf spot may cause the death of affected leaves but usually the spotting is minor and little cause for alarm. The best control is to pick off or rake up the diseased leaves and destroy them.
Nematodes can cause a decrease in plant vigor. Symptoms include frequent wilting, poor growth, small leaves, and nutritional deficiencies. Soils can be sterilized prior to planting, but no chemical control measures are available to treat established plants. Mulching or adding manure or peat under the plants can reduce nematode damage by creating good soil and thus encouraging their natural enemies.
Hibiscus is a lovely but high maintenance plant. Unless you are willing to commit time and money for this maintenance it should be used sparingly in your South Florida gardens.
Eileen Ward and her husband Peter have owned and operated Greensward of Marco, Inc., a lawn maintenance and landscaping company since 1981. Watch Eileen’s gardening videos on MarcoIsland-TV.com.