Question: I have a rose bush in my yard with many holes in the leaves, but why don’t I ever see any insects?
Answer: The holes in your leaves are most likely caused by a pale reddish-brown beetle called the Chinese rose beetle. This beetle was introduced to Hawaii before 1896 and is common in gardens today. To knowledgeably manage the Chinese rose beetle, you should know a few important facts about this pest. The beetle is nocturnal with peak feeding and mating activity 30 minutes after sunset. They hide during daylight hours. Furthermore, with our year-round growing season, your rose plants will most likely experience holes even though you try to control the beetles.
It is a good home gardening practice to establish a "threshold" for damage. This means that before taking extreme measures to irradicate the Chinese beetle from your garden, take into consideration that the beetle is part of the ecology of your garden and a little damage is tolerable.
Q: What can I do to control the Chinese rose beetle?
A: To keep their numbers at a tolerable level, here are a few management options: Hand-pick beetles after sunset and drop them into a jar of soapy water to discard. Since beetles avoid light, plant the rose bush near a light source, such as a street light or available porch light, to deter them. Keeping the plant as healthy as possible will also deter insect damage.
Q: I see black, circular spots on the leaves of my roses. What can I spray?
A: "Black spot" is a common fungal disease of roses that grows on the upper surface of the leaf and stems. It is caused by free-standing water on the surface of leaves. It requires this water to reproduce and grow. Once you see the black, fuzzy spots, it is best to practice sanitation by pruning infested leaves, placing them directly into a bag and discarding in the rubbish can to prevent the spores from spreading. To prevent black spot, it is important to water only the root system of rose bushes. Installing a drip hose that surrounds the plant is a good practice. Hand-watering or automatic sprinklers that spray the entire plant with water will encourage black spot to grow.
Neem oil has been found to be a good antifungal garden spray. Otherwise, visit your local garden center and search for a fungicide that lists "rose bush" and "black spot" on the label. Read the label carefully for precautionary statements and directions for application.
Q: Are there better varieties of rose bushes to plant in Hawaii?
A: A local grower who supplies roses can best answer this question because you will then know the variety available on-island. Make sure the rose bush is a heat-tolerant variety. The Honolulu Rose Society’s members have provided recommendations based on their experience. It may be useful to print out the list, which includes hybrid teas and grandifloras, climbers, floribundas and miniatures, and bring it with you to the local grower.
To find the list, click on "Roses for Hawaii" at www. honolulurosesociety.org.
You can also learn more from the Honolulu Rose Society at the University of Hawaii Urban Garden Center in Pearl City from 9 a.m. to noon Feb. 11, when HNL Rosarians will celebrate a newly planted rose garden and have roses for purchase in celebration of Valentine’s Day.
For more information, visit www.ctahr.hawaii.edu/ougc or call 453-6055 or 453-6050.
Jayme Grzebik is an extension agent with the University of Hawaii at Manoa College of Tropical Agriculture and Human Resources. She is the statewide coordinator for the UH Master Gardener program and can be reached at grzebik@hawaii.edu.
