This week I am going to answer two questions from readers relating to garden pests.
Hi Dr Kris
I have a house in Bali and only come there several times a year. I was there last week and noticed all the trees had disfigured leaves and on closer inspection noticed that they looked as if they were diseased with scale. We had to chop back an oleander, all the frangipanis are without leaves, even the and palms seems to have scale. I believe I need Pest Oil or White Oil. Can you recommend where I can buy something to treat the problem, and how often should the plants be sprayed. Thanks, Leonie
And…….the second question.
Hello we have black stuff all over the leaves of our various plants. What can we mix together and spray on them please. Thanks so much.
Claire
Villa Coco
The first question is about a common pest, scale insects. Scale can be hard to treat and can destroy your garden rapidly. The second question refers to sooty mould which is related to the presence of scale. Sooty mould is a black sticky coating that covers foliage, and is a result of honeydew secretions by the scale. In Bali you can often see this on frangipani and mango trees.
The armored scale insects are some of the most successful plant eating insects. They occur almost everywhere that perennial plants are found. Female scale insects look like small oval, legless bumps. The males are minute yellow-winged insects (a few millimeters in size). On many plants, scales are the most serious pests, and most plants are susceptible to scale insects. Scales cause damage by sucking the sap from the plants. Heavily infested plants appear unhealthy and produce little new growth. If the scales are not controlled, leaves will drop prematurely, sometimes killing entire branches and sometimes entire trees.
Female scale insects produce eggs, which hatch soon after laying, while others are live-bearers that lay crawlers. Within a few hours male and female crawlers move off in search of new sites to colonise. They usually settle within a few centimetres of the parent but can travel up to 100 metres. They can be dispersed by the hands and clothing. The crawlers settle on a new site within a day or two and begin feeding and start to grow a protective cover. Once in place, they do not move again. You can identify scale insects by looking on the undersides of leaves and around leaf joints. Scale-damaged plants look withered and sickly and may have sticky sap or a black fungus, known as sooty mould on the leaves and stems.
Scale tends to colonise most heavily on the underside of leaves and close to the trunk. On frangipani they can be found often on the trunk, as it is very soft. The scales have a hard protective shell which reduces the effectiveness of many surface contact sprays. Surface sprays, such as white oil, need to be applied so as to cover the entire plant, especially under the leaves.
The traditional remedy is white oil also called horticultural oil. It works by smothering and suffocating the insect, both in its adult and crawler stages. When dry, white oil remains effective for 1-2 weeks. The advantages of white oil include effectiveness, low toxicity to both the user and the environment, and low cost. Its main disadvantage is that white oil will damage flower buds and may also damage sensitive leaves. Repeat spraying may also be necessary.
Scale can be difficult to kill because their waxy hard shell coating prevents the white oil from penetrating. What you have to do is break the breeding cycle by spraying every 2-3 weeks to kill the emerging larval scale insects before they form their protective coating. The adult scale dies once it has bred and spawned. It is the young scale you need to target for 2-3 weeks and after, if there are no signs of more young you should keep spraying for at least another 3 week cycle to ensure you have got them all. One surviving scale insect will re-infest the plant.
If you want to buy white oil try the new Mitra 10 on the Jalan By- Pass Ngurah-Rai close to Kuta. You could also try at the many nurseries in Kerobokan, or Jalan Hayam-Wuruk Sanur, close to the Orange Hotel.
You can also use Neem Oil or Pyrethrin as a last resort. They kill by suffocation but scale insects will not drop off when they die. That shell they build over themselves stays attached to the stems even when dead. Those sprays are also toxic to the surrounding environment and beneficial organisms. Whichever solution you use, a week after a thorough spraying you’ll have to take a sponge and a weak detergent solution to wipe off the dead scale.
Safer alternatives are available. You can physically wipe scale from the plants leaves. Remove scale insects with a twig or your fingernail. They will scrape off of plant tissue easily. Personally, I just blend a cup of cheap vegetable oil with a cup of dish soap. About a tablespoon of this mixture added to a litre of water is my version of white oil. The dish soap is toxic to the scale and will break down the armor coated shell. The vegetable oil helps the soap to stick and smother the scale.
Soft scales excrete large amounts of honeydew, which is rich in nitrogen compounds and sugars. The honeydew excretion promotes the growth of a fungus known as sooty mold. This black/grey colored fungus coats the top side of leaves, interfering with photosynthesis and makes the plants unattractive. Armored scales do not excrete honeydew.
Black sooty mold is often a result of the scale as it is the secretion of honeydew from the scale that causes the sooty mold. The scale secretes honeydew as it feeds on the sap and that honeydew residue falls and is spread around the foliage of the plant. Once the scale insects are dead, the mould will dry and eventually wash off.
You do not need to remove foliage affected by sooty mould, as leaves will still photosynthesize even with heavy amounts of sooty mould present. The sooty mould is sticky, grey/black and it is very unsightly, but if you solve the scale problem the sooty mould will eventually clear up by itself. The scale is a much bigger problem than the sooty mould so concentrate on removing the scale first. Be diligent and examine infested plants for evidence of new scale every day.
It may take a while, but you will win this war! Good Luck and please email me if you have any questions relating to your garden.
