Black Olive Scale (Saissetia oleae)
|Occasionally a sap-sucking insect known as Brown or Black Olive Scale will be seen on olive trees. It is rarely a problem if the trees are in good health. We usually only spray our mature trees for scale every two to three years and only then if they need it. However, certain areas of Australia are more prone to the scale.
The adult females are very easy to recognise on the olive tree stems. They are dome shaped, dark brown to black in colour, and about the size of a match head.
The tiny eggs laid under the female, look like piles of very fine sand. Mainly during the summer, these eggs hatch into tiny, six-legged, cream coloured ‘crawlers’. The crawlers move up the stems and usually settle along the veins of young leaves. At this stage they don’t have the impervious shell of the adult and can usually be killed with one or two applications of white oil about two weeks apart. White oil should be used only as directed on the label by the manufacturers (and by your agricultural department) and never during the hot part of the day. It puts an oil film over the young ‘crawler’ and suffocates it. If applied in the hot part of the day it also stops the leaves from breathing properly and can be detrimental to the tree. The White oil application will also tend to rid the tree of ‘sooty mould’ as discussed soon.
If the crawlers are allowed to live, they will moult after about one month and then migrate to the young stems and twigs of the tree. Here they will mature and lay more eggs and their protective brown shells will be impervious to white oil. Squash the scale between your fingers to see if it is alive. If it is alive, then your fingers will be wet from the juices squeezed out. If it is dead then your fingers will be dry and dusty.
Bad infestations of live mature scale may need spraying with an insecticide such as Supracide. (Important: See note regarding “Treatment”) In Greece, Supracide is the main spray used for most olive problems. Once again, check with your local agricultural chemical supplier and the product label, for directions.
Probably the damage done by the scale itself to the tough olive tree is negligible compared with what happens next.
As the scale feeds, the ‘manure’ they excrete is a sweet, sticky, ‘honeydew’. This excreted sticky liquid can finally cover the leaves of the entire tree. A fungus known as sooty mould feeds on this food and multiplies until the entire tree may be covered with the black sooty mould. This is where the real problem lies.
The leaves are coated with the black deposit, so the sun’s light can’t penetrate the leaves properly. Therefore photosynthesis can’t take place efficiently. Therefore, ‘root producing’ food is not manufactured in the leaf. Therefore roots don’t develop properly. Therefore the poor root system can’t collect enough food and water from the soil to send up to produce more leaves, which in turn will produce more root. Once the vicious cycle begins, a stunted and unhealthy tree with poor crops is the result.
To make the problem worse, sweet ‘honeydew’ on the leaves also attracts large numbers of ants. It appears that as the ants constantly move over the scale, they frighten away the small wasp parasites which in normal cases would keep the scale under control.
Above: Adult scale on the underside of olive leaves
Above: overturned scale with orange crawlers showing.
Above: An olive branch covered in sooty mould.
Above: Closeup of sooty mould on olive leaf.
The good news is that healthy olive trees don’t get the scale, sooty mould and ant infestation to any great extent. More good news is that heavily infested trees are easily fixed.
Normally, one thorough spraying of the entire tree and soil below with a systemic insecticide will be adequate. Nevertheless, to be sure, a second spray about two weeks later may be worthwhile.
Now, if there is no more live scale, there is no more eating, therefore no more ‘honeydew’ excreta, therefore no more sooty mould and ants. Over a period of time the dead sooty mould deposit will peel off the leaves from exposure to the rain, wind and sun. The green leaf surface will be exposed and growth will continue as normal. Treat the tree to an occasional feeding of Seagold fertilizer/mulch and foliar application and some water and watch its health come back.
SCIENTIFIC NAME: Saissetia oleae
DESCRIPTION OF THE PEST
Black scale adult females are about 0.20 inch (about the size of a match head) in diameter. They are dark brown or black with a prominent H-shaped ridge on the back. Young scales are yellow to orange crawlers and are found on leaves and twigs of the tree. Often, a hand lens is needed to detect the crawlers. Black scale usually has one generation per year in interior valley olive growing districts. In cooler, coastal regions multiple generations occur. Black scale prefers dense unpruned portions of trees. Open, airy trees rarely support populations of black scale.
Young black scale excretes a sticky, shiny honeydew on leaves of infested trees. At first, affected trees and leaves glisten and then become sooty and black in appearance as sooty mould fungus grows on the honeydew. Infestations reduce vigour and productivity of the tree. Continued feeding causes defoliation that reduces the bloom in the following year. Olive pickers are reluctant to pick olive fruits covered with honeydew and sooty mould.
Pruning to provide open, airy trees discourages black scale infestation and is preferred to chemical treatment.
A number of parasites attack the black scale, the most common are Metaphycus helvolus, Metaphycus bartletti, and Scutellista cyanea. These parasites, combined with proper pruning, provide sufficient control in northern and coastal orchards. In other regions, biological control is often ineffective because the black scale’s development pattern hampers parasite establishment.
ORGANICALLY ACCEPTABLE METHODS
WHEN TO TREAT
If infestations are resulting in honeydew, treat the crawlers. In interior valleys, delay treatment until hatching is complete and crawlers have left protection of the old female body. Once crawlers have completely emerged, a treatment can effectively be made in summer, fall or winter provided the scales have not developed into the rubber stage (later second instar, which are dark, mottled grey, and leathery, with a clear H-shaped ridge on the back).
Due to the chemical nature of the treatments, Please check with your agricultural chemical supplier as to the suitability, application and safety precautions of your chosen scale treatment for olives. Some growers have used Summer or Petroleum Oil and Supracide. Californian olive growers use Oil Emulsions, Diazinon 50WP, Methidathion and Carbaryl. The use of chemicals reduces the microbial population in your soil and can inhibit the uptake of certain nutrients to your trees. Harmful residues of chemicals can also build up in your soil structure.
A new product Admiral has become available which acts as an insect growth regulator rather than a kill-on-contact pesticide, it has been quite effective and like any treatment of scale; timing is essential. Ants can be controlled with an Ant Bait suitable for Horticultural use. We suggest Distance Plus Ant Bait.
“Olives – Pest Management Guidelines” (UCPMG Publication 8, 1994). These guidelines cover the major olive problems found in Australia and California and are available for free from their website http://www.ipm.ucdavis.edu/PMG/selectnewpest.olives.html . (The information comes from California so all references to places, seasons, months and treatments are Californian). If you have any questions, please contact The Olive Centre, PH: 07 4696 9845, FAX: 07 4696 9914, Email: email@example.com