There is a lot of information on the web about treating scale, some relevant to a specific country, some accurate, some not and much that is contradictory.
The following information is based upon treatment methods that the Native Orchid Society of South Australia (NOSSA) growers have found most effective.
WHAT IS SCALE?
Scale are tiny sap sucking insects of which there are several species in Australia. The female adults build a shield-like cover for protection. The shields are often brown but can be white or red. The shield can be up to 3mm in size. Once a shield is built the adult does not move about but stays in the one position. Ants farm scale as they exude a honeydew sap, a food source for the ants.
Immature scale or crawlers do move about. These can be a different colour from the adult eg juvenile brown scale can be yellow, other species can have grey juveniles. They are lightweight no more than 1mm and easily windborne.
The life cycle is short, and for many species, within a month there is a new generation of scale. Scale multiply rapidly.
Scale tend to attack epiphytic orchids. Evergreen terrestrial orchids may be affected but not the deciduous ones.
EFFECTS OF NOT TREATING SCALE:
Apart from making the plants look ugly, scale left unchecked can
- infect other plants
- weaken the plant leading to death of the plant
- make the plant worthless for shows
- develop secondary infection of sooty mould
- treating the scale will treat the mould
Inspect and Monitor plants
- check leaves (both upper and underside), crevices, sheaths, pseudobulbs, stems, etc
Quarantine and treat new plants before introducing them to the orchid collection
- newly acquired plant can be a major source of scale infestation
- for thoroughness, use both a contact and systemic spray (see below Types of Sprays)
- schedule spraying 2 – 4 times a year
Consider relocating ferns if they are under orchid benches as this can often be a host for brown scale.
- If free-standing bench or hanging pots are free of scale and plants are not touching any other surface than applying Vaseline around each of the feet/lower part of the hooks will prevent ants and crawlers from moving into the area.
- Vaseline is waterproof and so will be effective for a long time.
Biological control alone appears to be ineffective but the following is a list of known predators
- Crypotlaemus montrouzieri Native ladybird feed on mealybugs and felt scale
- Mallada signata, Green Lace Wings, feed on aphids, spider mites, various scales, mealybugs, moth eggs and small caterpillars
- Chilocorus beetles
- Aphystis wasp species
Scale are hard to eliminate entirely. Vigilance and persistence are important factors in controlling scale.
Treatment works either by
- a direct contact spray whereby the insect is suffocated by smothering. This is effective for all stages of the life cycle but particularly for the adult under its shield.
- an application of a systemic chemical.
- or a combination of both.
For treatment to be effective the leaves (both upper and underside), crevices, sheaths, pseudobulbs, stems must be thoroughly drenched with the spray of choice.
Types of Sprays
Whatever type of contact spray used, treat every 2 weeks for three treatments
- Soapy Water (for those who like using home-made remedies)
- Using pure soap (not detergent), suds up a bar in a bowl of water, and pour into a spray bottle.
- Homemade Horticultural Oils
- See ABC Fact Sheet: Horticultural Oils for more details on oils.
- Eco-Oil (Pest oil)
- No petroleum derivatives; registered organic substance
- Do not spray on very hot days as it can cause burning.
- Always dilute (2.5 mls per 1 litre as per instructions on label), never use neat
- It is important to dilute, as the oil can also block the leaves stomata and so suffocate the plant.
- It is a foliage spray and should not be used on the plant roots.
Whatever chemical spray is used, repeat within 4 weeks of the initial treatment and then as often as necessary.
Ideally, it is best to use multiple systemic sprays to avoid the scale becoming resistant to the chemical.
- Active ingredient Imidacloprid, an insect neurotoxin, is absorbed by the plant and then ingested by the insect
- low toxicity to humans and pets
- but use heavier drenching rather than fine mist to avoid inhalation
- Effective for about 2 – 3 weeks
- broken down by light so remains in pots and soils for awhile
- Follow manufacturer’s instructions at the minimum recommended strength.
- Active ingredient acetamiprid
- Low toxicity to bees (although bees are not usually a problem in an orchid house)
- High toxicity
- Harder to obtain
- 80 – 95% kill rate for slugs and snails
NB: Do not use the contact and chemical spray at the same time
- Wait a week between using the different types of sprays.
- Spraying too soon will negate the effects of the first spray.
Despite what application is used, all need the following treatment
Rub the scale off taking care not to damage the leaves
- Use either toothpick, fingernail, toothbrush or cloth/tissue
- Rubbing the scale off allows for
- detection of new infestations
- eggs under the shield to be killed
Thoroughly disinfect any recycled pots
Dispose of all debris as eggs and scale can survive for weeks and reinfect the plants
Oliver D, Scale and its Control Oliver, Orchid Societies Council of Victoria, http://www.oscov.asn.au/articles3/scale.htm accessed 5/1/17
Johnson PJ, Scale, Orchids – The Bulletin of the American Orchid Society, September 2003, http://www.aos.org/orchids/orchid-pests-diseases/scale.aspx accessed 5/1/17
Manners A, Scale Insects: A difficult problem that can be managed, 2015, Agri-science Queensland, Department of Agriculture and Fisheries https://www.ngia.com.au/Category?Action=View&Category_id=682 accessed 5/1/17
Imidacloprid https://en.wikipedia.org/wiki/Imidacloprid accessed 5/1/17
Eco-Oil http://ecoorganicgarden.com.au/products/pest-disease/eco-oil/ accessed 7/1/17
Les Nesbitt, NOSSA, pers comm
Kris Kopicki, NOSSA, pers comm
Richard Austin, ANOS Vic, pers comm