Scale and how to treat it
Scale insects are sap-sucking bugs in the super family; Coccoidea: a group of insects comprised of about 8000 species worldwide but with much smaller numbers in New Zealand with less than 400 species.
These scale insects cling on to stems, branches, and even the leaves of plants they’re feeding sap from. But due to their appearance, they can often be mistaken for a plant disease instead. In many cases, heavy infestations build up unnoticed before plants begin to show damage. Large populations may result in poor growth, reduced vigor and chlorotic (yellowed) leaves. If left unchecked, an infested host may become so weak that it dies.
Understanding what they look like and taking action as soon as they’re first spotted is one of the most important factors when getting rid of scales on plants.
Scale insects can be divided into two groups:
Armoured (Hard): Secrete a hard protective covering (3mm long) over themselves, which is not attached to the body. The hard scale lives and feeds under this spherical armor and does not move about the plant. They do not secrete honeydew.
Soft scale: Secrete a waxy film (up to 13mm long) that is part of the body. In most cases, they are able to move short distances (but rarely do) and produce copious amounts of honeydew. Soft scale vary in shape from flat to almost spherical.
They have a shell-like bump appearance and tend to firmly attach to their host plant. This immobile nature of theirs further perpetuates the notion that it may actually be a disease, but it’s not. They’re often covered in an armour and are found in clusters.
The colours can also vary from the most common brown to white, tan, or even orange. Scale insects suck the sap out of plants which causes deformed leaves, yellowing leaves, brown marks, and will cause them to eventually fall off if not treated in time.
The appearance of sooty mold is another primary sign that indicates scale on plants. This happens when scale insects produce honeydew while feeding on the plants themselves. In turn, this attracts fungal organisms that promote the growth of sooty mold in some (but not all) scales.
This type of black-coloured fungus disrupts photosynthesis in plants and is a big indicator that scale insects might be feeding on your plant.
Healthy plants are less likely to suffer from insects like scale. Feed plants regularly and ensure that they are getting adequate water.
Early intervention is important in controlling infestations. If you have ants crawling about your plants then you are most likely to have scale or another sucking pest.
Boisduval scale (Diaspis boisduvali), an armored scale, can infect orchids in the greenhouse and the home, and should be guarded against. Boisduval scale is the most common type of scale found on orchids and can multiply rapidly.
Life Cycle of Scale Bugs
Adult females lay eggs underneath their protective covering which hatch over a period of one to three weeks. The newly hatched nymphs (called crawlers) migrate out from this covering and move about the plant until a suitable feeding site is found.
Young nymphs insert their piercing mouthparts into the plant and begin to feed, gradually developing their own armor as they transform into immobile adults. They do not pupate and may have several overlapping generations per year, especially in greenhouses.
Note: Males of many species develop wings as adults and appear as tiny gnat-like insects. They are rarely seen and do not feed on plants. Females often reproduce without mating.
Scale management is usually a protracted and serious effort, and never fun. Light infestations restricted to one or a few plants can be treated with household products rather than concentrated insecticides. When possible, immediately isolate infested plants from others to prevent the crawlers from moving among them.
The key to control is persistence. Management methods least toxic to people, pets and plants are the most time consuming and laborious. Chemical methods, including oils, soaps and synthetic insecticides are progressively more toxic and expensive, but less work. Regardless of method or chemical used, remain vigilant and expect to make at least two to three applications 10 to 16 days apart.
Persistent or heavy scale infestations may require the use of synthetic insecticides. Few insecticides are tested on or specifically registered for use on orchids, but several common, inexpensive, home-and-garden chemicals are labeled for ornamental plants. Using a systemic insecticide like Groventive will attack the surface dwellers you can see, as well as the other crawlers that are as yet hiding where surface sprays alone cannot reach.
Due to plant costs, owners' personal attachment to orchids, and many growers' desire to avoid insecticides when possible, they use Beneficial Insects.
Commercially available beneficial insects, such as ladybugs, soldier beetles, lacewing, and parasitic wasps are natural predators of the young larval or crawler stage.
This is where nymphs appear soon after the eggs hatch. At this stage, they have legs and actively crawl around to find a new spot to attach and feed on your plant. Bioforce Limited is one of the NZ companies that offer biological control agents, https://www.bioforce.co.nz/
One of the most important things to do is prepare your orchids for spraying or dipping.
Always try and clean as many scales off your plant as possible, wipe clusters off as chemicals will often kill the surface insects are they protect the ones underneath - usually the immature babies.
Take as many bracts (dead sheaths) off the orchids as possible as scale often live underneath them. For a bad or persistent case, its better to unpot the orchids as the scale live under the surface especially in bark. Treat the plant bare rooted. A longer dip is often better.
When using the dipping method, add a few drops of detergent in the mix. This breaks up the surface tension of the water, ensuring that there are no air pockets around the plant and creating better overall coverage. A dip of about 20mins is usually sufficient. Make sure the plants are completely submerged.
Be aware to kill scale you need an oily spray which on certain orchids will mark or discolour the leaves. When treatment is finished, wash the leaves to take off excess chemicals. You may still have discoloured leaves but hopefully no more scale which if left untreated will kill your orchid.