Ferns grown outdoors seldom have serious problems with insect damage. However, the same is not true for ferns grown in restricted environments indoors or under glass. Aphids, in all their hues, can be a real problem and scale insects can adversely affect the health of large plants and can kill small ferns. Scale insects can make a plant unattractive as they usually appear as ugly, brown lumps on the stipes and rachises, on the under surface of a frond. They can also produce copious excrement, fancifully called ‘honeydew’, which falls down onto lower fronds and then gets infected by black, sooty mould. Whilst the excrement can be washed off the plant, the scales remain firmly fixed to the frond even after the insects’ death. Apart from spoiling the appearance, the drain on the plant from this infestation of the fronds and rhizomes, causes lack of vigour, distortion and, eventually, death.
Scale insects have a thick waterproof, waxy scale or shell, like a limpet, that makes it difficult to dislodge or kill them. Some may be hermaphroditic and self-fertilising, whilst the females of other species require fertilisation by a male. Although the females have limited mobility and cannot fly, males are often winged and capable of short-distance flight. Once they settle and produce the scale the females do not move again. The females lay their eggs under their scale and die. When the eggs hatch, the young ‘crawlers’ emerge and move to new parts of the plant or migrate to new plants. Whilst seasonal effects are minimal indoors and under glass, it seems that spring is a time when new infestations of scale insects are particularly common.
Special mention should be made of the Fern Scale or Coconut Scale, Pinnaspis aspidistrae. This insect is one of only two of the forty-two species of Pinnaspis that present a horticultural problem. It is a member of the Hard Scale family Diapididae and is present on every continent except Antarctica. It is polyphagous, meaning that it can infest around sixty types of plants including Aspidistra plants and laurel and citrus
bushes. So, introduction of this pest into a fern collection could be via a plant in a different family altogether.
Fern Scale is a particular problem on Platycerium and other large ferns such as
Asplenium nidus. Usually, it is not the adult tan-coloured female scales that are seen but the white, 1–2 mm immature male insects (Figure 1). These can be present in their thousands and give the fern the appearance of having white, finely ground, desiccated coconut sprinkled over it. However, the name Coconut Scale comes not from the appearance of the insects but their predilection for damaging coconut palms. The insects are also very likely to settle on the lamina of the fern and produce pale ‘bleached’ spots where they suck fluid from the frond surface (Figure 2). The appearance of the plant is ruined, and heavy infestations can prove lethal (Figure 3). Prompt systemic treatment should be used.
Eradicating scale insect is not an easy task and requires vigilance so that infestations can be dealt with promptly. The scale protects the insect from topical treatments with powders or sprays. If a frond is very badly infested, remove it, and throw it away. If the infestation is less, then a systemic poison can be effective. This can be applied both to the fronds and to the roots. Acetamiprid is available commercially as a control of vine weevils both in the underground larval form and the adult weevil. This compound is also effective against scale insects, although the dead scales may remain attached to the fern and have to be rubbed off. However, it is important not to soak young, growing foliage in acetamiprid as this can be damaged, resulting in distorted, brittle fronds with brown edges (Figure 4). Acetamiprid is a neonicotinoid that is toxic to pollinating insects, birds and fish. Whilst acetamiprid is less toxic than many neonicotinoids, it should only be used indoors where these creatures cannot be affected. Horticultural oils may give adequate control without excessive phytotoxicity to ferns, meaning their growth won’t be adversely affected or inhibited. Two thorough treatments two weeks apart should give good control. Ferns are notoriously sensitive to pesticides and not all ferns will tolerate these chemical treatments or oils. It is best to ‘spot test’ a fern by treating one frond before treating the whole plant.
Biological controls are, not surprisingly, only partially effective because of the protective scale. However, in the commercial fruit-growing industry, the lack of mobility of the female insects allows some control to be achieved with parasitic nematodes, which have the time to penetrate under the scale. This experience may be relevant to the development of biological control of Pinnaspis aspidistrae in ferns, possibly with the
entomopathogenic (disease causing) nematode Steinerma feltiae.
In summary, infestation with Pinnaspis aspidistrae is common and occasionally serious. Treatment has to be a combination of removal of infected material, application of topical horticultural oils and systemic treatment with insecticides, most effectively neonicotinoids.
PETER BLAKE
Peter became interested in tropical ferns whilst making regular trips to India in the 1990s and 2000s. He retired from the NHS in 2012 and was able to develop his interest in growing tropical ferns from spores. He now lives in Norwich where he can provide much more space for their culture. He maintains a particular interest in the ferns of South India and Sri Lanka.
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