Two of the most persistent houseplant pests look nothing like insects. Mealybugs hide under cottony white wax, and scale insects sit still under shell-like covers that resemble small bumps on a stem. The Royal Horticultural Society (RHS) publishes separate pages on each, plus a page on sap-feeding insects on indoor plants. This article summarises those three pages. It reports RHS gardening advice for houseplants, conservatories and greenhouses in the UK, not a legal standard, and it is general information.
What each pest is
The RHS describes glasshouse mealybugs as sap-sucking true bugs typically covered in a white waxy material, closely related to scale insects and often considered unarmoured scale insects. Most species prefer moist, warm climates, and those in the UK are typically found in glasshouses and other protected places. They live in clusters in inaccessible spots such as leaf axils, leaf sheaths, between twining stems and under loose bark. Some species, mostly Rhizoecus, feed on roots, and the RHS says these are also confined to glasshouses and houseplants, apart from the golden root mealybug, which can survive on roots outdoors.
Scale insects are described as many species that suck sap from garden and glasshouse plants in Britain, ranging from under 1 mm to over 1 cm across. More than 25 species occur in British gardens. All have a waxy shell-like covering when mature. Several are confined to houseplants and to greenhouses and other sheltered places, while others live on ornamentals and fruit trees outdoors.
Signs to look for
For mealybugs the RHS says the usual first sign is fluffy white wax in leaf axils or other sheltered places, with the insects or their orange-pink eggs underneath. Large numbers cause an accumulation of sticky honeydew, which encourages black sooty mould. Heavy infestations can reduce vigour, stunt growth and cause premature leaf fall. Plants named as commonly affected include cacti and succulents, African violets, bougainvillea, citrus, fuchsia, grape vines, hoya, orchids (especially Phalaenopsis), oleander, passion flower, peach and tomato.
For scale insects the signs are shell-like bumps on stems and the underside of leaves, poor growth where populations are large, honeydew on upper leaf surfaces and, in early summer, eggs under a covering of white waxy fibres. The RHS warns that the white egg masses of cushion scales can be mistaken for mealybug or woolly aphid. On the indoor-plant page it adds that indoor scale insects often sit close to the midrib or main veins on the underside of leaves.
How they arrive
The RHS’s clearest message is that these pests usually arrive on new plants. Female mealybugs do not fly or crawl far, and the RHS says the same about the adult females of scale insects. Its advice is to inspect new purchases carefully, and where possible to quarantine them, keeping new plants apart from the existing collection. For mealybugs the mealybug page suggests isolation of at least a month before adding acquisitions to an existing collection. Quarantine gives eggs and nymphs a chance to develop and be recognised, and populations time to grow enough to be spotted without risk to other plants.
What the RHS says to do
The RHS approach is tolerance first, then proportionate action:
- Tolerate where possible. Plants can tolerate some mealybug, although populations can build quickly if left unchecked, and well-tended healthy plants tolerate light populations of scale insects.
- Monitor houseplants often so action can be taken before damaging populations develop.
- Separate affected plants from unaffected ones to limit spread.
- Remove adult scales and egg masses when seen, although the RHS says this may not reduce a large population.
- Clear dead leaves and pruning waste from greenhouses and around houseplants, since these may carry mealybugs or eggs.
- Consider moving plants that tolerate UK outdoor conditions outside, where the insects become accessible to natural predators or cooler conditions can slow their life cycles.
- Dispose of heavily affected plants. The RHS says replacing plants heavily affected by scale or mealybugs can be simpler than trying to eliminate the insects, and says the same in its mealybug advice.
Manual removal has limits. The indoor-plant page notes that mealybugs are hard to remove by hand because of where they hide.
Biological controls
For mealybugs the RHS says a ladybird, Cryptolaemus montrouzieri, can be released into greenhouses and that its larvae look like large mealybugs. Both adults and larvae find and eat mealybugs and eggs in confined spaces. Parasitic wasps of the genera Leptomastix, Leptomastidea and Anagrus are sometimes available, and can give control where population levels are fairly low. The ladybird and wasps need relatively high temperatures, so are only likely to succeed from May to September.
For soft scale the RHS says the nematode Steinernema feltiae can be bought from some biological control suppliers. Nematodes can infect non-target animals, so they should be used with care and only when there is a specific problem to treat. Outdoors, it says some ladybirds, parasitoid wasps and birds eat scale insects. On the indoor page it adds that all biological controls are susceptible to pesticides and cannot be used alongside most pesticidal controls, and that the RHS does not support pesticide use.
Greenhouse or living room
The RHS says scale insects in greenhouses can breed continuously throughout the year, whereas species on outdoor plants mostly have one generation a year. Glasshouse mealybugs are active all year on houseplants and in greenhouses.
Questions people ask
Do biological controls suit a flat with a few plants? The RHS says they are most suitable for a glasshouse situation.
The bottom line
The RHS treats mealybugs and scale insects as pests that arrive on new plants and then persist in warm, protected places. Inspecting and quarantining purchases, tolerating light populations, isolating affected plants and being willing to bin a heavily infested one are its main recommendations, with biological controls for greenhouses in the warmer months.
Sources
- Royal Horticultural Society, “Mealybug (Glasshouse): Identification & Control” (rhs.org.uk)
- Royal Horticultural Society, “Scale Insects: Identification and Control” (rhs.org.uk)
- Royal Horticultural Society, “Indoor plants sap feeders: Identify” (rhs.org.uk)