Parthenogenesis occurs in some scale

insect species and means females can

clone themselves to reproduce asexually.

Hermaphroditic species also exist,

particularly in the genus Icerya, and

possess both female and male sex organs.

FAST FACTSThere are approximately 20 global families of scale insect within the order Hemiptera, and some 8000 species of coccoid globally, with around 1100 described here

in Australia. Scale insects are sexually dimorphic. Females resemble grub-like larvae, with reduced legs that are hard to see with the naked eye, and remain in the same spot for most of their lives. Males are usually delicate, winged little creatures that could be mistaken for small flies. Males have short lifespans and rarely eat, but females can live for months by inserting their elongated straw-like mouthparts into a plant and sucking plant sap, all safely under their protective ‘scale’. As they feed, they release sweet sticky honeydew from their rear-end, providing an important sugar resource for other animals. The sugar composition of this resource varies greatly, depending on both the plant and insect species.

Many insects that feed on floral nectar, like bees, flies, butterflies, moths and ants, can also be found feeding on honeydew. It’s an especially valuable resource in winter or when flowers are scarce. And it’s not just other insects: geckoes, honeyeaters and other small birds, even sugar gliders, are all known to slurp sugar from scale insects.

Honey farmersNumerous bee species are honeydew foragers in many parts of the world. Honey bees are well-known honeydew collectors, making uniquely-flavoured honeys depending on the host tree of the scale insects. German Black Forest honey and New Zealand’s honeydew honey are famous examples of this specialty honey – plant sap that has been recycled through two different insect species!

We’re all familiar with aphids, mealybugs and scale insects in the superfamily Coccoidea as parasitic garden or agricultural pests. Coloured warty lumps or fluffy cotton-wool balls cluster on plant stems and in crevices. Sticky honeydew and black sooty mould coats trees and shrubs. Some species produce woody galls to live in (like Apiomorpha species on eucalypts), while others cover themselves with protective waxy shields. We mostly think of them as a nuisance and spend a lot of time and money trying to eradicate or control them. Yet not all species are pests, writes Manu Saunders, so what other roles do they play within ecosystems?

SCALE INSECTS: A SWEET RESOURCE FOR POLLINATORS

NATURE’S SHACK

A woolly mealybug (Monoplebulus sp.). ‘Hard’ scale insects are not directly connected to the waxy scale their soft bodies sit beneath. In contrast, the covering of ‘soft’ or ‘felty’ scale insects is attached to the invertebrate. Photo: Arthur Chapman [CC]

SUGAR

26 | Wildlife Australia | SPRING 2018

Plenty of solitary bee species around the world also forage on this sugary resource. It’s not clear how the bees locate the resource on plants where there are no floral cues to attract foraging pollinators. Ants are commonly associated with scale insects too. They act like ‘farmers’, tending their local scale populations and keeping away predators to keep the sugar flowing. It’s even possible these ant-scale relationships could benefit some plants by deterring other more damaging herbivores from settling down.

Sooty and sweetIn some forest systems, scale insect populations support complex webs of species interactions. In Australian tropical forests, honeydew is a key resource influencing the composition of ant communities. But New Zealand’s temperate native beech forests are a wonderful example of a complex food web built on a foundation of scale honeydew. The scale insects in question are the sooty beech scales (Ultracoelostoma species), whichlive off native Nothofagus trees, mostly on the South Island.These coccoids are considered a keystone species because they produce an abundant, year-round supply of sugar that supports the ecosystem. As their sticky secretions build up on the trees, a black sooty mould grows on it, coating the trunks.

In some forest systems,

scale insect populations

support complex webs of species.

This common tea-tree, Leptospermum scoparium, has formed a gall around an Eremococcus turbinata ‘felt scale’ insect. Photo: John Tann

A rattle ant (Polyrhachis australis) drinks honeydew from the straw of a scale insect. Photo: Deb Yarrow/aussiebugs.com.au

Ants and scale insects are said to have a ‘trophobiotic’ relationship – a kind of symbiosis in which one organism provides food for theother in exchange for a mutually beneficial service such as protection.Here, ants are seen herding aphids in order to ‘milk’ them for theirhoneydew. The ants milk the aphids by stroking their abdomen and, inreturn, keep the aphids safe from ladybug larva. Photo: Jenny Thynne

Birds, mammals, insects and reptiles feed on the sugar and fungi, as well as on the high numbers of other animals feeding there! Unfortunately, the honeydew is also attractive to invasive species, like aggressive European wasps (Vespula spp.).

Clearly, scale insects have a bigger influence on their ecosystems than we might think. The multitude of complex interactions they support, and the benefits they provide as food resources, make them a fascinating group of animals to learn more about.

READING Crane E, Walker P. 1985. Important honeydew sources and their honeys. Bee World, 66:105–112. Gardner-Gee R, Beggs JR. 2013. Invasive wasps, not birds, dominate in a temperate honeydew system. Austral Ecology 38:346–354. Meiners JM, Grisowld TL, Harris DJ, Morgan Ernest SK. 2017. Bees without flowers: before peak bloom, diverse native bees find insect-produced honeydew sugars. The American Naturalist 190:281–291.

Ecologist MANU SAUNDERS is a Postdoctoral Research Fellow at the University of New England, where she is currently working on ecosystem services research. She is particularly interested in insects, and how we are connected to them through agriculture. Manu regularly writes about a range of topics at www.ecologyisnotadirtyword.com

Vespula spp. wasps prey on and compete with other insects in New Zealand’s beech forests, with potentially damaging impacts on the honeydew ecosystem. They can dominate the honeydew resource, reducing the flow available for other animals, and they’ve also been observed driving away native bees. Photo: Jon Sullivan

Wildlife Australia | 27

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