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Entomological Society of Victoria N e w s l e t t e r No. 5 December 2014 INDEX Do plants really need insects or is it just vice versa? Do plants really need insects A short history of insects and plants or is it just vice versa?.................................1 An extract of a presentation to the Friends of Cranbourne Botanic Gardens, Melbourne Urban Bioblitz By Patrick Honan, November 2014 photos…………….…………5 This is a complex question with a history of about 450 million years, so only a Do plants really need insects very broad, potted glossy overview is possible. Insects appeared about 50 or is it just vice versa? million years after land plants, but the relationship really took off when (cont’d)……………….……..6 flowering plants appeared about 100 million years ago. The Patch Primary School Insects have utilised plants as a food source to the best of their abilities, and excursion photos…………..9 plants have done everything in their power to prevent being used. Meet your ESV Insects will cooperate with plants where it is in their interest, and vice versa, Council…………………….10 and as long as the reward is high enough. In extreme cases this develops into an obligate mutualism, where a species Articles of interest………..12 of plant cannot survive without the continued existence of a species of insect, and vice versa. ESV upcoming events......18 Around the Socities………18 Herbivory More than half of all insect species are herbivorous, and there are more ESV newsletter goes species of plant-eating insects than there are species of plants in the world. A global………………………19 single eucalypt tree may support more than 100 species of Can you help?...................19 herbivorous insects. Gippsland Lakes Bioscan Many insects are monophagous but photos……………………..20 some will feed on more than 150 plants species; however, ninety Prominent percent of all herbivorous insects entomologists….......……..21 feed on plant species within three Upcoming plant families or fewer. In any given conferences……….………22 habitat, the amount of plant material eaten by insects is 3-4 times that A female Goliath Stick Insect (Eurycnema Addendum – George Vernon goliath) tucking into a eucalypt leaf eaten by mammals. Hudson……..................….23 Defences Given the amount of plant material available on earth, it’s no wonder it’s such a heavily utilised resource. But a remarkable amount of it remains uneaten, due to: Plants being very low in protein – although some plants are up to 20% protein, much of it is not the right sort needed by animals, and insects are protein-rich; www.entsocvic.org.au 1 The majority of a plant is generally low in to the toxins, and specialists struggle because nitrogen – xylem is 0.05% and wood is 0.3% these plants can be hard to find and the nitrogen (although new growth is generally specialist, by its nature, can feed on little else. high in nitrogen); Birdwing butterflies are a good example, feeding Much of the plant is low in water (particularly only on plants of the one genus (Aristolochia). bark, wood and seeds) and insects that feed on these must find other water sources; Many of Cellulose is indigestible to all but a few insects the (eg silverfish and termites); chemicals Silica in grasses is not only indigestible but are not wears down the mouthparts; only toxic Plants are low in sodium, requiring butterflies to the for example to find sodium elsewhere and insects, but concentrate it in their bodies to the Secondary defensive compounds (at least plants 100,000 different types), which can be Wanderer Butterfly caterpillar (Danaus plexippus), also known as the Monarch, themselves, antifeedants or feeding deterrents, or block an feeding on Gomphocarpus cancellatus and so are insect’s ability to extract nutrients, or simply (formerly Asclepias rotundifolia) localised in act as toxins. specialised To help glands or pockets. As a side issue, many insects them that specialise on a toxic plant host adopt digest (sequester) the toxins and use them for their own the sea defence (birdwings, Wanderers, sawfly larvae) or of plants even as sex attractants, aggregation pheromones amongst or moulting hormones. which they live, In some cases, insects (eg arctiid moths) may use many compounds (eg cardenolides) from plants of one insects family (eg Asclepiadaceae) for defences, and A male Common Crow Butterfly (Euploea support a other compounds (eg pyrrolizidine alkaloids) from core) absorbing nutrients from animal dung range of plants of another family (eg Boraginaceae) for gut flora aphrodisiacs and flight arrestants. Although this is or house micro-organisms (often in mycetomes rare. on their bodies). Defences by large trees Defences by herbaceous plants Trees are large enough to support generations of Short-lived herbaceous plants defend themselves insects, giving the insects time to adapt to the against being eaten by apparency, or lack trees’ toxins, and they often grow in forests thereof. They are difficult to find and, once found, where trees of the same species are readily die back before the insects can build up a large available and cannot hide – they are highly enough population to take advantage of them. apparent. They are often pioneering after cyclones or fires, and then disappear, or grow rapidly from the These trees don’t rely so much on toxins, which base with a high tissue turnover, or contain silica may be overcome by the insects, but on (eg grasses). digestibility-reducing compounds such as tannins and on gums and resins. Although not particularly Many use complex toxins (such as alkaloids and toxic, they slow development, reduce the number cyanogenic glucosides), and the rarest toxin is the of generations of insects possible over time, most effective. If any particular compound increase the insects’ exposure to predators, and becomes too common it will attract a larger increase the energy required to consume the number of specialists able to detoxify it, which is plant – about the equivalent of the energy a disadvantage to the plant that produces it. required to find less apparent plants. As a rule, generalist herbivores cannot live on these plants because it takes a specialist to adapt 2 Trees in seasonal areas produce a flush of growth glyphopa), which, combined with the above compounds, sawflies (Perga overwhelm the insects with too much food to be species), consumed in one sitting. Christmas Beetles Some trees, such as eucalypts, supplement their (Anoplognathus digestibility-reducing compounds with extra toxins species) and in their young leaves, and reduce the ability of Eucalyptus insects to adapt to these toxins by ensuring each Lerps tree produces a slightly different toxin through Eucalyptus Lerps (Cardiaspina (Cardiaspina genetic heterogeneity (outcrossing). Genetic albitextura), perhaps the best albitextura). studied native insects in Australia variation is extremely important, resulting in The grazing individuals with the same parents exhibiting pressure is often such that competitive exclusion widely different levels of insect resistance (eg is prevented – no one species of eucalypt can Jarrah (E.marginata) to the Jarrah Leaf Miner dominate all others. (Perthida glyphopa)). Acacias, probably older than eucalypts, are even The level of toxicity in eucalypts, and the more species rich with a wider array of defences importance of coevolution, is demonstrated by (eg extra floral nectaries and myrmeconia), with the lack of introduced insect species that have very high levels of defensive tannins and gums. adapted to them in Australia, and the dearth of insects that attack eucalypts overseas. Another method of defence is to attract the predators or parasites of herbivorous insects. Despite Some plants do it through recruiting ants to the defend their leaves (myrmeconia) or with extra toxicity of floral nectaries. eucalypts, some Physical barriers such as waxes and hairs, groups especially hooked hairs that impale, or sticky hairs such as (glandular trichomes) or other substances that leaf trap small insects tend to be present in lieu of toxins. And lignin, silica and corky tissue in the beetles Eucalyptus Leaf Beetle larvae (Paropsis (eg wood, or tough compounds in the leaves, may species) feeding on their favourite plant Paropsis wear down insect mandibles. atomaria) are completely immune, almost regardless of the Communication level of compounds. The number of ‘eucalypt’ When under attack, many of the defensive species in Australia (more than 400) is possibly compounds produced by plants are highly volatile due to the arms race between this group of trees and are released into the air to be detected by and leaf beetles (Family Chrysomelidae). other plants, which then begin producing defensive compounds of their own, in preparation Try going into a stand of eucalypts and finding a for attack. single leaf undamaged by insects – it’s an It’s unlikely that the plant under attack is warning exercises that can take a very long time. Many its nearby conspecifics or competitors – it’s more native insect species can defoliate and kill entire likely that other plants are ‘eavesdropping’ and eucalypt preparing their own defences in advance. These forests, such compounds are not always species- or even as Spur- family-specific – a given plant will pick up on legged Stick chemicals from a range of other plant species. Insects (Didymuria These compounds are also picked up by parasitic violescens), wasps and some insect predators, which use Jarrah them to hone in one plant which support their Leafminers host caterpillars. Christmas Beetle, Anoplognathus species (Perthida 3 For some time there has been evidence that Problems caused: May cause actively-growing plants communicate with each other through shoots to wilt and die, premature ageing of vibrations, nanochemical oscillations generated leaves followed by leaf fall and reduce overall from within the cells. Again it’s not so much plant vigour. communication with each other, but plants reacting to the presence or activities of other nearby plants.