RHS Journals
The Garden
April 2008
Herbivores and pollinators
In the fourth part of his series on gardens as ecosystems, Ken Thompson looks at the many and varied organisms that feed on living plants, including those vital to fruit and seed production, the pollinators.
While it is true to say most plant material in gardens is usually recycled only after it is already dead, that does not mean there are not plenty of animals queuing up to eat your precious plants while they still live.
Herbivores are a vital part of all garden ecosystems, because it is the plant-eaters who are the prey of animals higher up the food chain. The vegetarian herbivores are the ‘primary consumers’ of the food energy made by plants (primary producers), converting this energy into a form – their own bodies – that feeds carnivorous animals, or ‘secondary consumers’. Larger animals may indeed prey on smaller carnivores as well as herbivores, but ultimately all animals rely on the organisms that feed on living or dead garden plants. (Carnivores and parasites will be the focus of Part 5 next month.)
Slugs and snails are top of most gardeners’ herbivore ‘hit list’ – surprising, really, as most molluscs are harmless consumers of dead plants and animals. Only a few species cause problems.
More than most garden animals, the importance of slugs and snails varies enormously from place to place and from time to time. This is because molluscs, rather like crustaceans (such as woodlice), are most at home under water, and even the terrestrial kinds are only really happy if it is cool and damp.
So while some gardeners on heavy soils are driven to despair by slugs, others on dry, sandy soils wonder what all the fuss is about. Before getting too mad with molluscs, gardeners should reflect that there are many plants they do not like to eat – in a recent study at the University of Sheffield, we found that common garden snails refused to eat almost half of the native British plants we tested. Slugs are often a problem in gardens largely because we tend to choose to grow the kind of tasty, fast-growing plants they like.
Less visible plant-eaters
Barring a handful of larger animals, most garden herbivores are insects, which fall neatly into two kinds based on their mouthparts – suckers and chewers.
The suckers are simpler, since almost all are in a single order: Hemiptera or bugs. This order is extremely diverse – approximately 1,700 species are native to Britain – but all bugs share mouthparts modified into a piercing tube, like a tiny hypodermic needle. Some species tap into plants, while others are predatory, and wield their syringes against other animals (there will be more on carnivores next month).
Bugs include the most abundant plant-eaters in the garden: aphids. Aphids graphically illustrate a key feature of most ecosystems, which is that plants are often short of nutrients, but nearly always have too much carbon. Thus aphids need to process large volumes of plant fluids (sap) to get enough protein, and often excrete surplus carbon-rich sugar as honeydew. Honeydew attracts ants, social wasps, flies and even bees, but much of it can end up on cars and garden furniture, much to the annoyance of gardeners.
Other bugs, froghoppers or spittle-bugs, have found another use for all that spare fluid: the nymphs blow bubbles out of their backsides and hide in the resulting froth, which we know as cuckoo spit. Aphids can reach plague proportions and have few redeeming features, but many larger bugs are handsome insects, and good (if rather noisy and erratic) fliers. Hawthorn shield bug (Acanthosoma haemorrhoidale), a glossy green and bronze leaf-eating bug with distinctive, broad, pointed ‘shoulders’, is a relatively common visitor to my own garden in Sheffield.
Biting off more Chewers are much more diverse than suckers, and often cause more obvious damage, leaving signs of their activity such as holes in leaves.
Among insects, the three major groups of chewers (each with species regarded as pests by most gardeners) are: beetles, sawflies and lepidoptera – moths and butterflies. In many cases, the adults do not eat plants at all, and even if they do, it is usually the larvae – caterpillars – that do most damage. This is the part of the life cycle that does the bulk of the growing, so naturally it needs the most food.
Caterpillars are abundant, but the majority are surprisingly hard to spot; they have plenty of enemies, and one of the best defences is simply to stay out of sight. If a caterpillar is brightly coloured, for example mullein moth, which is black, yellow and white, that is a sure sign it does not taste good. Other species may be protected by long, irritant hairs.
Many chewing insect larvae spend all or most of their lives concealed within plant stems, seedheads or even inside trees, living or dead. Wood is carbon rich but nutrient poor, so such larvae develop only slowly and may take many years to reach maturity. Their reward
is protection from most predators (but not all: woodpeckers for example). One group of specialist wood feeders sometimes seen is longhorn beetles: large, handsome beetles with long antennae. Most are quite rare, but black and yellow wasp beetles (Clytus arietus) are occasionally found in gardens.
All insect herbivores are relatively small, which opens up the possibility of lifestyles that simply are not an option for larger animals. The larvae of around 500 British moths, beetles and flies live inside leaves. Such larvae are known as ‘leaf miners’; although the adult insects are rarely seen, the mines may be conspicuous. One familiar garden example is holly leaf miner; another is a sporadic pest of beetroot (both these are flies).
Most native leaf miners are specific to native plants, but some exotic garden plants have brought their miners with them; one example is firethorn leaf miner (Phyllonorycter leucographella), a small buff and white-marked moth that was introduced into Essex in the 1980s. Its larvae can now be found making obvious mines on the upper surface of pyracantha throughout England.
Some other insect chewers have gone further, and have persuaded plants to provide a bespoke residence and food supply all rolled into one: these are the gall-formers. The insects responsible for most galls (including marble and spangle galls of oak and robin’s pincushion on wild roses) are gall wasps. Gall wasps are tiny and have weird and wonderful life cycles, involving alternating sexual and parthenogenetic generations (parthenogenesis is reproduction without sex, in which females produce exact copies of themselves; this is also found in aphids).
Plant galls are mini-ecosystems in their own right, supporting flies, moths and other gall wasps that do not make their own galls, but lay their eggs in those formed by others. This is not strictly parasitism, but they may cause the death of the rightful owner.
Pollination service
Plenty of insects may pollinate flowers from time to time, but there are three major groups of garden pollinators: hoverflies; moths and butterflies; and bees.
The first two have larval stages that do not depend on flowers. Hoverfly larvae exploit a range of food sources, but young moths and butterflies (that is, caterpillars) eat plants. It is a sad fact that if you like to see the adults in your garden, you usually have to put up with their delinquent offspring.
There are many more large moth species than butterflies; in fact the ratio is about 35 to 1. Most moths fly at night, but there are still more day-flying moths than butterflies. All moths and butterflies have a long proboscis and can extract nectar from deep flowers, so to keep them happy, grow buddleia, hebe, honeysuckle, verbena, red valerian, lavender, Michaelmas daisy, ice plant ( Sedum spectabile ) and teasel.
Adult hoverflies eat nectar and pollen, while their larvae are quite varied in their habits. Some eat rotting plants in dung, decaying matter or water; many are predatory; and one (narcissus fly) is a pest of garden bulbs. Although many are important aphid predators, the legless, maggot-like larvae of hoverflies probably go unnoticed (or unrecognised) by most gardeners. Hoverflies are extremely abundant in gardens, showing just how good gardens are for pollinators. In the 2000–2004 study Biodiversity in Urban Gardens in Sheffield (BUGS), hoverflies were the second most prolific group of flying insects, after parasitoid wasps.
Continued >