In the average garden, life-and-death struggles are a daily fact of life as tiny creatures battle to survive in an ever-changing environment. Keen gardeners, intent on keeping up with the latest fashions in plant selection and garden design, are forever digging, pruning, spraying, mulching, watering, planting, weeding and fertilising to keep their little patch of paradise looking at least as good as that of their neighbours. Many take great satisfaction from growing their own fruit and vegetables.
Waiting to spoil their efforts is an army of insects, seemingly poised to suck and chew every plant that the gardener lays out. Almost overnight, a perfect specimen can be reduced to little more than compost by almost invisible critters. But nature is not entirely opposed to gardeners. A rival insectile army holds the voracious vegetarians in its sights, regarding them as entrées and mains on its own menu.
Man has long used the parasites and carnivores of this second legion to control pests; indeed, many species have been deliberately introduced into New Zealand in an effort to reduce pests on food crops. Their effect is rarely total annihilation, for a predator that killed every last one of its prey would wipe itself out too. However, they can be effective in reducing damage to plants.
One pest that has been almost eliminated is the cottony-cushion scale insect. This Australian native can be very destructive in citrus orchards but was brought under control in New Zealand within two years of the introduction of the cardinal ladybird. From New Zealand, the ladybird was taken to California where it also saved their citrus industry. There are now many ladybird species in New Zealand, most of which do a great job of demolishing aphids.
For the last 10 years I have been photographing the deadly tussles among the smaller denizens of our garden in Katikati. Over this period, I have been surprised at how the insect population has changed. New species have arrived and thrived, while others that were common a decade ago are now rarely, if ever, seen. Natives seem to be the most vulnerable to aggressive newcomers. The native praying mantis (possibly an accidental import from Australia in the early days of European settlement) was abundant in our garden in 1993, half a dozen or more commonly being found on a single shrub. Next summer I spotted the first springbok mantis, a larger and more aggressive species from South Africa. This species has since multiplied rapidly and now seems to have largely replaced its native counterpart. Stick insects, also once abundant, are now another rarity. I suspect sparrows feast on the juveniles.
One of the few native insects to thrive in our garden in the face of foreign invasions was the tree weta. I encouraged them by drilling 20 mm holes in lumps of wood that I fixed to the fence under the dense foliage of some trees overhanging from neighbouring properties. At the colony’s most populous, there were upwards of 20 weta, but numbers fell to 3 or 4 after new neighbours reduced the trees to firewood. These insects are not exactly a gardener’s best friend as they have an appetite for numerous garden plants, particularly bromeliads.
Some predators, like wasps and mantids, are outright carnivores, grabbing and devouring a wide range of insects and their larvae. Others, like the brown soldier bug and the robber fly, seize their prey then suck it dry, siphoning off its body fluids with a long proboscis, which they wield like a syringe. Another group of parasitic insects inject an egg or eggs into their prey, the resulting larvae gradually gnawing away its insides as they grow. Somehow, most of the larvae eat round the critical organs required for survival, allowing the parasitised host to survive for a period of weeks before it finally collapses.
The Asian paper wasp first appeared in our garden in numbers in 1993 also. The Australian paper wasp had been resident in the district as far back as I could remember, but it never became as common as the Asian species now is. We find Asian paper wasp nests by the dozen every spring and summer, and though they prey helpfully on the numerous varieties of caterpillar that damage so many of our plants, they make it virtually impossible to maintain a population of monarch butterflies. They discover and tuck into every last caterpillar, even though other predators, birds included, appear to be warned off by the wrigglers’ black and yellow colouring.
The cabbage white is one of the few butterflies that seems able to survive in any numbers despite a plethora of predators. Several parasitic wasps have been introduced to defeat this foreign pest, but from earliest spring to the first frosts of winter we continue to suffer an endless stream of these pesky lepidopterans in our garden. The Asian paper wasps take a considerable toll of the plump green caterpillars, and it is not uncommon to find clusters of tiny, golden silk cocoons of the parasite Apanteles beside a dying caterpillar from which the black wasp-like creatures, just 3 mm long, have recently exited. Unfortunately, Apanteles—rarely noticed in the garden on account of its diminutive size—is itself prey to an even smaller wasp, Baryscapus galactopus, which lays its eggs in pupating Apanteles larvae, thereby killing them.
Such predating of predators is not uncommon. A minute Aphidiinae that parasitises other aphids is in turn killed by another parasite. You need a lens to see these tiny insects clearly, and some I have come to recognise only since sending slides to John Early, entomologist at the Auckland War Memorial Museum, for identification.
Among the parasitic insects are many black-and-yellow or black-and-orange wasp-like creatures, the females of which have long ovipositors at the end of their abdomens. They look quite intimidating but most are entirely harmless and make welcome residents in the garden. Especially colourful is the introduced yellow-banded leaf roller, 8 mm long and patterned in brilliant yellow with black spots and stripes. This has become very common in our part of the North Island in recent years, and is extremely useful as it parasitises a wide range of other, more destructive leaf rollers, which it has significantly reduced in numbers.
Another highly colourful species is the lemon-tree borer parasite, a native about the same size as a paper wasp but much brighter, with cream and black bands on its body and with bright-orange legs. It has a long ovipositor with which it drills through the tough exterior of a tree to lay its egg in the grub of beetles deep inside the wood. Ctenochares bicoloris is a colourful, self-introduced insect from Africa, 25 mm long and bright orange with black tips to its wings and abdomen. It is one of several useful parasites of the green looper.
Insects adopt a range of strategies to avoid becoming prey. Some harmless species are coloured so as to appear dangerous or unpalatable. Others resemble entirely different species, thereby going unrecognised by many a predator (not to mention the average human observer) despite being quite common. The drone fly looks like a bee, and even acts and sounds like one as it buzzes around the flowers feeding on pollen. Even more apian in appearance is the large, fat narcissus-bulb fly, a perfect replica of the common bumblebee. This little-noticed insect is a major pest to many bulb species, its maggots eating the insides of some of our loveliest flowering bulbs. They don’t necessarily kill their plant hosts but force them to multiply, the resultant bulbs being too small to support flower production.
The caterpillars of native moths can be every bit as damaging as those of the cabbage white butterfly and loopers. An infestation of cabbage-tree moth caterpillars can turn a large cabbage tree into an eyesore, every leaf scalloped with dozens of deep notches. Predators do not seem to find the culprits particularly palatable compared with many introduced species.
The kowhai moth is another native that can almost completely defoliate a shrub, especially the dwarf varieties of kowhai. The caterpillars are thin and greenish with a few bristles and dark spots and are very difficult to spot. At the slightest shaking of a twig they will drop to the ground. Our miniature kowhai is regularly almost stripped of foliage by these hungry leaf-eaters, though our larger specimen seems to suffer little—chiefly, it appears, thanks to the efforts of the local sparrows, which visit the tree daily in considerable numbers. In recent years, silvereyes have also become regular visitors, and between them the birds keep the caterpillars in check. Curiously, they rarely visit the dwarf kowhai nearby.
The cabbage-tree moth is an extraordinary insect, rarely seen on account of its highly effective camouflage markings. It seems to know that the brown stripes across its wings can be aligned perfectly with the striations on a dead cabbage-tree leaf, and is almost invariably found facing across a leaf, thus achieving this effect. Caterpillars of this species regularly deface the leaves of cabbage trees, gnawing large embayments along their otherwise straight margins. Most of the damage seems to occur when the leaves are tightly curled in the early stages of growth.
Common katydids can, to a degree, vary their colour according to what they feed upon. Those we spot in our garden are occasionally of a purplish hue rather than the more typical leaf green. I recovered one such individual from a variegated brush-box tree on which it was feeding, its red-blue legs blending with the reddish stems of the leaves. Green katydids can be very difficult to find on large shrubs and are much more common than they appear to be. On summer evenings their soft chirping calls flutter round the garden, but being ventriloquial these are extremely difficult to trace. Though katydids feed on foliage, they do not seem to accumulate to the extent that they cause overt damage.
One insect that seems to have few enemies is the bronze beetle. This extremely destructive native is only 4 mm long but has a voracious appetite for leaves and apples and appears almost immediately after fruit set in spring. Large numbers chomp into newly formed apples, on which they inflict huge scars. They also have a taste for rose flowers, especially our Rosa rugosa, the petals of which they shred. We are reluctant to use poisonous sprays on our edible crops, but trying to pick off these tiny villains by hand is virtually impossible as at the slightest disturbance of the foliage they drop to the ground. If your trees are small and you have lots of time, they can be shaken into a container.
This all just goes to show that, despite our size and intelligence, without pesticides or organisms that inadvertently take up cudgels on our behalf we are no match for insects.