And not just the forest. Infra-red video footage has caught a possum, eyes glaring green in the darkness, in the act of scaring a brooding kokako off its nest and taking an egg. The future of New Zealand’s rarer birds cannot be guaranteed in places where possums are rampant.
I didn’t know how long the possum had been caught, or how soon the trap would be cleared, but the notion of myself as the star in a Born Free scene, letting the creature go, wasn’t an option. Department of Conservation staff had put months of effort into monitoring kokako in the Hunuas, and into trapping rats and possums.
My only weapon was a glass juice bottle in my pack. This I used, in a painfully unsatisfactory way, to put the animal out of its misery.
In the long moments while I considered what to do, and calmed the possum in order to hold it still, biological control had never held more appeal for me.
As an environmental menace, the possum is in company with goats, deer, rats, ferrets, weasels and several other mammalian imports which have flourished in this country and become pests. Yet on a worldwide scale, the pests which cause the most concern are those that threaten human food production.
It’s a war out there. It is estimated that for every three tonnes of food laboriously wrested from the soil, only two tonnes is available for human consumption. The other tonne is lost, mainly to hungry insects. In the struggle to maximise production levels—and, increasingly, to provide “perfect” produce for picky consumers—chemicals have permitted humans to keep one step ahead of the enemy.
But dissatisfaction with aspects of chemical pest control is prompting a new interest in biological control—making use of nature’s weaponry to fight modern battles.
The idea is far from new. Which child hasn’t heard of the Old Lady who swallowed a fly, then a spider to catch it, a bird to catch the spider, a cat to catch the bird—until she bit off more than she could chew?
Biological control is about investigating nature’s existing relationships and putting them to human use. Its methods are interwoven with the historical fabric of agriculture, and even include such humble strategies as using ducks to pick insect pests off crops.
“Classical” biological control (biocontrol, for short) has to do with addressing an ecological imbalance. It means importing the missing natural enemies of a new immigrant, and thereby controlling the target organism. Its agents are either predators, which catch and eat their prey, pathogens (disease-causing organisms) or parasitoids (parasites which cause the death of their prey).
In its broader sense, biocontrol refers to any kind of control that is biological as opposed to chemical, such as plant breeding for resistance to pests, the use of pheromones to disrupt mating in insects, and immunosterilisation of male pests.
Parasitoids are particularly effective biocontrol agents though it has taken humans a while to appreciate these elegant assassins. Indeed, nineteenth-century naturalists felt their modus vivendi called into question the existence of a benevolent God. Take the ichneumon wasp. As big as two pinheads, ichneumons feed on nectar, sugar and water—harmless enough—but reproduce at the expense of other organisms. An adult female injects her eggs into the larva of a particular host species. The eggs hatch into hungry little ichneumon larvae, which steadily eat out the innards of the caterpillar. Intestines and reproductive system make up the first course, followed by muscles and nervous system (so that the meal remains mobile and in the freshest state possible) before the parasitoids break out of the host’s wrinkled skin like aliens in a sci-fi movie.
Descriptions of the ichneumon’s habits caused cries of revulsion, and invited the reader to pity the still walking and feeding host, twitching involuntarily as the internal gnawing sapped more and more of its strength.
The maligned ichneumon well illustrates the value of biological over chemical control. A specific host is targeted, rather than whole communities of beneficial insects suffering when broad-spectrum insecticides are used.
A further problem with the chemical approach is that many organisms, especially smaller ones that breed rapidly—bacteria, insects, some plants—quickly develop resistance to pesticides. Only the few individuals with some inherent ability to survive spraying are left to reproduce, effectively concentrating this resistance in subsequent generations until the sprays are no longer effective. Growers need to put off this evil day as long as possible, and so must spare the sprays.
Limiting pesticide use is desirable from a marketing perspective, too, as overseas buyers increasingly tolerate only minimal chemical residues on produce. At the same time, however, international quarantine regulations may stipulate zero tolerance of stowaway insects. Growers must walk a tightrope between not enough spraying and too much, either of which may mean a loss of income.
Biocontrol can circumvent these problems, and promises other benefits, too. Humans are spared the effort of chemical distribution, as agents self-disperse even into inaccessible terrain. Furthermore, the population size of the control agent mirrors that of the pest, flourishing and declining in tandem with it. There are no running costs and no residues, and because the destruction is done by nature itself, not humans, there are fewer ethical dilemmas and the killing seems more acceptable.
But expectations should not be raised too high. Biocontrol is not an instant panacea. Neither is it intended to completely eradicate any pest; rather—as in nature—merely to control it.
The major disadvantage of biological control is the time—often years—and money spent on exhaustive testing, aimed at ensuring that the proposed controller does not itself become a pest.
The irony of introducing a foreign species to control a foreign species is not lost on observers. There is no turning back after releasing a mass of tiny insects into the wild. Peter Cameron, an entomologist at Crop & Food Research in Auckland, is pragmatic. “Importation of these control agents will certainly change the environment in some way, but judgments can only be made by the standards of the day. The decision on new biological control agents comes down to the weighing up of two risks: one, of importing it, and the other, of not importing it.”
Researchers point out that with the more sophisticated ecological knowledge we have today, a “generalist” predator like a ferret would no longer be considered as a possible control agent—although ferrets were introduced here in their thousands in the 1880s to control rabbits.
The Old Lady belonged to the old school.
Australia was the setting for several early experiments in biological control. One, the vanquishing of the prickly pear cactus, was a huge success. Brought from Latin America to a land where it had no natural enemies, the cactus had run wild. From 1920, a search was made in its homeland for a natural control, and the result was a moth from Argentina with the appropriate name of Cactoblastis. It was imported to Australia for mass-rearing of eggs and subsequent larval release on to the plants—millions were needed to raze 12,000 square miles of sometimes shoulder-to-prickly-shoulder cactus. The caterpillars disappeared inside the fleshy “leaves” to consume the interior, changing the dense cactus stands into wilted brown stems, and leaving for posterity a dramatic series of before-andafter photographs far superior to those produced for any slimming advertisement.
A less successful case of biocontrol across the Tasman, concerns a generalist gone mad. The South American marine (cane) toad was introduced to Queensland in 1935 to control cane field beetles. Unfortunately, the toad’s diet is not tightly linked to any one item of prey. As a result, the venomous toad consumes many beneficial insects, poisons whatever preys upon it, and is steadily hopping towards the valued national parks of the Northern Territory.
The marine toad features in another cautionary tale, this time in Micronesia, pre-World War Two. Rats were becoming a problem on some of the islands. What a good idea it would be, someone thought, to import giant monitor lizards to eat them. But strategists had completely overlooked the fact that the monitors are diurnal, and rats nocturnal, so the two were never destined to meet. The hungry monitors turned to domestic poultry, which angered local people, so marine toads were brought in to control the monitors. Sure enough the poisonous toads soon caused a decline in monitor numbers, at which point it became apparent that monitors had been usefully controlling the grubs of the rhinoceros beetles (which had been damaging coconut palms), as well as feeding on coconut crabs (which had been preying on the giant African snail—itself introduced as a source of human food, but later becoming a pest).
As the monitors declined, toad numbers increased, but were eaten by domestic pigs, cats and dogs, which likewise died from the venom. The outcome? With hardly any cats or dogs left, there was a rat population explosion. The local people were back at square one, except with additional pests.
Such early mishaps forced scientists to think carefully about the qualities needed in a useful control agent. Like employers conducting an interview, pest controllers must now be convinced that the chosen candidate will fulfil the requirements of the job.
The first requirement is that the agent must attack only one pest species, or a very narrow range of species. The most common question asked of biological control researchers is “What will the imported insect eat or parasitise when it has finished off all the pests?” No one wants another toad fiasco.
Specificity means that when the favoured diet is no longer abundant, control agents will perish rather than switch to a different prey. In reality, providing such a guarantee is extremely difficult. Testing in Petri dishes and field cages cannot duplicate conditions in the wild, and it could take a lifetime to test every potentially threatened organism.
The second requirement is that the lifestyles of pest and control agent must be synchronised, so that, for example, an adult parasitoid is out searching for prey in which to lay eggs at exactly the same time as the pest larvae are developing. Also, the control agent’s reproductive ability must enable it to keep pace with increasing pest numbers.
Thirdly, the predator must actively seek its prey. Ladybirds make good control agents, because they put in the legwork catching aphids. The suitably rapacious praying mantis, on the other hand, is unsuitably relaxed in its hunting methods.
The fourth point is that control agents should be able to survive pest-free periods, perhaps by becoming dormant, or taking energy from a source such as pollen.
Finally, control organisms must be able to adapt to new environments. Many potentially useful insects remain locked into Northern Hemisphere day length and seasonal rhythms, and are unable to thrive in the south. Alternatively, an insect which needs winter chilling as part of its life cycle might flourish in Otago but be an ineffective control agent in Northland.
Even minute disease-causing organisms must adhere to this same employment code. A case in point is a bacterium recently called in, Red Adair-like, as group leader in New Zealand’s longest-running project, the fight against the crinkly grass grub. The target pest is the larva of an endemic scarab beetle, which can cause great damage to pasture by devouring the roots of grasses. In this instance, rather than an ill-advised insect introduction becoming a problem, it is human modification of the environment through modern farming systems and introduced grasses which has allowed the species to run wild. Estimates of the annual monetary loss to the country through grass grub range from $41 to 89 million.
Solutions to the problem have been sought for over 50 years. Chemical controls have been of only limited use, due to increasing resistance to the chemicals, unwanted residues on the grass and high cost. Up until now, the introduction of biological enemies has not worked, either. In the days when procedures and protocols weren’t so strict, around 20 species known to control relatives of the grass grub in other countries were introduced in the hope that they would cope with both a new host and a new habitat. None succeeded. Although there are several native enemies, such as threadwonns and tachinid flies, and also diseases caused by protozoan, bacterial and fungal pathogens, these seem to have only limited local effects.
Hope now lies with two species of bacteria belonging to the genus Serratia, which can fatally infect grass grub larvae. The bacteria occur naturally in soil, so are swallowed by grass grubs as they ingest roots and soil. However, the bacteria are not sufficiently abundant to cause extensive grass grub death. Trevor Jackson, an AgResearch scientist, and others have found that higher doses of bacteria can kill the grub, and hit upon the idea of growing Serratia in concentrated cultures in the laboratory, then injecting the bacteria back into the soil.
The remedy, commercialised in 1989 under the trade name Invade, can reduce grass grub numbers substantially. The manufacturers claim that fields treated in winter yield as much as 40 per cent more pasture than untreated fields following bacterial injection using a modified disc drill.
One of the most successful biocontrol stories concerns armyworm. Well known until the 1970s, this scourge has declined into obscurity following concerted attack by a suite of parasitoids, one of which proved highly effective. Armyworm caterpillars were once a feature of the countryside, swarming across roads and causing cars to skid as hordes searched for crops to defoliate. Now it is only occasionally seen. Estimates from the 1970s of the money saved through the work of the parasitoids were put at between $4.5 and 10 million per year.
Wasps are also under attack from parasitoids. In a 1991 survey of adult attitudes to pests and pest control methods carried out by Lincoln University’s Agribusiness and Economics Research Unit, results showed that a large majority of respondents considered rabbits, possums and wasps the most serious pests, and that fruit flies, grass grubs, flies, termites and rats were considered serious or very serious by about half of the population.
Wasps are especially numerous in South Island beech forests, where they forage for honeydew, a waste product excreted from scale insects feeding on beech sap. Honeydew is part of the diet of native birds such as tui, kaka and bellbird, and of native moths and geckos. At least some of these species are suffering under competition from wasps, and wasps also prey on many native insects directly.
“There is always a loud hum in February in the beech forests, and the wasps’ long sting gives people the hurry-along” says Peter Read of Landcare Research at Lincoln. “Forestry operations are curtailed, ripening fruit and beehives destroyed, and the whole ecology of native forests affected.” Conditions in this country—no natural enemies, mild winters, an abundance of food—permit some of the highest densities of social wasps in the world. “Nests here may be six times larger and produce 23 times as many new queens as those in the Northern Hemisphere,” Read says.
The solution to the wasp problem may lie with a parasitoid, imported from Switzerland and released at hundreds of sites since 1988. Wasps don’t defend the entrance to their nests as honeybees do, thus parasitoids may infiltrate with ease. Although the incidence of parasitised nests is increasing in parts of the northern South Island where the parasitoid was released, so far it is not having an impact on overall wasp numbers.
Comments Peter Cameron: “Biocontrol is long-term work. We have to be patient.”
Progress is much more apparent with two-spotted mites, a pest of many crops and plants, especially those in glasshouses. The mites can be successfully controlled by larger predatory mites, commercially available as a preparation called Mite-E.
Peter Workman of Crop & Food Research in Auckland, which developed the preparation, says that growers of a variety of flowers and vegetables (roses, orchids, strawberries, cucumbers, capsicums, apples, kiwifruit) are starting to use the biological control agent to at least reduce the need for spraying. “Glasshouse growers may still spray for aphids with a selective insecticide that won’t kill the predatory mite, or occasionally spray for fungus, though that can be largely controlled by attention to temperature and humidity. Distributing Mite-E every six weeks in summer and 12 weeks in winter will control all the two-spotted mites on glasshouse roses. Other crops need more or less frequent applications. In fact, control is so good that the predators will die out, and that is why they have to be reintroduced periodically.”
To raise the mites, technicians grow dwarf beans, stock them with the pest, and a week later introduce the predators. After another week there will be few two-spotted mites left, but plenty of predatory mites. The leaves are then harvested and sold to the grower, who spreads them around his crop. “You need about 4000 mites for a 1000 square-metre glasshouse,” says Workman.
Whitefly—the other main pest in glasshouses—can also be controlled by a biological agent, trade-named En-force.
A combination of biological control agents, attention to growing conditions so as to minimise pest problems, and judicious use of chemicals as a back-up is termed integrated pest management (IPM). IPM aims to minimise chemical residues and harm to the environment, yet give good control of pests. Over much of Europe, 90 per cent of glasshouse-produced vegetables are grown using this approach. “In New Zealand we are lagging behind,” Workman says.
Integrated pest management has been adopted by Watties and Cedenco in their large-scale outdoor growing of tomatoes around Gisborne and Hawkes Bay. The main target is tomato fruitworm, the caterpillar stage of the moth Helicoverpa armigera, which chews holes in tomatoes and nibbles at the kernels of sweetcorn, as well as damaging lucerne, soybeans and a variety of other crops. To control it, a small parasitoid wasp originating in Kazakhstan was imported from Romania into New Zealand in 1977 and released here the same year. This species lays its eggs only in small (3-10 mm) caterpillars of Helicoverpa, and, in the parasitoid manner, eventually destroys them. No sign of the parasitoid was seen until 1981, but it is now widespread and is reducing Helicoveipa numbers by about 70 per cent.
However, in commercial growing zero damage is the target, so in 1987 Crop & Food Research scientists imported a second larval parasite, one which prefers larger caterpillars as a repository for its eggs, and attacks any individuals the Kazakhstan wasp has missed. Between them, the pair of parasites have reduced tomato fruitworm damage to 1 per cent in early unsprayed tomatoes, but they are less effective in sweetcorn, and Helicoveipa may still cause some damage to late-season tomatoes.
Watties and Cedenco now place pheromone traps in their fields to attract male moths, and when monitoring shows that moth density has reached a threshold level, trained scouts are sent out to carefully examine a selection of plants for the presence of caterpillars. If an average of more than one caterpillar is found per plant, the field is sprayed. (A field may be 30 hectares in area and contain $200,000-worth of tomatoes).
Before the introduction of IPM, spraying was carried out routinely every week or two. The number of spray treatments required for a crop of tomatoes has now been reduced from eight (before the introduction of the parasites), down to between one and three in the last few years.
Habitat manipulation to enhance the numbers of natural enemies of insect pests is another biological approach that is proving successful. Insect ecologist Steve Wratten of Lincoln University has studied the overwintering habits of useful arthropods such as beetles and spiders in order to find ways of increasing their abundance. He has found that some require specific microclimates, perhaps in tussock-forming grasses, while others need nectar and pollen to supplement their diet of insect pests.
Drawing on the British hedgerow heritage, Wratten has developed “beetle banks,” which have attracted international attention because of their simple efficacy. Who wouldn’t like a system with minimal running costs (the cost of seed plus the loss of production of a thin strip of land) and with complete reversibility at any time?
Double-ploughing one row in the field creates a bank, which is sown with grasses such as cocksfoot and Yorkshire fog. Wratten says that densities of predators on beetle banks at Lincoln reached 2000 per square metre only 18 months after the banks were made.
For other predators such as hoverflies and parasitic wasps, it is pollen and nectar which are in short supply on farmland in the spring. Phacelin, a blue-flowered annual from California, and buckwheat can remedy this problem. The predators which these plants attract require the amino acids from pollen for the maturation of their reproductive systems. Recent results at Lincoln show that Micelin can enhance hoverfly predation of aphids on cabbages, while buckwheat sown below apple trees can double the proportion of leaf-roller caterpillars killed by parasitic wasps in the trees.
While great densities of natural insect enemies are highly beneficial, high densities of brush-tailed possums in this country present an enormous problem. The possum’s presence here is a result of misplaced 19th century enthusiasm for species importation to a land deemed biologically deficient. Scientist George Thomson, writing in 1922, described the mindset behind the introductions. “The settlers found themselves in a land . . . which seemed to them to reproduce many of the best features of the homeland . . . . Here, in a land of plenty, with few wild animals, few flowers apparently . . . with streams almost destitute of fish, with shy songbirds and few game birds, and certainly no quadrupeds but lizards, it seemed to them that it only wanted the best of the plants and animals associated with these earlier memories to make it a terrestrial paradise . . . . I have been on the council of an Acclimatisation Society, and I know the enthusiasm, unalloyed by scientific considerations, which animates the members.”
Along with species as diverse as ‘nooses and mules, wapiti and widgeons, salmon, trout, pheasants, quail and partridge, rabbits, hares, tahr and chamois, came “this valuable and harmless animal,” the possum.
As it happens, the possum has proved neither valuable nor harmless. Recognition of the seriousness of the impact of this consumer of native forests and vector of bovine tuberculosis can be seen in the government’s National Science Strategy Committee’s designation of possum control as a priority research area, along with climate change and sustainable land management.
In order to develop biocontrol strategies, every aspect of the possum’s lifestyle is being scrutinised in even more detail than the British Royal Family’s. “The net is being cast so wide because we know so little about possum ecology,” says Phil Cowan of Landcare Research. “We’re working on a three-tier priority: in the short term we must improve the way we control possums, with better toxins and baits. In the medium term we are looking for suitable possum parasites, and in the long term we hope that some form of reproductive control can be achieved.”
So far, two possum-specific species of intestinal roundworm have been discovered that at least debilitate infected adults. Their current distribution is very limited (none are in the South Island) giving hope that if these worms could be more widely disseminated, they might cause extensive ill-health and death among possums. A naturally occurring disease which causes “wobbliness” in some captive possum populations in New Zealand is also being studied.
The long-term option involves research into possum reproduction, with a view to developing sperm vaccines which would induce the body of a female possum to reject sperm—an immunocontraceptive method. The gene for a suitable sperm protein could conceivably be spliced by genetic engineering into an organism that already infects possums—perhaps a virus, the Leptospirosis bacterium or the intestinal worms.
Another possible control method is disruption of possum lactation, by somehow locating and then switching off the gene(s) controlling the production and composition of the mother’s milk.
David Heath, a scientist with AgResearch at Wallaceville (where much of this research is being undertaken), explains that researchers are looking for low-level pathogens. “You are better off not killing the animal—which stops the spread of your infectious agent—but infecting it with something debilitating. Then the possum will produce just a single offspring instead of the usual five during its lifetime.”
One thorny aspect of possum research is the fact that possums are protected in Australia. If a suitable biocontrol method was found here, it might make its way back across the Tasman. Accidents happen. Just last year the disease-causing rabbit calicivirus escaped into the wild rabbit population in Australia. Furthermore, scientists cannot rule out the possibility that a virus might change hosts and bring about the demise of non-pest species.
As the search for biological control agents becomes more and more high-tech, with genetics and microbes supplanting the ancient Chinese farmer’s simple method of setting down ant nests to combat citrus pests, the risks, as well as the rewards, may be increasing. Microorganisms, with their minute size and generation times measured in minutes, have a greater propensity for unpredictable change than do larger creatures.
The big worry, as Francis Howarth of Hawaii’s Bishop Museum noted in 1991, is that “adequate data have not been collected to defend the notion that the introduction of agents is environmentally safe and risk-free . . . . Biological control introductions are part of the much larger problem of the invasion of new areas by alien species, which are recognised as a major factor in species extinctions.”
Alien organisms invariably disrupt established populations, and since the introduction of new organisms is usually irreversible, and the chances that an introduced agent will negatively affect non target species increases over time as the agent spreads and comes into contact with increasing numbers of species, we can’t be too careful.
Graeme Ramsay, a retired entomologist, is concerned that today’s research bodies, the Crown Research Institutes, have to concern themselves largely with short-term profitmaking. “There needs to be enough time and money to develop the science adequately, before biocontrol agents are released,” he cautions.
It is already accepted that the parasitoid introduced to control cabbage white butterfly in 1932 is parasitising the native yellow admiral butterfly, and other species may be in decline as well. George Gibbs, entomologist at Victoria University, Wellington, makes the point that little is known about the undesirable side effects that may have resulted from earlier releases of insect biological control agents, “because we monitor only for the results we expect.”
The importance attached to biological introductions is to be seen in new legislation—the Biosecurity Act and the Hazardous Substances and New Organisms Act—which deal in part with risk assessment and procedures for importing new organisms, be they plants, sheep or genetically engineered microbes. Such legislation is a far cry from the days when the only obstacle to importing an organism was whether it would survive the long boat trip.
But while the new Acts are all well and good for intentional introductions, they cannot deal with the increasing number of invasion incidents: mosquitoes arriving in used tyres, voracious new caterpillars at Mission Bay, fruit flies in Mt Roskill, new eucalypt pests probably being blown here from Australia.
Vigilance may be our best defence, but it is unlikely that biocontrol scientists will be out of work any time soon.