Biocontrol: natural pest management

Nature is an ongoing war.  A battle for survival, for dominance, and resources. ‘Pests’ are when one species are winning and are having a detrimental effect on our desired outcome. Charles Merfield explains how we can change the balance and allow nature to regulate itself.

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Biocontrol is the ultimate in organic pest management. As an alternative to using the agrichemical pesticides, biology and ecology control pests such as aphids and plant rusts.  

There are three sub-types of biocontrol: importation, augmentation and conservation.

Importation

New Zealand is a world leader in importation biocontrol. Also called classical biocontrol, it is when an exotic pest is managed by importing one of its natural enemies from its native range. Natural enemies are organisms that attack the pest organism, e.g., they eat or parasitise it. 

New Zealand is a world leader in importation biocontrol because our unique native ecosystems have been isolated from the rest of the world for millions of years. Humans have, relatively recently, imported huge numbers of exotic organisms from the rest of the world. Indeed, all of NZs productive crops; trees, fruits, grasses, livestock, etc. are imported, mostly from northern Europe. With these inevitably (and sometimes purposely) came pests, pathogens and weeds. There are only a few natives in our farming system, such as grass grub and lemon tree borer. 

These exotic organisms sometimes become pests because their natural enemies have not come with them when they were deliberately or accidentally introduced. This releases them from what ecology calls top-down population regulation. This is where an organisms’ population is controlled by organisms higher up the food chain that attack it, e.g., eat it. The textbook example is zebra populations on the African plains are controlled by predators such as lions. This is the opposite of bottom-up regulation where an organism’s population is limited by its food supply, e.g., the amount of pasture limits the zebra population and in turn the zebra population limits the lion population. So, unless you’re a plant at the bottom of the food chain or an apex predator like a lion, your population is regulated both top-down and bottom-up. 

Importation biocontrol aims to re-establish top-down regulation by importing natural enemies that attack the pest. And it can be spectacularly effective. One of the classic examples is the near elimination of prickly pear cactus in the 1920s from Australia’s rangelands by the caterpillar of the Cactoblastis cactorum moth. However, only about 10% of introductions completely control the pest, some 40% have some level of control and 50% have no effect, so while it can be amazingly effective, the effect is sadly often limited. 

There is also the risk that the imported natural enemies become a pest itself, with ferrets and stoats here and cane toads in Australia being well known examples. Importation biocontrol is therefore the preserve of governments and is heavily regulated to ensure that new introductions are not going to have undesired outcomes. Due to the huge cost (millions of dollars) and that it takes several years to get a new biocontrol agent through the regulations, about one species is imported every few years. 

The Environmental Protection Authority has approved 40 biocontrol agents since 2000.  Eleven for insect pests (codling moth, tomato potato psyllid, wasp etc.) and the rest for weed control (broom, ragwort, Californian thistle, wandering willie…)  

Augmentation

Augmentation biocontrol is where the pest and its natural enemies both exist in the environment but the natural enemies are not achieving sufficient control, so their populations are augmented by adding more of the natural enemies. There are two sub-approaches: inoculation and inundation. 

With inoculation, the environment is inoculated with small numbers of the natural enemies, which are now called biocontrol agents, with the aim that they will breed and multiply increasing their populations. This is the main form of biocontrol used in glasshouses. There are companies that specialise in producing these biocontrol agents and selling them to growers. In NZ there are some fifteen biocontrol agents for sale, but overseas where there are fewer biosecurity issues there are hundreds of products. 

Inundation involves large numbers of the biocontrol agent being released, i.e., to inundate the pest.  Inundation is where biopesticides fit in biocontrol. Biopesticides are the biological alternative to a chemical pesticide, i.e., their active ingredients are biocontrol organisms or extracts. For example Bacillus thuringiensis for the control of caterpillars and pyrethrum extracted from Chrysanthemum cinerariifolium for the control of insects. Biopesticides is the most rapidly expanding area of biocontrol, due to the multiple problems being faced by the chemical pesticides and also because they are easily applied by sprayer. 

Conservation

Finally, conservation biocontrol, like augmentation, is where both the pest and biocontrol agent exist in the environment, but the biocontrol agent’s population is too low to control the pest. Instead of adding biocontrol agents as in augmentation, the ecosystem is manipulated to boost the biocontrol agent. 

Typically, this is done by adding plants to provide resources for the biocontrol agents. This includes food such as pollen, nectar and alternative prey and shelter, e.g., tussocky grasses to hibernate it. To me the name conservation biocontrol is a bit of a misnomer as we are manipulating ecology, i.e., the interactions among organisms, so it’s really ecological control. 

There are lots of plants that can be used for conservation biocontrol, but anything that provides shelter and food is good. Typically, this means more wild type plants rather than those highly bred for showy flowers as these often contain little nectar and pollen. The late great Prof. Steve Wrattens ‘holy trinity’ is phacelia, buckwheat and alyssum. One of these he found would often come up as the best source of nectar and pollen when doing research. Phacelia and buckwheat are quick growing annuals so best suited to annual crops like vegies. Alyssum is a perennial and flowers year-round so well suited for perennial crops and also dotting round the garden. I think every polytunnel and greenhouse should have an alyssum plant in the corners. Wrattens found the cultivar ‘Benthamii’ was the best. 

In commercial situations diverse mixtures of herbaceous (pasture) species including grasses, legumes, and forbs such as plantain, chicory and yarrow are used. It is important that they are not regularly mown, instead they need to go to flower to provide nectar and pollen. This includes the grasses, as their wind-blown pollen is much smaller, so is important food for tiny biocontrol agents such as predatory mites. Only cut the mixture when it is starting to go to seed, and ideally only cut sections at a time so that some area is in flower over the whole of summer. Also don’t cut low, cut 10-15 cm high. 

For home gardeners and commercial growers, conservation biocontrol provides the biggest opportunity. This is because importation biocontrol is controlled by government, augmentation involves continually spending money buying biocontrol products, while conservation biocontrol is all about making your garden more friendly to beneficial species, and therefore boosting overall biodiversity. So go on, set up an area of your lawn to be your diverse pasture conservation biocontrol spot and marvel at all the insects visiting it and plant some of Wrattens ‘holy trinity’ round the rest of the garden. 

Growing the Holy Trinity

These are quick-growing, easy-care annuals that flower indeterminately throughout summer.  They prefer full sun but will tolerate partial shade.    

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