Biological control is the idea that a pest or weed can be controlled by its ‘natural enemy’. Introductions of such natural enemies have been tried in New Zealand many times, often based on scientific investigation or advice, but sometimes against such advice.
By the 1880s hordes of insect pests were infesting orchards and chewing through crops. Biological control was first tried against a scale insect, Icerya purchasi, which was ravaging citrus trees. An orchardist tried importing parasitic flies to control it, but they did not survive. But in 1888 an Australian ladybird species, which arrived accidentally, proved so effective at hunting down the pest that it has not been a problem since.
In the 1880s insect pests were devouring apples and other crops. In 1881 an infestation of what is thought to have been cosmopolitan army worm, probably Mythimna separata, made headlines around the world. In Rangitīkei, a huge mass of them moving across a railway track actually brought a train to a standstill, with its wheels spinning on their crushed bodies.
One of the earliest successes, about 1920, was the introduction of a parasitic wasp to control Eriosoma lanigerum, an aphid pest of apple trees. More recently a species of naturally occurring bacteria, Serratia entomophila, has been cultured and can be applied to reduce grass-grub populations in pasture.
Parasitoids have been successfully introduced to control pests such as sitona weevil on lucerne, Argentine stem weevil in ryegrass and clover root weevil. These parasitoids are small braconid wasps (Microctonus spp.) that lay eggs in the adult weevil and render it sterile. M. aethipoides has effectively controlled sitona weevil since it was introduced in 1982 and M. hyperodae has been successful against Argentine stem weevil since 1991.
Biological control agents for ragwort and nodding thistle have reduced but not eliminated these weeds.
Breeding pasture and orchard plants
Initially British grasses and clovers were planted in New Zealand, but from 1912 varieties were selected and bred specifically for local conditions by staff of the joint Department of Scientific and Industrial Research/Department of Agriculture Plant Research Station. In the early 1930s Grasslands Ruanui ryegrass and Grasslands Huia white clover, which provided more persistent and productive pasture, were selected. Ruanui was later superseded by Nui, another ryegrass with better summer growth, but Huia remained dominant for over 50 years. Although new varieties have since been bred, it remains the baseline standard.
In 1982 it was found that the endophytic fungus that lives within ryegrass plants produces alkaloids which may be deterrent or toxic to insects. A strain of endophyte, AR1 from Europe, which does not cause toxicity to grazing animals but protects the plant against Argentine stem weevil and pasture mealybug, was commercially released in New Zealand.
Independent breeders included Hutton Kidd, an orchardist who bred new apple varieties in the 1920s, and Bruno Just and Hayward Wright, nurserymen who selected kiwifruit varieties in the 1930s.
One of the major successes of the DSIR fruit breeding programme was Gala, which became one of the world’s most popular apples. Pacific Rose, from a Gala–Splendour cross, has also been particularly successful. A later breeding programme by HortResearch produced the Jazz variety from a Braeburn–Royal Gala cross.