DDT (Dichloro-Diphenyl-Trichloroethane) became available for agricultural pest control in the 1940s. It was a relatively cheap way to rid the soil of grass grubs, porina and other insects, and farmers used it liberally, often applying it indiscriminately by plane. By the late 1960s, grass grubs were showing resistance to DDT, and evidence that it accumulated in the soil and entered the food chain led to it being banned in 1970. In the next few years, farmers experienced a resurgence of grass grubs and other pests, probably as a result of the long-term suppression of their natural enemies caused by frequent application of DDT and its persistence in the environment.
DDT was first manufactured in 1874 but was not used as an insecticide until 1939, when it was used to control mosquitoes spreading malaria and typhus. After the Second World War it was widely applied as an agricultural pesticide. Rachel Carson’s 1962 book Silent spring highlighted the dangers of DDT to the environment, and the chemical was banned in most countries by the 1980s. It is still used in low dosages to control malaria, although mosquitoes are becoming increasingly resistant.
After the banning of DDT and all organochlorine insecticides, it became less economic to control pasture pests with insecticides, although they are still used occasionally.
Natural control of grass grubs
Research has focused on finding natural ways to control insect pests. Grass grub larvae are affected by a range of bacterial and protozoan diseases, which can reduce populations but do not prevent serious outbreaks of the grubs.
The bacteria Serratia entomophila has been used as a natural insecticide to reduce the build-up of grass grub populations.
Three parasites are used to control weevils:
- Small braconid wasps (Microctonus spp.) lay their eggs in adult weevils and cause sterility. They are effective against weevils on lucerne, clover and ryegrass.
- M. aethiopoides has helped control sitona weevils since it was introduced in 1982. Another strain was released in 2006 to help control clover root weevils.
- M. hyperodae has been successful against Argentine stem weevils on ryegrass since releases began in 1991.
Plant resistance and tolerance
Some plants are tolerant to insect pests, or are avoided by them. Grass grub larvae do not eat lucerne, birdsfoot trefoil or phalaris, although these plants are not widely used by farmers. Tall fescue grasses can survive grass grub attack, unlike ryegrass and white clover.
In 1982 a fungus was discovered in ryegrass that protected it from Argentine stem weevils. This endophytic fungus lives entirely inside the ryegrass plant and is transmitted only via ryegrass seeds. It produces various alkaloids which may benefit the plant by being toxic or repellent to insects. However, the alkaloids in an endophyte strain brought into New Zealand in ryegrass seeds cause ryegrass staggers and other health problems in grazing livestock.
A strain of endophyte, AR1, which occurs naturally in European ryegrass, was commercially released in New Zealand after inoculation into local cultivars. It protects plants from Argentine stem weevils and pasture mealybugs, but is not toxic to grazing animals.
The endophyte AR37, released in 2007, reduces ryegrass damage caused by several insects including Argentine stem weevils, African black beetles, pasture mealybugs, porina and root aphids, with minimal harm to livestock.