AgResearch and Bio-Protection Research Centre scientists Trevor Jackson, Richard Townsend, Jessica Dunbar, Colin Ferguson, Sean Marshall and Sue Zydenbos studied the effect of flipping land at Cape Foulwind near Westport, the Amuri irrigation scheme in Canterbury and converting to pasture areas of forestry on the North Island volcanic plateau. In all three cases, large insect populations were reported several years after pasture was sown, affecting production.
The scientists said large areas of land use change caused a new environment to be created, providing insect pests with huge food supplies, allowing them to multiply rapidly without pressure from natural enemies.
At Cape Foulwind, several dairy farms were built by flipping pakihi swamp. Since 2001, about 3000ha was sown into pasture and although it grew vigorously initially it was soon hindered by porina and, in 2004, manuka beetles.
Manuka beetles are a native scarab beetle. Their larvae feed on pasture roots, causing similar damage to grass grubs. Adults have a bright, metallic green colouring and swarm during the day in early summer.
One of the two species of manuka beetle found at the Cape Foulwind conversions, Pyronota setosa, had been considered rare previously.
By 2009, all of the dairy pastures were infested, with average populations ranging from 100 to 1340 larvae/m2. Damaged pasture became choked with weeds following the loss of sown grass.
Despite an extensive and costly control programme, farmers estimate the beetles are still causing a 30% loss in production at the cape.
In Waiau Valley border dyke irrigation began in 1980 and by 1984, 13,606ha were part of the scheme. Before then, the valley was used for dryland sheep farming. There were regular summer droughts and low insect pest numbers. For the irrigation scheme land was moulded into border dykes, new pasture sown and fertiliser applied. By autumn 1988, grass grub larvae were found in high densities and destroyed pasture had to be resown.
It also took three to four years in the central North Island for pasture to become infested with grass grub following a land use change from pine forestry to dairy farming. Some pasture averaged more than 700 larvae/m2, with patches as high as 2000/m2. New pasture sown in areas that were previously deep in the forest block took longer to become infested but, once established, grass grub populations grew quicker and the larvae were larger and healthier than those elsewhere.
However, areas monitored have shown diseases to build up slowly and control the insect populations.
After 10 years the Beauveria fungi, a microsporidean and a proteobacterial pathogen, might be starting to have an affect on the beetle population at Cape Foulwind. Amber disease, milk disease and protozoan diseases have slowed grass grubs in other areas.
The scientists advised farmers involved in large-scale pasture renewal or land use changes that they should have knowledge of insects in their region and have monitoring systems in place to give an early warning of population increases.
Although insecticides could be used to control pests, they could also cause a rebound in insect numbers because they could prevent natural diseases from spreading.
