Biocontrol agents are increasingly replacing sprays to control Northland's most challenging weeds.
Entomologist Dr Jenny Dymock, of Doubtless Bay in the Far North, works with the Northland Regional Council to provide biocontrol services throughout Northland.
She helps to distribute biocontrol agents and monitors their distribution and effectiveness.
Northland's semi-tropical climate provides a warm welcome for weeds and controlling them can be daunting and expensive.
"We grow weeds very well.''
Replacing herbicides is a tiny army of insects, fungi and plant diseases that attack and weaken their host plants to recreate the environmental balance of their home countries.
The NRC has imported 50 agents over the past 90 years.
Dymock said one of the biggest success stories is the ragwort flea beetle, a light golden-brown beetle about 2-3mm long. Its muscular back legs allow it to jump like a flea when it is disturbed.
Imported from the US in 1981, the beetle is now "in every nook and cranny of New Zealand", she said.
"It has decimated ragwort. With biocontrol there is usually no silver bullet but this beetle is doing a great job."
Ragwort is a member of the daisy family that produces bright yellow flowers and a huge number of seeds that spread easily to invade paddocks, open forests, riverbeds and coastal areas to outcompete native plant seedlings and grass. It is poisonous to cattle, horses, pigs and chickens.
"We get regular calls from people concerned when they see the yellow flowers of ragwort but it is part of a cycle and the beetle numbers will build up naturally to match the infestation.
"There is no need to spray for ragwort anymore,'' she said.
Farmers who insist on using herbicides should only spray from July to August when the beetle larvae are pupating.
"However, spraying can disrupt the population of flea beetles and end up making the ragwort problem worse,'' she said.
Dymock said other biocontrol agents in Northland include several targeting gorse.
The gorse spider mite (Tetranychus lintearius) is now established everywhere in Northland, although it favours dry conditions. The mite colonies get knocked back whenever it rains.
Gorse thrips (Sericothrips staphylinus) can be found around Kerikeri and the gorse seed pod moth (Cydia ulicetana) and gorse seed weevil (Exapion ulicis) are everywhere but ineffective at reducing gorse populations.
Gorse soft shoot moth (Agonopterix uticetella) is established in the gorse in the dunes behind Uretiti Beach at Ruakaka.
"I have just re-released the gorse soft shoot moth and the gorse colonial hard shoot moth (Pempelia genistella) in the Far North which I collected from Canterbury,'' she said.
Several agents targeting alligator weed have had mixed results. Alligator weed beetle (Agasicles hygrophila) is widespread in Northland and occasionally reduces alligator weed on ponds but is largely ineffective when the weed invades pasture. Alligator weed moth (Arcola malloi) is also widespread but ineffective as a biocontrol agent.
The woolly nightshade/tobacco weed lacebug (Gargaphia decoris) is found all over Northland and is most effective in defoliating leaves that are in shade.
Dymock said the Chinese privet lacebug (Leptoypha hospita) has established at three sites in Northland but is not spreading very far nor quickly so she is distributing more at as many new sites as possible this summer.
Agents targeting mist flower have been so successful that the weed is now rarely seen.
Other agents have been introduced to target tradescantia, lantana, Mexican devil weed and boneseed.
"The boneseed leaf roller Tortrix s.l.sp. ("chrysanthemoides") did not establish anywhere except at Mangawhai Heads. I don't know why it survived there and nowhere else in New Zealand,'' she said.
The Tutsan beetle (Chrysolina abchasica) released at Hokianga and Otangaroa has survived its first winter and the moth plant beetle (Freudeita cupripennis) released at Awanui also survived the winter and was recently released at Whangarei Heads.
"I have high hopes for this beetle as a biocontrol agent as its larvae feed on the roots and crown of the plant,'' she said.
Dymock said she is working on establishing more insect biocontrol agents, including a parasitoid wasp of tomato potato psyllid (TPP), which is a devastating pest of tamarillo, potatoes and tomatoes in Northland.
"I have released the wasp Tamarixia triozae but would like to release more and confirm establishment."
Other parasitoid wasps target codling moth, the green vege bug and giant willow aphid, which will be particularly helpful for orchardists and gardeners.
Insect biocontrol agents already successfully released in Northland include ladybirds, Cotesia parasitoid wasps for armyworms in maize, greenhouse thrips parasitoid wasp, clover root weevil and Argentine stem weevil parasitoid wasps, gumleaf skeletoniser Cotesia wasp, white butterfly parasitoid, passionvine hopper egg parasitoid, lacewings and predatory mites.
NRC weeds manager biosecurity Joanna Barr would like landowners and holidaymakers to keep an eye out for two unwanted organisms that have not yet been found in Northland.
They are the serious cropping weed velvet leaf, which can be spread by rural contractors through dirty maize harvesting machinery. The yellow-flowered plant with soft velvet leaves and spiky seed pods is resistant to many herbicides and its seeds can survive for 50 years. A single plant can drop up to 17,000 seeds.
The other is a highly invasive coastal sea spurge. It is a euphorbia with multiple stems that are often reddish at the base and has green flowers. Its milky sap is toxic to people and animals.
"This one is an invader from Australia that would have a serious effect on our dune systems if it was to become established."
Anyone seeing these pest plants should let the NRC know on the Environmental Hotline, 0800 504 639.