Could Dwarf Mistletoe Help Solve the Giant Wilding Conifer Problem?
A dwarf mistletoe Arceuthobium sp. on Pinus in USA. Photo credit: Stan Shebs CC BY-SA 3.0
Waves of wilding conifers are currently moving across the New Zealand landscape and there seem to be few long-term options available to control them, or even limit their spread. Almost all regions have got problems with self-sown wilding conifers originally introduced for silvicultural purposes. Not only do they invade productive agricultural land: they also change iconic grassland habitats, and are even invading beech forest, where they alter forest composition as well as ecological processes.
One of the main culprits is lodgepole, or contorta pine (Pinus contorta), which was planted extensively for shelterbelts and forestry in the past. Although plantations are now rare, they were major sources for the current wilding contorta problem. The previous government recently awarded the NZ Wilding Conifer Management Group $16 million over 4 years to try to tackle the problem in some of the worst areas, which include the Mackenzie Basin, Molesworth Station, Central Otago, Mid Dome in Southland, and areas of the Central Plateau, North Island. However, management tools for this invasion are poorly developed and long-term solutions are required.
In its native range of North America, Pinus contorta falls into four sub-species: shore pine (Pinus contorta subsp. contorta); lodgepole pine (Pinus contorta subsp. latifolia); tamarack pine (Pinus contorta subsp. murrayana) and Bolander’s beach pine (Pinus contorta subsp. bolanderi). All have been introduced into New Zealand, with the most invasive probably being shore pine, which is native to coastal California, Washington and Oregon.
In 2001 Pinus contorta was declared an unwanted organism under the Biosecurity Act 1993 and currently has no economic value. “This opens the door for biocontrol to be considered as an option,” said Simon Fowler. In the past the possibility of cone or seed-feeding insects has been suggested. However, this has met with scepticism from the forest industry, which is concerned about non-target effects and transmission of diseases such as pitch canker to the more valuable conifer species like radiata pine (Pinus radiata). Simon Fowler believes there are three potential options (not necessarily mutually exclusive) for progressing biocontrol of P. contorta, which has no commercial value in New Zealand:
- conduct further research to quantify the risk that insect agents might exacerbate disease transmission
- switch attention to other seed-feeders that are less likely to create entry points for, and/or vector, pine pitch canker
- investigate the potential for agents that attack other parts of the plant.
With these options in mind Simon has suggested that dwarf mistletoes (Arceuthobium spp.) could hold the answer. “The idea is novel but has merit for several reasons,” said Simon. “Dwarf mistletoes are serious parasites of coniferous forest trees in western North America. These widespread parasites retard growth of infected trees and cause extensive timber losses through direct and indirect mortality. In addition, the dwarf mistletoes reduce seed production and wood quality of the host plants. Plus the host-specificity of Arceuthobium spp. is well documented, and some species are known to be highly specific,” Simon explained.
One potential advantage of using mistletoes is that even if there is potential for spill-over non-target attack on rare hosts, mistletoes are unlikely to become a problem in commercial plantations. They disperse too slowly and would be destroyed in a typical 25- to 30-year P. radiata forestry cycle in New Zealand. Furthermore, A. americanum, a species affecting P. contorta, is dioecious (with separate male and female plants required for establishment), and generation times are typically slow, limiting spread in their native range to 30–60 cm per year.
Simon has established links with researchers at the University of Oregon, where groups work on dwarf mistletoes and weed biocontrol, and is planning a fact-finding visit soon. “We want to establish the genetic provenance of the invasive contorta pine in New Zealand, and analyse the existing literature and expert opinion from the USA/Canada as to what the best dwarf mistletoe species would be, or the best combination,” he said. The host range of the contorta pine dwarf mistletoes, as well as their potential impacts given different projected dispersal rates, will need to be thoroughly evaluated. Also, since the pine dwarf mistletoes are currently classified as unwanted organisms in New Zealand, we would need to apply to have that changed before we could import and study them.” Simon concluded by saying, “We will keep the forest industry well informed of our plans.”
This project is funded by the Ministry for Business, Innovation and Employment as part of the Winning the War on Wildings programme.