Landcare Research - Manaaki Whenua

Landcare-Research -Manaaki Whenua

Tiny Insects to Tackle Giant Reed

Giant reed wasp ovipositing

In January the Environmental Protection Authority approved the release of two new biocontrol agents to tackle a new target in New Zealand, giant reed (Arundo donax). Northland Regional Council fronted the application to release a gall-forming wasp (Tetramesa romana) and a scale insect (Rhizaspidiotus donacis), since the Northland region currently has the worst problem with this plant.

“The opportunity to utilise these agents arose opportunistically when they were imported into containment in Auckland as part of the Cook Islands’ project,” explained Quentin Paynter. However, a search for release sites on Rarotonga found that giant reed had been confused with the similar-looking elephant grass (Pennisetum purpureum) and is rare enough to warrant an eradication effort, rather than biocontrol.” Although giant reed is not a major problem in New Zealand yet, it is well beyond eradication and is among the 100 ‘World’s Worst’ invaders, so the opportunity to act sooner rather than later was too good to miss.

Giant reed is a tall, clump-forming grass resembling bamboo, with thick rhizomes. Under optimal conditions this species can accumulate biomass quickly, with stems growing up to 10 cm a day, and it forms dense stands over 5 m tall. The hollow stems are up to 4 cm across, with bamboo-like leaves.

Plume-like flower heads are produced at the top of stems in late summer. However, fertile seeds are not produced in its invasive range, and it spreads via plant and rhizome fragments.

Giant reed is found in the Old World from the Iberian Peninsula of Europe to south Asia, including North Africa and the Arabian Peninsula. It has been intentionally distributed around the world (possibly for millennia) as a source of fibre and roofing material, as a garden ornamental, and for erosion control. It has escaped cultivation and has become naturalised and invasive in many tropical, subtropical and even cool-temperate regions of the world, including much of the Pacific region.

Giant reed was commonly planted in Northland as a soil stabilisation plant in the 1960s–80s. The plant is currently mainly problematic in the north of the North Island, but occurs in many regions and is steadily expanding its range. Isolated infestations have been found as far south as Queenstown and below Haast, so it is clearly not limited by cooler climates here.

“Giant reed poses a severe threat to New Zealand waterways, wetlands and riparian areas,” confirmed Quent. It commonly invades moist banks along river and stream margins, but is very tolerant of a wide range of eco-climatic conditions. As well as displacing native plant and animal species and providing a habitat for pests like rats and possums, giant reed alters the vegetation structure and ecological processes of invaded habitats, such as the hydrology and nutrient cycling and the fire regime. The bulky plants narrow channels and increase sedimentation, and falling debris accumulates in waterways. This increases the risk of floods, which in turn can assist with the dispersal of giant reed by dislodging plant fragments, which take root downstream. Floating masses can also damage infrastructure.

This species is a serious concern overseas in arid and semi-arid habitats because it uses more water than native plants, lowering groundwater tables. In Mexico it is called el ladron de agua, the water thief, and is stated to consume three times more water than typical native vegetation.

Giant reed is also extremely flammable, and the development of large stands increases the likelihood and intensity of fires. In some habitats giant reed has the potential to establish an invasive plant–fire regime, since it increases the risk of fire but also recovers from fires more quickly than native plants. While it thrives on disturbance, giant reed can also invade intact habitats.

Control of giant reed using conventional means is problematic. Small infestations can be eradicated by constant removal of fronds or by repeated herbicide application, but this requires persistent effort for many years, and is therefore often unsuccessful as well as uneconomic. Although its distribution is still limited in New Zealand, giant reed is already considered too difficult or expensive to eradicate using conventional means.

Giant reed has become an environmental weed in a number of southern hemisphere countries, including South Africa, where a biocontrol programme is currently underway. In North America giant reed is regarded as a major weed in California and Texas, and in adjacent areas of Mexico. “Both of the giant reed biocontrol agents now approved for release here have been released in the USA and Mexico, following extensive studies in Europe,” explained Quent. The gall wasp was released first, and assessment trials have shown that it began to reduce the vigour of giant reed after only 5 years. The scale insect is also now established there, but it is too early to judge its impact.

The gall wasp is about 5 mm long and attacks the stems. It is native to the Mediterranean region, where it is widespread and common on giant reed. Females produce eggs parthenogenetically (no males required) and deposit them into growing shoots. The plant responds by producing gall tissue, which distends the stem within 2 weeks. At 27°C in the laboratory the gall wasp can complete a generation in 33 days, so multiple generations per year can be expected in the field during the warmer months. Emerging adults leave characteristic emergence holes in damaged stems.
Following a huge mass-rearing effort, over 1 million gall wasps were released in Texas, and researchers there are comparing the performance of giant reed before and after their release at 10 sites. They have already recorded clear differences in plant health. By 2014 (5 years after release), above-ground biomass had decreased by an average of 22%, with 10% mortality of main shoots and 17% mortality of lateral shoots. The decline in biomass was correlated with increased total numbers of gall wasp exit holes in main and lateral shoots per site. Estimated live biomass declined a further 32% between 2014 and 2016 (giving an overall 45% reduction in 7 years). The amount of damage caused by the gall wasp is expected to increase as their populations build further.

The scale insect sucks the sap from young stems and rhizomes of giant reed, reducing the rate at which the plant grows, accumulates biomass and occupies space. It is the first armoured scale (Diaspididae) to be used as a weed biocontrol agent, and like the gall wasp is native to the Mediterranean region, where it is widespread and commonly found on giant reed. “Armoured scales can cause significant damage to their hosts, and many are important orchard pests worldwide, including New Zealand,” said Quent.

An insecticide exclusion experiment in Spain found that side-shoots infested with the giant reed scale grew at less than half the rate of uninfested shoots. This experiment also found that the rhizomes of giant reed in nine sites where the scale was present were only half the weight of rhizomes from sites where the scale was absent. In Europe, the scale has only one generation per year. The females are immobile, and after around 3 months of continued feeding and expansion reach reproductive maturity and produce mobile juveniles, known as crawlers. The males are considerably smaller (c. 0.5 mm) than the females (c. 1.2 mm) and have wings. The crawlers disperse aerially, and can also attach themselves to larger insects to disperse to new plants.

Laboratory trials have shown that the two agents are complementary and can together have a major negative impact on giant reed growth. The gall wasp forms terminal galls that stop growth and induce the plant to produce fresh side shoots, reducing overall height and depleting rhizome reserves. These side shoots are highly suitable for colonisation by the scale crawlers, which further deplete plant vigour.

Both insects are highly host specific and are only expected to attack giant reed in New Zealand. “Because of the significant amount of work done before these agents were released in the USA, we only needed to check that three additional grasses were unsuitable hosts for the scale for New Zealand (the endemic Isachne globosa and Zoysia minima, and the indigenous Spinifex sericeus). No further testing for the gall wasp was needed,” confirmed Quent.

Permission to remove the gall wasp from containment has recently been granted by the Ministry for Primary Industries, and mass-rearing is underway in Auckland with the aim of making the first field releases of this new agent in spring 2017. It is hoped that permission to remove the scale from containment will also be granted soon.

Funding for the process to gain approval to release these agents was provided by the Northland Hawke’s Bay, Horizons and Greater Wellington regional councils. We are grateful to John Goolsby, United States Department of Agriculture, for providing colonies of both insects as well as lots of useful information and advice.