Surveillance crucial to tracking aphid invader
GroundCover™ Issue: 124 | Author: Dr Sharyn Taylor
Unwelcome, but not unexpected, Russian wheat aphid is making itself at home in south-east Australia
Less than two months after the exotic cereal pest Russian wheat aphid (RWA) (Diuraphis noxia) was first discovered in Australia it was acknowledged that it is not technically feasible to eradicate the pest due to the widespread detection and the aphid’s ability to disperse rapidly.
Emergency use permits for chlorpyrifos and pirimicarb to control RWA in cereal crops have been activated and further emergency permits may be prepared as the season progresses and research becomes available to support other chemicals.
A national management plan, currently being updated, has been developed to help minimise the impact of RWA in Australia, with technical advice from state government departments of agriculture, the GRDC, CSIRO and the Centre for Environmental Stress and Adaptation Research (cesar).
Monitor, monitor, monitor
Surveillance is essential to track the path of RWA’s spread, and the rate of spread. Maps of RWA’s known distribution are updated regularly by Plant Health Australia (http://bit.ly/RWADist).
Check crops for signs of RWA and report any suspected detections to the Exotic Plant Pest Hotline 1800 084 881 or directly to the relevant state department of agriculture (see box).
Where the aphid has been confirmed, keep a close watch on crops to determine the population density and whether control options are needed. If RWA hasn’t been officially confirmed in a specific state or territory, it remains a notifiable pest and must be reported.
What is Russian wheat aphid?
RWA is a soft-bodied insect present in all the world’s major grain-growing areas. In other countries, it has caused significant damage to crops, resulting in up to 70 per cent yield losses.
Many populations of RWA are asexual and do not need males to breed. Adult aphids give birth to live nymphs which, after the fourth moult, develop into either wingless or winged adults. Wingless adults are a light green colour with an elongated shape, making them quite distinct compared with other aphid species. They are about two millimetres long with short antennae and possess a ‘double tail’.
Based on observation of overseas populations, the aphid takes about three weeks in winter and 10 to 14 days in mid-spring to reach maturity. Adults then continue to produce about two nymphs a day for two to four weeks. One female can produce 30 to 60 nymphs, which means aphid numbers can increase rapidly.
Reproduction rates are expected to increase as the temperature increases, so keeping a close watch on aphid levels will be necessary through the season. Further research will be required to determine the impact of local environmental factors on RWA populations.
Wheat and barley are the main hosts, although oats, rye and triticale will also support adult aphids. Brome grass is likely to be a major grass weed host but further information is required on the hosting status of grass weeds generally.
Symptoms of RWA damage include longitudinal rolling of leaves where the aphids shelter, and whitish, yellowish to pink-purple streaks along the length of the leaves. These symptoms can be confused with nutrient deficiency or herbicide damage from bleaching herbicides such as diflufenican.
RWA differs from other common cereal aphid species by injecting salivary toxins into the leaf of the host plant during feeding, which causes wheat streaking and death of infested leaves. In cases of heavy infestation, this can kill the plant. The toxins only affect infested leaves and once a RWA infestation is controlled new leaf growth is unaffected.
Infestation of the flag leaf may trap the awn and prevent the head from completely emerging. This produces a gooseneck head and, as a result, the grain does not properly mature. Heads can also appear bleached.
How it spreads
RWA is easily spread by the wind and on live plant material. Winged adults can spread through crops by flying and using wind currents. Long-distance dispersal also occurs by aphids ‘hitchhiking’ on machinery, clothes, vehicles and live plant material.
In grain-growing regions overseas a key management strategy for RWA is to use a threshold approach to determine if sprays are required. Current international advice suggests an economic threshold of 20 per cent of plants infested up to the start of tillering and 10 per cent of plants infested after tillering. These thresholds serve as a guide and need to be considered based upon factors such as:
- aphid numbers;
- crop growth stage and time of season;
- crop yield potential;
- cost of the control option to be employed;
- beneficial insect populations;
- yield loss under Australian conditions;
- forecast weather conditions; and
- other insect pest species present.
The national technical group says an integrated pest management approach should be used in Australia to foster populations of the aphid’s predators and other beneficials to reduce aphid damage.
Spraying populations of RWA is likely to kill predators and other beneficials. Early reduction of predators could contribute to a spike in numbers of RWA, and other aphids, in spring when temperatures increase.
Blanket spraying could also increase pressure on the development of pesticide resistance in other key pests such as green peach aphid or redlegged earth mite.
Overseas data indicates that RWA is susceptible to heavy winter rainfall and the combination of cold and wet weather will help check its build-up during mid-winter. However, aphid activity is likely to increase with rising temperatures later in the year and later infestations are predicted to have a larger impact on yield if they attack the major yield-contributing leaves (flag, leaf 2 and leaf 3).
Monitoring aphid levels, particularly from the start of stem elongation and through flag leaf development and ear emergence, will be critical.
The aphid’s habit of living in rolled leaves can make it difficult for chemicals to reach the pests, but overseas experience has shown that both foliar and seed treatments can be successful.
Chlorpyrifos and pirimicarb are currently listed for control under emergency use permit. Pirimicarb has a six-week withholding period for both harvesting and grazing. Chlorpyrifos has two-day grazing and 14-day harvest withholding periods. These permitted chemical sprays, particularly chlorpyrifos, will kill foraging honey bees. It is essential to coordinate spray activity with local beekeepers. See the BeeAware website for more information on pesticides and honey bees.
The GRDC Paddock Practices region e-newsletter June 2016 (southern region) provides information on spray rates and management. Trial work is underway to confirm the relative effectiveness of a broader range of insecticide options to better inform control under Australian conditions.
Good farm hygiene may help prevent the entry and movement of RWA and other pests. Movements of people, vehicles and equipment can pose a risk in carrying the aphid from existing infestations to new sites. Simple steps to protect your property include having a designated visitor area and using farm vehicles to visit production areas wherever possible.
RWA is attacked by a range of beneficial insects. Those that commonly occur in Australia include: parasitoid wasps (Aphidius colemani, Aphidius ervi, Diaeretiella rapae), ladybird beetles, lacewings, damsel bugs, hoverflies, carabid beetles and fungi, which have already been seen affecting RWA in South Australia.Integrated pest management of grains
More informationFarm biosecurity
GRDC Project Code PHA00015, CSE00003
Region North, South, West