Russian wheat aphid - current investigations and recent findings

Take home messages

● Development of regional economic thresholds for Russian wheat aphid (RWA) is in progress at trial sites throughout SA, Victoria, Tasmania, and NSW. Use of international thresholds are still advised until Australian thresholds are developed.

● RWA has been detected as far north as the Liverpool Plains, NSW. Growers can track changes in RWA distribution on a recently developed interactive map.

● Barley grass is a major host supporting RWA survival over summer. Symptoms have been rarely observed in non-crop grasses. Looking for symptoms is, therefore, not a good strategy for monitoring for RWA presence in weeds.

● Insecticide seed treatments, while efficacious, should be used with caution and given priority in areas of high risk of infestation of RWA in 2019.

Background

Russian wheat aphid (RWA) is one of the world’s most economically important and invasive pests of wheat, barley and other cereal grains. Since first being discovered in SA in 2016, RWA has been found widespread in cereal growing regions of SA, Victoria, NSW and Tasmania. Their small size, green colour, elongated shape, very short antennae and apparent lack of siphuncles readily distinguish RWA from other pest aphids found in Australian cereal crops. Unlike other aphids, which cause damage through feeding on plant nutrients, RWA injects salivary toxins during feeding that cause rapid, systemic phytotoxic effects on plants, resulting in acute and observable plant symptoms, as well as potentially significant yield losses.

The first detection of RWA in Australia occurred on cereal crops in May 2016. Within one month, a combined industry-government biosecurity committee determined that an eradication attempt for RWA was unlikely to be successful. RWA is now a management concern for grain growers in regions where it has been found.

In a recent study by Avila et al. (2019) the potential spread and establishment of RWA in Australasia was assessed using a re-parameterised CLIMEX model that took into account currently known distribution records of the aphid and the presence of irrigated crops. According to the model results, RWA has the potential to establish in all key grain growing regions in Australia. However, since RWA has not been previously detected in Australia, it is not yet known what effect local agro-climatic conditions will have on the ability of RWA to establish and feed on hosts, which include wheat, barley and a large range of cultivated and wild grasses. A new GRDC investment, ‘Russian wheat aphid risk assessment and regional thresholds’ (investment 9176535) has been launched to investigate regional risk and management tactics for RWA.

Current research

The South Australian Research & Development Institute (SARDI) and cesar are assessing the regional pressure of RWA with the aim of developing regional economic thresholds and gaining a better understanding of the role that green bridges are playing in supporting RWA populations between cereal cropping periods.

Currently only provisional intervention thresholds for RWA are available, which are based on US research (Pike and Alisson, 1991). This research recommends control at the following points: >20% of all plants infested up to GS30 and >10% of tillers infested from late stem elongation (following GS30). Since initial detection of RWA in Australia, growers have been advised to use these thresholds as they represent the best current knowledge.

Development of regional economic thresholds

In 2018, 15 trial sites were set up throughout SA, Victoria, NSW and Tasmania by Dr Maarten van Helden and Thomas Heddle (SARDI) in collaboration with regional organisations. Sites were chosen in regions where RWA was known to be established. Cereals tested at each regional trial site included spring wheat and barley. Winter wheat, durum wheat and oats were also tested at some sites. A subset of these trial sites was artificially inoculated with the aphid at a specific time point to ensure thresholds could be developed. This trial site work builds on the SA Grains Industry Trust (SAGIT) Time of Sowing trials conducted by SARDI in 2017 and 2018 in three regions – Bool Lagoon, Roseworthy and Loxton.

Each 2018 trial site included the following treatments – Gaucho® seed treatment, chlorpyrifos treatment, seed treatment plus chlorpyrifos, and no treatment. Yield data were collected for each treatment at each trial. Data on RWA abundance, presence of beneficials, and RWA migration times were also collected throughout the season at these sites.

As we currently have only one season of trial site data, no inferences can yet be made. However, these trials will be repeated in 2019, which will strengthen our data set and enable further investigation into the relationship between RWA numbers, plant symptoms and yield loss across regions, as well as allowing for development of regional economic thresholds.

Green bridge surveillance and risk assessment

There are many factors that will influence RWA survival during times when its favoured hosts are not available for nourishment, including the local climate, land use (e.g. vegetation on roadsides, irrigated public spaces), availability of alternate hosts, abundance of volunteer cereals, and predation by beneficials.

Surveillance for RWA over spring and summer from October 2018 to February 2020 is generating data about types of vegetation the aphid is surviving on between cropping, and what environmental conditions support its survival over this period, as well as collecting valuable information about beneficial species predation of RWA. Once enough green bridge data is collected, use of modelling algorithms will allow us to predict aphid population growth over this critical period.

The ultimate aim of the project is to develop additional guidelines for RWA management that are regionally specific. While trial site results are not discussed here, due to limited data so far, there is information included on preliminary findings of green bridge surveillance and a RWA population growth modelling tool under development that will make use of trial site data.

Where has RWA been found?

Our most current data indicates that RWA is present in a large and still expanding area covering all cereal growing regions of SA, Victoria, Tasmania and most of NSW. Our spring sampling shows that the aphid is widespread across these regions in at least low numbers, however it is not known how typical this spring distribution is as we have only sampled for one season. In late 2018, the aphid was detected at Coonabarabran and the Liverpool Plains (NSW), which is a northerly extension of range for this aphid.

A distribution map that is still commonly used to understand and demonstrate RWA distribution in Australia is derived from AusPestCheck (Plant Health Australia), which collected monitoring data from state governments when RWA was under active surveillance by biosecurity authorities. Through the current project, we have produced an RWA Portal which includes an up-to-date map, which sources data from 2018 green bridge surveillance and adviser reports to PestFacts services. This map updates in real time, approx. every three hours, and lists information sources for each data point, evidence of absence data, and allows users to toggle with the timeframe between 2016 and 2019. It can be found on the RWA Portal.

Figure 1. RWA Interactive Map. Detections span 2016-2018 and with data sourced from 2018 green bridge surveillance and adviser reports to PestFacts. Red (light) icon indicates RWA detection in that area. Green (dark) icon with cross out denotes no RWA found during summer surveillance (map developer – Dr James Maino, cesar)

What we know about the environmental conditions under which RWA will thrive

Despite few RWA issues reported to PestFacts services during the 2018 cereal growing season, our spring sampling detected RWA in all cereal growing regions where RWA has been reported previously. The presence of RWA in an area does not automatically mean it will cause damage to crops. RWA needs to infest cereals in early autumn in order to develop into damaging population levels in spring during booting and flowering.

While we are still accruing data about conditions that support RWA survival and can give limited advice, the following is what we can say:

● Hot and dry summer conditions reduce over-summering populations of the aphid, with RWA likely to persist where there is available moisture and green material (from rainfall or irrigation).

● Higher than average temperatures are unfavourable for RWA survival.

● Localised summer rainfall events resulting in germination of weeds like barley grass can provide refuges for the aphid.

● Field observations and experiments over the past three seasons indicate that RWA abundance and development on crops is much higher in low rainfall zones (<400mm per year) and on drought stressed crops.

● This year’s field trial observations support international research findings that indicate mature crops (GS40 or higher) are less attractive and are less likely to be invaded by RWA in spring.

This work is ongoing – RWA is still a very new pest to Australia and we are continuing to learn about its biology as the current investment progresses. More pertinent information about environmental influences is likely to be gained at crop establishment, particularly in regard to area-wide aphid abundance and flight timing. Significant early infestation of a crop will only occur through a combination of abundant green bridge and good flight conditions that would aid RWA migration to cereal paddocks during the seedling stage in early autumn. Good flight conditions for aphids are calm, warm days over 20°C. During the 2018 season, these conditions were not met in southern Australia.

Influence of region, season and local conditions on RWA populations

SARDI and cesar have been sampling RWA over spring, with summer time follow up surveys currently in progress to determine what conditions support survival of the aphid leading into autumn sowing. Data analysis is ongoing, but some early conclusions can be drawn regarding RWA abundance across region and season.

During the spring, RWA populations were found to be widespread in Victoria, NSW and SA, consistently appearing in randomly selected roadside stops, regardless of proximity of cereal crops or sources of water. The limiting factor for their presence seemed to be the presence of preferred host species, in particular barley grass. At most sites, populations of RWA were found residing in weeds outside of crops, however these populations were generally smaller than those found within crops. Within crops, large populations, causing visible symptoms, were most commonly observed in young tillers, particularly on paddock edges.

Populations in southern Victoria and Tasmania were comparatively sparse, while in Tasmania they were largely restricted to crops despite the abundance of green host weeds that were observed to be preferred in Victoria and NSW (such as barley grass). It is unclear if the lack of positive detections of RWA from the northwestern regions of Tasmania and the southern regions of Victoria are due to populations being too diluted among the plentiful green vegetation to detect or are due to unsuitability or incomplete dispersal.

While summer abundance data is still preliminary as surveys are currently in progress, early insights from the Victorian sites suggest that RWA populations have declined dramatically over the summer, however they are still present throughout Victoria (Figure 2). The few active populations that were detected appear once again strongly dependent on preferred hosts such as barley grass, which had become far less common, but also appeared more dependent on persistent sources of summer water that could maintain green host plants. Three of the five active populations detected during the summer in Victoria were found within city limits, in areas that would receive relatively consistent summer watering, including weedy lawns and ovals. Over-summering RWA populations were also detected on regrowth within cereal paddocks in the cooler, southern regions of Victoria (interestingly, in areas where they were not detected during the spring).

Figure 2. RWA population distribution and abundances across Victoria in the spring (left) compared to summer (right). Presences are represented by red (light) aphids, and absences by green (light) aphids with cross out.

What weeds and summer pasture species supported RWA over summer?

During the 2018 spring sampling, RWA was found on a variety of non-crop grasses, with barley grass appearing to be the preferred host, followed closely by brome grasses (including prairie grass) (Figure 3). Small colonies were sometimes found on wild oat grasses, and alates were occasionally found on phalaris grasses. Very sparse populations were found on young rice crops. These results are consistent with previous SARDI findings regarding possible host plants in SA (GRDC DAS000170 project), with Bromus species and barley grass being of highest preference of all weed grasses tested.

Symptoms (curled and striped leaves and trapped heads) were rarely observed in non-crop grasses, and when they were observed, were more subtle in appearance than those observed in cereal crops (Figure 4). Looking for symptoms is therefore not a good strategy for monitoring RWA presence in weeds.

Figure 3. The proportion of individuals within each host plant represented during random plant searches that were positive for RWA (the number of plants searched per species is displayed above the bar). Please note these are preliminary data from Victoria, NSW and Tasmania during the spring sampling – formal plant identification is still required for some specimens.

Figure 4. RWA and damage symptoms on barley grass (left), brome (middle), and wheat (right), observed during spring monitoring. The brome grass pictured showed feeding symptoms (striping on the leaf), which was unusual among weeds (Credit: Dr Elia Pirtle, cesar). The wheat shows the striping symptoms that were frequently observed in younger tillers supporting RWA colonies.

Beneficial control of RWA

A diverse range of beneficial insects are known to predate on RWA and these populations will build in response to the presence of aphids throughout the season. Growers are encouraged to consider control options that will have minimal impact on beneficial populations. This investment is also investigating beneficial predation of RWA during the summer period, which will add to our knowledge of how to manage RWA at a regional level.

Beneficial invertebrates observed actively feeding on RWA populations during spring 2018 surveys included adult and larval ladybird beetles, larval brown lacewings, and parasitoid wasps. Other beneficial invertebrates commonly detected around RWA populations included spiders, hoverfly larvae, and predatory hemipterans. Large RWA populations frequently showed signs of heavy parasitism by wasps in the form of mummified aphids (Figure 5).

Figure 5. A parasitoid wasp and several RWA mummies on a cereal (Credit: Dr Elia Pirtle, cesar).

Using modelling to predict RWA population growth throughout the season

To extend the power of data collected through field trials, green bridge surveillance, combined with climate data, we can use modelling techniques to make inferences about how RWA populations will behave in the near future region by region.

However, robust predictions are achieved when additional data is incorporated on top of that which will be collected during the current project. For example, since RWA is an exotic pest we are using existing international research on RWA biology wherever possible. For example, a study conducted by Ma and Bechinski (2009) on RWA feeding on barley revealed that population growth (and thus damage potential) is strongly dependent on environmental temperature and crop growth stage. This overseas study found that earlier crop growth stages nearly always favour higher RWA population growth potential, while higher temperatures (generally) favoured higher growth rates.

Using experimental data collected by Ma and Bechinski (2009), a model was developed to predict RWA population growth and damage inflicted on the crop in relation to time of infestation during crop development. To forecast crop growth rates, we leveraged an existing model on cereal growth (Keating et al. 2003) that has been developed over decades of research and development - this is a model known as the Agricultural Production Systems sIMulator(APSIM). APSIM crop growth data was then linked to the RWA population growth model and the underlying assumption was made that a negative impact on biomass was related to the number of RWA on the plant.

As a preliminary test of the model data from one of the 2018 RWA trial sites (Birchip), one wheat variety (cv. Scepter) was considered in order to model population growth based on a variety of colonisation dates (Figure 6). The exact date of colonisation was known because this trial was artificially inoculated with RWA and when the actual date of RWA colonisation was inputted, the model generated a trend line that closely matched real RWA abundance data for the trial site (solid line), which was an encouraging proof of concept for the model. Additional colonisation dates were then inputted into the model (dashed lines).

By using APSIM, RWA abundance may then be transformed to predict impact on biomass (and thus, yield). However, as this is an early stage test of the model, based on limited trial data, it is important to not over generalise findings. In addition, natural events, such as heavy rain or immigration of beneficial species, can impact aphid numbers, and this model does not account for such events that may reduce populations at this stage. As the RWA research progresses, we will continue to test and refine this model, in the expectation that it may be developed into a useful prediction tool for assessing RWA population growth and likely yield impact on-farm.

Figure 6. Actual RWA abundance data collected from the 2018 Birchip trial site (left) and RWA population growth as predicted by the new RWA-APSIM module based on variable colonisation dates (right) (module developer: Dr James Maino, cesar)

Completed research

Efficacy and length of protection provided by seed treatments

In a recent study led by cesar researcher, Lisa Kirkland, the efficacy and length of protection afforded by several insecticide seed treatments against RWA and oat aphid (Rhopalosiphum padi) were tested.

The experiment was conducted under ‘semi-field’ conditions using closed artificial microcosms with added aphids (Figure 7). As such, caution should be taken in applying the results of the experiment to field conditions – in closed microcosms watering is abundant and plant roots are limited to the container. Nonetheless, the results have revealed some insightful trends.

Figure 7. Semi-field conditions using closed artificial microcosms (Image credit: cesar).

The seed treatments (shown in Table 1) were tested on wheat (cv. Trojan) grown for sixteen weeks. The aphids were introduced two weeks after emergence. Each species was designated its own treatment and aphid populations were counted fortnightly. To simulate aphid colony establishment at different growth stages, after counting, the populations were ‘reset’ back to a level of 30 aphids per microcosm by removing or introducing individuals.

The results of this study are summarised in Figure 8. They show all insecticide seed treatments currently registered for use in Australian cereal crops are effective against RWA and oat aphid. Specifically, higher label rates of Cruiser® and Cruiser® Opti increased the length of protection by 2-3 weeks in this trial, while the addition of the synthetic pyrethroid lambda-cyhalothrin in Cruiser® Opti did not provide clear benefits over Cruiser® in protection against these aphid species. Interestingly, oat aphid was able to persist and reproduce on wheat at an earlier growth stage than RWA. This indicates the oat aphid is more tolerant to certain insecticides and may therefore re-infest insecticide-treated wheat fields earlier than RWA.

Table 1. Rates and details of chemical treatments examined (Source: Kirkland et al. 2018).

APVMA minor use permits are in place for the use of imidacloprid PER82304 and thiamethoxam PER86231 based seed dressing treatment for the Russian Wheat Aphids in cereals are in winter cereals. Minor use permit can be obtained here.

Figure 8. Average numbers of RWA (pale grey bars) and oat aphid (dark grey bars) at weekly intervals after wheat emergence for each chemical treatment. Counts are plotted against the week when aphids were introduced to tubs (Source: Kirkland et al. 2018).


Future research

Regional threshold trial sites will be run again in 2019 throughout SA, Victoria, Tasmania, and NSW. These trial sites will give us two seasons worth of data for the verification of currently used international thresholds, or development of new, regionally specific thresholds.

Continuation of the green bridge surveillance over 2019/2020 will also add to our understanding about climatic (and biological) factors that influence RWA survival between harvest and sowing. Importantly, further verification of green bridge host plants favoured by RWA will allow us to develop guidelines for green bridge control to limit the risk of RWA moving into crops. Further, identification of key beneficials impacting RWA will continue, and this information may be used to update or develop integrated pest management (IPM) strategies.

Data collected from trial sites and green bridge work, particularly assessment of local climates that favour RWA population growth and RWA abundance data gathered throughout the year, will support us in further developing forecasting tools, such as the early stage RWA-APSIM module described here.

Current advice

● Monitoring for the aphid itself on green bridge hosts is advisable as classic RWA symptoms have been rarely observed on graminaceous species over spring and summer.

● Volunteer cereals and weedy grasses found within next season’s cereal paddocks should be controlled at least 2-3 weeks prior to sowing. This will aid in reducing local numbers of the aphid pre-production.

● Registered neonicotinoid insecticide (mode of action Group 4A) seed treatments are very effective to avoid autumn infestation of crops if RWA are migrating (however, over the 2018 season, migrations into crops did not occur in most areas where RWA is present, most likely due to unfavourable conditions for aphid survival over summer and unfavourable flight conditions).

● To ensure seed treatments remain a long term viable control option for grains pests, industry stewardship and good resistance management are vital. Growers are urged to use neonicotinoid seed treatments judiciously, according to the regional risk, and using the Find, Identify, Threshold, Enact (FITE) approach.

● RWA is easy to detect in autumn and winter before yield is impacted. If RWA is present in potentially damaging numbers, it can be controlled efficiently by insecticide sprays around growth stage 32-40, eliminating the aphids before there is a risk of yield loss. The overseas threshold is >20% of all plants infested up to GS30 and >10% of tillers infested from late stem elongation (GS30 or later).

Useful resources

The RWA Interactive Map

GrowNotes Tips & Tactics

Russian Wheat Aphid Tactics for Future Control

Russian Wheat Aphid Dynamics in 2017 (research update)

References

Avila, G., Davidson, M., Van Helden, M., and Fagan, L. (2019). The potential distribution of the Russian wheat aphid (Diuraphis noxia): An updated distribution model including irrigation improves model fit for predicting potential spread. Bulletin of Entomological Research, 109(1): 90-101.

Bellati, J., Mangano, P., Umina. P., and Henry., K. (2012). I SPY Insects of Southern Australian Broadacre Farming Systems Identification Manual and Education Resource. Department of Primary Industries and Resources South Australia (PIRSA), the Department of Agriculture and Food Western Australia (DAFWA) and cesar Pty Ltd.

Keating B., A, Carberry., P.S., Hammer., G.L, et al. (2003). An overview of APSIM, a model designed for farming systems simulation. European Journal of Agronomy, 18: 267–288.

Kirkland, L.S., Pirtle, E.I., & Umina., P.A. (2018). Responses of the Russian wheat aphid (Diuraphis noxia) and bird cherry oat aphid (Rhopalosiphum padi) to insecticide seed treatments in wheat. Crop and Pasture Science 69: 966-973.

Ma., Z., and Bechinski., E.J. (2009). Life tables and demographic statistics of Russian wheat aphid (Hemiptera: Aphididae) reared at different temperatures and on different host plant growth stages. European Journal of Entomology 106:205–210.

Pike KS, and Allison, D. (1991). Russian wheat aphid. Biology, damage and management. Pacific Northwest Cooperative Extension Publication PNW371.

van Helden, M. (2018). Testing the suitability of Australian native and introduced grasses as host plants for Russian wheat aphid. GRDC DAS000170 SARDI Entomology Report.

Acknowledgements

Project 9176535 is a GRDC investment that seeks to deliver information on Russian wheat aphid management for grain growers. This project is being undertaken by the South Australian Research & Development Institute (SARDI) and cesar. The project team would like to acknowledge 2018 trial site contractors in SA, Victoria, NSW, and Tasmania.

Seed treatment research was supported by GRDC and made possible with the support of Advanta Seeds, Syngenta Australia and Bayer CropScience. Untreated Trojan wheat seeds were provided by Advanta Seeds via Longreach Plant Breeders. The authors acknowledge the assistance of Ken McKee, David Landmeter, Shane Trainer, Jenny Reidy-Crofts, Colin Edmondson, and Samantha Ward.

Contact details

Jessica Lye
cesar Pty Ltd, 293 Royal Parade, Parkville Vic
03 9349 4723
jlye@cesaraustralia.com
@cesaraustralia

Elia Pirtle
cesar Pty Ltd, 293 Royal Parade, Parkville VIC
03 9349 4723
epirtle@cesaraustralia.com
@cesaraustralia

Maarten van Helden
South Australian Research and Development Institute
08 8429 0642
Maarten.VanHelden@sa.gov.au

GRDC Project code: 9176535 and CES00004