PestFacts May 2021
How to use the new Russian Wheat Aphid action threshold calculator
Last year, researchers from SARDI and Cesar Australia developed a predictive Russian wheat aphid calculator to help decide if spraying is economically justified. Here we explain the calculator and how to use it to assist with making control decisions.
Russian wheat aphid
In south eastern Australia, the Russian wheat aphid (RWA, Diuraphis noxia) is now widespread. Its lime green colour, elongated shape, very short antennae, apparent lack of siphuncles and feeding damage symptoms readily distinguish it from other pest aphids found in Australian cereal crops. Contrary to Oat and Corn aphids, RWA is not a major vector of viruses, damage is purely due to aphid feeding and the plants’ reactions.
What the calculator does
The predictive calculator uses economic and agronomic data inputs to calculate the action threshold at which control should be taken in order to prevent an increasing RWA population from reaching the economic injury level.
The calculator can be applied to winter and spring cereal varieties.
Economic Injury Level
The ‘Economic Injury Level’ determines when it becomes cost effective to undertake control measures. The economic injury level is reached when the cost of pest control equals the value of yield loss resulting from feeding. So, when pest densities are higher than the economic injury level, or if they are predicted to exceed the economic injury level, it is more economical to control the pest. When pest densities are lower or are predicted to be lower, than the economic injury level, pest control would be more expensive than the cost of the yield lost.
Because crops are not always inspected immediately before pest control is undertaken, the calculator takes this delay into account and predicts the change in aphid pressure in the lead up to head emergence.
How to use the calculator
The calculator is used at GS30 (start of stem elongation) to determine the action threshold (the maximum percentage of tillers with RWA is usually reached between GS40 (start of booting) – GS50 (start of head emergence).
To use the calculator, you will need to have estimations for cost of control ($/Ha), cereal market price ($/t), yield potential (t/ha), and days until GS50 (start of head emergence). You will also need to input RWA abundance data (percentage of tillers with RWA), collected based on a standardised monitoring technique. Monitoring at GS30 will allow prediction of aphid density leading up to head emergence, and therefore if action is needed during that time period. The main effect of RWA feeding on yield is during flowering and grain filling and that is when aphid numbers are usually highest. Monitoring earlier is not reliable because the number of tillers increases strongly between GS20 and 30 depending on fertility and rainfall.
How to monitor for Russian wheat aphid
Monitoring at GS30 (start of stem elongation)
- At GS30 count the number of tillers in 50 cm row lengths (choose at least 5 areas of the paddock) (e.g. 433 tillers counted).
- In the same row, lengths count the number of tillers with symptoms (e.g. 43 tillers with symptoms, so you have 10% (0.1) tillers with symptoms (43/433).
- In or around each of the five areas check 20 tillers with symptoms for the presence of RWA, total 100 tillers to be checked. (e.g. results = 54 tillers with symptoms and RWA). At this stage, you have 54% (0.54) tillers with symptoms that also have RWA.
- Multiply the percentage of tillers with symptoms with the percentage of symptomatic tillers that are hosting aphids. This will result in the percentage of tillers with RWA. In this example 0.1*0.54 = 0.054 = 5.4% of Tillers with RWA
No results were found
Management considerations
The growth stage at which RWA establishes has a strong effect on yield loss potential. If establishment occurs after GS30 there is low aphid growth potential. Even when aphid establishment occurs at early growth stages, large yield losses can be mitigated through monitoring and timely control.
A control decision should be made based on RWA population densities between GS30 and GS50. Predation and parasitism play an important role in controlling the aphid population growth rate, so keep an eye on natural enemies present in paddocks.
Wetter and cooler summers or early breaks will increase the risk of early season infestation by Russian wheat aphid in mainland grain growing areas. In low green bridge risk years (a dry, hot summer and autumn), infestations reaching economically damaging levels are unlikely.
Acknowledgements
The research initiative that supported the development of the calculator was a GRDC investment that sought to deliver information on RWA management for grain growers. This project was undertaken by the South Australian Research & Development Institute (SARDI) and Cesar Australia. Thank you to Dr Jess Lye for contributing to this article.
Original article written by Lizzy Lowe (Cesar Australia) for PestFacts south-eastern, modified for SA.
What pests to look out for in emerging crops
As crops emerge, a range of invertebrates also become active. Young crops are particularly susceptible to feeding damage for the first 3-5 weeks following emergence. Where possible, use agronomic practices to ensure rapid establishment, after which crops can often outgrow moderate damage.
Best practice pest management in emerging crops involves regularly monitoring crops to ensure early detection of pest issues, and correct identification to inform the selection of appropriate controls as required. It is important not to assume that pre- and post-emergent insecticides will adequately control pest issues – different pests require specific management, and monitoring is the only way to be sure!
SARDI Entomology is running a SAGIT funded trial in the next two years to monitor nocturnal invertebrate activity in broadacre paddocks on multiple crops. We will keep you updated on the results.
Earth mites
At least four earth mite groups feed on seedling crops. Correct earth mite identification is crucial for effective control; different mite species vary in susceptibility to insecticide, either naturally or through insecticide resistance.
Hatching of redlegged earth mite (RLEM; Haylotydeus destructor) occurs following autumn rainfall (>5 mm) and cool temperatures (range <16°C to <20.5°C). So far, few regions of SA have received sufficient rainfall and mites seem to still be dormant, but activity has been reported from NSW and Victoria (PestFacts SE, Twitter). RLEM often aggregate (groups of 30 or more) when feeding. Resistance to synthetic pyrethroid and organophosphate insecticides has been detected in RLEM around the Upper South East and Fleurieu Peninsula regions. To help manage resistance, see the GRDC resistance management strategy for RLEM.
Blue oat mites (BOM; Penthaleus spp.) are similar in appearance to RLEM but have a distinct orange-red patch on their back that is visible under a hand lens. Unlike RLEM, BOM typically feed singularly or in small groups.
Bryobia or clover mites (Bryobia spp.) are relatively small mites with a flattened body, and front legs up to 1.5 times their body length. They prefer warmer conditions and are often seen earlier in autumn than other species. If treatment is necessary, organophosphates may provide better control than synthetic pyrethroids. Populations decline during cold winter conditions.
Balaustium mites (Balaustium medicagoense) are relatively large, slow-moving mites often observed feeding at the tips of leaves. Pyrinex Super® is the only registered product for this species (and only in canola).
Your observations of mites are essential for the PestFacts newsletter. SARDI is also currently undertaking RLEM research and is looking to collect populations of all species of mites. If you find a population or would like more information, please contact:
- Maarten van Helden: 0481 544 429 or maarten.vanhelden@sa.gov.au
- Kym Perry: 0421 788 357 or kym.perry@sa.gov.au
Lucerne flea
Lucerne flea (Sminthurus viridis) hatch following adequate rainfall and thus this season we have received no reports yet. Lucerne flea feeds on a range of crops and pastures, causing a characteristic ‘windowing’ of leaves. Lucerne flea occurs in hotspots in paddocks, so it is important to monitor paddocks entirely. If treatment is necessary, avoid synthetic pyrethroids; organophosphates are recommended.
Beetles and weevils
The larvae and/or adults of several beetle species can be sporadic pests of seedling crops in autumn. No beetles have been reported yet, but typical species to look out for are:
Mandalotus weevils (Mandalotus spp.) are a group of native Australian weevils, with at least ten species known to feed on crops in parts of south-eastern Australia. Adults are 3-5mm long and emerge onto the soil surface during late autumn. They are often difficult to find; search at night using a torch, or during the day in the top 1cm of soil at the base of weeds or damaged seedlings. Adults lop stems, cotyledons or seedlings. In canola, check problem areas (history of Mandalotus damage) for damage one week after emergence. If needed, insecticidal spot treatment to control adults is the only management option. See the SARDI Mandalotus weevils in canola factsheet for more information.
The vegetable weevil (Listroderes difficilis) is a sporadic pest widespread in cropping areas. Adults and larvae feed on foliage at night during early winter. Adults are 8mm long with grey-brown bodies, a prominent weevil snout and a distinctive pale-coloured ‘V’ on their back. Larvae are legless, yellow to green in colour with an orange-brown head. Check emerging canola crops for damage and weevils along crop edges or near areas with host weeds, particularly capeweed.
Larvae of the southern false wireworm or vegetable beetle (Gonocephalum spp.) are minor pests of winter cereals. Larvae can feed on germinating seeds underground, while adults can feed on emerging canola at ground level. Larvae are cream, yellow or tan with cylindrical elongate bodies. Adults are small, matte dark grey-black coloured beetles.
Larvae of bronzed field beetle (Adelium brevicorne) are often active in autumn, and feed on canola at ground level, causing seedling losses. Removal of surface trash is an effective management strategy for this pest in problem areas. Adults are shiny black beetles up to 11mm long with a slight bronzed colour. Larvae are up to 12 mm in length, dark and shiny with two upturned spines at the tail end.
Earwigs, millipedes and slaters
The Portuguese millipede (Ommatoiulus moreletti) and slaters are primarily detritus feeders however in recent years, higher densities of these invertebrates have been associated with crop damage. In particular, the pill bug (Armadillidium vulgare) has been reported feeding on lentils and canola in recent seasons. Note, other slater species are often present in paddocks and are unlikely to cause damage. It is currently unknown why slaters and millipedes switch from feeding on detritus. No reports of these species causing any issues have currently been made to the PestFacts SA team.
The European earwig (Forficula auricularia) is a sporadic pest of broad-acre crops, and generally most abundant when paddocks provide adequate habitat for population build up. We received reports of high numbers of European earwigs earlier in the year in the Mallee living in and around stubble. Prevention such as removal of trash and reducing stubble retention is the main form of control to minimise these pests. There are no registered insecticides for European earwig; methiocarb snail baits and fipronil-based seed treatments may offer some control but may not protect seedlings against high densities. European earwigs are also a beneficial insect as they can predate many soft bodied insects, such as aphids.
How to report
PestFacts SA always wants to know what invertebrates you are finding in your crops and pastures, whether it is a pest, beneficial or unknown – and even the ‘usual’ pests. Please send your reports or identification requests by using the PestFacts Map online report form, tagging @PestFactsSARDI or using the #PestFactsSA hashtag on Twitter, or by contacting:
- Rebecca Hamdorf: rebecca.hamdorf@sa.gov.au or 0429 547 413
- Maarten van Helden: maarten.vanhelden@sa.gov.au or 0481 544 429
