Reduce pests by controlling green bridges

By Cropping Officer Tim Bartimote

February 2020

Rain across various parts of our region is great news. However, it is important to keep in mind that while rain will assist in pasture regeneration and increase stored moisture in our fallows, it will also germinate weeds. Weeds are known to rob paddocks of valuable moisture and soil nutrients, costing producers yield and production over the course of the season. On top of this these weed species can act as hosts of a variety of important pests, giving them a habitat to thrive in until the time they can transition to nearby developing crops. In this article I will outline some key pests and weeds species to monitor for that may harbour them over the summer period.

What is a Green Bridge?

A ‘Green Bridge’ refers to weeds and crop volunteers that serve disease and insect pests by providing a habitat to survive on from one cropping season to the next. This vegetation can grow in the paddock, alongside fence lines, headlands, roadsides and non-crop land. Green bridges can be germinated seed from the previous crop, local weeds germinating from summer rainfall or germinating weeds blown in from nearby areas. Grasses and broadleaf weeds are both significant, however particular pest species are known to have preferred hosts to use as green bridges over the summer. It takes a community effort to control green bridges, as rust spores, aphids and mites all travel on the wind, easily crossing farm boundaries. Ideal timing for control, so as to interrupt the life cycles of a number of pests, is at least 2-3 weeks before sowing begins. Failure to control green bridges increases pressure on insecticides, fungicides and genetic resistance in crops. As it allows potentially resistant populations to continue to breed over the summer and pass on their resistance to following generations.

Insect Pests

Mites

Large populations of mites can breed on green bridges. Mites species such as the clover mite (Bryobia spp.) and balaustium mite (Balaustium medicagoense) are capable of maintaining populations throughout the year when habitat is available. Populations can skyrocket if green bridges of clover are nearby and there is a warm start to the growing season. In contrast blue oat mites (BOM) (Penthaleus spp.) and red legged earth mites (RLEM) (Halotydeus destructor) require 5 consecutive days of temperatures less than 20°C to hatch. RLEM is commonly associated with capeweed so monitor the emergence of this weed as temperatures cool and early season rain appears. Mites can also be vectors of viruses, such as the wheat curl mite (Aceria tosichella, WCM) which is the primary source of the Wheat Streak Mosaic Virus (WSMV). WCM breeds very quickly between 24°C and 27°C, making early sown crops an ideal target. Mites are often found on volunteer cereals, broadleaf and grass weeds. WCM, in particular, is known to be carried by barley grass, great brome, annual ryegrass, couch grass, liverseed grass, pigeon grass and rats tail fescue. Having an available green bridge to survive on will greatly increase the likelihood of a large population getting a head start to then be able to transfer to a nearby crop.

Aphids

Aphids pose a threat to cereals, pulses, canola and pastures. Damage can occur when large populations feed on plants or when they act as vectors for various plant viruses. For example, oat aphids (Rhopalosiphum padi), corn aphids ( (Rhopalosiphum maidis) and rose grain aphids (Metopolophium dirhodum) are known vectors of Barley Yellow Dwarf Virus (BYDV). The Russian Wheat Aphid (Diuraphis noxia, RWA) can also be limited if green bridges are controlled effectively. As part of a GRDC investment, RWA populations were monitored on various grass species in November 2018 to see which species are preferred as a habitat over summer. Large numbers were identified in barley grass, brome grasses and volunteer wheat and barley. Monitoring during the warmer months in January and February noticed that RWA populations significantly decreased as these preferred hosts became rarer. Broadleaf weeds need to also be managed as they are a green bridge for significant pests like the Green Peach Aphid (Myzus persicae, GPA). This insect is known to be found in turnip weed, marshmallow, Lincoln weed and other cruciferous plants. Over the summer period aphids can also survive in perennial grass such as perennial ryegrass, kikuyu, couch and African love grass. It may also be required to control perennial grasses which are near to or surrounding paddocks where cereals will be sown.

Diseases

Cereal Rusts

A number of cereal rust pathogens (Puccinia species) are able to survive on hosts in the summer period, to then re-infect the following winter crop. They require a living host to survive as they can only persist on dead plants for a short time. Their success as pathogens is attributed to their short generation cycles of roughly 2 weeks. Green bridges for rusts often include volunteer wheat, barley and triticale, as well as on grassy weeds such as barley grass, brome grass and phalaris. It is important that these green bridges are controlled before the following cropping season begins. Often severe epidemics appear when green bridges overlap with early cereal sowing opportunities.

Rhizoctonia

Known as Rhizoctonia solani, this disease causes the development of bare patches in cereals, crop legumes and pastures. Surviving in organic matter, this fungus will emerge and infect the roots of young seedlings. The severity of the pathogen is reduced significantly when it is unable to infect weeds or volunteer crops. Common green bridges are the roots of volunteer cereals and grassy weeds. Rhizoctonia can survive on live and dead plant tissue. This is why it is important to control green bridges 2-3 weeks before sowing. As it will provide enough time for the disease to deplete its food source before a crop is sown. Summer rainfall without green bridges present, will also aid in reducing inoculum levels.

Root Lesion Nematodes (RLN)

RLN are endoparasitic worms which feed and reproduce inside of plants roots. They have a large impact on cereal, pulses, oilseeds and pastures. Confined to the Pratylenchus genus, two important species in NSW are P. neglectus and P. thornei. These nematodes have two phases, they either parasitise plant roots or undergo induced dormancy due to a lack of moisture in the soil. Unable to survive on dead roots or organic matter, RLN rely on being able to parasitise an extensive range of weed and crop species. Summer rain provides moisture to reactivate RLN out of dormancy, enabling them to attack nearby roots of green bridges. Common green bridges include; clovers, medics, volunteer crops and a mixture of weeds e.g. barley grass. After 4-6 weeks they are able to complete their life cycle and after several cycles will have a population which can be quite damaging to the following crop. Without a green bridge, RLN are unable to reproduce effectively because there are no roots to parasitise, causing significant population decline.

More Information

For more information on green bridges and the pests they can harbour feel free to contact a member of Central West LLS Ag Advisory Team or your local agronomist.

References

CESAR (2018) PestNotes - Green Peach Aphid (Myzus persicae). Accessed at: http://cesaraustralia.com/sustainable-agriculture/pestnotes/insect/green-peach-aphid

CESAR (2019) PestFacts - Not many Russian wheat aphids in the green bridge. Accessed at: http://cesaraustralia.com/sustainable-agriculture/pestfacts-south-eastern/past-issues/2019/issue-2-22-may-2019/not-many-russian-wheat-aphids-in-the-green-bridge/

Depart of Primary Industries and Regional Development - Agriculture and Food (2015) Diagnosing bryobia mite. Accessed at: https://www.agric.wa.gov.au/mycrop/diagnosing-bryobia-mite

Grain Research Development Corporation (2010) Green Bridge - Fact Sheet. Accessed at: https://grdc.com.au/__data/assets/pdf_file/0027/201978/green-bridge.pdf.pdf

Grain Research Development Corporation (2013) Barley Yellow Dwarf Virus - Fact Sheet. Accessed at: https://grdc.com.au/~/media/documents/resources/publications/fact-sheets/grdc-fs-bydv-pdf.pdf

Grain Research Development Corporation (2009) Plant Parasitic Nematodes - Fact Sheet. Accessed at: https://grdc.com.au/__data/assets/pdf_file/0018/224424/plant-parasitic-nematodes-south-and-west.pdf.pdf

Park R, Cuddy W, Hollaway G (2015) Green Bridge to Rusty Crops. Groundcover Issue 118.Accessed at: https://grdc.com.au/resources-and-publications/groundcover/ground-cover-issue-118-sep-oct-2015/green-bridge-to-rusty-crops