Control of green bridge for pest and disease management

Page last updated: Wednesday, 26 April 2023 - 10:27am

Please note: This content may be out of date and is currently under review.


The greatest impact of virus diseases to crops occurs when they are infected early (at seedling stage) in the growing season. This is more likely to occur when a green bridge is present prior to sowing, it allows both the virus to survive and vector populations (such as aphids and wheat curl mite) to have increased at a time when new crops are just developing. Destroying the green bridge (cereal and canola volunteers and weeds) well in advance of sowing minimises local threat from virus diseases.

A green bridge can encourage the early build up of aphids that may lead to early virus infection of crops, especially canola (Turnip yellows virus, TuYV syn. Beet western yellows virus) and cereal (Barley and cereal yellow dwarf virus, BYDV). TuYV and BYDV also rely on the green bridge for survival between growing seasons. Wild radish and volunteer canola are hosts for TuYV while grass weeds and volunteer cereals are hosts for BYDV and need to be removed prior to sowing. For TuYV and BYDV, it is vital to prevent spread to crops by aphids in first eight weeks after emergence.

Volunteer cereals and grass weeds can harbour wheat streak mosaic virus and wheat mosaic virus (syn. High Plains virus) and the wheat curl mite (WCM) vector. Warm temperatures favour increases in WCM populations and virus spread. In areas with abundant cereal volunteers, warm temperatures and when crops are sown early, the risk of virus infection in crops can be minimised by spraying out volunteers at least four weeks before sowing so these viruses are not spread to emerging seedlings. When volunteer cereals are not completely removed prior to seeding localised crops can become 100% infected with reduced yield and shrivelled grain.

Other diseases

Other pathogens, well adapted to surviving the non-cropping phase, can also be advantaged through developing population pressures earlier than normally possible. Survival of other cereal root diseases such as take-all (Gaeumanomyces graminis) and pythium root rots (Pythium spp.) is likely to be enhanced by opportunities to parasitise host roots over summer and autumn.


Summer rainfall in the WA wheatbelt can have major implications for the survival and development of insect pests. The responses of particular insect pests vary from enhanced survival and reproduction to sharp increases in mortality and a rapid decline in numbers. Regular surveillance is the key to the management of most insect pests. If summer rain produces out of season weeds and crop volunteers just prior to seeding, these paddocks should be surveyed for the presence of pests. Paddocks can be surveyed by close inspection of plants, the use of a sweep net to collect any insects present, and by removing the surface layer of soil for observation of subterranean species.

If large infestations of insects that pose a potential threat to the planned crop are found in summer or autumn up to a month prior to expected seeding date, the destruction of their habitat may be considered. However, summer populations may not carry over into the growing season if conditions deteriorate.

If large infestations are found around the time of seeding, an insecticide treatment can be applied very economically along with herbicides. It is common agronomic practice to apply a prophylactic synthetic pyrethroid insecticides at the same time as a herbicide application, this will control mites such as balaustium mite, weevils and lepidopterous pests but will not control European earwigs, molluscs or soil dwelling pests such as desiantha larvae.

Trials have shown that if a green bridge is controlled at least two weeks prior to seeding, then there is a decreased risk of pests damaging emerging crops.


Several species of aphid are pests of cereals, pulses, lupins, canola and pastures. Not only can they cause significant damage to crops and pastures due to their feeding when they are present in very high numbers, but they can also inflict considerable damage in low numbers as vectors of a number of viruses, as outlined above. While aphids have several generations in a season and their numbers can fluctuate widely, they survive by taking refuge in small, isolated pockets of suitable habitat and summer and autumn rainfall is critical to the initial development and migration of a number of aphid species into crops and pastures. Destroying the green bridge well in advance of sowing minimises local threat from aphids and aphid vectored diseases.

Slugs and snails

Presence of a green bridge provides habitat for slugs and snails to survive summer and a food source in autumn. Baiting is the only chemical control for slugs and snails. It has limited success if there is a green bridge present in the paddock, as the baits compete with the weeds to attract the molluscs.

Pest mites

A green bridge allows pest mites such as balaustium mite and bryobia (clover) mite to survive in summer. If the green bridge is not controlled prior to sowing, these mites can move from the dying weeds onto the emerging crop. Balaustium and bryobia mite do not have a cold temperature requirement so can be present all year round.

Earth mites, including redlegged earth mite and blue oat mite, only hatch from over-summering eggs once there has been sufficient moisture and seven days of temperatures below 20°C. Controlling a green bridge does not control these mites unless conditions for hatching have been met.

Diamond back moth (DBM)

Populations of DBM can build up if a green bridge is present over summer. However, it is on those years where there is a mild autumn and winter that DBM cause damage to canola seedlings. In most years, cold autumn and winter temperatures suppress populations and they do not cause any seedling damage. Removing a green bridge from on-farm paddocks decreases the risk that the pests will move onto the germinating crop.

Other pests

Other pests may take advantage of summer rain and green bridge to build up in numbers, these may include webworm, cutworm, lucerne flea and beetle larvae.

Minimising the threat and maximising the opportunity

The same wet summer conditions that favour the build up of diseases and pests can also lead to opportunities for early sowing when effective weed management is achieved. Timely sowing with direct drilling can provide optimal crop performance through the avoidance of end of season drought stress. But direct drilling also shortens the interval between autumn pest and disease loads and development of new crops.

Maximising the pre-plant fallow period with early weed control is important to reduce potential disease and pest activity before cropping. Early weed control rather than delayed sowing should be used to maximise the pre-plant fallow period. Adequate chemical fallow (minimum six weeks) is most important in seasons where summer or autumn rains promote prolific pre-season weed growth.

Summer weed control is a progressive strategy for optimum crop production. Some benefits include:

  • Opportunities for early sowing by keeping cropping paddocks manageable and conserving soil moisture.
  • For root diseases, including RLN, early control of summer weeds should assist new crops to establish roots in a less hostile soil environment, favouring early crop growth and vigour.
  • For leaf diseases, particularly rusts, early control of summer weeds will delay the commencement of disease, reduce disease impact and therefore expense of subsequent control measures.
  • Other pests of young crop such as webworm, cutworm, mites, lucerne flea, slugs and beetle larvae will also be diminished.

Also consider:

  • Weeds are more readily killed when small. Large weeds can transplant particularly in zero-till crops.
  • There are opportunities for summer crops in preference to leaving weeds uncontrolled for water utilisation.
  • There are opportunities and implications of summer herbicide use in herbicide resistance management.
  • Grazing, burning and cultivation can also provide good weed control if compatible with other farming practices.

More information on weed control is available at Crop weeds: controlling small weeds or browse the information available on the crop weeds page.