Summer ‘green bridges’ – holiday retreats for aphids

With the addition of Russian wheat aphid to the suite of aphid pests in broadacre crops, green bridge control is more important than ever

 

The term ‘green bridge’ is often used to describe the role of weeds and crop volunteers in helping pests (and diseases) that threaten crops, cross from one cropping season into the next. This mass of vegetation grows on paddocks, headlands, roadsides and non-crop land after summer rain – although it can also refer to summer crops. A green bridge can include crop volunteers usually from last year’s crop, and sometimes from crops grown one or two years before.

Many summer weeds are hosts of crop pests and the viruses they sometimes carry. For example brassica plants (such as the turnip weeds, wild radish, mustards and Lincoln weed) are preferred hosts for green peach aphid (Myzus persicae) and Beet Western Yellows Virus (BWYV). BWYV has an extraordinarily wide host range of summer weeds and pasture species. The same principle applies to the summer grass hosts of cereal aphids and Barley Yellow Dwarf Virus.

Controlling these host plants in the vicinity of next year’s canola, legume and grains crops is an important tactic that will lessen the likelihood of aphid and virus next season, especially if late summer rains re-occur, and early sowing is undertaken. Applying herbicides to weeds and volunteer plants at sowing or after may be too late to prevent the movement of pests and diseases into the emerging crop. 

 

Let’s not forget Russian wheat aphid

Shortly after Russian wheat aphid (Diuraphis noxia, RWA) was detected in Tarlee (South Australia), there was uncertainty as to how the aphid would handle Australian conditions and what impact it would have. The primary concern centred on the aphid’s ability to inject salivary toxins during feeding, resulting in a suite of symptoms that could be detrimental to yield.

While RWA stole the frontline act on the invertebrate pest scene during winter and early spring, our view is that concerns have been eased due to

a)   populations declining through the wetter and cooler than average conditions in much of Victoria and southern NSW through winter and early spring, and

b)   the surprising degree of control by natural enemies (including predators and parasitoids).

However, it will be important for growers and advisors to be on the front foot for RWA prevention and control for 2017. A key component of this is controlling the green bridge. The host range of RWA includes more than 140 species of cultivated and wild plants within the family Graminae (grasses). These include wheat, barley, triticale, rye, oats, pasture grasses and wild genera including Poa, Bromus, Hordeum, Lolium, Phalaris and others.

The US literature suggests that winged RWA are very efficient in locating isolated host plants, particularly wheat and barley volunteers. RWA infestations in the ‘initial’ Tarlee outbreak appeared to have commenced on wheat volunteers. Likewise, many of the earlier Victorian infestations were first detected on volunteers.

For further information on green bridge control, see GRDC Green Bridge Fact Sheet.