Rutherglen bug infestations of grain sorghum
This season we are seeing large infestations of Rutherglen bug (Nysius vinitor) (RGB) in sorghum crops from Central Queensland to the Downs, and further south. The large infestations are most likely a result of storm activity and the growth of weed hosts on which they breed up. RGB are very mobile, with large numbers of bugs appearing in (or disappearing from) crops before or after storms. There doesn’t seem to be particular stage of crop development at which RGB start to infest sorghum. The timing of infestation is determined by storm activity, or by hot dry weather that causes weed hosts to dry off, forcing bugs into nearby crop.
What we are seeing in the field this season
Where RGB are active, early sorghum crops, or early heads in a crop, are most affected. Damaged grain is red where bugs have fed on the portion of the seed not covered by the glumes. Dark spots are also visible on the grain. Grain is small and shrivelled, and has not continued to fill beyond the point at which it was damage. Cutting open the damaged grains, there is fungal and bacterial infection of the seed, making it black and mushy. This grain will not continue to fill, and is likely to be lost at harvest.
Sampling for RGB
Because RGB populations can start to build up at any stage of crop maturity, and our data show that RGB can significantly impact on seed set during flowering and early seed development, monitoring for RGB should start at mid flowering. Practically, RGB can be sampled for along with Helicoverpa; beating individual heads into a bucket. The distribution of RGB is typically patch across the field. A few heads with high numbers, lots of heads with lower numbers. This means that more, rather than fewer, heads need to be sampled to get an idea of the overall level of infestation in a field.
RGB and their damage potential
….. in sunflower
There has been little research on RGB in any crops other than sunflower. In sunflower they are known to be a major pest, feeding on the seed as it sets and matures, resulting in reduced yields and oil quality.
…….in sorghum
In the USA there is a related species, the false chinch bug (Nysius raphanus), which causes yield loss and reduced seed viability in sorghum. Yield loss is caused not only by the direct feeding of the bugs, but also through allowing a fungus to infect the seed through the feeding wounds, causing further deterioration and discolouration of the grain. Threshold trials showed that chinch bug caused a reduction in seed set when present on grain from flowering through soft dough stages. Where seed set was reduced, overall grain weight increased. In other words, the plant compensated with fewer, heavier grain in damaged heads.
DPI&F research on RGB in sorghum – what we know
RGB feeding post flowering will reduce grain set
Preliminary research on RGB has been done in sorghum and has shown that adult RGB will reduce seed set by around 20% at densities of 50-100 bugs/head. If bugs infest after grain is set, adult RGB will feed on the seed. Affected seed looks spotty and red externally (see photo), and hollowed out internally. Fungal infection also occurs resulting in further deterioration of the developing grain and ultimately seed that is small, shrivelled and light – and likely to be lost at harvest.
Early instar nymphs do not damage grain
No evidence of grain feeding was found for first, second and third instar nymphs. Older nymphs did feed on developing grain as do adults.
There are no soft-options for RGB control
Fipronil, indoxacarb and dimethoate were compared with alphacypermethrin in a replicated field trial on the Downs. None of the products provided control of nymphs statistically equivalent to the commercial comparison, alpha-cypermethrin. Compared to the unsprayed, alpha-cypermethrin was highly disruptive to predatory invertebrates.
What we don’t know
Can RGB damage grain from hard dough through to harvest?
The data for the impact of RGB on maturing sorghum does not provide a clear picture of whether grain continues to be damaged. We know that large populations of nymphs can be present in crops right up to harvest, but whether they are feeding on the plant, or on the grain are unknown.
RGB economic threshold for sorghum
Further work is needed to determine the density-damage relationship. This includes work to understand compensation when seed set is reduced early, the direct damage to grain during filling and maturation, and potentially indirect damage caused by RGB feeding on the sorghum plant which may impact on grain fill.
Soft options for RGB
Biopesticides, in this case the fungal disease Metarhizium is an option for RGB. It is currently being evaluated by DPI&F, and if effective will be an important addition to sorghum IPM.
Based on what we know, the following strategies can be suggested for minimising RGB damage to sorghum:
Start checking for RGB from flowering
Developing and filling seed are susceptible to damage from RGB adults, so protect the crop during these stages.
A threshold of 20-50 bugs/head is suggested. Treating populations of adults does not guarantee there will not be further infestation. RGB can reinfest a field overnight if the conditions are right.
RGB will breed in sorghum, so infestations of adults will result in increasing numbers of nymphs. Large populations of RGB can cause problems with machinery at harvest. Continue to monitor populations through to harvest as a decision to treat will need to take into account withholding periods for harvest.