Will Rutherglen bug damage sorghum post grain fill?

As many of the early sorghum crops reach physiological maturity, and approach harvest, questions are being asked as to whether it is necessary to control large populations of nymphs in these crops. More specifically, whether these RGB will cause any damage to the maturing grain between physiological maturity and harvest.

Photo: Dave Murray (BigBug) out looking at a sorghum crop at Glen Ogden's property on the Darling Downs.

In summary, the best available information suggests that sorghum is will not suffer yield loss as a result of RGB feeding from physiological maturity (hard dough) through to harvest.

DPI&F trials to examine the impact of RGB on grain as it matured, did not provide a conclusive answer to this question. In 2008 we are undertaking field trials to try and answer this question.

However, if we look at what is happening with the sorghum plant from physiological maturity on, we can draw some conclusions about the potential for RGB to cause damage to maturing grain. Importantly, once grain reaches physiological maturity it has reached its full potential weight, and from then on starts to lose moisture as it matures. This means that even if large numbers of RGB continue to feed on the sorghum plant (on stems and leaves) their feeding will not impact on the development of filling of the grain at this stage.

Photo (left): Physiological maturity is reached when a black layer appears at the base of the seed, near to where the seed is attached to the stem

Another question that we do not have a definative answer to is whether RGB continue to feed directly on the maturing seed, or if they feed only on the sorghum plant once the grain reaches hard dough. In trials where plants at hard dough were exposed to RGB we did not see the evidence of feeding damage to grain that we saw when heads were infested at earlier stages of grain development (see earlier posting on RGB damage).

Crops that have had early infestations of RGB, even some of those that have been treated, now have moderate to large populations of nymphs and adults in them. The adults are likely to be newly emerged, having developed from nymphs in the crop. The nymphs will have emerged from eggs that were laid by an earlier infestation.

Controlling RGB to prevent the problems associated with infestations at harvest remains an issue. The inclusion of an insecticide along with the herbicide when the crop is being sprayed out prior to harvest is a practical approach. Be alert to the witholding period of any insecticide used. Treatment with insecticide prior to harvest is not a guarantee that the crop will be free of RGB at harvest. There remains the possibility of reinvasion by adults at any time.

Rutherglen Bug in sorghum

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.

Rutherglen bugs are everywhere!

Rutherglen bug (Nysius vinitor)(RGB) is one of the insect species that arrives in crops in spring in large numbers, usually in association with storm activity. You may also have seen them on your windows and screens (and around the lights) at home in recent days. It is likely that the bugs are moving around in the environment, perhaps even transported from some distance away on storm fronts. You will probably find RGB in all crops and weeds at the moment, not just the winter cereals.

Because RGB are moving so much in the environment, and probably migrating in on storm fronts, we are likely to see ongoing infestations of crops for some time. This means that any decision to control RGB needs to be done with full knowledge that the treated crop may be infested again in a short period of time.

(Photos: RGB adult (top) and RGB late instar nymph (bottom). Keith Power)

Rutherglen bugs in wheat and barley
At this stage in the season, RGB is not going to have any impact on yield in winter cereals. Grain that is hard will not be damaged by this sucking bug.

The main issue with RGB around harvest time is contamination of harvested grain. When RGB are in very large numbers they can cause a number of issues at harvest:

• Live bugs in the sample can result in rejection of a load at the delivery point
• Large numbers of bugs (and bits of bugs) in the grain can elevate grain moisture. This problem is probably worst when RGB are breeding in the crop, and there are large numbers of nymphs - this is unlikely to eventuate in wheat.
• There are no insecticides registered for RGB in winter cereals. If you are controlling armyworm, or helicoverpa (except with NPV) then some of these options will control RGB to some extent. But be very aware of the WHP of any insecticide used this late in the season.

Treating a crop now for RGB is no guarantee that there will not be a reinfestation before harvest

What can you do if you have large numbers of RGB at harvest?

The best approach is to try and limit the number of RGB that end up in the harvested grain. Some of the suggestions for doing this include:

• Harvesting at night
• Fitting screens to the header
• Leave the tarp off the load for as long as possible to allow RGB to escape post harvest

If you are storing grain, RGB will not cause damage to grain in storage, and there is no need to treat grain to kill them. Large numbers of crushed RGB in harvested grain have been identified as tainting the grain with the oily exudates from their scent glands. It is unclear over what period this tainting persists.

What about RGB in other crops?

Seedling and vegetative crops
RGB is primarily a seed-feeding species, and have the capacity to damage crops during grain filling – but we know very little about how much damage in any crop other than sunflower.

In very large numbers, RGB can damage seedling crops purely by weight of numbers feeding on seedlings. In more advanced vegetative crops they will not cause any impact as long as the crop has adequate moisture and is growing actively. Be alert to RGB in sunflower at budding and flowering, and in sorghum from flowering through to soft grain; these infestations may warrant treatment.

RGB in sunflower
Whilst RGB numbers may be high in vegetative sunflower now, it is important to weigh up the decision to spray with the following:
Actively growing plants, with adequate moisture, will not be greatly impacted on by RGB feeding
Reinfestation is a real possibility, and if treating during the vegetative stage, it is likely that insect control in the crop will run to 3 sprays (vegetative, budding and flowering) if RGB pressure remains high and helicoverpa infest the crop as well.

In sunflower there are two critical periods during which RGB can cause significant crop damage:
Budding: bugs congregrate on the upper stem and bud. Bug feeding on the stem behind the head may cause the stem to wither and the bud droop.
Flowering: eggs are laid in the head and nymphs emerge in about a week and start feeding on developing seeds. Adult numbers are often minor in comparison with the size of the population once nymphs start to emerge.
Feeding on developing seeds causes yield loss, and a loss of oil content and quality of grain.

Thresholds for sunflower:

Budding: 10 bugs per head
Flowering: 20-40 bugs per head
If it is necessary to treat at flowering, do so before the heads turn down, otherwise it is difficult to get good contact with the bugs in the flower.

Control considerations in sunflower

Synthetic pyrethroids (SP) are the most effective option for controlling RGB

• If RGB are in large numbers at budding and flowering, but there are a few helicoverpa present, consider an SP/NPV mix. Steward™ (for use in sunflower under permit to control helicoverpa and RGB) will provide, at best, suppression of RGB and will not provide adequate control of a large population.
• The impact of insecticides on bees is an important issue in sunflower, particularly if there are hives nearby. Spraying later in the day, when bees are less active, will reduce the impact on them.