RUTHERGLEN BUG, SORGHUM MIDGE AND DIAGNOSING POOR SEED SET IN GRAIN SORGHUM

Take home message

Timely action to protect profits from insect pests 

The management of Rutherglen bug (Nysius vinitor) (RGB) has emerged as a concern for growers and agronomists in the past 2-3 seasons. The concern is whether RGB are causing crop loss and need to be controlled. Until recently, there had been no research on RGB damage potential in sorghum, perhaps because RGB has generally been controlled incidentally by midge sprays.

 

Typically RGB are detected in sorghum crops in large numbers prior to desiccation or harvest. However, in 2007-08, early flowering sorghum was infested at flowering, resulting in significant reductions in seed set, and consequently yield. Whilst we had seen this type of damage in our trials, we had not seen it before in the field. What this experience shows is that RGB is a pest that needs to be monitored from head emergence until the crop reaches hard dough.

 

Current recommendations for RGB in sorghum are based on trials that have examined the impact of different RGB populations on sorghum at different stages of head maturity. We are proposing working thresholds that reflect the damage potential of RGB in sorghum in our trials, but this requires further refinement and testing in the field.

 

Our current data suggest that flowering-milky dough stages are most susceptible to RGB, and the potential for yield loss reduces as the grain fills and matures. The data we have is still preliminary and needs further validation in the field.

However, we are suggesting a working threshold of 20-25 bugs per head during the flowering-milky stages, and 30-50 bugs/head at soft dough stage. No yield loss occurs with infestations post physiological maturity (black layer). A range of densities reflect the potential spread of chemical control costs which determine the breakeven point at 3-5 % reduction in yield (around $15-25 in control costs @ $250/t and 2 t/ha).

 

Over the past two seasons, RGB control trials have demonstrated that synthetic pyrethroids are effective for controlling large populations of RGB (deltamethrin and alpha-cypermethrin are recommended as they have known MRLs and export slaughter intervals). Other chemical options have been tested, and will suppress moderate populations. Because the use of synthetic pyrethroids poses a significant threat to the management of Helicoverpa and aphids in sorghum, it is important that the treatment of RGB is only undertaken when it is warranted.

 

Samples of grain from heads infested with different densities of RGB have been analysed by NIR for changes in grain quality. The only key parameter that changed in the samples was a reduction in starch content as RGB density increased above 100 bugs per head at the milky dough stage. The implication of this result needs to be explored further in relation to feed quality traits and possibly ethanol production.

 

While breeding for midge resistance has been successful, current selection is based on one mechanism of resistance; ovipositional antixenosis. This is essentially a ‘barrier method of resistance’ by the plant, which makes it difficult for the female midge to lay eggs. It appears to be controlled by genes in a number of locations, which consequently makes it difficult for breeders to simultaneously select for this trait while breeding for other important agronomic traits. In order to overcome this problem and maintain a high level of midge resistance, research began nearly ten years ago on the introduction of a new source of midge resistance into current elite germplasm. Antibiosis is a resistance mechanism that can cause up to 60-70% larval mortality and therefore allows the grain to develop with little or no damage.  It also appears there is a possible associated tolerance resistance mechanism, which causes selective feeding in larvae - larvae tend to move away and feed elsewhere, before returning to the developing grain at a later date, when both grain and larvae are able to successfully develop. The benefit of the antibiosis trait is that it is believed to be carried by a single gene, thus making it easier to select for. There is however, a major difficulty with breeding for this trait, and it is in the field screening process. As a result, the incorporation of this resistance within the DPI&F sorghum breeding program is being aided by the use of marker-assisted selection (MAS). Using MAS linked to the region containing the gene, the need for expensive and time consuming field screening trials will be substantially reduced. It will also allow breeders to accurately distinguish between antixenosis and antibiosis (something previously not easily possible with traditional field screening techniques), thus allowing breeders to be able to use both mechanisms of resistance either alone or in combination.

 

Although all current commercial hybrids have some level of resistance, this new antibiosis mechanism means that hybrids with some antixenosis resistance, previously rated a 3, could end up rating as high as 8+, providing resistance to the point of ‘practical immunity’ under most Australian conditions. It is quite probable that in the next 5-10 years, we could say ‘so long’ to the associated economic and environmental impacts of the sorghum midge. Until then, managing for midge through the selection of appropriately rated varieties, timely monitoring, and control where necessary remain important components of successful midge management.

 

The table below illustrates the contribution that midge resistance can make to the economics of growing sorghum, both early-mid and late season, and emphasises the importance of selecting high midge rated hybrids for later plantings.

 

Economics of midge resistance under Australian conditions (without chemical control).

 

 

Every season there are numerous reports of poor seed set on sorghum heads. Where full heads of grain are generally expected, occasionally there is a disappointingly low seed set on many sorghum heads. It is not always easy to identify the underlying reason or reasons for the missing or damaged grain, particularly when several weeks elapse between when the damage actually occurs and when the damage is finally noticed.

 

The purpose of this section is to provide some advice to assist in the diagnosis of sorghum crop loss. While some of these causes are easily identified, in many cases a number of factors may contribute to the grain loss and apportioning the relative damage to each cause can be much more difficult.

 

The typical question is ‘What caused this to my crop?” The best advice is to prevent the damage happening in the first place. This means understanding what the potential problems are, and how losses to these problems can be minimised.

 

The main causes of grain loss are sterility, sorghum midge, corn earworm, Rutherglen bug, mice and birds. Each of these will be considered from the perspective of identifying what caused the loss after the event.

 

 

Sterility is usually very obvious because the glumes are empty, there are no tell-tale signs of feeding, and it is usual for a band of the head to be devoid of set seed. It is not uncommon for the tip of a head to have good seed set while seed set lower down is very poor or absent. This ‘band’ of poor seed set often represents the portion of the head that was in flower, or about to flower, when the adverse conditions were experienced by the crop. One of the major causes of sterility is very hot weather experienced during flowering. High temperatures can overheat those sections of heads still wrapped in the boot leaf. The result is non-viable pollen or damage to the female flower parts. Persistent rainy weather at flowering can also reduce pollination and seed set.

 

 

Sorghum midge is a common pest of grain sorghum, but with the widespread adoption of midge resistant hybrids, the amount of insecticide spraying to control midge is greatly reduced. The main problems with midge are encountered later in the summer growing season when midge numbers have built up on successive crop plantings. The experiences of the past two seasons have highlighted these conditions, and in many cases multiple sprays were required to prevent midge damage in late sowings.

 

Female midge lay their eggs into glumes at flowering. Larvae (maggots) hatch from the eggs and feed on the grain, preventing further grain development. The end result is empty glumes wherever midge have been feeding. Depending on the level of midge pressure, the damage can vary from low to severe.

 

It is not easy to tell if midge is the cause of poor seed set. There are a few tell tale signs that can help. The first is the presence of white pupal cocoons at the apex of the glume, but these will not be apparent until about two or three weeks after eggs were laid. The cocoons are easily dislodged, so only a low proportion of midge-affected grain will have intact pupal cases. Secondly, developing grain can be squeezed to see if midge larvae are present. While some grains with feeding midge larvae will exude a reddish liquid when squashed, red exudate is not a certainty if midge are present. One of the key points to note with midge is that there will be no frass or grain residues on the ground under midge-damaged heads.

 

While parasitic wasps will kill developing midge larvae, they do not prevent the midge from damaging the grain. Instead of a midge emerging from the grain, a small black wasp will exit; but the damage is done.

 

 

Most corn earworm eggs are laid on the sorghum head during the first few nights after head emergence from the boot leaf. Thus on any one head, larvae tend to be of the same age and size. Within a crop, flowering may spread over 2 or 3 weeks, so the earliest heads to flower can have large larvae on them while the last heads to flower may have just eggs or very small larvae.

 

Up to the fourth instar (less than 13 mm in length), larvae are feeding mostly on anthers. Larger larvae feed on developing grain and are voracious feeders, causing  about 85% of their feeding damage during the last few days of larval life. Where larvae have completed their development on a sorghum head, there are tell tale signs indicating where they have been. With high larval infestations, the plants and ground beneath them are littered with frass which turns white when dry. Often there will be part grain left on the heads and on the ground below.

 

Occasionally high densities of medium and large larvae may be present during the late vegetative stage of crop development, and as heads protrude from the boot leaf, larvae move onto the heads and can cause serious losses. Fortunately this type of damage is not very common.

 

 

Recent experiences with RGB have increased our awareness of this potential pest problem. RGB are very mobile and can invade a crop overnight under favourable conditions. Spring and early summer is the time when these invasive movements are more likely to occur.

 

RGB feed by inserting needle-like stylets into the plant and sucking out sap. High numbers of RGB during flowering can reduce seed set, or cause reduced grain size if feeding during the grain fill period. The individual feeding sites on developing grain will discolour dark brown. Where extensive feeding by large numbers of RGB has taken place, the exposed top of each grain can be fully discoloured.

 

There are also some vertebrate pests that can cause losses in the field. Again there are some signs that are indicators of their activity, and appropriate action should be taken to prevent losses.

 

·         Plenty of whole grain on ground

·         Holes and burrows evident, particularly after rain.

 

·         Heads ripped to shreds, with plenty of whole and part grain on ground

·         Heads broken from weight of birds

·         Usually on field borders, particularly near tree lines and waterways

 

Contact details:  

Melina Miles, DPI&F Toowoomba. Ph: 07 46881369

e-mail: Melina.Miles@dpi.qld.gov.au

David Murray, DPI&F Toowoomba. Ph: 07 46881326

e-mail: Dave.Murray@dpi.qld.gov.au

GRDC Program code: DAQ00074