Making decisions about Rutherglen bug in maturing sorghum crops

Mating pairs of RGB on a sorghum head

Mat­ing pairs of RGB on a sorghum head

Ruther­glen bug (RGB) num­bers are per­sist­ing in many sorghum crops as they start to reach phys­i­o­log­i­cal matu­ri­ty. Queens­land Depart­ment of Agri­cul­ture and Fish­eries (DAF) research has shown no evi­dence of yield loss as a result of direct feed­ing on grain once it reach­es phys­i­o­log­i­cal matu­ri­ty (black lay­er). How­ev­er, because most crops this year have stag­gered head emer­gence, there are a num­ber of con­sid­er­a­tions to take into account in mak­ing a deci­sion about whether fur­ther treat­ment of the crop to reduce RGB num­bers is war­rant­ed.

RGB damage

RGB dam­age sorghum yield and qual­i­ty by feed­ing direct­ly on devel­op­ing and fill­ing grain. Not only does this reduce grain weight, but it allows entry of com­mon fun­gi and bac­te­ria that fur­ther dete­ri­o­rates the grain. If infes­ta­tions of RGB are present from flow­er­ing, feed­ing can pre­vent grain from fill­ing. This very ear­ly dam­age looks very much like midge dam­age. Infes­ta­tions that occur once grain is fill­ing (milky-hard) will result in grain that is heav­i­ly spot­ted with feed­ing injury to the seed coat and lighter in weight.

more severely damaged grain as a result of prolonged feeding

More severe­ly dam­aged grain as a result of pro­longed feed­ing

Sorghum grain showing early damage from RGB feeding

Sorghum grain show­ing ear­ly dam­age from RGB feed­ing

Heat can also impact on sorghum heads caus­ing dark­ened and pinched grain. This type of dam­age may be con­fused with RGB feed­ing dam­age. The pres­ence of feed­ing spots on the seed is char­ac­ter­is­tic of RGB dam­aged grain.

Sorghum head that has not sustained RGB damage during early grain fill, but is now infested with adults; showing full grain and no spotting of seed

Sorghum head that has not sus­tained RGB dam­age dur­ing ear­ly grain fill, but is now infest­ed with adults; show­ing full grain and no spot­ting of seed

IMG_3108

Sorghum head that has had sus­tained RGB feed­ing from ear­ly grain fill, show­ing char­ac­ter­is­tic spot­ting of the seed

Sampling

RGB are typ­i­cal­ly clumped in their dis­tri­b­u­tion. Typ­i­cal­ly, when beat­ing indi­vid­ual heads in a buck­et, you will find heads with no, or very few RGB and oth­ers with hun­dreds. RGB release a chem­i­cal that results in aggre­ga­tions of indi­vid­u­als.

When sam­pling it is use­ful to con­sid­er:

  • The pro­por­tion of heads infest­ed, and whether nymphs are present
  • The age/maturity of infest­ed heads
  • The dis­tri­b­u­tion of infes­ta­tions
    • are there ear­li­er areas of the crop that are more heav­i­ly infest­ed?

Infes­ta­tions are typ­i­cal­ly ini­ti­at­ed by move­ment of adults into the crop (often with winds asso­ci­at­ed with storms), so the youngest heads in the crop are at great­est risk of yield loss from pro­longed RGB infes­ta­tion.

Nymphs

RGB females lay eggs in the sorghum heads once they have fed on devel­op­ing grain. RGB eggs and nymphs devel­op rel­a­tive­ly slow­ly. Eggs take around 4–5 days to hatch and then the nymphs will take 2–3 weeks to devel­op from 1st instar to adult. This is why nymphs are gen­er­al­ly only seen in matur­ing heads. Heavy infes­ta­tions of nymphs will move up and down on the plant, feed­ing on both the leaves, stem and in the seed heads. Very small nymphs can­not dam­age seed, but larg­er nymphs (3rd-5th instar) can cause dam­age. Nymphs do not appear to move from plant to plant, so unin­fest­ed heads near heav­i­ly infest­ed ones will not auto­mat­i­cal­ly be infest­ed.

Threshold

If the crop has pre­dom­i­nant­ly adults present, and infest­ed heads are sus­cep­ti­ble to dam­age, use the rec­om­mend­ed eco­nom­ic thresh­old:

  • Flow­er­ing to soft dough:  20 bugs per head
  • Hard dough to phys­i­o­log­i­cal matu­ri­ty to har­vest: RGB have no impact on yield at these stages.

Large pop­u­la­tions of adults and nymphs are usu­al­ly seen in heads that are start­ing to colour, through to har­vest.

Deci­sions about whether these lat­er infes­ta­tions war­rant treat­ment should take into account:

  • The stage of grain matu­ri­ty (phys­i­o­log­i­cal­ly mature (i.e.black lay­er) seed is not at risk)
  • Grain that is still soft dough:
    • treat if adult num­bers are over thresh­old
    • if adult num­bers are below thresh­old but nymph num­bers above thresh­old, then treat­ment is war­rant­ed, but can be delayed if nymphs are small. This may allow time for the crop to reach phys­i­o­log­i­cal matu­ri­ty.
  • The poten­tial for large pop­u­la­tions to cause har­vest and deliv­ery issues (clog­ging, excess mois­ture, deliv­ery of live insects).

Control options

Rein­fes­ta­tion by adults is com­mon in RGB. Check­ing treat­ed fields at 2–3 days after spray­ing will help in deter­min­ing if the treat­ment was effec­tive. Longer re-check inter­vals may be too long to dis­tin­guish between poor effi­ca­cy and rein­fes­ta­tion.

When RGB den­si­ties are extreme, it is chal­leng­ing to get high lev­els of con­trol. Keep in mind that 90% con­trol of 400 RGB per head will still leave 40 RGB per head. Nymph pop­u­la­tions can often be much high­er than this.

The chal­lenge of good con­trol is exac­er­bat­ed by large pop­u­la­tions of nymphs that may be mov­ing up and down the plant, and more dif­fi­cult to con­tact direct­ly with insec­ti­cide. For this rea­son, insec­ti­cides with some resid­ual may be more effec­tive against large, mobile pop­u­la­tions of nymphs.