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.