Insect pest management in sorghum – a refresher

Sorghum is most sus­cep­ti­ble to crop loss from insect pests dur­ing flow­er­ing and grain fill, and this arti­cle deals most­ly with these species (sorghum midge, heli­cov­er­pa and Ruther­glen bug).

How­ev­er, some years there may be pest infes­ta­tions in veg­e­ta­tive sorghum.

Vegetative sorghum

Dur­ing the veg­e­ta­tive stages you may see a few army­worm or heli­cov­er­pa lar­vae caus­ing shot-holes in the leaves, but this dam­age is gen­er­al­ly just cos­met­ic. Corn aphids tend to dis­ap­pear by the time heads have emerged; they are very vul­ner­a­ble to pre­da­tion and par­a­sitism, par­tic­u­lar­ly once they are no longer pro­tect­ed with­in the whorl.

armyworm damage veg sorg

Ear­ly leaf dam­age by army­worm

shot holes_sorg

Cater­pil­lar shot-hole dam­age does not usu­al­ly impact on yield

Helicoverpa damage to developing grains

Heli­cov­er­pa dam­age to devel­op­ing grains

Corn aphid typically colonise the whorl until the head emerges

Corn aphid typ­i­cal­ly colonise the whorl until the head emerges

On rare occa­sions, large heli­cov­er­pa lar­vae can move from leaf feed­ing (or feed­ing on weeds) up onto heads at late flow­er­ing-ear­ly grain fill. Because these lar­vae are unex­pect­ed, and larg­er than can be effec­tive­ly con­trolled by NPV (Vivus®), they can cause sig­nif­i­cant dam­age. Thor­ough and reg­u­lar crop mon­i­tor­ing will pick these up. Large heli­cov­er­pa lar­vae tend to graze on fill­ing grains, and the dam­age can be a clue to their pres­ence.

Reproductive sorghum

Mon­i­tor for Heli­cov­er­pa, sorghum midge and Ruther­glen bug (RGB) from head emer­gence.

Key points:

  1. Egg par­a­sitism in heli­cov­er­pa can be very high, par­tic­u­lar­ly in fields that have not been sprayed
  2. John­son grass abun­dance in late spring can be an indi­ca­tor of like­ly midge pres­sure in ear­ly crops.
  3. Lat­er plant­ed sorghum is at greater risk of midge attack as it is exposed to larg­er pop­u­la­tions that have built up in ear­li­er crops
  4. Sorghum is most sus­cep­ti­ble to crop loss from RGB dur­ing ear­ly grain fill. Seed crops are par­tic­u­lar­ly vul­ner­a­ble as feed­ing reduces both yield and seed via­bil­i­ty.
  5. It is crit­i­cal that RGB adult num­bers are record­ed dur­ing flow­er­ing and grain fill. In some sea­sons, repeat­ed immi­gra­tion of RGB adults can make con­trol deci­sions dif­fi­cult.

More detailed infor­ma­tion on pest sam­pling and man­age­ment options (on this page):

See also the Beat­sheet video on Sam­pling insects in sorghum.


Helicoverpa

Monitoring egg lays

The major­i­ty of heli­cov­er­pa egg lay­ing occurs on heads as they emerge, and before they start flow­er­ing. Eggs can be detect­ed by shaking/beating/spinning sorghum heads into a buck­et to look for heli­cov­er­pa eggs. Whilst egg counts are not used in the eco­nom­ic thresh­old deci­sion, they are an indi­ca­tion of the lev­el of moth activ­i­ty, and poten­tial­ly sub­se­quent lar­val pres­sure. Be aware that many of the eggs laid will not pro­duce dam­ag­ing lar­vae.

Eggs are high­ly sus­cep­ti­ble to pre­da­tion by preda­to­ry bugs (damsel bugs, pirate bugs), ants and bee­tles. Eggs are also fre­quent­ly par­a­sitised by the tiny Tri­chogram­ma wasp. Par­a­sitised eggs turn black and are eas­i­ly dis­tin­guished from new­ly laid eggs (white) or eggs with devel­op­ing lar­vae (brown ring). Egg par­a­sitism can be very high, par­tic­u­lar­ly in fields that have not been sprayed.

Trichogramma para vs heli develop_Scholz

Unpar­a­sitised vs par­a­sitised eggs (image by Brad Scholz)

Small lar­vae can be par­a­sitised by larg­er wasps like Micropli­tis, which kill the lar­va before it gets large enough to do sig­nif­i­cant dam­age.

microplitis heli

Micropli­tis cocoon beside heli­cov­er­pa lar­va from which it has emerged


 Sampling for larvae

Beat/shake/twirl five sorghum heads into a buck­et. Count the num­ber of lar­vae, cat­e­goris­ing the lar­vae by size (less than 7 mm is the crit­i­cal size for a well tar­get­ed NPV treat­ment). Repeat at a num­ber of sites across the field.

Control of helicoverpa

Nucle­opoly­he­drovirus (NPV)  is the most wide­ly used con­trol option for heli­cov­er­pa in sorghum. It is high­ly effec­tive, and has the ben­e­fit of pre­serv­ing ben­e­fi­cials, which then con­tribute to the sup­pres­sion of sur­viv­ing heli­cov­er­pa and aphids in the crop. Peak con­trol is 4–9 days after appli­ca­tion, and ongo­ing infec­tion usu­al­ly con­tin­ues through the dura­tion of the crop.

To deter­mine whether the heli­cov­er­pa infes­ta­tion is like­ly to cause sig­nif­i­cant eco­nom­ic loss, and treat­ment is war­rant­ed, use the thresh­old cal­cu­la­tor [https://thebeatsheet.com.au/economic-threshold-calculators/].

Getting the best possible result with NPV

Timing

Apply NPV sprays before 50% brown anthers, par­tic­u­lar­ly if the spread of flow­er­ing is large. By going ear­ly, the ear­ly flow­er­ing heads will be ful­ly pro­tect­ed and sec­ondary infec­tion will con­trol most cater­pil­lars on the late flow­er­ing heads. Tar­get cater­pil­lars less than 7 mm in length. Apply NPV when lar­vae are active­ly feed­ing (25–35°C) to ensure rapid inges­tion.

Water quality and volumes

Water used in spray mix­es should have a pH of 7. Alka­line water will seri­ous­ly reduce the per­for­mance of NPV, so buffer water with Li700 or equiv­a­lent to neu­tralise pH.

For high-vol­ume, water-based sprays, a min­i­mum of 30 L water/ha is rec­om­mend­ed for aer­i­al appli­ca­tion, and 100 L water/ha for ground rig appli­ca­tion.

Coverage

NPV must be ingest­ed to be effec­tive. Achiev­ing good cov­er­age means pay­ing par­tic­u­lar atten­tion to water vol­umes, noz­zles, oper­at­ing pres­sure, weath­er con­di­tions. Spread NPV over as much of the head as pos­si­ble to ensure cater­pil­lars have a high chance of pick­ing up a lethal dose as they feed.

Other options for control of helicoverpa in sorghum

When well tar­get­ed, NPV his­tor­i­cal­ly per­forms very well on grain sorghum, usu­al­ly achiev­ing greater than 90% con­trol with­out the use of addi­tives. How­ev­er, if lar­val pres­sure is extreme, even 90% con­trol may not reduce the lar­val pop­u­la­tion below the eco­nom­ic thresh­old (e.g. 10 larvae/head with 90% con­trol still leaves 1 larva/head, which may be above thresh­old).

So is there anoth­er prod­uct that will do a bet­ter job? The answer is ‘No’. All heli­cov­er­pa in sorghum are H. armigera, with high lev­els of resis­tance to car­ba­mates and syn­thet­ic pyrethroids. The use of these prod­ucts is like­ly to pro­duce a result infe­ri­or to NPV, and impact adverse­ly on ben­e­fi­cial pop­u­la­tions.

Other considerations

Weath­er, par­tic­u­lar­ly tem­per­a­ture, can slow the rate at which lar­vae suc­cumb to NPV. If lar­vae don’t seem to be dying as fast as expect­ed, con­sid­er whether the dai­ly aver­age tem­per­a­ture has been cool­er than nor­mal.

On aver­age, at 30°C, it takes 4.5 days after infec­tion for a 6-day old cater­pil­lar to die. This com­pares with 6.2 days at 25°C and 7.5 days at 20°C. Below 20 C, it may take longer than 8 days for lar­vae to start dying.

Heliconsumptiongraph

Con­sump­tion of arti­fi­cial diet by healthy and NPV-infect­ed 6-day old cater­pil­lars and time to death from NPV at three tem­per­a­tures (Data from Chris Mon­sour).

Note that although infect­ed lar­vae do not die imme­di­ate­ly, their con­sump­tion (and thus poten­tial dam­age) declines sig­nif­i­cant­ly.

For tech­ni­cal infor­ma­tion on the use of NPV vis­it the AgBitech web­site.


Sorghum midge

Female sorghum midge laying eggs in sorghum floret.

Female sorghum midge lay­ing eggs (image by Chris Free­bairn)

The first gen­er­a­tion of sorghum midge occurs in John­son grass, with midge mov­ing to sorghum in ear­ly sum­mer. The abun­dance of John­son grass through Novem­ber and ear­ly Decem­ber can be an indi­ca­tor of like­ly midge pres­sure, par­tic­u­lar­ly in ear­ly crops.

Female midge lay eggs into the flower spikelet and the lar­va feeds on the devel­op­ing ovary, pre­vent­ing nor­mal seed from devel­op­ment. The life­cy­cle of a midge takes just over 2 weeks, so it is pos­si­ble to have more than one gen­er­a­tion in a crop that has flow­er­ing stag­gered over this length of time. Lat­er plant­ed sorghum is at greater risk of midge attack as it is exposed to larg­er pop­u­la­tions that have built up in ear­li­er crops.

Varietal resistance

One of the main tools avail­able for man­ag­ing sorghum midge is midge resis­tance vari­eties. The table below shows a rat­ing com­par­i­son of poten­tial yield loss under low (1/panicle) and high (3/panicle) midge pres­sure.

sorghumratings

Eco­nom­ic val­ue of midge dam­age for a range of sorghum midge rat­ings

These yield loss esti­mates assume that (i) spray­ing results in a 100% kill, (ii) there is no midge dam­age pri­or to chem­i­cal appli­ca­tion, and (iii) aver­age midge pres­sures remain the same over 4–5 days. In real­i­ty, research has shown that one well-timed insec­ti­cide for midge (put on from pan­i­cle emer­gence and before midge even enter the crop) will still only pre­vent 70–80% dam­age pro­tec­tion in low­er-rat­ed sorghum hybrids. In 8 rat­ed hybrids, yield loss­es can be reduced by over 90%.

Midge rating for current commercial grain sorghum varieties

Vari­etyCom­pa­nyMidge Rat­ing
HGS-102Her­itage Seeds7
HGS-114Her­itage Seeds6
Crac­kaNuSeed3
Rip­paNuSeed5
EnforcerNuSeed6
Liv­er­tyNuSeed4
TigerNuSeed3
Dom­i­na­torNuSeed5
MR BusterPacif­ic Seeds4
MR Baz­leyPacif­ic Seeds4
MR Tau­rusPacif­ic Seeds6
MR Scor­pioPacif­ic Seeds6
MR Apol­loPacif­ic Seeds7
84G22Pio­neer4
85G33Pio­neer6
85G44Pio­neer4
Agi­ta­torRadi­cle Seeds4
BrazenRadi­cle Seeds5

Sampling for midge

Often the first sign that midge are active is midge caught in spi­ders’ webs in the field.

Gen­er­al­ly, peak midge activ­i­ty occurs between 9 and 11 am, and this is the best time to look. Mon­i­tor for midge over 10 metres of row in at least 4 dif­fer­ent loca­tions in your crop.

Changes in weath­er can bring midge into a field from sur­round­ing areas (John­son grass, ear­li­er sorghum crops) at any time of day. Midge num­bers can vary wide­ly both with­in a crop and between plants; thor­ough sam­pling is crit­i­cal to esti­mate midge abun­dance. Mon­i­tor crops dai­ly whilst sus­cep­ti­ble.

Sorghum heads are most attrac­tive to midge at mid flower. It is not uncom­mon to see dou­ble or triple the num­ber of midge on pan­i­cles at ear­ly-mid flower com­pared with the end of flow­er­ing. At low­er midge den­si­ties, adult flies will move around and lay almost exclu­sive­ly on the flow­er­ing por­tion of the pan­i­cle.

Midge flies are only 1–2 mm long, and it is very easy to under­es­ti­mate midge num­bers if you are not care­ful. The eas­i­est way to ‘get your eye in’ is to look at the top half of mid flow­er­ing pan­i­cles and look for MOVEMENT of the small red flies against a still sorghum pan­i­cle look­ing from side on and slight­ly above side-on one sec­tion of the sorghum pan­i­cle at a time. Keep your eyes focused over a cou­ple of branch­es of flo­rets for sev­er­al sec­onds at a time to detect female midge walk­ing around the branch or bob­bing up and down prob­ing their ovipos­i­tor into each flo­ret. On windy days you may have to hold each head still and shel­ter the pan­i­cle with your body.

As the sea­son pro­gress­es, you may also start to see the black midge par­a­sitoid, Eupel­mus spp. Whilst it can be present in rea­son­ably large num­bers, this par­a­sitoid does not occur ear­ly enough to pre­vent midge from caus­ing dam­age. It can also be con­fused with the midge, so be sure to look for the red­dish abdomen of the midge, not just lit­tle black ‘flies’.

Managing midge in sorghum

Insec­ti­cides only kill the adult midge as they move about the crop and do not kill the eggs or hatched lar­vae that are already present inside the sorghum flo­rets. While midge adults only live for one day, they do most of their egg lay­ing (and sub­se­quent dam­age to the crop) in the morn­ing. Use the eco­nom­ic thresh­old cal­cu­la­tor for sorghum midge to deter­mine if treat­ment is war­rant­ed.

Syn­thet­ic pyrethroids and car­ba­mates are the only effec­tive prod­ucts cur­rent­ly reg­is­tered for sorghum midge con­trol. Note that they are high­ly dis­rup­tive to ben­e­fi­cial insects that pro­vide con­trol of heli­cov­er­pa eggs and lar­vae, aphids and Ruther­glen bug.


Rutherglen bug

Ruther­glen bug (RGB) reduce grain yields by feed­ing on the devel­op­ing seed. Crops are most sus­cep­ti­ble to crop loss if RGB are present through ear­ly grain fill. Once grain reach­es phys­i­o­log­i­cal matu­ri­ty, RGB impact is neg­li­gi­ble. Sorghum seed crops are par­tic­u­lar­ly vul­ner­a­ble to RGB dam­age as feed­ing not only reduces yield, but sig­nif­i­cant­ly reduces the via­bil­i­ty (ger­mi­na­tion) of seed.

It is crit­i­cal that RGB adult num­bers are record­ed if they are detect­ed whilst sam­pling for midge and/or heli­cov­er­pa dur­ing flow­er­ing and grain fill. In some sea­sons, repeat­ed immi­gra­tion of RGB adults can make deci­sions about the need for, and tim­ing of con­trol, quite dif­fi­cult.

Recent research on the effi­ca­cy of insec­ti­cides on RGB con­trol has shown that syn­thet­ic pyrethroids have good knock­down con­trol, but lim­it­ed resid­ual effi­ca­cy. Tar­get­ing RGB adults ear­ly, before they lay eggs may give a bet­ter con­trol out­come than wait­ing until there are larg­er pop­u­la­tions of adults and nymphs. RGB females start to lay eggs once the sorghum starts fill­ing grain. RGB adults and nymphs will be inci­den­tal­ly con­trolled by appli­ca­tions of insec­ti­cide for sorghum midge.

RGB damage

RGB feed­ing direct­ly on devel­op­ing and fill­ing grain reduces sorghum yield and qual­i­ty by reduc­ing grain weight and allow­ing entry of com­mon fun­gi and bac­te­ria that fur­ther dete­ri­o­rates the grain. Infes­ta­tions of RGB present from flow­er­ing 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.

Sorghum grain showing early damage from RGB feeding

Sorghum grain show­ing ear­ly dam­age from RGB feed­ing

more severely damaged grain as a result of prolonged feeding

More severe­ly dam­aged grain as a result of pro­longed 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

Developing sorghum head showing signs of RGB damage; undeveloped seed, and spotting on developing seed.

Devel­op­ing sorghum head show­ing signs of RGB dam­age; unde­vel­oped seed, and spot­ting on devel­op­ing seed.

 

Sampling

RGB are typ­i­cal­ly clumped in their dis­tri­b­u­tion. 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 and age/maturity of heads infest­ed
  • Whether nymphs are present
  • Infes­ta­tion dis­tri­b­u­tion: 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 leaves, stem, and 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.

Thresholds

If the crop has pre­dom­i­nant­ly adults present, the eco­nom­ic thresh­olds are:

  • Flow­er­ing to soft dough:      20–25 bugs per head
  • Hard dough to har­vest:         no impact on yield.

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).

For 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.

Also con­sid­er 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 deter­mine 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 thus more dif­fi­cult to con­tact direct­ly with insec­ti­cide. For this rea­son, insec­ti­cides with some resid­ual may be more effec­tive against large, mobile pop­u­la­tions of nymphs.