An infestation of Rutherglen bugs that may have ridden recent storms into the northern grains region is causing headaches for growers from northern NSW to Central Queensland.
Dr Melina Miles, Queensland Department of Agriculture, Fisheries and Forestry (DAFF) says large numbers of Rutherglen bugs have migrated into cropping regions since early November.
“While the exact origin of the bugs is unknown, it is likely they are being carried on storm fronts from inland regions where they have bred up over winter and spring on native host plants,” Dr Miles said.
“Large numbers are affecting seedling establishment, simply by weight of numbers feeding on the emerging seedlings.
“In some instances the seedling crops are invaded by large numbers of nymphs walking out of weedy fallows into establishing crops.”
Dr Miles says ploughing a deep furrow between the seedling crop and the source of bugs, or a border spray may be sufficient to prevent ongoing infestation.
She says sorghum is vulnerable to Rutherglen bug from flowering to soft dough stage.
Grains Research and Development Corporation (GRDC)-supported research shows sorghum crops infested during flowering will fail to set seed, and infestations at milky dough stage will result in seed covered in small, dark feeding wounds.
Dr Miles says developing grain affected by Rutherglen bug feeding is light in weight, with poor germination. Under wet conditions, fungal and bacterial infections further degrade Ruthergllen bug-damaged grain, infecting through feeding wounds on the seed.
She says growers should monitor sorghum crops from booting to milky dough stage.
“This sampling can be done at the same time as checking for Helicoverpa,” Dr Miles said.
“Shake the sorghum head into a bucket and count the number of bugs present.
“Repeat for at least 10 heads from across the field and average the Rutherglen bug count.
“Rutherglen bug infestations can vary greatly between heads so it is important to take samples from the earliest flowering sections of the field as these will incur damage first.”
Management recommendations for Rutherglen bug in sorghum include:
- Flowering – milky dough: control warranted if more than 20 bugs per head
- Soft dough: 20-25 bugs per head
- Hard dough – physiological maturity: Rutherglen bug has no impact on yield at this stage
Dr Miles says there are two critical periods during which Rutherglen bug control may be necessary to prevent crop loss in sunflowers:
- 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. These heads do not continue to develop and set seed normally.
- Flowering and seed fill: eggs are laid in the florets when they open, 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,” she says.
“Preventing the subsequent population of nymphs is the key driver of Rutherglen bug control prior to petal drop.
“Feeding on developing seeds causes yield loss, and a loss of oil content and quality of grain.”
Dr Miles says reinfestation is a real possibility, with continuing influxes of Rutherglen bug into crops in December and January.
“It is likely that spring crops will require multiple sprays to keep populations below threshold and prevent the build up of large populations of nymphs through the grain filling stage.
“Being very clear on what critical stages need to be protected will enable better targeting of RGB populations.”
For more details visit the Beat Sheet blog at http://thebeatsheet.com.au and for more information on GRDC-supported research, visit www.grdc.com.au/pestlinks.
Caption: Growers are urged to be on the lookout for Rutherglen bug.
Media releases and other media products can be found at www.grdc.com.au/media
Dr Melina Miles, Principal Entomologist & Team Leader (Grains Lead)
Ph: 07 4688 1369
Rachel Bowman, Cox Inall Communications
(07) 3846 4380
0412 290 673