Experts beetle away on scarab research
Grains Research and Development Corporation (GRDC) funded research is paving the way for more effective control of scarabs across the northern cropping belt.
While reports of scarab damage have tended to be sporadic and associated with wetter seasons, in certain areas, persistently affected paddocks have suffered up to 80% loss in sorghum crops prompting on-going research into a variety of potential control methods.
Scarabs feed on roots, impacting on plant growth and ability to tolerate moisture stress, which is visible as slowed crop growth, plant death (often in patches), delayed maturing and lodging.
Scarabs generally have a one to two-year lifecycle which can be longer if growing conditions are unsuitable such as if it is too dry or food sources are inadequate. This means larvae can be present in fields for 12 months of the year.
Recent in-field trials have focussed on cultivation, comparing the impact of a single offset disc and chisel plough on scarab larvae densities, as well as insecticide treatments at sowing, both in-furrow treatments and seed treatments.
Queensland Department of Agriculture and Fisheries (DAF) principal entomologist Dr Melina Miles said both the disc and chisel plough cultivation trials resulted in a significant reduction in larvae numbers but also resulted in a full disturbance of the soil surface and correspondingly high reductions in soil moisture.
“The impact of cultivation on soil moisture is a major impediment to the potential uptake of cultivation to manage high density infestations,” Dr Miles said.
“However it may be possible to be more targeted with cultivation and achieve the same outcome. Examination of the distribution of larvae across the plant row and inter-row shows a concentration of larvae on the plant row in the majority of fields.
“This pattern of distribution opens up the possibility of more targeted tillage with reduced disturbance.”
Trials assessing the efficacy of in-furrow and seed treatments have also been undertaken given that there is little information on the impact of these treatments on the scarab larvae that are causing damage to establishing crops.
Dr Miles said although preliminary results suggested that insecticide treatments at sowing could potentially provide some crop protection, it was also probable that the insecticides would deter larvae from the zone in which they are active but as roots grow out of the treated zone they could then be damaged by larvae.
“It is also possible that the use of seed or in-furrow treatments will have little longer term effect on the density of the population, essentially locking growers into a program of using these treatments to ensure a crop,” she said.
“Techniques for evaluating the efficacy of seed dressings, in-furrow treatments and possibly side-dressed insecticides are under development which will allow for evaluation at different times post-application to test the longevity of the treatments and their potential toxicity to scarab larvae.”
Work is also continuing into the definitive identification of larvae species – at least four species have been collected from fields in the northern region and it’s believed that the most common species is Othnonius batseii, the black soil scarab.
At the same time, research teams are undertaking additional investigation into the within-field distribution of scarab larvae which will be critical to developing a reliable monitoring technique for growers and agronomists.
“In the absence of a defined threshold, observations of crop loss and infestations indicate that densities above 15 larvae per square metre (to 15cm depth) are associated with significant crop loss,” Dr Miles said.
Dr Miles encouraged growers and advisors to monitor for scarab larvae during the growing season and if larvae are found, contact her at DAF Toowoomba on 13 25 23 or via email.
DAF Manager (Media)
13 25 23,
Sarah Jeffrey, Senior Consultant Cox Inall Communications
0418 152 859,
GRDC Project Code DAQ00196