Queensland, Australia
March 30, 2012

Soybean loopers have taken to southern Queensland soybean and sunflower crops, defoliating plants and potentially reducing yields.

Dr Melina Miles, Department of Employment, Education and Innovation (DEEDI) principal entomologist says reports of caterpillars causing defoliation have been coming in for several weeks but there is some confusion over identification.

“The caterpillar seen on sunflower crops recently has been identified as the soybean looper, and is the same species reported on a couple of weeks ago defoliating soybeans,” Dr Miles says.

“There are two colour forms of the soybean looper, although the colour differs, they are both the same species and behave the same way.”

Dr Miles says distinguishing loopers from Helicoverpa lies in the in way loopers move along the plant and videos of this can be viewed at DEEDI’s blog, The Beat Sheet at www.beatsheet.com.au.  

“The way the larvae move is an important clue,” Dr Miles says.

“Loopers loop – as they move along they draw their rear legs right up to the front, resulting in the body forming a loop.

“Examining the larvae closely, you can see that loopers have three pairs of small true legs at the front, then a big gap without legs, and then two pairs of ventral prolegs at the rear (not counting the anal prolegs at the very rear of the larva).”

She says Helicoverpa move without raising their bodies far above the leaf, rather they move with a “slinking” action.

Helicoverpa larvae have three pairs of small true legs at the front, and four pairs of ventral prolegs at the rear.

“There is no local trial data on looper damage to sunflowers or looper thresholds because severe looper infestations occur so rarely,” Dr Miles says.

“In the US, looper control in sunflower is recommended if defoliation reaches 25 per cent of leaf area, and larvae are still small and the crop flowering or grain filling.

“When defoliation exceeds 50pc yield losses were incurred.”

Growers are urged to take the following recommendations from Grains Research and Development Corporation (GRDC)-supported research into account when making decisions:
• Powdery mildew data shows yield loss will occur if the top third of leaves is not retained through budding and grain fill.
• One assessment of Helicoverpa defoliation of sunflowers shows 20pc defoliation did not result in yield loss.
• Loopers will consume 80pc of their total food intake in the final two instars before pupating. Consequently, the rate of defoliation will speed up as larvae reach maturity.
• Loopers do not feed on the buds or developing heads in sunflower
Large larvae (25-40 millimetres) will be close to maturity and likely to stop feeding and pupate within a few days.

If control is warranted, loopers are susceptible to Bt (Dipel®) up to about 15mm in length, Dr Miles says.

“If the bulk of the population is larger than 15mm long, Bt may not control the population effectively, particularly where there is heavy pressure, such as more than 20 larvae per square metre.

“However, at least 20pc defoliation can be tolerated with no yield loss, so there is no need to achieve near perfect control to protect crop yield.”

Deltamethrin is the only other registered insecticide for looper control in sunflower.

Dr Miles says loopers NPV (Vivus Max®) will not control loopers as it is only active against Helicoverpa.

Check larvae on leaves and on the heads to see if there are Helicoverpa present as Helicoverpa only require control if there are more than 20 larvae per head, she says.

“Good coverage is important, particularly as the loopers are likely to be concentrated on the undersides of leaves.”

For more information, visit www.thebeatsheet.com.au or www.grdc.com.au/pestlinks.