New Helicoverpa thresholds in vegetative soybeans

The new economic threshold for Helicoverpa in vegetative soybeans is 8 larvae per sqare metre and replaces the old 33% defoliation threshold. The new threshold is based on field trials conducted by John Rogers (formerly with DPI&F at Kingaroy). These field trials show that approximately 7.5 larvae per square metre can be tolerated with no yield loss, but that severe yield losses can occur once this critical population (the inflection point) is exceeded.

The new threshold (8 larvae/m2) is based on the maximum number of larvae that can be tolerated before there is an economic reduction in yield. The closeness of the threshold and the inflection point is a measure of the severity of the yield losses that can occur once this critical population is exceeded.

Previous thresholds were based on the maximum defoliation (33% and widely cited in the scientific literature) that can be tolerated without reducing soybean yield. In John Rogers’ trials, Helicoverpa populations equivalent to the new threshold (8/m2) inflicted significantly less than 33% defoliation. Note that the threshold may be influenced by crop size, with fewer larvae tolerable in very early or very small crops, and more larvae acceptable in larger more vigorous late-vegetative crops.

The reason yield loss occurs below 33% defoliation is because of Helicoverpa’s feeding behaviour - they are not called budworms for nothing. As well as feeding on leaves, they also feed on the soybean plant’s vegetative terminals and auxiliary buds, the latter which are the precursors to floral buds.

Previous vegetative thresholds allowed for vegetative terminal loss (tipping) with 25% terminal loss the cited critical level above which action was required. The new thresholds are below the old terminal-loss guidelines as populations of 8 larvae/m2 destroyed fewer than 25% of terminals in John Rogers’ trials.

The crop’s ability to tolerate 7.5 larvae/m2 during the vegetative stage without yield loss, means that Helicoverpa nucleopolyhedrovirus [NPV] (e.g. VivusMax®) can still be safely used prior to flowering, provided it targets appropriately small larvae (<7 mm long). This is because NPV only has to keep populations below this critical level, rather than achieving ≥90% control that would be required if yield loss commenced as soon as populations exceeded 0/m2.

Immediate intervention with a more robust larvicide may be required against extremely high populations (e.g. > 20/m2). While indoxacarb (Steward®) could be used at this stage, only one application is allowed per field per crop growth cycle, and this product is best saved for later in the season when it is most needed.

Until data to the contrary is available, the 33% defoliation vegetative threshold is still valid for
loopers and cluster caterpillars which are primarily foliage rather than bud feeders. However, cluster caterpillars are more likely to attack soybean pods than loopers, but not as savagely as Helicoverpa.

John Rogers’ studies illustrate the link between a pest’s feeding behaviour and its impact on crop yield. The studies also highlight the importance of having ‘species specific’ data, and that a ‘one threshold model fits all’ approach is not always appropriate. Further trials are planned to study the feeding behaviour and damage potential of cluster caterpillars and all the major looper species attacking soybeans. However, such detailed research is likely to take at least 3-4 years to complete.


Helicoverpa damage in soybeans

A- vegetative damage
B - damage to terminals results in
C - reduction in pods and yield








Article by Hugh Brier (DPI&F Kingaroy), John Rogers (formerly DPI&F Kingaroy and Kate Charleston (DPI&F Toowoomba)

Bring on NPV against grubs on grain sorghum

Every year caterpillars of the corn earworm (helicoverpa), Helicoverpa armigera, cause losses to sorghum crops. Regular inspection during flowering is important to detect caterpillar infestations and properly time control measures.

Pre-flowering heads of grain sorghum are very attractive to egg-laying moths of the corn earworm. On any individual head, most eggs are laid prior to the start of flowering, as indicated by the presence of yellow anthers.

By the end of flowering, when brown anthers are present at the base of the head, eggs will have hatched and most larvae will be less than 7 mm in length.

A timely spray application of the naturally occurring biopesticide NPV (nucleopolyhedrovirus) remains the best control option for grain sorghum crops under attack from corn earworm.


NPV performs exceptionally well on grain sorghum, with well timed sprays usually achieving greater than 90 per cent control while leaving beneficial parasites and predators to mop up survivors.

If the spread of flowering in a crop is large, it may be better to spray earlier rather than wait until 50% of the crop is at the brown anther stage. This is because caterpillars on the earliest flowering heads may be larger than the ideal size to target with NPV, and they will cause some damage if not adequately controlled with NPV.

Research has shown that early application of NPV creates a disease outbreak and secondary NPV infection will control most caterpillars on late flowering heads.

Other issues to ensure good results with NPV

Water quality
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.


Water volumes
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, so the challenge is to achieve good coverage of the target. This means paying particular attention to water volumes, nozzles, operating pressure, weather conditions, etc. You want to 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 on the head.

Additives
Additives such as Amino Feed, etc. are not recommended when NPV is applied to grain sorghum.

Paying attention to the detail will ensure the best results from NPV.

Article by Dr. Dave Murray


New Beat Sheet contributors

The beat sheet blog team has been expanded and includes two new contributors, Kate Charleston and Zara Ludgate.

Kate is the development extension officer with the entomology team in Toowoomba. Her role is to provide information about IPM in field crops as well as training to growers and industry in managing insect pests according to IPM principles.


Kate joined entomology in June 2008 and has previously worked as a research scientist, agronomist, plant health inspector and extension officer. She started her career with the Department of Primary Industries in Tasmania and joined the Queensland Government in 1999 as an extension officer in a sugar project at South Johnstone in Far North Queensland. Kate has worked with sugarcane, cotton and pulse crops and has considerable training and extension experience.






Zara has just started her career in entomology research. She comes from a rural background and completed a degree in plant and soil science at St Lucia, Brisbane in 2007. For her honours year she investigated the effect of a plant defence compound on the fitness of diamondback moth.


While in Brisbane she provided technical support for research into bio-pesticide production for helicoverpa and green vegetable bug management. She is now based in Toowoomba with the crop protection systems - entomology unit. Her current research interests include insecticide resistance in whitefly and integrated pest management in grain crops.