The Beat Sheet » soybeans

Managing Helicoverpa softly in vegetative soybeans

admin — Wed, 13 Jan 2010 22:51:38 +0000

While soybeans are more likely to be attacked by helicoverpa from flowering onwards, severe infestations can occur as early as the seedling stage. In recent years, severe early infestations have been reported on the Downs and in the coastal Burnett with the affected crops experiencing significant yield losses.

The latest published thresholds for helicoverpa in vegetative soybeans (Rogers and Brier, 2010) show that while soybeans can tolerate damage inflicted by moderate helicoverpa populations up to 7 larvae/m2 without yield loss, severe yield loss is inflicted by populations >7 larvae/m2 at a rate 4-5 times greater than during the pod-fill stage. The conundrum therefore is that while vegetative soybeans are far more tolerant of low to moderate helicoverpa populations (< 7/m2) than podding soybeans, they are markedly less tolerant of populations > 7/m2 than are pod-filling soybeans (see Figure 1).

The reason for this severe yield loss is that unlike most leaf-feeders such as loopers, helicoverpa also attack the plant’s auxiliary buds and vegetative terminals, completely destroying these structures.

Damage to auxiliary buds potentially reduces yield as these structures are the precursors to the plant’s flowers and (subsequently) pods. Damage to vegetative terminals is potentially bad for yield. The reason for this is that while plants may compensate by setting additional side branches, pods formed on these are often closer to the ground and are more difficult to harvest.

Where helicoverpa populations are not excessive, damage is spread over a number of plants and the crop is able to recover and compensate without yield loss.

However, once populations exceed critical level in vegetative crops (about 7/m2), damage per plant reaches a critical level beyond which the subsequent plant growth is severely affected.

To reduce the risk of severe early helicoverpa damage, the sampling guidelines have been revised to “sample crops twice weekly from the seeding stage onwards”. This more-intense sampling regime maximises the chance of helicoverpa larvae being detected while they are still small (ideally <7 mm) and thus able to be controlled with a Helicoverpa virus biopesticide, eg VivusMax or Gemstar.

Remember that the use of biopesticides in pre-flowering soybeans is a key in the “Go Soft Early” strategy to minimise the risk of silverleaf whitefly (SLW) attack in soybeans. The “Go Soft Early” strategy also promotes the build up of beneficial insects attacking other pests such as helicoverpa and loopers.

Vegetative soybean crops can tolerate populations up to 7 larvae/m2 and it is not necessary to kill every helicoverpa larva in a crop. Assuming only 70% control, even populations as high as 20 larvae/m2 can be reduced to below the critical 7 larvae/m2 level in vegetative crops.

Very small plants have fewer nodes and hence fewer auxiliary buds and a given helicoverpa population will damage a greater proportion of auxiliary buds per plant. Severely damaged plants will also be more susceptible to subsequent helicoverpa attack.

Note that small larvae often feed in leaf terminals, so inspect these and look for the tell tale damage symptoms, small holes in the leaflets and frass. For the latest guidelines about applying helicoverpa virus, refer to the previous blog of 8th January 2010. In seedling and early vegetative crops, pesticide costs can be halved by banding the spray over the crop, and blocking off nozzles over the bare inter-row.

References:
Rogers D.J. and Brier H.B. (2010). Pest-damage relationships for Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on vegetative soybean. Crop Protection 29(1): 39-46.
Rogers D.J. and Brier H.B. (2010). Pest-damage relationships for Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) on soybean (Glycine max) and dry bean (Phaseolus vulgaris) during podfill. Crop Protection 29 (1): 47-57.

Article by Hugh Brier. Images by Hugh Brier and Joe Wessels

What is eating my soybean pods?

admin — Fri, 06 Mar 2009 04:22:00 +0000

Field crickets
High numbers of field crickets have been reported across the Darling Downs in the last couple of weeks, with some large aggregations of adults attracted to lights at night. Are these crickets doing damage to crops? The answer may well be yes.

. Field crickets are generally a pest of pastures but can also attack soybeans, cotton, sugarcane and sunflowers. There are two species commonly found – the brown field cricket and the slightly larger black field cricket. The brown field cricket is the most prevalent at present. Crickets hide during the day in cracking soils or under clods of dirt or crop residue and emerge at night to feed on crops. They usually feed on seedlings but high populations in late summer may feed on more mature plant structures such as sunflower heads and soybean pods.

How do I know that crickets are damaging my soybean pods?
Crickets are not the only pest of soybean pods. Mice can also do considerable damage to pods, with soybeans often the last of the summer crops to mature and as a consequence the only source of food on offer for mice.

Cricket adults and large nymphs chew into pods to reach the seeds but this damage looks similar to damage done by mice. So how do you determine what pest is doing the damage?

The best time to check for crickets is to inspect crops at dusk or later into the night when crickets are most active. Field cricket activity can also be monitored with light traps. Another way to determine whether crickets are present is by using hessian bags placed out overnight at regular intervals across the paddock. In the morning check for the presence of crickets sheltering under the bags.

The best way to determine whether mice are damaging the crop is to go out at night to check for their presence or use mouse bait cards (as described on the DPI&F website).

Mice damage to soybeans can be an ongoing and costly problem as soybeans provide good groundcover for mice and excessive grain losses immediately prior to or during the harvesting operation are likely to increase mouse populations given an ongoing food source. This will also impact on early follow-on cereal crops such as wheat and barley.

For further information on how to manage mice in your crop please visit the DPI&F website through the link provided below:

http://www.dpi.qld.gov.au/cps/rde/dpi/hs.xsl/4790_8283_ENA_HTML.htm

How do I control the crickets if they are damaging my soybeans?
Past experience has shown that foliar insecticide applications do not provide control of crickets. They shelter by day and are found low down under dense canopy at night, making spray contact difficult.

Chlorpyrifos-treated cracked grain baits are registered for cricket control in soybeans, but this is mainly used to protect seedling crops. The bait is prepared by mixing 100 mL chlorpyrifos (500 g/L EC formulation) and 125 mL sunflower oil, and adding this to 2.5 kg of cracked wheat or sorghum/ha. The bait is broadcast evenly by air or ground.

Article by David Murray and Kate Charleston

Soybean Moth causing major damage in soybeans

admin — Thu, 12 Feb 2009 22:16:00 +0000

Soybean moth Aproaerema simplexella is a very common pest of soybeans but is usually only present in very low numbers. This season high numbers of moths and caterpillars were found especially in the Wide Bay Burnett region with some fields sustaining extensive damage. Similar leaf miners attack many horticultural crops, but are species other than A. simplexella. Weed hosts of soybean moth include emu foot (Psoralia tenax)

Identification
The soybean moth is a small narrow winged moth, up to 6 mm long. The forewings are dark brown/grey, each with a white bar across them, and pale brown hind wings. In soybean crops with large infestations, moths can be seen flying up from the foliage when disturbed.

Small elongated eggs are laid on both the top and underside of leaves, generally near leaf veins. Larvae of soybean moth are pale green/grey and grow up to 7 mm (in length).

Soybean moth infestations are favoured by hot, dry weather, with crops under severe moisture stress most at risk. Large populations can cause extensive damage by stripping all leaves from crops and so reduce photosynthesis and grain production.

Monitoring and thresholds
Monitor crops regularly for the early warning signs of rare plague events. Look for numerous small, pale patches (leaf-mining) on the leaves and large numbers of soybean moths in the crop or around lights at night.

DPI&F plans to investigate the effectiveness of a number of pesticides registered in soybeans (against other pests,) as part of GRDC-funded pulse IPM research project DAQ00086. The hope is to identify at least a moderately selective pesticide to preserve soybean moth parasites such as Temclucha sp., a small Ichneumonid wasp (8 mm). This species has been observed in very high numbers in some crops infested with soybean moth.

Article by Kate Charleston and Hugh Brier

The indicative threshold is based on defoliation, with. 33% pre flowering and 15-20% defoliation during early to mid pod-fill.

Control
In most years control is not required but large infestations in the Bundaberg region will need chemical control to prevent total crop loss. Check thoroughly before spraying, as larvae may have already pupated (as black pupae within the webbing) or reached full size (7 mm) and stopped feeding.

There are no specific registrations for the control of soybean moth. However pesticides effective against Helicoverpa (except Helicoverpa virus and Bt), and targeting that pest in soybeans will most likely also give reasonable control of soybean moth.

Other caterpillars that mine and web soybean leaves include soybean leafminer Lithocolletis aglaozona, a much smaller and less abundant species, and legume webspinner Omiodes diemenalis, common in coastal regions, but which is much larger (up to 15 mm) and is bright green. Legume webspinners also web leaves together, rather than mining leaves (feeding inside them).

Similar leaf miners also attack many horticultural crops, but are species other than A. simplexella. Weed hosts of soybean moth include emu foot (Psoralia tenax).

Damage
Larvae initially feed inside leaves (i.e. in leaf mines) and emerge after approximately four days to feed externally, folding and webbing leaves together to form a protective shelter. Infested leaves are often crinkled and pulled together in the middle and this together with webbing of leaf is the most obvious symptom of damage. In low numbers the larvae only cause cosmetic damage. While larvae normally feed on leaves only, extremely high populations will also graze on the surface of pods, after they have denuded the crop of leaves.

New Helicoverpa thresholds in vegetative soybeans

admin — Thu, 15 Jan 2009 01:13:00 +0000

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.lds 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.
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.
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.
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)

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 thresho

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.

Until data to the contrary is available, the 33% defoliation vegetative threshold is still valid for

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.