As the summer cropping season winds down, there are some key observations in pulse crops made by leading consultants from New South Wales’s Northern Rivers and Queensland’s Hervey Bay regions that haven’t been covered in recent Beatsheet articles.
1: Indoxacarb failures linked to poor coverage
The high Helicoverpa pressure encountered this summer resulted in many indoxacarb (e.g. Steward®) sprays being applied to soybeans in the Casino region, with varying degrees of success.
While the initial reaction was “This must be due to resistance”, closer examination of the spraying protocols used revealed this not to be the case. In all reported cases, spray failure could be linked back to poor nozzle selection, and/or low spray water volumes.
Indoxacarb (Group 22) is an ingestion insecticide, as are chlorantraniliprole (Group 28—e.g. Altacor®), emamectin benzoate (Group 6—e.g. Affirm®), and the biopesticides Bt and helicoverpa viral products. For ingestion products to be effective, thorough spray coverage is essential.
Spray volumes should be at least 100 L/ha for ground applied sprays, and 30L/ha for aerially applied sprays (see individual labels for any specific exceptions), and a medium droplet size applied (as opposed to the coarse to very coarse droplets specified for herbicides).
Therefore, correct nozzle selection is paramount (note: air-induction nozzles are often linked to poor pest control when using ingestion insecticides). For more information, refer to GRDC’s Nozzle selection guide.
2: Late soybeans hit by lucerne crown borer
Late planting rains resulted in a lot of late (post-Christmas) soybean plantings around the Northern Rivers.
In a normal season, late plantings would be equated with a low lucerne crown borer (LCB, Zygrita diva) risk, as it is usually the earlier planted crops that are most heavily infested. As a result, even in regions with a history of significant LSB activity, these crops were not treated with fipronil at planting (seed dressing or in-furrow sprays under permit).
Some late crops have subsequently experienced significant LCB damage (30% plant girdling), with plants being girdled before the pods were filled. This scenario may have been exacerbated by the dry finish to the season, as the LCB response to a crop drying down is the same as a crop approaching harvest maturity; i.e. they girdle the stem, before pupating in the tap root. The earlier this girdling occurs during podfill, the smaller the seeds in pods at harvest, and greater the yield loss.
While nothing can be done to control larvae in the stems, management strategies such as late irrigations may slow the drying down of the crop, and will delay the onset of girdling.
To the obvious question of “Should all late plantings be seed treated?” the answer is no, as in most years with late spring/early summer rain, it will be the earlier crops that are greatest risk, and in most years, the cost of seed treatments or in-furrow sprays for late crops will not be recouped. The other reason not to routinely treat late crops is to reduce any potential fipronil residues.
For more information on crown borer see previous Beatsheet articles:
- Don’t let lucerne crown borer overwinter in your soybean stubble
- Lucerne crown borer on the move in soybean in 2015
3: No sprays needed in many Hervey Bay/Maryborough soybean crops
Soybean crops within the Hervey Bay and Maryborough regions had a particularly active beneficial insect population this season which resulted in very few sprays.
Natural enemies that were particularly active included predatory bugs and parasitic wasps keeping helicoverpa, loopers and grass blue butterfly larvae in check. Soybean aphids were visible in many crops but were held at sub-threshold levels by ladybirds.
Podsucking bug (PSB) populations were also low, possibly due to the dry start to the season, a factor that may also have reduced PSB numbers in other regions compared to previous seasons.
All in all, the ‘no need to spray’ scenarios were good for farmers’ wallets, for the regions beneficial insects, and for the region’s go softer image.