With harvest underway across Northern NSW and the Northern Rivers, reports are coming in of Kernel Grub (Assara seminivale) damage being detected during the early stages of this season’s crop. For growers cracking nuts and finding hollowed-out shells packed with webbing and frass instead of usable kernel, it’s a frustrating and commercially costly discovery – particularly when the damage isn’t visible from the outside.
It’s worth taking a closer look at what Kernel Grub is, how it gets established and what practical steps can reduce its impact.
What Is Kernel Grub and How Does It Get In?
Kernel Grub larvae don’t bore through sound, undamaged shells. They are opportunistic – gaining entry primarily through two pathways: an open micropyle (for instance the variety 246) or through shell damage caused by other insects, particularly Macadamia Nut Borer and Fruit Spotting Bug.
Adult moths lay eggs directly onto nuts in the field prior to harvest. Once hatched, larvae enter the nut, consume the entire kernel and fill the shell cavity with a webbed mass of insect frass. Larvae grow to around 15 mm before pupating inside the damaged nut or husk and emerging as adult moths. Where infested nuts are brought into the shed, the pest can continue its life cycle in storage – making timely delivery to the processor an important consideration
What’s Being Observed This Season
Incidences of Kernel Grub have been recorded during the early part of this season’s harvest across some properties in the region – including orchards in the Lismore, Ballina and Yamba districts. While it is difficult to determine an exact cause, the damage does appear to be occurring on farms where pre-harvest cleanup operations – particularly sweeping and mulching – have been less thorough than ideal. These operations, when done well, physically remove old nuts and associated insect populations from the orchard floor before the new season’s crop begins to fall.
Observationally, orchards that maintain good hygiene through the season – and in particular those that prioritise cleanup leading into the new season, rarely experience meaningful Kernel Grub pressure. The pattern is consistent enough to be worth noting.
Orchard Hygiene Matters
Old nuts left on trees or on the orchard floor between seasons provide carry over sites where larvae can complete development, pupate and emerge as adults ready to infest the incoming crop. Out of season nut set that goes unmanaged creates a similar carry over opportunity.
Kernel Grub is, in many respects, a downstream problem. When Nut Borer and Spotting Bug are well controlled and shell integrity is maintained, entry points for Kernel Grub larvae are limited. When core pest and disease management programs are less than ideal or when pre-harvest hygiene is overlooked, the conditions for Kernel Grub to cause commercial damage are created.
Good macadamia agronomy in Northern NSW recognises that pest scouting throughout the season is not optional. Regular monitoring allows problems to be identified and addressed before they compound. Kernel Grub is a good example of a pest that rarely becomes a serious issue in orchards where pest scouting and pre-harvest management are taken seriously.
Practical Steps for Growers
Sound macadamia orchard management to reduce Kernel Grub risk includes:
- Thorough pre-harvest sweeping and mulching to remove old nuts and reduce insect carry over
- Maintaining tight control of Nut Borer and Spotting Bug throughout the season to limit shell damage
- Removing old nuts from trees and the orchard floor after the end of the harvest season
- Managing out of season nut set where practical
- Delivering harvested nuts to the processor promptly to limit post-harvest development in storage
For absentee orchard management situations where owners are not regularly on farm, these pre-harvest practices can easily be overlooked without a structured macadamia management program in place. Having a farm consultant conduct regular property visits and provide clear agricultural direction ensures that nothing critical falls through the gaps.
Worth Keeping an Eye On
If Kernel Grub damage is showing up in your Kernel Recoveries it’s worth reviewing pre-harvest hygiene practices and the broader pest management program across the orchard. In most cases, the answer lies in one or both of those areas.
Growers or farm managers across Northern NSW and the Northern Rivers wanting an independent assessment of their pest and disease management program or pre-harvest management practices are welcome to contact Allen Agri Consulting to discuss their situation. Whether you manage your own property or require professional macadamia farm management support, we’re available to help.