I&I NSW entomologist Craig Maddox says macadamia growers are generally surprised by the level of damage fruit spotting bugs can do when the shells have hardened later in the season.

Macadamia nut shells are like a two millimetre wall of concrete when mature, but that doesn’t stop fruit spotting bugs from using a gift of nature to penetrate the rock-hard shell and damage the kernel inside.

“Growers are generally surprised by the level of damage these bugs can do when the shells have hardened later in the season,” I&I NSW entomologist Craig Maddox said.

“The bugs have a filament-thin proboscis which exudes enzymes that break down the shell over about 24 hours - leaving a tiny hole that is barely noticeable.

“It’s like using a needle to push through a slab of concrete - one of those amazing feats of nature.

“But the damage they cause to the kernel inside is significant and easy-to-see - leaving those affected nuts as reject quality.”

Mr Maddox said late season damage to macadamia crops from fruit spotting bugs (FSB) had been increasing dramatically in recent seasons and had caused losses of more than 20pc in some orchards this year.

Growers have probably been less likely to act to control FSB late in the season for three main reasons:

• The damage is hard to see and affected nuts don’t drop pre-maturely from the tree.
• Many growers find it hard to believe the bugs can penetrate the hard shell, but if you grow A series nuts or 849 variety they have no trouble getting through and it is highly likely to occur.
• The increasing use of bio-control for nut borer means treatments which used to go out late in the season - and as a secondary consequence reduce FSB populations - are no longer going out.

Mr Maddox said growers need to plan their approach to FSB control next year to limit the damage.

“At this stage, monitoring for the pest and treatment with an insecticide when bug levels become unsustainable is the main strategy,” he said.

“But we are investigating novel approaches to control fruitspotting bugs, one of the most important insect pests in a range of subtropical and tropical horticulture including macadamias, avocados and custard apples.

“We have been looking at a holistic approach to managing fruit spotting bugs through biological control, improved pest monitoring using lures and traps, companion planting and ‘soft’ sprays.

“There are some very promising signs that we can reduce the use of chemical sprays significantly and achieve sustainable long-term control with this new approach.”

Mr Maddox said a fly which parasitises fruit spotting bug nymphs and adults is a potential biological control as well as egg parasitoids.

“Both of these potential bio-controls require further research, particularly to see if they can be mass-reared for commercial release into orchards.

“Pheromone compounds which lure the pest to traps in the field could also play an important role.

“New ‘soft’ pesticides can be screened for the effectiveness as well.” Trap crops such as Murraya panniculata are favourable sites for FSB where the pest can be monitored and controlled. All these options provide avenues for improvement in the control of FSB.