Finally we have a new preventative for the infuriating gall wasp that has been decimating our citrus, lemon trees in particular, across Victoria.
‘Overhaul’ is an organically* rated kaolin clay (used in papermaking and ceramics) and has been used in broad-acre agriculture to reduce heat stress and sunburn in tree and horticultural crops (e.g. tomatoes) for 18 years; in that time an unexpected secondary benefit has become apparent: the fine coating of clay resulted in less insect damage to crops. It is hypothesised that the clay works in a variety of way depending on the insect: repelling, reducing egg laying, impeding grasping, restricting movement, altering behaviour, inducing paralysis and mortality, and camouflaging the plant. Whichever way it works, trials by the NSW Dept. of Agriculture in the Riverland and Sunraysia have found it significantly reduces the incidence of galls (from Citrus Gall Wasp) in their citrus trees. Both number and size of galls are reduced (70-90%).
Using ‘Overhaul’ on your citrus trees will help reduce the incidence of gall wasp attack, in addition, its primary function is to reduce heat stress in the tree (the white clay reflects the heat). This stops the shutdown of photosynthesis and fruit drop in response to heat stress, and allows the tree to continue to grow and do well in hot weather. Just remember that by still photosynthesising, it will be using water at a higher rate than if it had shut down to prevent water loss, so water the root zone well.
Do not spray when flowering, as you don’t want to interrupt the action of the insect pollinators. Most lemon trees have their major flower bloom in early spring, so there is no conflict – but some lemon trees (e.g. Eureka), flower sporadically during the year, so you will need to take care to avoid spraying the flowers.
Spray from multiple directions to get an even coverage and spray to point of run off – called ‘near drip’ in North America. Do first spray around September / October and repeat monthly (at half strength for repeat sprays) to maintain coverage. Make sure trees are well covered in mid to late November** when hatchings from the galls are high.
We are hoping that the coverage will also deter leafminer attack and white fly, but have no specific information on that yet. In the USA Kaolin Clay is used to deter Codling Moth and Oriental Fruit Moth, Leafhoppers, Powdery Mildew, Stink Bugs, and Thrip, so we see multiple benefits for it in the home produce garden. It is completely chemically inert, so safe to use on apples to prevent codling moth and sun scorch, just wash the clay off after harvest.
‘Overhaul’ is available from the nursery in 500g or 1kg packs which should last several years – it has almost infinite shelf life.
*Organically rated in USA and Canada by OMRI
** This date is Melbourne specific
Photo 2: NSW Department of Primary Industries