The Department of Primary Industries and Regional Development, nurseries and rose growers have been inundated with photos and specimens of roses that are suffering from all sorts of symptoms which include:
The culprit causing the damage are Chilli Thrips known as Scirtothrips dorsalis, a tiny insect less than 2mm in length. These thrips have a punch and suck action, they puncture a hole in plant cells and suck up the sap, this is what causes the mottled effect on the leaves and brown scarring on the rose buds.
Chilli thrips are most active in spring, summer and early autumn if it’s warm and dry. This is their preferred weather and ideal for breeding. Thrips are hard to see with the naked eye but if you get a plain sheet of paper and tap the leaves or flower you may see them moving on the paper. The life cycle of Chilli thrips is complete within 15 days and includes egg, 2 instar larval stages, prepupa, pupa and adult. Eggs will hatch between 2-7 days.
These pesky little insects are polyphagous; meaning they will feed on many different plants. They are now a widespread pest found in Pakistan, Japan, the Solomon Islands, Australia, South Africa, Israel, the Caribbean and America.
Before you reach for the systemic insecticide its worth knowing that they have many predators like lacewings, ladybirds, predatory thrips and predatory mites. Thrips like to be dry and undercover, so spraying the underside of your roses with a blast from the hose will help dislodge them.
An organic alternative is Nature’s Way Vegie and Herb spray or Natrasoap but gardeners will need to reapply it every two weeks. Pyrethrum is also effective as a direct contact spray. Some rose growers have resorted to Baythroid, Conguard, Confidor, or Imidacloprid but remember systemic sprays with Imidacloprid will be absorbed throughout the plant including the pollen, when bees collect the pollen they die.
By far the best way of controlling any pest or disease is prevention, spray roses in September when Chilli thrips are in their early stages of development before they grow into plague proportions.
Don’t use any oil-based sprays when temperatures are above 32 degrees as the leaf tissue on plants may burn.