As the name suggests the wattle tick scale is a coccid usually only found on wattles. However Persian Silk Tree is in the wider Fabaceae super family and I gather the scale can occasionally switch to infest the silk tree. Persian Silk Tree, in Canberra at least, tends to have a short life of no more than 20-30 years. If yours is at this age then perhaps it is nearing the end of its life and its defences are down allowing the wattle tick scale to thrive.