The flat-bodied bronze orange bugs in the photos below are also known as stinkbugs because of the vile odour of the secretion they release when disturbed. These bugs can do a lot of damage to citrus trees as they suck sap from new shoots and flower stems, causing shoots to wither, and crop losses. The secretion is very caustic and can burn foliage. Stinkbugs are commonly seen on citrus trees from winter onwards where a prolonged dry spell has caused water stress.
The young nymphs are green, (about 6 mm long) and can be found in groups on the underside of leaves. As they grow to adults, they can reach 2.5 cm in length, and change in colour through grey-green, to orange or pink-orange, to almost black. Older nymphs and adults are more obvious, clinging in groups to upper surfaces of foliage.
No organic sprays or treatments seem to be effective for these pests. We have found the safest organic way to remove them is to don some goggles or sunglasses and some long rubber gloves (to avoid the secretion burning or staining your skin), and then use a stick to knock them into a fruit tin containing some methylated spirits. It is best to de-bug your tree/s before the weather becomes too warm as, on hot days, the bugs leave the foliage and congregate around the base of the tree. The bugs will die off in hot, dry weather but by then they will have done a lot of damage.
Some gardeners use a vacuum cleaner to remove stinkbugs, but this method is only practical if your cleaner uses disposable paper bags so that you can dispose of the critters in a sealed bag, and if your vacuum cleaner and extension leads are suitable for outdoor use.
In summer, have a look under citrus leaves for stinkbug eggs. The eggs are spherical and fairly large for insect eggs (3 mm), and they are laid in rows. Remove the leaves carrying eggs and dispose of them in a sealed bag.
To prevent future attacks, make sure your citrus trees receive adequate water and are mulched to keep soil moisture more consistent. Also check that soil pH is between 6 and 7, and that your trees have received adequate complete fertiliser for best pest resistance.
The Brisbane Insects website has a lot of photos of stinkbugs (a.k.a. bronze-orange bugs) at all stages of their life cycle, including one of their mating position, and a close-up of exactly which part of their body releases the vile liquid.