My backyard has recently been invaded by countless thousands of Plague Soldier Beetles. I’ve seen them before but never in such vast numbers. A couple of trees in my backyard were covered with them. They seem to spend much of their time copulating (apparently 92% are having it off at any time) and sucking nectar from flowers. Despite their name they are quite harmless to humans and plants and even eat aphids and other garden pests such as Elm Leaf Beetles. They have some nuisance value just given their vast numbers but left alone they gradually disappear. Why they periodically appear in such vast numbers is unclear.
The PSBs release a chemical to repel their predators – I notice that, apart from some Noisy Minors, birds seem to leave them alone. This chemical repellant has anti-cancer properties that are being studied.
Update: I have been puzzling today about the standard description of the orange coloration on parts of these beetles as a warning to predators that they are poisonous. Is it a warning? Or is it, as Wallace (the cofounder with Darwin of the theory of natural selection) wrote in his letters, that bright colours signify to predators that these particular beetles are not mistaken to be of the standard edible neutral colour type? Excerpts from the Wallace letters – including these comments on the role of bright colours in insects - are in the latest New Scientist. (1849)