The firebug, Pyrrhocoris apterus, is a common insect of the family Pyrrhocoridae. Easily recognizable due to its striking red and black coloration, but may be confused with the similarly coloured though unrelated Corizus hyoscyami (cinnamon bug, squash bug) (see comparison). Pyrrhocoris apterus is distributed throughout the Palaearctic from the Atlantic coast of Europe to northwest China. It has also been reported from the US, Central America and India. It has been reported as recently expanding its distribution northwards into mainland UK and eastward on to the coast of the Mediterranean sea.  They are frequently observed to form aggregations, especially as immature forms, with from tens to perhaps a hundred individuals.
Firebugs generally mate in April and May. Their diet consists primarily of seeds from lime trees and mallows (see below). They can often be found in groups near the base of lime tree trunks, on the sunny side.
They can be seen in tandem formation when mating which can take from 12 hours up to 7 days. The long period of copulating is probably used by the males as a form of ejaculate-guarding under high competition with other males.
P. apterus was the subject of an unexpected discovery in the 1960s when researchers who had for ten years been rearing the bugs in Prague, Czechoslovakia attempted to do the same at Harvard University in the United States. After the 5th larval instar, instead of developing into adults, the bugs either entered a 6th instar stage, or became adults with larval characteristics. Some of the 6th instars went on to a 7th instar, but all specimens died without reaching maturity.
The source of the problem was eventually proven to be the paper towels used in the rearing process; the effect only happened if the paper towels were made in America. The researchers could replicate these results with American newspapers such as the New York Times, but not European newspapers such as The Times. The cause was found to be hormones found in the native balsam fir tree (Abies balsamea) used to manufacture paper and related products in America, and in some other North American conifers. This hormone happened to have a profound effect on P. apterus, but not on other insect species, showing the diversification of hormone receptors in the insects. The most potent chemical component was later identified as juvabione, the methyl ester of todomatuic acid, which is produced by the trees in response to wounding; it mimics juvenile hormone closely at the chemical level, defending against vulnerable pests.
A firebug feeding on mallow seeds (Hibiscus syriacus).
- Brickfields Country Park: Cinnamon Bug – Corizus hyoscyami
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