In my home town of Dalby, Queensland there exists a monument in a small park by Myall Creek that has always seemed curious to me: it is a cairn dedicated to the cactoblastis cactorum (a moth originally found in Argentina).
Here are the details about it (from the Dalby Council website):
CACTOBLASTIS MEMORIAL CAIRN
A cairn was erected in Marble Street in 1965 to record the indebtedness of the people of Queensland (and Dalby in particular) to the Cactoblastis Cactorum. This tiny moth saved the Darling Downs from infestation by an introduced plant, the Prickly Pear.
A single, yellow flowering prickly pear was brought to Australia in 1839. By 1925 over 50 million acres of land in Queensland and New South Wales were covered with prickly pear, the greatest example known to man of any noxious plant invasion. The Dalby District was then heavily infested. It was impossible to effectively eradicate the weed either by sprays or cultivation. The land was rendered unusable and drove many from their farms.
The first eggs of the Cactoblastis Cactorum moth were imported from Argentina early in 1925 and were bred in very large numbers and liberated throughout the prickly pear territory. Within 10 years the insect had destroyed all the dense mass of prickly pear.
The cairn is located on Myall Creek as a lasting monument to the Cactoblastis Cactorum and its victory over the prickly pear menace.
For a long while I had wondered whether it was the only monument in the world to an insect, although I have subsequently learnt about the Boll Weevil Monument in the town of Enterprise, Alabama (which is, as you can see from the link, a grander monument than the humble cairn in Dalby).
The prickly pear cactus apparently was orginally brought to Australia with the First Fleet, as host cacti for the cochineal insect, exploited to produce a distinctive red dye (highly prized in Europe in the early 19th century and used, amongst other things, for the the British Red Coats). There was a strong economic imperative for the British Empire to establish an alternative source for cochineal dye at the time of colonisation, as it was produced solely in Mexico (which was under Spanish control). [Incidentally cochineal dye is still used today as a food colouring, E120]
Unfortunately, as the history of Australia has demonstrated several times, using introduced species can be a blessing and a curse, as cactoblastis cactorum is now poised to wreak havoc in Mexico. Following the successful Australian example of biological pest control, the moth has been used around the world to eradicate infestations of cacti on agricultural land, including in the Caribbean, which allows a short hurricane-blown trip to the Mexican shore… where there is plenty of nice, juicy cactus for caterpillars to chew on. The saviour has become the pest.