When I emptied a bucket of washing-up water onto the tree ferns yesterday evening, I disturbed a swarm of planthoppers. They flew around in a blue-grey blizzard and then settled on the mint bush and cordyline, which were both dry. The insects looked miffed.
Planthoppers and leafhoppers are sap-sucking bugs. They are spread over several families, including Flatidae, Membracidae and Eurymelidae. Some look like thorns. Some like miniature cicadas. (The resemblance is reflected in their family name — Cicadellidae). Others are altogether stranger. Many lantern bugs (Fulgoridae) have a strange forward-pointing extension on their heads, which was initially thought to be luminescent. It's not, unfortunately. Australia only has a few species of fulgorids and they are all outweirded by species found elsewhere.
Ants often hang around with planthoppers to harvest the honeydew that exudes from the bugs' rear ends. In Madagascar, at least five species of geckos also engage in similar behaviour. These lizards signal to planthoppers to release the sugary fluid by vibrating their heads. After 'milking' a bug for a few minutes, a gecko then approaches the next one and repeats the process. Male geckos appear to be very territorial about their honeydew farms and only allow females of the same species to visit. They will even chase away any ants that try to get in on the act.
David Attenborough's 'Life in Cold Blood' featured footage of the exchange.
Although some geckos will feed on the honeydew from scale insects, planthopper milking is only known from three genera in Madagascar (Phelsuma, Lygodactylus, Homopholis). If you live in an area with tree-climbing geckos, it might be worth watching out for this behaviour. You never know … Sadly, the neighbours' cats have killed off the marbled geckos (Christinus marmoratus) in my garden, so I won't get the chance.
Folling, M, Knogge, C & Bohme, W. (2001) Geckos are milking honeydew-producing planthoppers in Madagascar. Journal of Natural History 35(2): 279 – 284.