Why Is That Bump on the Stem Alive? An Insect That Gave Up Moving
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Tiny insects that settle on a plant, lose their legs and cover themselves in wax look like part of the bark. Two of them coloured empires and varnished the world's furniture.
What the animal gives up
The young hatch as mobile crawlers with legs and eyes, disperse over the plant or on the wind, and then settle at a feeding spot and insert a long mouthpart into the plant to drink from its sap. At that point the female of most species undergoes a drastic simplification, losing her legs, eyes and antennae, and secreting a protective covering of wax or a hardened shield that she lives beneath for the rest of her life. She never moves again. Males in many species develop differently, becoming small winged insects with no functioning mouthparts that live a day or two, find a female and die.
Why they are hard to deal with
The lifestyle produces a pest that resists ordinary control:
- •The waxy or shelly covering repels contact sprays entirely
- •Only the mobile crawler stage is really vulnerable
- •They feed inside the plant's transport system, out of reach
- •Many reproduce without males, so one individual starts an infestation
- •They travel worldwide on nursery stock, looking like bark
- •They excrete sugary honeydew that grows sooty mould on the leaves
The two that changed economies
Two species were commercially enormous. Cochineal, farmed on cactus in Mexico and Central America, yields a brilliant and stable red dye from the bodies of the females, and after the Spanish conquest it became one of the most valuable exports from the Americas, second only to silver, with the source kept secret for two centuries and the trade defended against espionage. Lac insects in India and southeast Asia secrete a resin in such quantity that it can be scraped from branches, and processing it gives shellac, which varnished furniture, sealed documents, insulated early electrical equipment and was the material of gramophone records until vinyl replaced it.
How they are controlled
Because sprays fail against the covering, control relies on other approaches and the successful ones are instructive. Timing a treatment to the crawler stage, which is brief and can be predicted by accumulated temperature or detected with sticky tape around a branch, is the standard chemical approach. Horticultural oils smother the insects by blocking the pores through which they breathe, which works through the covering where a poison does not. Biological control has some of its greatest successes here, most famously a ladybird introduced to California in 1888 that eliminated a devastating infestation of citrus within two years and is still cited as the founding case of the whole method.
The relationship with ants
Many species live in a working partnership with ants that is well documented and easy to observe. The insects excrete honeydew containing far more sugar than they need, since they must process a great deal of sap to obtain enough protein, and ants collect it directly from them, in some species stroking them to prompt the release. In exchange the ants drive off ladybirds, parasitic wasps and other predators, clean away excess honeydew that would grow mould, and in some cases carry the insects to fresh parts of the plant or take them into the nest over winter. Controlling the ants is therefore a recognised way of controlling the insects, since an unprotected colony is quickly eaten.
The takeaway
Females settle, insert a mouthpart into the plant and then lose their legs and eyes under a wax or shell covering, never moving again, which is why sprays fail and only the mobile young stage is vulnerable. Cochineal supplied a red dye second only to silver among Spanish American exports, and lac insects supplied the shellac that varnished furniture and made gramophone records. Ants defend colonies in exchange for honeydew.