Cochineals turn juices into bright colors

By Jeff Mitton

Some of the prickly pear cacti are encrusted with a tangle of waxy, white silken threads.

At first, the patches of white look like fungus or blotches of bleached, dead tissue. But closer inspection reveals pink dots and dribbles of brilliant red. An insect is hiding beneath the shroud of waxy silk, a scale insect that has colonized the cactus to suck its juices.

Cochineal, Dactylopius coccus, sucks juices from virtually all of the 200 species of prickly pear cactus. It is native to the tropical and semitropical regions of the Americas, but it is now established in Europe, India, Africa, Australia and the Canary Islands.

Adult females are sessile, for they lack appendages and are attached to the cactus pad that supplies fluids and nutrients. They cover themselves with waxy silk threads for protection against desiccation and sunlight and give birth to tiny red nymphs, or crawlers, with six legs.

Nymphs exude a waxy covering for protection, and they disperse by crawling to the edge of the pad and secreting filaments that act as a sail (similar to spider dispersal lofted by threads of silk) to carry them to nearby cacti. Adult males have legs and wings but die after mating, so they are rarely seen.

Cochineals appear to be well defended, for they hide beneath a canopy of filaments, beneath a taller canopy of cactus glochids (hair-like spines) and spines. But glochids and spines are not the deterrent to insects that they are to humans, so cochineal's primary defense is chemical. They synthesize carminic acid, which effectively deters ants and most other insects.

Carminic acid is colloquially referred to as cochineal red or carmine; it colors crawlers pink to red and makes adult females brilliantly red to deep purple. Carminic acid constitutes 17 to 24 percent of the dry weight of adult females.

No defensive structure or chemical is perfect, including carminic acid. Three predators, all larvae, are immune to the effects of carminic acid, prey upon cochineals, consume carminic acid and then use it in their own defense. But each of the predators has a different way of wielding the stolen defense.

Caterpillars of the scale feeding snout moth, Laetilia coccidivora, eat large numbers of cochineals, and although they ingest a lot of carminic acid, they do not incorporate it into their tissues. Instead, they store it in their crop and vomit it at any predator that threatens. Larvae of the ladybug Hyperaspis trifurcata eat cochineals, absorb the carminic acid into the blood system and then reflexively bleed when threatened, leaking toxic blood from their dorsal surface.

A larval fly of the genus Leucopis passes carminic acid from its gut into its blood system and then deposits it into large rectal pouches. The maggot defensively defecates its recycled, brilliant chemical defense when predators approach.

Perhaps the most dangerous predator attracted by cochineal red is mankind. For many centuries before the arrival of Columbus in the New World, Aztecs and Mayans farmed cochineals and dyed fabric with cochineal red.

When Hernan Cortes met Montezuma, he admired the vivid red of Montezuma's robe and he learned that conquered cities sent, in addition to gold and silver, cochineals as tribute to their ruler. Cortes took bags of dried cochineals home with him, and for the next 200 years Spain profited from a monopoly of the popular dye. Initially, cochineal red was used principally to dye fabrics, but today it is used most extensively to color foods and drinks.

In the early 1600s cochineal was one of the most valuable exports from the New World, so that even uneducated pirates valued it as a close third behind gold and silver on their list of coveted plunder. Cochineal farming fell off precipitously in the late 1800s when synthetic dyes became available.

Today, with the realization that some chemical dyes are carcinogenic, cochineal red is making a comeback and is being farmed in Mexico, Peru, the Canary Islands and Africa. At the same time, several highly publicized cases of anaphylactic shock and the horror of having insect derivatives in frappuccinos are creating a backlash against cochineal red.

Jeff Mitton, mitton@colorado.edu, is a professor in the Department of Ecology and Evolutionary Biology at the University of Colorado. This column originally appeared in the Boulder Camera.

Dec. 12, 2012