Nice smells are more appealing than bad smells. Many people spend large amounts of time in the shower to reach this end, adding on expensive lotions and perfumes to top it all off. We sniff appreciatively when a nice odour wafts our way – and wrinkle our noses at stinky ones. Smell is also crucial to our sense of taste. It turns out that humans are not alone in our emphasis on smells.
New research has found that smell plays an intriguing role in the lives of certain insects as well. While it is well established that odour is important to finding mates and food, this is a new twist on the topic. The larval form of certain large blue butterflies (Genus Maculinea) is a brood parasite. Like some birds, famously the cuckoo, these caterpillars trick a host into accepting them. This is an unusual strategy in insects, although many other butterflies also in the Family Lycaenidae have some relationship with ants.
Once in the ant nest, caterpillars are cleaned and fed while the host larvae are abandoned. In this case, the hosts are red ants of the Genus Myrmica. These ants accept the caterpillars because the caterpillars mimic the smell of their ant hosts. Some of this smelliness is due to plant foods. The butterfly eggs are first laid on specific plants – for the Alcon blue butterfly it is the marsh gentian – and the first phases of the caterpillar feed on flowers and seeds. But the fourth instar chews through its flower home, lowers itself with a silk thread, and shifts to the ground. There it waits to be adopted by an ant colony. This happens when a passing red ant of the right species picks up the still-tiny caterpillar and returns home. The caterpillar then spends the remainder of its larval phase inside the ant nest, where it is fed and also may eat ant larvae. This takes place over the autumn, winter, and following spring – while the caterpillar balloons up to about 12 mm long. It then pupates in the nest. The emerging adult butterfly must leave quickly, as it lacks the immunity of the caterpillar.
It also helps that these caterpillars tend to have thicker cuticles, to prevent being harmed by ant bites. They also show modified movement patterns, so that ants are not cued into behaving aggressively despite the familiar smell. And, amazingly, they make noises that resemble the ants’ own communication, further attracting caretaking attention.
Over evolutionary time, it is possible for parasitized species to change their species-typical smell, thereby thwarting any parasitic attempts. However, if there is interbreeding with other ant groups, this resistance can be lost if the other ants are not also hosts. The study did find that some ant colonies distant from gentian patches had no parasitism – and no resistance to being parasitized.
Oddly, the caterpillar faces its own possible parasitic fate. An ichneumon wasp (Ichneumon eumerus) seems to home in on the presence of caterpillars in the ant nest. It enters the nest, creating its own smelly signal that confuses the ants. It then injects its own eggs into the caterpillar. While the caterpillar is pupating, it serves as the food for larval wasps. Instead of a butterfly, a wasp leaves the pupa.
The Alcon blue is facing population declines due to habitat degradation. These butterflies do not move around very much, and their preferred environment is heathland, such as damp meadows and bogs, which tend to be low in nutrients. Also, host plants are also less available. Populations across Europe, from the Netherlands to Turkey, are at very low levels and the species is listed as endangered. The peculiarities of its development may not make it easy for populations to rebound, as the species prefers the marsh gentian and requires the ant care phase. However, the interaction between ants, butterflies, and evolution is a fascinating glimpse into the intricacies of nature.