Harmonia axyridis, a colorful beetle that is turning into an invasive species (Image: Wikipedia Commons)

When the Europeans discovered the "New World", they infamously brought with them diseases which that world had never before encountered. Infectious agents like smallpox, typhus and cholera were generously shared with the local population - often deliberately so - and were responsible for significantly decimating the natives' numbers. It's a common theme; a species colonizes an ecosystem occupied by another and whether by accident or design, spreads invasive pathogens which quickly overwhelm the untrained immune system of the native species. Whether it's humans or ladybugs, invasive pathogens have always been a reliable weapon to bring about genocide.

And speaking of ladybugs, something similar is being observed with the colorful species named Harmonia axyridis. Everyone likes beetles - I regularly caught them and observed their life cycle as a teenager - but it seems they don't necessarily like everyone. At the beginning of the twentieth century, a species of ladybug named Harmonia was introduced from Russia and Central Europe into parts of Europe and the United States as a pest control agent. Farmers relished its ability to destroy aphids and other scale insects which fed on valuable crops. Thought to have disappeared in the 1920s, Harmonia reemerged in Louisiana in the late 80 and was encouraged to spread elsewhere. Not surprisingly, since then the Law of Unintended Consequences has taken over and assured collateral damage. While the ladybugs are taking care of invasive aphids, they have turned invasive themselves, killing off several other species of ladybugs and insects and especially a dominant species named Cocinella which is an important part of ecosystems.

In order to find out what was killing the native species, researchers from the University of Giessen took samples of the clear fluid called hemolymph that is secreted from the bugs' legs. Chemical separation and analysis revealed the presence of a molecule which was named harmonine. Harmonine is a simple compound; chemists would recognize it as a diamine. It seemed the puzzle was solved; not only was harmonine thought to kill off Cocinella, but it most intriguingly proved fatal to the tuberculosis bacterium and the malarial parasite. In a theme that has been gratifyingly repeated throughout history, a chemical weapon from an insect's armamentarium gave rise to a promising lead against human disease.

Except for one glitch. While the hemolymph itself did kill the Cocinella ladybugs, pure, synthetic harmonine made in a laboratory did nothing to them. The implication was clear; there must be something else in the hemolymph besides harmonine that was destroying Harmonia's competitors. In a recent report in Science the same group has found the culprit. An obligate fungus - technically called a microsporidium - proliferates throughout the hemolymph. The fungal spores cause no harm to Harmonia probably because through time Harmonia has acquired resistance to them. In fact Harmonia seems to have made its peace with the fungus so well that the researchers located it in both the eggs and larvae of the beetle.

The fungus is catastrophic to Cocinella, however. Harmonia's eggs and larvae kill Cocinella competitors when they hungrily snack on them. Injection of fungal spores into the rival beetle causes death within two weeks. This time the authors were careful to eliminate other factors; injection of hemolymph in which the microsporidia was eliminated caused no ill effects, confirming the causal role of the fungus. In addition the researchers found the dying Cocinella beetles swarming with fungal spores (poor bastards).

The study shows that over the ages Harmonia has started to wield a pathogenic fungus like a finely honed biological weapon. One interesting aspect which the researchers don't explore in this study is whether the co-evolution of the beetle and fungus began when it was introduced widely as a pest control agent or whether the two have been friends from early on in Harmonia's evolutionary history. But whatever the history between these two symbionts, it's clear that nature has been using biological weapons since much before we ever thought of putting anthrax spores in an envelope.

Reference: Invasive Harlequin Ladybird Carries Biological Weapons Against Native Competitors, Vilcinskas et al. Science, 2013, 340, 862.