A conqueror extraordinaire among bugs
This map represents a fascinating thing I learnt today: as of June 2015, the harlequin ladybird (Harmonia axyridis) had been found in 59 countries outside of its native range (11 countries more than the last time anyone checked, which was in 2011). That’s a lot of new ground for a bug to have expanded into naturally, including landmasses separated by oceans.
Obviously, it had help. According to this study, whose authors also produced the map, humans either accidentally imported the bug from different locations (e.g. with foodstuff) or deliberately introduced it into certain ecosystems so the bug could invade them and displace other nuisance-bugs. But come 2016, and the harlequin ladyird has itself become a nuisance-bug – mostly to native Coccinellidae (the same family as all ladybugs) and particularly to the two-spot ladybug. According to a study published in 2000,
Ironically, many biotic invasions are apparently facilitated by cultivation and husbandry, unintentional actions that foster immigrant populations until they are self-perpetuating and uncontrollable. Whatever the cause, biotic invaders can in many cases inflict enormous environmental damage: (1) Animal invaders can cause extinctions of vulnerable native species through predation, grazing, competition, and habitat alteration. (2) Plant invaders can completely alter the fire regime, nutrient cycling, hydrology, and energy budgets in a native ecosystem and can greatly diminish the abundance or survival of native species. (3) In agriculture, the principal pests of temperate crops are nonindigenous, and the combined expenses of pest control and crop losses constitute an onerous “tax” on food, fiber, and forage production. (4) The global cost of virulent plant and animal diseases caused by parasites transported to new ranges and presented with susceptible new hosts is currently incalculable.
In May 2013, Ed Yong had written for Nature about how the harlequin ladybird gets to be so deadly:
Some scientists had previously thought that the harlequin owed its success to harmonine, a toxic antibacterial chemical found in its blood (or haemolymph). Harmonine allows the harlequin to resist certain diseases and to poison native ladybirds that eat its eggs. However, Vilcinskas and his team found that high concentrations of harmonine do not kill seven-spot ladybirds — but a dose of the harlequin’s haemolymph does. When Vilcinskas and his colleagues looked at the harlequin’s haemolymph under a microscope, they found the culprit: a microsporidian parasite. These exist in the eggs and larvae of all harlequin ladybirds, but in a dormant and apparently harmless state.
Per another study published in 2007, in six locations sampled within a 40-hectare area at the North Florida Research and Education Center, Monticello, the number of predator-bugs had reduced from sixteen to two in nine years. Researchers attributed this dramatic decline exclusively to the fact that the harlequin ladybird had been introduced in these areas at the start of the period. They wrote:
Following its arrival, H. axyridis appears to have become the dominant predator and populations of the [yellow pecan aphid complex] and native predator species were dramatically reduced. No native coccinellids, syrphids, mirids or anthocorids were detected in these “after” samples. Brown or green lacewing larvae or adults were rarely recorded in pecan. Only spiders and reduviids, some species of which likely engage in intraguild predation on H. axyridis, were present in any detectable numbers.
Introducing one species to displace another from the latter’s native range makes the former a bio-control agent. Such agents are considered ‘classical’ if they are natural enemies of the target species (but not necessarily in all environments). Ecologists are generally favourable towards the use of classical agents, especially if they are selected and introduced carefully. There have been many success stories wherein bio-control agents have been used to suppress target populations so humans continue to have access to natural resources they consider valuable. However, as the authors of the 2000 study have acknowledged, the “prevention of invasions is much less costly than post-entry control”, such as when an agent is introduced that doesn’t specifically suppress one other species but goes after a large number of them. The finest example of this is Australia’s cane-toad experiment. Even CRISPR/Cas9 offers no easy way out.
Featured image: A harlequin ladybird. Credit: spacebirdy/Wikimedia Commons, CC BY-SA 3.0.
