Imagine a species that spreads across continents — its range extending by 300 km a year. It overwinters in homes and buildings in massive numbers, and sometimes bites people. An invasion in a hospital in Austria led to the evacuation of the intensive care unit. Although the insect does eat pests, it also consumes beneficial insects and crops, including grapes, stone fruit, apples, pumpkins and berry crops. And, worst of all, this species was invited into our homes and gardens. This is the story of Harmonia axyridis, the harlequin ladybird.
The harlequin ladybird is native to Asia. The ladybug was first introduced to North America in California in 1916, but was introduced throughout the continent in the 1980s as a biological control for aphids and other soft-bodied pests. By 1994, the species was found in the wild in Canada. Wild populations are now found throughout the U.S. and Europe and parts of South America and Africa.
A team of 56 scientists from around the globe studied the beetle’s invasion and impact. The harlequin ladybird does what is expected of it— it eats aphids. Its voracious appetite doesn’t stop there. It also eats predators and parasitoids of aphids. The result is a decline in biodiversity. Native ladybird species, in particular, have suffered by predation and competition from the introduced species.
As for the aphids, their numbers can be controlled by any ladybird species including the harlequin, but scientists are concerned about the effect on native species. According to the research team, “the ‘Insurance Hypothesis’ predicts that [pest] control will, in the long term, be better achieved with a diverse array of natural enemies.” Having only one species to control a pest leads to a vulnerable ecosystem. If a disease or parasite wipes out the harlequin ladybird, aphids will become out of control. In contrast, if there are several aphid predators and one suffers a decline, the others will thrive and take over the slack. Researchers are investigating ways to control the harlequin before native species are driven out completely.
Source: The harlequin ladybird, Harmonia axyridis: global perspectives on invasion history and ecology. Helen E. Roy et al. Insect Invasions: Biological Invasions. Volume 18, Issue 4, pp 997-1044. April 2016.
- Janet Wallace