A sure sign of Spring in these parts is the layer of ladybug shells one inevitably encounters the first time it gets warm enough to open a window. These are not the sweet, friendly ladybugs of childhood memory: benign foe of aphids, farmer's friend. They are aliens, an introduced species with rusty orange shells intentionally released as a biological control for crop and landscape pests. Multicolored Asian lady beetles (Harmonia axyridis) are indeed very effective predators, but they also display the disconcerting ability to rapidly expand and naturalize in varied environments, an attribute that they share with invasive species. They are partial to ripening fruit, especially wine grapes.
The USDA made multiple introductions of this species in the Southeast beginning in 1979, and the lady beetle was well established in four southern states within 15 years. It is now widespread across the country, and most often encountered when it invades human space, congregating en masse to overwinter in our houses. It emits a pheromone that alerts other beetles to congregate at favorable overwintering sites - attics, windowsills and other living areas. Older homes are particularly susceptible to lady beetles seeking shelter from the storm, clustering around window panes like flies on sunny days. Swatting them produces a foul, yellow liquid that stains, and there is no insecticide currently listed to control them on fruit.
So are they invasive, or just an introduced species with certain admirable qualities but annoying habits? A quick search of the web shows a number of sites expressing concern about this species as a nuisance for people. It clearly has the ability to naturalize in native (and built) habitats, including agricultural areas, disturbed areas, natural forests, planted forests, riparian zones, scrub/shrublands, urban areas, and wetlands. It is not a restrictive predator but rather enjoys a wide variety of food sources, from garden pests to peaches - Asian lady beetles have been observed swarming spills at picnics. But to be considered invasive, it should also displace native species.
The Wisconsin Council on Invasive Species says they do.
"Multicolored Asian lady beetles have negative impacts on human health and aesthetics, on the fruit industry, and on native lady beetle populations."
Those are fighting words. The impact on human health is a surprise, but apparently there is growing concern that all those dead lady beetle shells in our homes constitute a significant allergen, comparable to cockroaches and cat hair for people with allergic sensitivities. Not surprisingly, people with Asian lady bird allergies more frequently live in rural areas. Symptoms of Asian lady beetle sensitive include cough, rhinitis, conjunctivitis and acute asthma.
As for displacing native species, the Global Invasive Species Database has this to say:
"Intraguild predation by H. axyridis is seen as a potential mechanism leading to displacement of native coccinellid species. H. axyridis appears to be a top predator in the guild of aphidophagous insects. Many studies indicate that H. axyridis uses other members of the aphidophagous guild as a food source (Koch, 2003). The intensity of predation by H. axyridis on other guild members appears to be inversely related to aphid density (Burgio et al. 2002).
In a study conducted by Koch et al. (2003) the authors found that H. axyridis can act as a potential stressor to populations of monarch butterflies (Danaus plexippus)."
So when there aren't enough aphids to go around, the Asian lady bird beetle goes after the competition. It preys on eggs of Coleoptera and Lepidoptera species, including Monarch butterflies. What preys on it, and are there sufficient natural checks on its populations in North America? Thus far, the species is rapidly expanding and now the most widespread lady bird in North America.
Whether they become as ubiquitous as dandelions but as ecologically benign, or turn out to be a major stressor on native diversity as well as a nuisance, is an answer that will likely unfold in the next decade or so. We can always look to Europe for its experience with H. axyridis. It is now widespread across much of Europe and recently invaded the British Isles.