The Asian Harlequin ladybug, Harmonia axyridis, eats aphids like they’re Popplers, and it’s been repeatedly introduced into the U.S. and Europe to do exactly that. But since it was first introduced, H. axyridis has spread of its own accord, and displaced native ladybugs. This isn’t just because the Harlequin ladybug eats more aphids, or breeds faster, than the locals; it looks like part of the Harlequin’s success is due to the fact that it eats its native competition.
Although they’re known for eating aphids, most ladybugs are perfectly willing to engage in intraguild predation—that is, to eat other insects that are themselves primarily predators. Including other ladybugs. So a team at Wageningen University in the Netherlands set out to see whether H. axyridis might engage in a different kind of intraguild predation than its native competitors—do the Harlequins preferentially attack ladybugs of different species, and, when they do, are they more likely to win?
The team tested this in what they call a “semi-field” experiment, by creating encounters between ladybug larvae on individual leaves of small potted lime trees. They chose two other ladybug species, Coccinella septempunctata and Adalia bipunctata, for comparison to, and competition with, H. axyridis. Then, on the leaves of small potted lime trees, the researchers set up larval ladybug death matches.
Death matches for science, mind you.
The experiment was, basically, this: put two ladybug larvae on the same leaf, and watch what happens. The team paired up every possible combination of pairs of larvae from the three different ladybug species, so they ended with observations of each species interacting with (1) another member of its own species and (2 and 3) members of each of the other species.
In a majority of these larval ladybug death matches, the paired larvae didn’t actually interact; either they failed to come into contact before the experiment timed out (the researchers gave the larvae 1000 seconds to start rumbling) or one or both larvae jumped off the leaf or crawled back onto the nearest branch. Across all the different possible species pairings, the larve actually interacted in between 23 and 43 percent of the trials.
However, when the larvae did manage to make contact … they also didn’t attack each other that frequently. Out of hundreds of trials, some of the larval pairings only resulted in one or two aggressive interactions. Most of the time the larvae failed to react, or just turned around and departed the leaf. So, okay, “Larval Ladybug Deathmatch” is probably not coming to next year’s reality TV lineup.
However however, out of the small fraction of trials in which the larvae did interact, and did interact aggressively, Harlequin ladybug larvae were clearly the meanest ladybugs on the leaf: when they attacked the larvae of the other species, they went for the ladybug jugular, and ate what they killed a little more than half the time. (The study’s authors use the word “predate” to describe this kind of interaction, a usage for which I do not care.) Harlequin ladybug larvae would sometimes attack members of their own species, but they never ended up eating them.
In comparison, the other two species hardly engaged in any aggression, and the team recorded only two instances of ladybug-on-ladybug predation in which Harlequin larvae weren’t the predators.
So the authors conclude that Harlequin ladybugs successfully invaded Europe and North American, in part, by eating the larvae of species that would otherwise stand in their way. Based on this data set, though, it’s a bit hard for me to believe this could be a major contributor.
Even on the experimental leaves, the larvae either failed to make contact or didn’t interact aggressively most of the time. Then, there’s some reason to think that the larvae don’t come into such close contact on a regular basis when left to their own devices: the research team also tried to set up death matches by placing the larvae on different leaves of the same tree, and then never saw the larvae wander onto the same leaf. So unless ladybug larvae hang out in groups in nature—and maybe they can, if they happen onto the same aphid-ridden tree—Harlequin ladybugs are probably not chowing down on the competition very often.◼
Raak-van den Berg, C. L., H. J. De Lange, & J. C. Van Lenteren (2012). Intraguild predation behaviour of ladybirds in semi-field experiments explains invasion success of Harmonia axyridis. PLoS ONE, 7 : 10.1371/journal.pone.0040681