Why is Harmonia axyridis the Culprit in Coccinellid Biting Incidents? an Analysis of Means, Motive, and Opportunity

Abstract

The tendency of aphidophagous coccinellids to bite humans is a subject that has seen very little study in the scientific community. With the introduction of adventive species, anecdotal reports of biting lady beetles have greatly increased in frequency and seemed to warrant further investigation. Coccinellids of three native species [ Coccinella novemnotata Herbst, Coccinella trifasciata L., and Adalia bipunctata (L.)] and the two most common introduced species [ Harmonia axyridis (Pallas) and Coccinella septempunctata L.] were collected in the wild and reared separately in laboratory colonies. In caged biting trials, 40 individually marked coccinellids of a single species that were given either no source of food or water or aphids ad libitum were exposed to the experimenter's hand and forearm and the movement, bite frequency, and severity with which they bit into the skin were recorded. All species tested were shown to bite humans. Feeding treatment did not have a significant effect on biting propensity or bite duration in most species. Large adventive species such as Coccinella septempunctata were distinguished by a lower propensity to bite than the smaller native species. All species except H. axyridis and C. novemnotata exhibited a startling blood-feeding behavior that is not yet clearly understood and was heretofore undocumented in this family of beetles. In two trials affording A. bipunctata and C. novemnotata adults a choice between cotton wicks soaked in cow blood or wicks soaked in water, both species preferred blood, with Adalia bipunctata preferring blood to a much greater extent. Furthermore, survivorship of hematophagic coccinellids was tested as well by monitoring the survival and weight change across individuals afforded the following five treatments: 21 aphids per day, 3 aphids per day, 3 aphids per day in addition to blood, blood only, and water only. Coccinellids given 3 aphids per day exhibited very low mortality with only three dead individuals, while the treatment given the same number of aphids in addition to bovine blood exhibited a significant drop in survivorship very similar to that of the treatment given only blood. This remarkable trend is likely a result of oxidative stress and other hardships involved with feeding on vertebrate blood. Based on the level of mortality of coccinellids that overwinter within a building (the location of greatest exposure to humans) and their lack of adaption to human blood and tissue feeding, we predict that the currently rare behavior of biting humans should decline.