The Harmonia axyridis (Pallas) or Asian ladybird invasion was accompanied by a decline in several native ladybird species, including the very common two-spot ladybird, in Belgium, England and Switzerland. Because of its predatory behaviour vis-à-vis species that feed on the same prey (i.e., aphids), the Asian ladybird could account for both the decline and its highly invasive nature. In an attempt to assess the impact of this intraguild predation on native ladybirds, a doctoral thesis was undertaken at the Biological Control and Spatial Ecology Laboratory, Brussels Free University. The predatory behaviour of H. axyridis larvae in Petri dishes and on plants was described. It was observed that in the absence of aphids H. axyridis is a predator of the eggs and all the larval stages of the two-spot ladybird. Where aphids are present, according to their density the predatory behaviour can be modified with respect to the larvae, but not the eggs. Resistance to H. axyridis predation was also investigated by exposing larvae with the dorsal spines removed to an indigenous predator to test the role of the spines as a means of physical defence against an intraguild predator. It was found that the spines significantly reduced the number of bites, thus supplementing the other means of defence used to hold their ground in resources already being tapped by other predators. In addition to these laboratory studies an original method was developed and used to monitor intraguild predation in semi-natural conditions. The method relies on the use of GC-MS to detect prey alkaloids in H. axyridis. Predation on native ladybirds was thus confirmed in the case of several species in semi-natural conditions. In view of the number of sites where predation was detected it can be considered to occur frequently. Furthermore, a single Asian ladybird larva may ingest several species of native ladybird, undeniably confirming its status as a ladybird predator. We can therefore conclude that intraguild predation by H. axyridis on native ladybird species most certainly plays a part in their decline and probably has the same impact as having to compete for resources.
The Asian ladybird, an invasive ladybird predator
Who hasn’t heard of Harmonia axyridis (Pallas), that highly polymorphous ladybird which clusters prominently on buildings before hibernating? Introduced in the nineteen-nineties as a means of biological control, it quickly spread throughout northern Europe.