Abstract
The Multicolored Asian Ladybird, Harmonia axyridis, is an extremely successful invasive species. Here we suggest that, in addition to many other traits, the dorsal spines of its larvae contribute to their success, as suggested by behavioural observations of agonistic interactions between H. axyridis and European coccinellids. In coccinellids, the role of dorsal spines in these interactions has been poorly studied and they could be a physical protection against intraguild predators. Dorsal spines of second instar H. axyridis larvae were removed with micro-scissors, which resulted in spineless larvae after moulting (spineless group). These larvae were then exposed to starved Coccinella septempunctata larvae. Two control categories were also submitted to interactions: H. axyridis larvae with all their spines (control group) and with their spines, but injured by pin stings (injured group). Spine removal at the second instar did not hamper H. axyridis development. The bite rate by C. septempunctata was significantly higher on the spineless H. axyridis and more dorsally located compared to the control and injured groups, while no bite rate difference was observed between the injured and the control group. Our results suggest that in addition to behavioural and chemical defenses, the dorsal spines play a significant protective role against bites. Therefore, spines in ladybirds could be considered as a morphological defense against intraguild predation. In H. axyridis, these defenses might contribute to its success in food resources already exploited by other guild members and thus further facilitate the invasion of new areas.
Highlights
The Multicolored Asian Ladybird, Harmonia axyridis Pallas, is an extremely successful invasive species (Koch et al, 2006; Brown et al, 2011; Roy et al, 2016)
The relatively hard exoskeleton of coccinellid pupae and the hard exoskeleton of adults can display aposematic visual signals often linked to chemical defense (Majerus, 1994)
The day following surgery, 40% of the larvae in the spineless group died whilst one larva died in the injured group and no larva died in the cold control and the control groups (Figure 4)
Summary
The Multicolored Asian Ladybird, Harmonia axyridis Pallas, is an extremely successful invasive species (Koch et al, 2006; Brown et al, 2011; Roy et al, 2016). In other species the dorsal part of the exoskeleton is covered by protuberances of different lengths and forms (Gage, 1920; Hodek, 1973) which, as assumed by several authors (Richards, 1980; Dixon, 2000; Yasuda et al, 2001; Ware and Majerus, 2008), might provide protection against predation and against intraguild predation. This form of protection has only been tested with Curinus coeruleus Mulsant (Michaud and Grant, 2003). They observed that the dorsal spines of this species have a minor defensive value against coccinellids and chrysopids
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