Technical Comments

Comment on “Invasive Harlequin Ladybird Carries Biological Weapons Against Native Competitors”

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Science  20 Sep 2013:
Vol. 341, Issue 6152, pp. 1342
DOI: 10.1126/science.1241745

Abstract

We comment on the implications that Vilcinskas et al. (Reports, 17 May 2013, p. 862) attach to the finding that the exotic, invasive ladybird Harmonia axyridis carries microsporidia to which this species is insensitive but that is lethal to species that are native to the invaded areas. The authors suggest that these microsporidia might serve as “biological weapons” against the native competitors, but we cast doubt on the importance of this suggestion in the field.

The invasive ladybird, Harmonia axyridis, has a negative impact on comembers of the aphidophagous guild in the invaded areas (1). One of the main reasons that have been suggested is its voracious character, making this invader dominant in competition for food and a strong intraguild predator as well (2). Vilcinskas et al. (3) propose another factor contributing to Harmonia’s negative effect: Harmonia carries spores of parasitic microsporidia, which, when experimentally injected into the indigenous Coccinella septempunctata, cause a strong reduction in survival. Harmonia itself seems to be insensitive to the microsporidia. The authors suggest that these microsporidia act as biological weapons: They spread from Harmonia to native ladybird species by intraguild predation and kill them. Although we do not cast any doubt on these experimental results, we question their importance in the field.

The proposed mechanism for transfer of microsporidia from Harmonia to native ladybirds is that heterospecifics prey on Harmonia. This is most likely to happen at the immature stages (2). Two problems arise: (i) Vilcinskas’ experimental work (3) focuses on microsporidia isolated from the hemolymph of Harmonia; microsporidial abundance is neither quantified nor differentiated between adult and larval hemolymph. Moreover, for the eggs—the stage most vulnerable to intraguild predation (2)—only microsporidial presence, rather than abundance, is reported. (ii) More important, published evidence shows that Harmonia usually is the intraguild predator instead of the intraguild prey (2, 4), also under field conditions (5). Even Harmonia eggs deter heterospecifics (2, 6). This makes us wonder whether Harmonia’s “biological weapons” are deployed to a sufficient extent to contribute substantially to the negative impact on native ladybird communities, above the already demonstrated impact of the highly asymmetric intraguild predation. Another mechanism, co-hibernation, may lead to conspecific transmission of microsporidia (7), but because hibernating ladybird beetles usually form monospecific clusters (8), which is also likely the case for H. axyridis (9), this mechanism is unlikely to play a role in the transfer of microsporidia to native ladybirds. Therefore, we argue that, at present, there is no proof for the conclusion in Reynolds’ accompanying Perspective that “the almost worldwide invasive triumph of the harlequin ladybird Harmonia axyridis depends on the presence of a coexisting pathogen within the invading insect....” (7).

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