Root elongation test on seeds of Sinapis alba reveals toxicity of extracts from thirteen colour forms of the Asian multi‐coloured ladybird, Harmonia axyridis

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AbstractLadybirds (Coleoptera: Coccinellidae) use toxic compounds such as alkaloids in their hemolymph to defend against predators and pathogens. Direct measurement of their toxicity to predators is unavailable as predators show various avoidance reactions without eating the beetles. Because inhibition by harmonine – the alkaloid of the alien invasive ladybird Harmonia axyridis (Pallas) – was observed in many organisms, its toxicity can also be tested on various non‐target organisms. This is the first study examining ladybird alkaloid toxicity to plants. We used a quick, inexpensive, and easy‐to‐perform method to quantify the phytotoxicity of 13 strains of H. axyridis, representing natural colour forms and spontaneous laboratory mutations. We used the seed germination rate and root elongation of seedlings of Sinapis alba L. (Brassicaceae). Following a logistic curve, root length decreased with increasing concentration of the extract from adult beetles. The effect of extracts from the cowpea weevil, Callosobruchus maculatus (Fabricius) (Coleoptera: Chrysomelidae) (negative control) did not differ from the control (water). Extract from the lesser mealworm Alphitobius diaperinus (Panzer) (Coleoptera: Tenebrionidae) (positive control) decreased the root length to about half of that of the negative control. Extract from young ladybirds (1 week old) decreased the root length to about one‐third of the control and the extract from 3‐month‐old ladybirds to one‐10th. Differences in toxicity among all 13 strains were minor and not consistent for the two age cohorts.