Intraspecific and interspecific variation in prickly poppy resistance to non-native generalist caterpillars

Abstract

<p><strong>Background:</strong> Prickly poppies (genus <em>Argemone</em>, Papaveraceae) epitomize well-defended plants. With high prickle densities and exudation of bright yellow latex from glaucous leaves, there are few reports of herbivore damage on these plants. Yet, little ecological work has examined within-plant or among-species variation in levels of anti-herbivore defenses in prickly poppies.</p><p><strong>Questions:</strong> Are prickly poppies well defended against generalist herbivores? Does chemical defense vary within prickly poppy plants in a pattern consistent with optimal defense theory?</p><p><strong>Species study:</strong> <em>Argemone glauca, A. mexicana</em>, <em>A. ochroleuca, A. platyceras</em> were examined in bioassays using generalist caterpillars, <em>Agrotis ipsilon</em> Hufnagel and<em> Chrysodeixis eriosoma </em>Doubleday (Noctuidae).</p><p><strong>Study sites and dates:</strong> Seeds were collected from field populations in Mexico (2015) and Hawaii (2011); experiment was conducted at the University of Hawaii at Manoa, Fall 2015.</p><p><strong>Methods:</strong> Using a pair of no-choice bioassays, variation in the quality of whole-leaf and chemical extract infused artificial diets for caterpillar development was assessed.</p><p><strong>Results:</strong> Survivorship was lowest on the control lettuce diet, and varied little between poppy diets although pupal mass was marginally lower for caterpillars reared on <em>A. platyceras </em>than <em>A. ochroleuca</em>. Isolating the effects of plant chemistry, <em>C. eriosoma </em>performance was most strongly reduced by seed extracts, with an extension in pupal development time by one week compared to herbivores reared on other diets, as well as a significant reduction in mean adult mass.</p><p><strong>Conclusions:</strong> These results are consistent with optimal defense theory, reflecting greater allocation of defense in tissues of high fitness value (<em>i.e.,</em> seeds). Future work with native herbivores, chemical analyses, and investigations into alternative functions for these plant traits would shed new light on this interesting group of plants.</p>