To duck or not to duck: resistance advantages and disadvantages of the candy‐cane stem phenotype in tall goldenrod, Solidago altissima

Abstract

Summary• Solidago altissima populations consistently contain a minority of 'ducking', or 'candy-cane', stems. The goals of this study were to investigate whether these candy-cane stems may be an adaptation to resist herbivory, and to look for costs (namely, resistance tradeoffs or reduced reproduction) that might constrain the spread of the ducking trait.• In this study, herbivory and seed set were recorded for 272 erect and 272 candycane stems in a field population of S. altissima.• Candy-cane plants were twice as resistant as erect plants to two common apexattacking gall midges, but were 26% more susceptible than erect plants to a more abundant apex-boring caterpillar. The two stem morphs were equally resistant to all other herbivores surveyed. Candy-cane plants were 11% less likely to experience apex damage and 3% more likely to set seed than erect-stemmed plants. Damaged candy-cane stems were 9% more likely to produce seeds than damaged erect stems.• Although ducking is an effective means of resisting apex-galling herbivores, its spread may be constrained by susceptibility to an apex-boring caterpillar, which may enjoy enemy-reduced space on candy-cane stems. The evolution of ducking does not seem to be constrained by a reduced likelihood of sexual reproduction, or reduced tolerance of apex damage.

Resistance and tolerance to herbivory in Solidago altissima (Asteraceae): Genetic variability, costs, and selection for multiple traits

“…Goldenrod resistance and tolerance -Many studies have demonstrated signifi cant genetic variability in goldenrod resistance to insect -herbivore attack ( Maddox and Root, 1987 ;McCrea and Abrahamson, 1987 ;Maddox and Root, 1990;Cronin and Abrahamson, 2001b ;Wise, 2009 ), but few have examined costs and/or selection for resistance ( Uriarte et al, 2002 ;. These few studies suggest that there is generally strong selection for increased resistance, but there is low broad-sense heritability for resistance.…”

The role of ducking stems in plant anti-herbivore defense and its exploitation by herbivorous insects

“…Candy-cane genets initially grow erect stems, but their apices begin to nod after approximately 4 weeks until they point downward to look like candy-canes. Candy-cane genets are resistant to tephritid and cecidomyiid gallers that induce galls in apical meristems (Wise and Abrahamson 2008;Wise 2009). When the stems of candy-cane genets are straightened through exposure to strong shade, their resistance to herbivores disappears, suggesting that the nodding architecture of the candy-canes is responsible for the resistance (Wise et al 2010a).…”

“…Recently, a novel plant architectural trait was demonstrated to provide resistance to insect herbivory (Wise and Abrahamson 2008;Wise 2009;Wise et al 2009Wise et al , 2010a. In the tall goldenrod Solidago altissima in the United States, most genets have erect stems, while some genets have stems with nodding apices ('candy-cane' genets).…”

“…If these hypotheses are true, then plants and herbivores may use similar architectural defensive mechanisms to enhance their survival or growth. Although a series of studies by Wise and colleagues(Wise and Abrahamson 2008;Wise 2009; …”

“…If all the stems of a plant species are erect or nodding, experimental manipulation such as the bending or straightening of stems using wires or threads would be of practical use. Because nodding stems may be resistant to some herbivorous insects but susceptible to other herbivores (Wise 2009, but see Wise et al 2010b), both specific and total herbivore damage should be evaluated. Since a few herbivorous insects prefer stressed, weak plants to vigorous ones (plant stress hypothesis; White 1984White , 2009, it should be examined whether nodding stems are attacked by those herbivores.…”