Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore

Abstract

., "Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore" (2005). Faculty Publications in the Biological Sciences. 58. http://digitalcommons.unl.edu/bioscifacpub/58 AbstractSpatial and temporal variation in insect floral herbivory is common and often important. Yet, the determinants of such variation remain incompletely understood. Using 12 years of flowering data and 4 years of biweekly insect counts, we evaluated four hypotheses to explain variation in damage by the Eurasian flower head weevil, Rhinocyllus conicus, to the native North American wavyleaf thistle, Cirsium undulatum. The four factors hypothesized to influence weevil impact were variations in climate, weevil abundance, phenological synchrony, and number of flower heads available, either on wavyleaf thistle or on the other co-occurring, acquired native host plant (Platte thistle, Cirsium canescens), or on both. Climate did not contribute significantly to an explanation of variation in R. conicus damage to wavyleaf thistle. However, climate did influence weevil synchrony with wavyleaf flower head initiation, and phenological synchrony was important in determining R. conicus oviposition levels on wavyleaf thistle. The earlier R. conicus was active, the less it oviposited on wavyleaf thistle, even when weevils were abundant. Neither weevil abundance nor availability of wavyleaf flower heads predicted R. conicus egg load. Instead, the strongest predictor of R. conicus egg load on wavyleaf thistle was the availability of flower heads on Platte thistle, the more common, earlier flowering native thistle in the sand prairie. Egg load on wavyleaf thistle decreased as the number of Platte thistle flower heads at a site increased. Thus, wavyleaf thistle experienced associational defense in the presence of flowering by its now declining native congener, Platte thistle. These results demonstrate that prediction of damage to a native plant by an exotic insect may require knowledge of both likely phenological synchrony and total resource availability to the herbivore, including resources provided by other nontarget native species.

Cumulative herbivory outpaces compensation for early floral damage on a monocarpic perennial thistle

“…In addition, the Eurasian flower head weevil (Rhinocyllus conicus Frölich) invaded the floral-feeding guild in 1993 (Louda et al 1997;Louda 1998). Previous studies document both the feeding damage (Lamp and McCarty 1981) and the population-level impacts (Louda et al 1990;Louda and Potvin 1995;Rose et al 2005;Russell and Louda 2005) by these six floral herbivores.…”

“…(Platte thistle). This monocarpic perennial species is known to have strong population-level interactions with floral herbivores (Louda and Potvin 1995;Rose et al 2005;Russell and Louda 2005;Rand and Louda 2012), and would be expected to benefit from tolerance strategies. Apical meristem damage is generally a common occurrence imposed by physical or biological factors (Gruntman and Novoplansky 2011;Adhikari and Russell 2014;Klimešová et al 2014) and represents a disproportionately large potential early-season fitness risk for C. canescens.…”

“…The effect of floral herbivory on C. canescens seed production is influenced by the synchrony of its floral herbivores with plant-flowering phenology (Russell and Louda 2005). Distributing reproductive effort over longer time periods may increase the probability that a subset of flower heads avoid damage.…”

Community heterogeneity and the evolution of interactions between plants and insect herbivores

“…For example, Chew (1979) speculated that the selective pressures on the Brassicaceous host plants she studied would likely differ following invasion by the exotic crucifer Thlaspi. Several other examples of invasive plants that apparently alter attack on natives have recently emerged (Zimmerman 1960;Thomas et al 1987;Carroll and Boyd 1992;Solarz and Newman 1996;Rand and Louda 2004;Lau and Strauss 2005;Russell and Louda 2005;Lau 2006). We believe that the study of plant-herbivore interactions in both invaded versus non-invaded sites and in introduced versus native habitats are "natural experiments" that provide interesting research opportunities.…”

Evaluation of the field impact of an adventitious herbivore on an invasive plant, yellow toadflax, in Colorado, USA

“…It is important to note, however, that if B. pulicarius has no effect on L. vulgaris in dense, established sites, then B. pulicarius may be damaging sites most intensely where it may be most ineffective. Finally, because ramet density did not explain all of the local spatiotemporal variation in B. pulicarius damage, other factors may also be important in influencing the probability that sites (and ramets) experience B. pulicarius damage in given years, including climate, phenological synchrony between weed and insect, and the availability of other potential host plants (reviewed in Russell and Louda 2005).…”