Scarlet gilia resistance to insect herbivory: the effects of early season browsing, plant apparency, and phytochemistry on patterns of seed fly attack

Abstract

Pollen-limited plants are confronted with a difficult tradeoff because they must present showy floral displays to attract pollinators and yet must also minimize their apparency to herbivores. In these systems, traits that increase pollinator visitation may also increase herbivore oviposition and overall plant resistance may therefore be constrained to evolve largely as a correlated response to selection on plant apparency or vigor. We used a familystructured quantitative genetic experiment to evaluate the importance of ungulate browsing, flowering date and plant height (traits that are related to overall vigor), and variation in a putative phytochemical defense (cucurbitacin production) on patterns of seed fly attack in a scarlet gilia population. We found significant genetic variation in the amount of insect damage plants experience in the field, providing evidence that resistance may evolve. In addition, we found that browsing reduced seed fly attack and that oviposition is strongly related to plant size and flowering date; large, early flowering plants experience high attack. In addition, we found that high cucurbitacin production was correlated with low seed fly damage, although this effect was relatively weak.We found directional selection on final plant height and flowering date; tall, early flowering plants had the highest reproductive success. In addition, we found negative directional selection on cucurbitacin production, which may indicate a high cost of cucurbitacin or other functions of this phytochemical. Although seed fly herbivory arguably decreases plant fitness, we found an unexpected positive relationship between damage and fitness. A negative relationship between fitness and damage may be masked in this system through strong positive indirect correlations between patterns of damage and levels of pollinator visitation. Finally, we found significant genetic variation in flowering date, plant height, and cucurbitacin production. Resistance to seed flies may evolve in this population, but largely as a non-adaptive correlated response to selection on overall plant vigor. Phytochemicals may play a more important role in defense in years with high seed fly attack, or when pollen-limitation is less severe.

Additive effects of pollinators and herbivores result in both conflicting and reinforcing selection on floral traits

“…An exception may be traits involved in the placement of floral rewards (e.g., nectar spurs, location of anthers), which could influence both morphological fit to the pollinator and the likelihood of nectar robbery (Lara and Ornelas 2001) and florivory. Flowering phenology can be expected to influence interactions with both pollinators and antagonists, and may be a target of selection in both types of interactions (Juenger et al 2005, Sandring and Å gren 2009, Kawagoe and Kudoh 2010. This could result in either conflicting or reinforcing selection, depending on the seasonal variation in intensity of interactions with mutualists and antagonists in relation to flowering phenology.…”

Escape from floral herbivory by early flowering in Arabidopsis halleri subsp. gemmifera

“…Second, floral herbivory may directly reduce the availability of reproductive organs that would otherwise produce mature seeds (McCall 2008). Finally, temporal changes in the intensity of floral herbivory have been observed (Evans et al 1989;EnglishLoeb and Karban 1992;Bishop and Schemske 1998;Pilson 2000;Mahoro 2002;Juenger et al 2005;Tarayre et al 2007;Oguro and Sakai 2009), and this can serve as a source of selection on flowering time to avoid floral herbivory (Elzinga et al 2007). …”

Habitat Structure Affects Reproductive Success of the Rare Endemic Tree Syzygium mamillatum (Myrtaceae) in Restored and Unrestored Sites in Mauritius

“…A trade‐off between maximizing floral display to attract pollinators and minimizing visibility to herbivores has been stressed by other authors ( e.g. , Fenner et al 2002, Juenger et al 2005) and may play a role in our system. The positive correlation between the attack rates and number of buds per tree suggests positive density dependence.…”