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

A family-structured quantitative genetic experiment was used to evaluate the importance of ungulate browsing, flowering date and plant height, and variation in a putative phytochemical defense on patterns of seed fly attack in a scarlet gilia population, and found an unexpected positive relationship between damage and fitness.

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 family-structured 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.