Disentangling herbivore impacts in primary succession by refocusing the plant stress and vigor hypotheses on phenology

Abstract

The plant stress and plant vigor hypotheses are widely used to explain the distribution and abundance of insect herbivores across their host plants. These hypotheses are the subject of contentious debate within the plant herbivore research community, with several studies finding simultaneous support for both hypotheses for the same plant–herbivore interaction. We address the question of how such support is possible using dynamic site‐occupancy models to quantify the attack dynamics of Cryptorhynchus lapathi (poplar‐willow weevil) on Salix sitchensis (Sitka willow), a dioecious shrub colonizing Mount St. Helens, Washington, USA after the 1980 eruption, in relation to host plant stress, vigor, and sex. We also introduce several scaling criteria as a rigorous test of the plant vigor hypothesis and demonstrate why modeling insect detection is important in plant–insect studies. Weevils responded positively to water stress associated with seasonal dry‐downs, and this response was phenologically compartmentalized by larval feeding mode. Weevils preferentially attacked large and/or flowering stems, imposing an ecological cost on willow reproduction via increased stem mortality and susceptibility to future attack. We propose that the dual response to host plant stress and vigor is due to the synchronization between young weevil larval feeding and willow nutrient pulses that are mediated by environmental stress. In turn, this process drives successional dynamics, causing the juvenilization of upland willow plants and possibly delaying establishment of a willow‐dominated upland sere. These results highlight the common, but often overlooked, phenological basis of the plant stress and plant vigor hypotheses, which both focus on how stress changes the quality of plant resources available to immature insects.

Plant sex influences aquatic–terrestrial interactions

“…Interestingly, there are spatial patterns in weevil infection that may also be influencing this aquatic-terrestrial interaction. Our results corroborate those of Che-Castaldo et al (2019) which showed that female riparian willows were more frequently attacked by the weevil. We also found a significant sex 9 stream interaction with a higher proportion of female willows attacked by weevils at Goose Creek than at Geo-West Creek.…”

“…Despite being of lower quality, female litter accumulates nitrogen faster than male litter once incubated in the stream, possibly due to differential colonization by aquatic fungi and bacteria. We found no statistically significant differences in condensed tannins between males and females, and it is thought that weevil attack is not strongly influenced by willow defensive compounds (Che-Castaldo et al 2019). In addition, litter quality differences could be due to differential resource allocation and female requirements to make flowers, fruits, and seeds that have been shown to decrease allocation to defensive compounds like tannins and salicortin (Elmqvist et al 1991).…”

“…In addition, litter quality differences could be due to differential resource allocation and female requirements to make flowers, fruits, and seeds that have been shown to decrease allocation to defensive compounds like tannins and salicortin (Elmqvist et al 1991). We found no statistically significant differences in condensed tannins between males and females, and it is thought that weevil attack is not strongly influenced by willow defensive compounds (Che-Castaldo et al 2019). In this system, weevils may be keying into differences in willow cambium nutrient content (Che-Castaldo et al 2019) or olfactory cues based on other chemical compounds as has been shown for this weevil on poplars (Broberg and Borden 2005).…”

“…have been one of the major colonizers of riparian habitat on the Pumice Plain at Mount St. Helens (Lawetlat'la in Cowlitz; Washington, USA; del Moral and Jones 2002). The weevil attacks the base of willow stems, which often causes leaf death and sometimes leads to whole branch dieback or whole-plant mortality (Che-Castaldo et al 2019). The weevil attacks the base of willow stems, which often causes leaf death and sometimes leads to whole branch dieback or whole-plant mortality (Che-Castaldo et al 2019).…”

“…Willows are heavily infested with the invasive stem-boring weevil (Cryptorhynchus lapathi L.; Coleoptera: Curculionidae; Broberg et al 2002). The weevil attacks the base of willow stems, which often causes leaf death and sometimes leads to whole branch dieback or whole-plant mortality (Che-Castaldo et al 2019).…”