Academic journal article The American Midland Naturalist

Mowing Frequency Influences Number of Flowering Stems but Not Population Age Structure of Asclepias Viridis, an Important Monarch Host Plant

Academic journal article The American Midland Naturalist

Mowing Frequency Influences Number of Flowering Stems but Not Population Age Structure of Asclepias Viridis, an Important Monarch Host Plant

Article excerpt

INTRODUCTION

There is a well-documented decline in the size of the eastern North American migratory population of the monarch butterfly (Danaus plexippus L.) in overwintering sites in central Mexico over the last 20 у (Brower et al., 2012; Vidal and Rendón-Salinas, 2014; Semmens et al, 2016). A decline in milkweed (Asclepias spp.) availability in the Midwest is often cited as contributing to the population decline (Pleasants and Oberhäuser, 2013; Pleasants, 2017; Thogmartin et al, 2017). Milkweeds are the sole host plant for monarch larvae (Malcolm et al, 1993) and also provide nectar for adult monarchs during spring and summer breeding periods, as well as during the spring and fall migrations (Alonso-Mejia et al, 1997; Brower et al, 2006).

Because of the decline of milkweed in the Midwest region primarily due to agricultural practices (Zaya et al, 2017), understanding how management practices influence the structure of milkweed populations in other land use contexts is important for maintaining and enhancing milkweed availability for monarch butterflies. Natural and semi-natural grasslands and roadsides represent a majority of current milkweed habitat (Kasten et al, 2016; Zaya et al, 2017). Mowing is a common management practice in grasslands and roadsides, and may influence monarch-milkweed interactions. For example monarchs prefer new growth (or regrowth) compared to older growth (Zalucki and Kitching, 1982; Fischer et al, 2015). The occurrence of regrowth (resprouting) depends on the timing of mowing or other disturbance (Baum and Mueller, 2015; Fischer et al, 2015). In addition, though monarchs are considered nectar generalists (Robertson, 1928; Tooker et al, 2002), milkweed flowers are an important nectar source for monarchs. The timing and frequency of mowing may influence flowering, given the amount of reproductive effort is influenced by the availability of carbon resources in a plant (Southwick, 1984; Bazzaz et al, 1987; Kozłowski, 1992), especially when a disturbance removes a large portion of reproductive biomass (Howe, 1994; Iwasa and Kubo, 1997; Nofal et al, 2004). Moreover, continual removal of biomass could deplete carbon reserves, negatively affecting plant longevity and, consequently the mean age of a population (Dietz and Ullmann, 1998; Hautekeete et al, 2002). Given the timing of peak reproduction in a plant's life history, younger populations may have reduced overall nectar availability compared to populations with older individuals (Pergi et al, 2006).

Managed milkweed habitats in Texas and Oklahoma represent important spring and late summer/early fall breeding sites for monarch butterflies in which the availability of milkweed biomass and nectar are likely important for overall monarch success. Recent population modeling suggests the spring migration through Texas and Oklahoma is a critical time for the monarch's annual life cycle given the importance of the first generation to the overall population size (Flockhart et al, 2015; Inamine et al, 2016; Oberhäuser et al, 2017). Recent stable isotope research also indicates monarchs produced in this region in the late summer/early fall can be an important component of the overwintering population (Flockhart et al, 2017). In the Texas and Oklahoma area, A. viridis mirrors the importance of A. syriaca in the Midwest as a host plant (Malcolm and Brower, 1989; Baum and Mueller, 2015). Our previous research has shown the condition of A. viridis throughout the growing season, peak nectar availability, and plant regrowth all have important impacts on opposition, adult food availability, and support of monarch butterfly larvae during critical stages of development and different monarch generations across seasons (Baum and Sharber, 2012; Baum and Mueller, 2015). In light of these findings, a more holistic consideration of management effects on milkweed reproduction and population age structure is required. For instance mowing after peak flowering could promote nectar availability in early summer and reduce stress for A. …

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