Interspecific Variation in Size, Diapause Intensity, and Moisture Responses of First-InstarSpeyeria(Lepidoptera: Nymphalidae) Larvae

Abstract

Egg weights of Speyeria (Nymphalidae) species from California were measured to estimate relative sizes of first-instar larvae. First-instar larvae were compared for diapause intensity and for their physiological and behavioral responses to atmospheric humidity and free water. Diapause intensity, measured by number of days between first instar and molt to second instar, ranged from 7.6 d (Speyeria nokomis) to 40.7 d (Speyeria callippe). Among species, diapause intensity was uncorrelated with egg weight, but within three species (S. nokomis, Speyeria mormonia, and Speyeria zerene), diapause intensity was positively correlated to egg weight. There was significant interspecific variability in the desiccation tolerance of diapause larvae to low (11% RH) humidity. The least and most desiccation-tolerant species, respectively, were S. nokomis (LT50= 1.9 d) and S. callippe (LT50 = 13.6 d). Interspecific desiccation tolerance was uncorrelated with egg weight. Larvae of S. nokomis moved from lower to higher humidity within a humidity gradient, whereas larvae of S. callippe did not move toward higher humidity. Desiccated larvae of S. callippe and S. zerene rehydrated by imbibing free water. Exposure of S. callippe and S. zerene larvae to 100% RH in the absence of free water did not result in a body weight increase, but high humidity conditions reduced the rate of water loss.

Reproductive Strategies and Life History Evolution of Some CaliforniaSpeyeria(Nymphalidae)

“…The optimal reproductive strategies of Speyeria appear to involve other life history adaptations, less related to egg or larval size, but strongly related to larval habitat. Adaptations of diapausing first instars for surviving heat and drought are a significant component of the reproductive strategies; larvae show significant interspecific variation in desiccation resistance and diapause strength that is positively correlated to the severity of their habitat (Sims & Shapiro 2014).…”

“…Larvae have a weak diapause and are very susceptible to desiccation but the duration of larval exposure to desiccating conditions is brief. One species, S. nokomis, has larvae that actively move toward high humidity in a gradient (Sims & Shapiro 2014).…”

“…This period can extend four months or longer for populations of species such as S. callippe. Documentation of larval adaptations, and speciesspecific differences, were presented in a previous report (Sims & Shapiro 2014). Important physiological adaptations of first instars to summer habitats include desiccation resistance and diapause strength.…”

“…The relationship between RRE and mean monthly precipitation during the summer periods was not significant, but summer precipitation in many habitats may not have predictable impacts on diapause larvae. Larvae can imbibe water (Sims & Shapiro 2014) but episodes of brief or light precipitation might not reach the diapause larval habitat and thus would not be available for drinking. Species inhabiting dry habitats, from low to middle elevations, had higher RRE than wet meadow and alpine species.…”

“…In California, desiccation stress is especially severe among species occurring in summer hot and dry habitats where adults of species such as S. callippe (Boisduval) often emerge in May and conclude oviposition before July (Brittnacher et al 1978). The probability of individual larvae surviving through diapause to successfully complete development the following year is almost certainly very low and life-history strategies should reflect selection for reproductive patterns and larval adaptations that maximize larval survival and overall reproductive success (Zalucki et al 2002, Sims & Shapiro 2014. Relatively little is known about the comparative reproductive strategies of most Speyeria spp.…”