Abstract
Studying the drivers of host specificity can contribute to our understanding of the origin and evolution of obligate pollination mutualisms. The preference–performance hypothesis predicts that host plant choice of female insects is related mainly to the performance of their offspring. Soil moisture is thought to be particularly important for the survival of larvae and pupae that inhabit soil. In the high Himalayas, Rheum nobile and R. alexandrae differ in their distribution in terms of soil moisture; that is, R. nobile typically occurs in scree with well‐drained soils, R. alexandrae in wetlands. The two plant species are pollinated by their respective mutualistic seed‐consuming flies, Bradysia sp1. and Bradysia sp2. We investigated whether soil moisture is important for regulating host specificity by comparing pupation and adult emergence of the two fly species using field and laboratory experiments. Laboratory experiments revealed soil moisture did have significant effects on larval and pupal performances in both fly species, but the two fly species had similar optimal soil moisture requirements for pupation and adult emergence. Moreover, a field reciprocal transfer experiment showed that there was no significant difference in adult emergence for both fly species between their native and non‐native habitats. Nevertheless, Bradysia sp1., associated with R. nobile, was more tolerant to drought stress, while Bradysia sp2., associated with R. alexandrae, was more tolerant to flooding stress. These results indicate that soil moisture is unlikely to play a determining role in regulating host specificity of the two fly species. However, their pupation and adult emergence in response to extremely wet or dry soils are habitat‐specific.
Highlights
Current estimates of insect diversity range as high as 30 million species, and a large fraction of these species feed on plants (Jaenike, 1990)
According to the preference/performance hypothesis (Thompson, 1988), if the environmental conditions of oviposition sites affect offspring performance, the difference in environmental conditions may play an important role in choice of host plants by adult insects (König et al, 2016)
Similar to previous reports on the effects of soil moisture on performance of larva–pupal stage in some insects (Hou et al, 2006; Hulthen & Clarke, 2006; Jackson et al, 1998; Vargas, Chang, Komura, & Kawamoto, 1987), our laboratory experiments showed that soil moisture did have significant effects on larval and pupal performances in both species and extremely wet or dry soils significantly decreased their survival
Summary
Current estimates of insect diversity range as high as 30 million species, and a large fraction of these species feed on plants (Jaenike, 1990). According to the preference–performance hypothesis, the range of soil moisture at which a plant species occurs may influence the oviposition preference of adult flies on this plant species for maximizing the development and survival of their offspring with low mobility (Bonebrake, Boggs, McNally, Ranganathan, & Ehrlich, 2010). Both Rheum nobile and R. alexandrae are perennial herbs endemic to the high eastern Himalayas, with large translucent cream-colored bracts covering the entire inflorescence (Figure 1a,e). In the laboratory, we determined (1) the effect of different soil moisture on larval and pupal survival in the two fly species and (2) larval and pupal survival of the two fly species after short-term submergence in water
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