Frances S. Chew

Phytochemical correlates of herbivory in a community of native and naturalized cruciferae

Abstract Oviposition and larval feeding behaviors of the crucifer specialist Pieris napi macdunnoughii correlate with leaf glucosinolate profils of plant species in a natural community. Profiles are species-specific in this group of eight Cruciferae, but particular glucosinolates are shared by subsets of the community. Pieris accepts two lethal naturalized weeds whose glucosinolate profiles resemble that of an indigenous foodplant. The results suggest that specific glucosinolates constitute insect behavioral cues which are only loosely linked evolutionarily to foodplant suitability, and further suggest that allelochemically similar community associates influence the coevolution of individual plant species with insect herbivores.

Escaping an evolutionary trap : preference and performance of a native insect on an exotic invasive host

Exotic plants may act as population sinks or evolutionary traps for native herbivores. The native butterfly Pieris oleracea lays eggs on garlic mustard, Alliaria petiolata, but larvae develop very poorly on this exotic invasive plant. We examined oviposition preference of individual females and larval performance of their offspring for individuals from one area where garlic mustard is well established and one where it is absent. These data were used to assess whether garlic mustard is being incorporated into or excluded from the diet. Females from the area without garlic mustard showed a wide range of preference, families had low larval survival on garlic mustard, and larval survivorship showed no correlation with mothers’ preferences. Females from the area with garlic mustard preferred it to the native host, and larval survivorship on garlic mustard was positively correlated with the mother’s preference. Individuals surviving on garlic mustard took longer to pupate and weighed >30% less compared to pupae reared on normal hosts. Our results suggest that where garlic mustard is well established P. oleracea may be adapting to this plant by both improved larval performance and increased adult female oviposition preference for it.

Foodplant preferences of Pieris caterpillars (Lepidoptera)

SummaryLarvae of five Nearctic Pieris butterflies accept a wide range of native and naturalized crucifers under laboratory test conditions. Preferences among crucifers are usually statistical rather than absolute. Caterpillars do not necessarily reject plants that do not support larval growth in favor of those that do. Preferences are not significantly altered by larval development or prior experience with specific foodplant species. Progeny of different females do not exhibit significantly different preferences. Retention of behavioral flexibility by Pieris may enable older larvae to exploit alternative resources not suitable for young larvae.

Coexistence and host use by a large community of Pierid butterflies: habitat is the templet

SummaryWe report studies on the butterfly-hostplant communities in the species-rich area of west central Morocco. Pieridae feeding on Capparales form two distinct ecological guilds: inflorescence feeders and folivores. Several members of each guild may synchronously occur in sympatry. Substantial levels of cannibalism and inter-specific predation occur amongst the inflorescence feeders. No evidence was obtained for segregation of butterfy species on different hosts. Host plants included in the diet of specialists were also used by generalists. Despite substantial differences observed in laboratory trials of larval survivorship on different hostplants, results were congruent for all Pieridae, suggesting that little one-to-one insect-host coevolution has occurred. Host specialization was instead related to the year-to-year stability of host numbers in an area. High levels of pierid infestation occurred on host species with numerically stable populations. Host numerical stability was correlated with habitat type. There is little evidence for segregation of competing inflorescence feeders by hostplant species, but some evidence for segregation by habitat type (particularly by shading levels). We interpret our results as indicating that the hostplant affiliations of Moroccan Capparales-feeding Pieridae are subject to (at best) diffuse coevolutionary effects from hosts and competitors, and are strongly influenced by habitat characteristics.

Oviposition stimulants and deterrents control acceptance ofAlliaria petiolata byPieris rapae andP. napi oleracea

SummaryDifferential acceptance of garlic mustard,Alliaria petiolata byPieris rapae L. andP. napi oleracea is explained by their differential sensitivities to oviposition stimulants and deterrents in the plant. Fractions containing the stimulants and deterrents were isolated by solvent partitioning between water and n-butanol and by open-column chromatography followed by HPLC.P. napi oleracea showed no preference when offered a choice ofA. petiolata or cabbage, but was strongly stimulated to oviposit by post-butanol water extracts ofA. petiolata. The most abundant glucosinolate in this extract was identified as sinigrin, which could explain the high degree of stimulatory activity.P. rapae preferred cabbage plants overA. petiolata, and the relatively low stimulatory activity was also associated with the glucosinolate-containing aqueous extract. However, this species was strongly stimulated by a fraction that contained small amounts of glucotropaeolin along with unknown compounds. Deterrents to both species were found in the butanol extract fromA. petiolata, andP. napi oleracea was more sensitive thanP. rapae to these deterrents. Some HPLC fractions from the BuOH extract were strongly deterrent toP. napi oleracea, but were inactive toP. rapae. The ecological significance of these behavioral differences between the twoPieris species is discussed.

Habitat requirements and local persistence of three damselfly species (odonata: coenagrionidae)

Habitat requirements and population persistence were investigated in three damselfly species, all coastal plain pond specialists: Enallagma recurvatum, E. laterale, and E. pictum. Because of geographic restriction, two are of special concern to conservation, E. recurvatum and E. laterale. We surveyed more than 70 ponds on Cape Cod, Massachusetts, and collected adult presence-absence data during the summers of 1999 and 2000. We achieved a detection rate approaching 100% for each species by visiting each pond up to three times. We looked for relationships between the presence of each damselfly species and presence of specific aquatic vegetation, the presence of the other Enallagma species, and the number of ponds within various distances of the 72 surveyed ponds. Using stepwise logistic regression, we found the following significant associations: E. recurvatum with the rush Juncus militaris; E. laterale with water lilies (Nuphar variegatum and Brasenia schreberi) the damselfly E. pictum, and the number of ponds within 2 km; and E. pictum with the water lily Nymphaea odorata, the damselfly E. laterale, and the number of ponds within 1.5 km and 2.5 km. Presence-absence data were used to calculate turnover and local extinction rates for each species between the two years. E. recurvatums turnover and local extinction rates (33.3% and 41% respectively) were much higher than either E. laterale (9.8%, 11.5%) or E. pictum (7.7%, 10%). These results suggest that E. recurvatum occurs in a metapopulation, and that patch colonization rates might be important to local population persistence.

Variable glucosinolate profiles of Cardamine pratensis (Brassicaceae) with equal chromosome numbers.

A novel glucosinolate, 3-(hydroxymethyl)pentylglucosinolate, was present at high levels in Cardamine pratensis L. from eastern North America and in commercially obtained seeds, but not in C. pratensis plants from southern Scandinavia. Glucosinolates in a number of accessions of C. pratensis included glucosinolates with the side chains 1-methylethyl, 1-(hydroxymethyl)ethyl, 1-methylpropyl, 1-(hydroxymethyl)propyl, 3-methylpentyl, 3-(hydroxymethyl)pentyl, benzyl, 4-hydroxybenzyl, 4-methoxybenzyl, indol-3-ylmethyl (as well as its 1-methoxy, 4-hydroxy, and 4-methoxy derivatives) and the rare side chain 1,4-dimethoxyindol-3-ylmethyl. Substantial variation was observed for four biosynthetic characters: (i) extent of chain elongation of Ile-derived glucosinolates; (ii) biosynthesis of Phe/Tyr-derived glucosinolates in general; (iii) hydroxylation of branched-chain glucosinolates; and (iv) O-methylation of 4-hydroxybenzylglucosinolate (sinalbin). Cytological analysis of pollen mother cells and root tip cells in meiosis and mitosis established the chromosome number to be 2n = 30 for all accessions, irrespective of glucosinolate profile.

Specific glucosinolate analysis reveals variable levels of epimeric glucobarbarins, dietary precursors of 5-phenyloxazolidine-2-thiones, in watercress types with contrasting chromosome numbers.

Watercress obtained in food stores in the United States contained significant levels of epiglucobarbarin [(R)-2-hydroxy-2-phenylethylglucosinolate] and low levels of the 2S-epimer glucobarbarin identified by an HPLC+NMR+MS/MS approach. Typical combined levels were 4-7 μmol/g dry wt. The hydrolysis product, 5-phenyloxazolidine-2-thione (barbarin), was detected at similar levels as the precursor glucosinolates after autolysis of fresh watercress in water. Fragmentation patterns in MS(2) of reference desulfoglucosinolates were side chain specific and suitable for routine identification. Watercress was of two main glucosinolate chemotypes: Material from U.S. food stores had a complex profile including glucobarbarins, gluconasturtiin, indole glucosinolates and high levels (6-28 μmol/g dry wt.) of long-chain methylsulfinylalkyl and methylthioalkyl glucosinolates. Material from European food stores had a simple profile dominated by gluconasturtiin, with low levels of epiglucobarbarin and moderate levels of indole glucosinolates. Some wild U.S. material was similar to the U.S. food store type. Both types were found to be Nasturtium officinale by floral parts morphology. Cytological analysis of one U.S. food store accession indicated that it represented a chromosome-doubled variant within N. officinale. The nutritional consequences and invasive potential of the U.S. food store chemotype are discussed.

Modeling the decline and potential recovery of a native butterfly following serial invasions by exotic species

Population sizes and range of the native butterfly Pieris oleracea declined after habitat loss and parasitism by an exotic braconid wasp (Cotesia glomerata) introduced to control the exotic invasive butterfly Pieris rapae. Further declines are attributed to the invasive exotic weed garlic mustard (Alliaria petiolata), an oviposition sensory trap on which P. oleracea larval survival and growth are very poor. But a population of P. oleracea has adapted to garlic mustard over the past several decades, coincident with the introduction of a second parasitoid, C. rubecula, a specialist on P. rapae that is competitively dominant to C. glomerata. We used stochastic simulation models to assess the plausibility of a hypothesis that reduced parasitoid pressure over this time period enabled P. oleracea to adapt to A. petiolata. We simulated scenarios of trait proliferation via spontaneous mutation or immigration of the trait, and residual variation in the trait following the butterfly’s isolation in North America. Results indicate that the most likely scenario for the population that has adapted to garlic mustard includes (1) a change in selection following garlic mustard invasion to favor previously neutral residual variation in the population, (2) release from parasitism, and (3) evolution of improved larval survival on garlic mustard, which allowed an increased host range, and potentially, population size.

Transient abiotic stresses lead to latent defense and reproductive responses over the Brassica rapa life cycle

Plants can alter physiological and developmental trajectories in response to environmental cues by means of phenotypic plasticity. While cases of immediate plastic responses to different environments are well studied, phenotypic changes can also be delayed and occur in later life cycle stages. In this study, we investigated latent phenotypic plasticity in the development and chemical profile of Brassica rapa plants exposed to transient stress as seedlings. Four different stresses were applied to germinating seedlings: salinity, drought, nutrient deficiency, or acidity. Growth, reproduction, and glucosinolate chemical defenses (in leaves and seeds) were measured over the plants’ life cycles after normal conditions were restored. Despite initial stunting, B. rapa individuals recovered in total stem length and seed count compared with unstressed controls. There were, however, latent responses in flowering time, which was delayed in salinity-, drought-, or acid-stressed plants. Reductions in total flower count and total seed pod count were also observed in nutrient-deficiency stressed plants. Strikingly, previously stressed plants also showed latent differences in glucosinolate chemical defenses. Acid-stressed plants had higher concentrations of the plants’ major glucosinolate, gluconapin (3-butenylglucosinolate), in leaves and seeds, while nutrient deficiency-stressed plants had lower seed levels of gluconapin. Our experiments show that, despite outward recovery of growth, previously stressed B. rapa plants alter later defense and reproduction, leading to a plastic response delayed across life cycle stages. Thus, even transient, recoverable stress can have latent consequences for ecologically important chemical traits.