Cardenolide‐defended milkweed bugs do not evoke learning in Nephila senegalensis spiders

Antipredator defense of herbivorous insects often relies on the potential toxicity of defensive chemicals sequestered from their host plants. The colorful Lygaeinae (Heteroptera: Lygaeidae) store a concentrated mixture of toxic cardenolides (cardiac glycosides) in specialized storage compartments of the bugs' integument, from which they are released upon attack. Larvae and adults of the large milkweed bug Oncopeltus fasciatus (Heteroptera: Lygaeinae) are specialized to feed on cardenolide‐containing milkweeds in the plant genus Asclepias and display a conspicuous red and black colorations. To investigate whether O. fasciatus gained improved protection by feeding on a toxic host plant (Asclepias syriaca), compared to a nontoxic alternative (sunflower seeds), we fed nymphs and adults of O. fasciatus to the golden orb‐weaver Nephila senegalensis. While visually oriented vertebrates, such as avian predators, have been intensively investigated for their reaction to defensive compounds and aposematic coloration, less attention has been paid to invertebrate predators. Their different perceptual abilities can provide important opportunities for testing hypotheses on warning coloration and chemical defenses. The predation trials showed that the bugs fed on Asclepias were significantly less likely to be killed than the bugs reared on a cardenolide‐free diet. This suggests that sequestered cardenolides in O. fasciatus nymphs and adults represent a significant fitness advantage on an individual level against this invertebrate predator. Yet, when testing for avoidance learning in the spiders, negative experience did not change the way how similar prey was attacked at the next encounter. In this case, visual or chemical aposematism thus does not seem to matter for predator learning.     

Growth dynamics of a specialized milkweed seed feeder (Oncopeltus fasciatus) on seeds of familiar and unfamiliar milkweeds (Asclepias spp.)

The effect of different host plant diets on the growth dynamics of the large milkweed bug, Oncopeltus fasciatus, was investigated by rearing this specialized seed feeder on the seeds of seven milkweed species occurring in central Missouri. Growth rate, growth efficiency, and : biomass ratio proved to be the most sensitive measures of growth in detecting significant differences between diets. Asclepias syriaca L., A. verticillata L. and A. hirtella (Pennell) Woods, supported the most rapid and efficient growth, while A. incarnata L., A. purpurescens L., A. viridiflora Raf., and A. quadrifolia Jacq. were less suitable host species. In general, O. fasciatus grew best on the host species it utilizes most commonly in the field, thus supporting the proposed host familiarity hypothesis. The most commonly exploited plant species shared three characteristics of seed production and quality which accounted for 87%–96% of the variability in growth dynamics of O. fasciatus. The most important of these was the mean number of pods produced per plant. Seed nitrogen content was the next best parameter in further accounting for variability in growth rate and : biomass ratio but seed cardenolide content was the second parameter for growth efficiency.

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Résumé L'élevage d'Oncopeltus fasciatus sur les graines de 7 espèces d'Asclepias de la région centrale du Missouri a permis d'examiner l'influence de l'alimentation sur différentes plantes hôtes sur la dynamique du développement de cet insecte.La survie, la durée du développement, la biomasse des adultes et les calories de l'aliment consommé ou de la biomasse produite, présentent peu de différences significatives suivant les aliments.Par contre, 3 autres paramètres: le taux de croissance, l'efficacité de la croissance, et le rapport des biomasses /, sont plus sensibles pour la mise en évidence de différences. Asclepias syriaca, A. verticillata et A. hirtella permettent la croissance la plus rapide et la plus efficace, tandis que A. incarnata, A. purpurescens, A. viridiflora et A. quadrifolia sont des hôtes moins favorables. En général, O. fasciatus se développe mieux sur la plante qu'il consomme le plus fréquemment dans la nature, ce que était l'hypothèse des hôtes familiers. 87% de la variabilité du développement sur les différents régimes peut être expliquée par 3 paramètres liés à la production et à la qualité des graines. Le nombre moyen de gousses par plante intervient le plus dans la détermination de la variabilité du taux de croissance. La teneur des graines en azote est le paramètre qui intervient ensuite pour le taux de croissance et le rapport de biomasses /, mais la teneur des graines en cardenolide est le second paramètre pour l'efficacité de la croissance.

     

ZUR FRAGE DER WIRTSSPEZIFITÄT BEI ONCOPELTUS FASCIATUS (HETEROPTERA: LYGAEIDAE)

Zusammenfassung Die Frage der Wirtsspezifität von Oncopeltus fasciatus Dall. wurde unter Verwendung der Samen von Asclepias syriaca, Helianthus annuus und Arachis hypogaea untersucht. Die eindeutige Bevorzugung von A. syriaca gegenüber den Samen von Helianthus und Arachis beruht auf einer oder mehreren Substanzen in der Samenschale von Asclepias, die den Anstich auslösen. Diese Substanz(en) läßt (lassen) sich extrahieren und auf Helianthus-Samen übertragen. Milchröhreninhalt von A. syriaca-Pflanzen zeigt denselben Effekt. Die Lygaeide Oncopeltus fasciatus kann jedoch mit H. annuus-Samen ernährt und erfolgreich gezüchtet werden. Entwicklungsgeschwindigkeit und Reproduktion sind zwar vermindert, Lebensdauer und Eigewichte aber auf beiden Wirten identisch. Zuchtversuche mit Samen von Arachis hypogaea dagegen verliefen erfolglos.

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Summary The host specificity of the lygaeid Oncopeltus fasciatus feeding on seeds of Asclepias syriaca, Helianthus annuus and Arachis hypogaea has been investigated. The bug clearly prefers seeds of A. syriaca over seeds of A. hypogaea and H. annuus though the latter are also accepted. Stylet penetration is stimulated by unidentified chemical factors occurring in the milkweed seed coat. It is assumed that this stimulation increases the frequency of feeding acts and thus the volume of uptaken food. An extract of the seed shell of A. syriaca was coated on the surface of Helianthus seeds. After this treatment the bug was stimulated to penetrate the latter. The same effect was obtained with the milky latex of Asclepias plants. O. fasciatus can be reared on seeds of H. annuus successfully. The speed of development and the reproduction capacity are diminished but the longevity and the weight of the eggs on both hosts (A. syriaca and H. annuus) are identical. Seeds of A. hypogaea, on the contrary, appeared to be totally unsuitable as a rearing substrate.

     

Monarch butterfly host plant (milkweed Asclepias spp.) abundance varies by habitat type across 98 prairies

The decline in migratory monarch butterflies (Danaus plexippus) over the past 20 years has been attributed to several drivers, including loss of their host plants (milkweeds Asclepias spp.). This has sparked widespread interest in milkweed ecology and restoration. We developed a model on environmental and habitat‐type variables to predict milkweed abundance by sampling 93 prairie plantings (47 conservation plantings and 46 roadsides) and 5 unplowed prairie remnants throughout the state of Iowa, United States. Milkweeds were censused in 10–25 random locations within each site, and data on plant diversity, age of planting, soil characteristics, and management were tested as predictors of abundance. Milkweed densities of all species combined were highest in remnant prairies (8,705 stems/ha), intermediate in roadside plantings (1,274 stems/ha), and lowest in conservation plantings (212 stems/ha). Most milkweeds were common milkweeds Asclepias syriaca, which were more abundant in roadside than conservation plantings. Remnants contained the most milkweed species. Total milkweed and common milkweed abundance were both predicted by higher soil pH, a more linear site shape, and lower soil bulk density across restorations. Our results indicate that common milkweed is maintained by disturbance, and establishes readily in rural roadside habitat. Remnants are important as reservoirs for multiple milkweed species and should be protected.     

Host specificity and variation in oviposition behaviour of milkweed stem weevils and implications for species divergence

1. An herbivore's life-history strategy, including optimization of resource use, is constrained by its evolutionary history and ecological factors varying across the landscape. 2. We asked if related and co-distributed herbivore species maintain consistency of host preference and oviposition behaviours along the species' range. We surveyed two putative species of milkweed stem weevils, Rhyssomatus lineaticollis and R. annectens, which co-occur alongside their hosts, Asclepias syriaca and A. incarnata. 3. We confirmed the two species status of weevils, supported by differences in morphology and a bilocus gene phylogeny. Furthermore, we found that species divergence recapitulated the weevils current host plant use. 4. We found oviposition variation within and between species. R. annectens poked the stem haphazardly or girdled it before oviposition. Meanwhile, R. lineaticollis primarily trenched stems in the north, but poked or girdled in the south. Variation in oviposition patterns could be a response to variation in host plant defenses. 5. In nature, weevils strictly oviposited on their respective host plants, while in bioassays, R. lineaticollis exhibited strong preference for A. syriaca and R. annectens fed equally on both host plants. 6. Overall, our results support that milkweed stem weevils are strict specialists but might be undergoing changes in host use. R. lineaticollis specializes on A. syriaca but has two distinct modes of oviposition. Meanwhile R. annectens seems to be more accepting of other hosts. We hypothesize that these weevils might be shifting host use associated with changes in host plant distributions.     

Natural food requirements of the large milkweed bug,Oncopeltus fasciatus (Hemiptera: Lygaeidae), and their relation to gregariousness and host plant morphology

Summary The life ofOncopeltus fasciatus centers on the seeds of milkweeds (Asclepiadaceae,Asclepias syriaca in this study). Adults reproduce prolifically on these seeds, but they engage in only half as much copulation and lay only a few eggs when fed milkweed buds and flowers instead. They can not maintain body weight on a diet of just vegetative plants. Vegetative shoots support only slow growth and produce adults only in certain circumstances.Seeds are often inaccessible to nymphs in the three youngest instars, since their mouthparts are too short to penetrate the thick walls ofA. syriaca pods. Nymphs feeding from the outside of closed pods develop more slowly than those feeding on exposed seeds. Since mortality occurs at a constant rate, slow growth results in fewer surviving. Even adults, which can feed through almost any pod wall, prefer to feed where it is thinnest. Thus the pod wall effectively protects many seeds from this herbivore.Nymphs in larger groups (20 individuals) suffer much lower mortality than those in small groups (5 individuals), when they are feeding from the outside of closed pods. However, group size does not affect survival when nymphs are fed seed. Gregariousness apparently partly compensates for the less nutritious diet attainable from the outside of pods and thus is related to this bug's extreme specialization of diet.Other characteristics coordinated with the seed requirement include the timing of migration and egg laying, the female's choice of oviposition site, and the nymphs' balanced tendencies to be sedentary and to disperse. Among the milkweeds, such traits as thick pod walls, wide spacing, and rapid seed dispersal could have been selected for by a seed predator such asO. fasciatus.As a specialist and a probable agent of such selection,O. fasciatus is committed to keeping up with changes in its host plant. This requires maintaining some flexibility in its behavior or gene pool. K. Evans (personal communication) finds thatO. fasciatus in California onA. fascicularis lays eggs so early that the first nymphs hatch as the first flowers are opening. In that more equitable climate and on that host, adult reproduction is apparently not as dependent on pods being present in the colonized patch, and the nymphs must rely on non-seed food more often than they do in the association withA. syriaca in the East and Midwest. This very specialized species apparently retains enough adaptability to exploit milkweeds of various forms and phenologies living in a wide range of climates. This adaptability could promise its continued success in the coevolutionary race with its host plants.     

Oviposition preference and larval performance of North American monarch butterflies on four Asclepias species

Monarch butterflies, Danaus plexippus L. (Lepidoptera: Nymphalidae), occur world‐wide and are specialist herbivores of plants in the milkweed family (Asclepiadaceae). In North America, two monarch populations breed east and west of the continental divide in areas populated by different host plant species. To examine the population variation in monarch responses to different Asclepias species, we measured oviposition preference and larval performance among captive progeny reared from adult butterflies collected in eastern and western North America. Host plant use was evaluated using two milkweed species widely distributed in eastern North America (A. incarnata and A. syriaca), and two species common to western North America (A. fascicularis and A. speciosa). We predicted that exposure to different host plant species in their respective breeding ranges could select for divergent host use traits, so that monarchs should preferentially lay more eggs on, and larvae should perform better on, milkweed species common to their native habitats. Results showed that across all adult female butterflies, oviposition preferences were highest for A. incarnata and lowest for A. fascicularis, but mean preferences did not differ significantly between eastern and western monarch populations. Larvae from both populations experienced the highest survival and growth rates on A. incarnata and A. fascicularis, and we again found no significant interactions between monarch source population and milkweed species. Moreover, the average rank order of larval performance did not correspond directly to mean female oviposition preferences, suggesting that additional factors beyond larval performance influence monarch oviposition behavior. Finally, significant family level variation was observed for both preference and performance responses within populations, suggesting an underlying genetic variation or maternal effects governing these traits.     

Host-associated fitness trade-offs do not limit the evolution of diet breadth in the small milkweed bug Lygaeus kalmii (Hemiptera: Lygaeidae)

Theoretical models of evolution in a temporally variable environment predict that genotypes with low variance in fitness across generations will be favored. When host use varies temporally and fitness trade-offs exist among hosts, such that an increase in performance on one host results in a correlated decrease on the other, selection for low variance in fitness across generations will favor genotypes which are generalists. Before predictions such as this can be extended to natural herbivore populations, however, it is necessary to understand the extent to which performance trade-offs limit simultaneous adaptation to multiple hosts. The experiment reported here compares two populations of the common milkweed bug, Lygaeus kalmii (Hemiptera: Lygaeidae) which differ in patterns of host usage. One population is largely restricted to milkweed (Asclepias spp.) when milkweed seeds are available, but becomes a scavenger on a large assortment of available seeds when milkweed seeds are unavailable. The second population is restricted largely to dandelion (Taraxacum officinale), without access to milkweed. We examine these populations to test for host-associated genetic trade-offs between specialization on dandelion (Taraxacum) and two species of milkweed, Asclepias fascicularis, which is low in cardiac glycoside content, and A. speciosa, which is high in cardiac glycoside content. Despite the difference in patterns of host use of the two L. kalmii populations, the populations did not differ in their performance on any of the host plants. Within each population, bugs performed nearly as well on each host, except that bugs had significantly lower survivorship on dandelion than on either milkweed species. Trade-offs in performance among hosts were not present in either population: estimated genetic correlations across hosts were strongly positive. The inability of this study to detect host-associated fitness trade-offs is consistent with most published data on this topic.     

Isolation, characterization and cross-species amplification of polymorphic microsatellite loci in Asclepias (Apocynaceae)

Ten polymorphic loci were isolated and characterized from the milkweed, Asclepias syriaca L., of North America. These loci successfully cross-amplified in A. exaltata L. Polymorphism ranged from two to 16 alleles per locus per species in 68 individuals of A. syriaca and 56 individuals of A. exaltata. Expected heterozygosities ranged from 0.017 to 0.851 and significant deviations from Hardy–Weinberg equilibrium were found for two and three loci in A. syriaca and A. exaltata, respectively. No linkage disequilibrium was detected. These markers should prove useful for assessing population genetic structure and interspecific gene flow in these and other species of Asclepias.     

The effect of host plant phenology on reproduction of the milkweed bug,Oncopeltus fasciatus,in tropical Florida

Summary A field study of the relationship between host plant phenology and the reproductive pattern of the large milkweed bug, Oncopeltus fasciatus, was conducted in south Florida. Since O. fasciatus need seeds of either milkweed or Nerium oleander plants to reproduce, reproduction takes place on only those host plants that are producing seed pods.Two of four major host plants, Asclepias incarnata and Sarcostemma clausa fruit seasonally, producing pods in early autumn and early winter, respectively. The third milkweed host, Asclepias curassavica, produces almost no pods midsummer (although it flowers abundantly) and few pods midwinter. Nerium oleander (Apocynaceae) produces some pods all year but is only used by O. fasciatus in the summer when milkweeds are not producing pods. Correspondingly, reproduction of O. fasciatus has been observed year round, but relatively few females reproduce in midwinter, coinciding with decreased pod production and low temperatures. This pattern is consistent with the hypothesis that a photoperiodic cue of short day lengths under conditions of cool temperatures may cause adult females to enter diapause and delay reproduction in the field.A comparison of plant phenologies and rainfall between 1976, a very dry year, and 1978, a year with normal rainfall, showed that extreme dryness disrupted the seasonal fruiting of the milkweeds and consequently the reproduction of O. fasciatus.     

Limited dispersal and its effect on population structure in the milkweed beetle Tetraopes tetraophthalmus

Summary The movement patterns of adult milkweed beetles, Tetraopes tetraphthalmus, were monitored via a mark-recapture technique. Movement or dispersal patterns were studied in two natural populations, one in which the host plant, Asclepias syriaca, was nearly continuously distributed over a 250×90 m area and another where Asclepias was distributed in 17 small discrete patches. In both populations dispersal distances resulting from the flight patterns of the adult beetles were quite short, averaging less than 40 m from the point of first encounter 10 days after marking. Males were shown to be more vagile than females. The distribution of dispersal distances collected from one of the populations was fit to three statistical distributions cited in the literature as expected from dispersal by many small-scale movements or observed in other species. It was found that an equation describing an exponential decay gave the best statistical fit to the data collected here for milkweed beetles. The data is discussed in the context of the effects of the limited dispersal power of the beetles and the distribution of suitable habitat on the population structure of Tetraopes.     

Some consequences for a parasitic herbivore,the milkweed longhorn beetle,Tetraopes tetrophthalmus,of a host-plant shift from Asdepias syriaca to A. verticillata

Summary The impact of differences in host plants on individuals and populations of insect herbivores was investigated using the milkweed longhorn beetle, Tetraopes tetrophthalmus (Forster), larvae of which feed parasitically on host rhizomes. One host, Asclepias syriaca L., was larger in stem and rhizome diameter and grew in cooler soil than the other host, A. verticillata L. The major effects on beetles were retarded phenology at the cooler site and reduced size on the smaller host. Reduced size of beetles was correlated with several important individual attributes: reduced length of life, number of ovarioles, egg size, and a reduced probability of mating with large beetles. The population consequences of these characteristics, largely inferred from these observations, were little or no outbreeding in the small population on A. verticillata.     

Aggregation and Mate Location in the Red Milkweed Beetle (Coleoptera: Cerambycidae)

We examined behavioral mechanisms underlying aggregation and mate location in the red milkweed beetle, Tetraopes tetrophthalmus (Forster) (Coleoptera: Cerambycidae). Larvae of this species feed on rhizomes of common milkweed, Asclepias syriaca L., and adults feed on the flowers and foliage, aggregating on individual stems within milkweed patches. Adults preferred to aggregate on milkweeds that had multiple, large inflorescences. Males actively searched for females, often flying between host plants. Mate location did not appear to involve long-range pheromones or vision, but rather males landed on milkweed stems arbitrarily, whether or not females were present. Males remained for longer periods, and so tended to accumulate, on milkweed stems that had female-biased sex ratios. We conclude that aggregation of T. tetrophthalmus is cued by host plant characteristics but dynamically influenced by the sex ratio of conspecifics present on individual stems.     

Chemical egg defence in the large milkweed bug,Oncopeltus fasciatus,derives from maternal but not paternal diet

Many herbivorous insects sequester defensive compounds from their host‐plants and incorporate them into their eggs to protect them against predation. Here, we investigate whether transmission of cardenolides from the host‐diet to the eggs is maternal, paternal, or biparental in the large milkweed bug, Oncopeltus fasciatus (Dallas) (Hemiptera: Lygaeidae). We reared individual bugs on either milkweed seeds [MW; Asclepias syriaca L. (Apocynaceae)] that contain cardenolides, or on sunflower seeds [SF; Helianthus annuus L. (Asteraceae)] that do not contain cardenolides. We mated females and males so that all four maternal/paternal diet combinations were represented: MW/MW, MW/SF, SF/MW, and SF/SF. Using larvae of the common green lacewing, Chrysoperla (Chrysopa) carnea (Stevens) (Neuroptera: Chrysopidae), we conducted two‐choice predation trials to assess whether maternal, paternal, or biparental transmission of cardenolides into the eggs of O. fasciatus increased protection against predation. Furthermore, we used high performance liquid chromatography (HPLC) to assess putative cardenolide content of eggs from the various parental diet treatment groups. The predation trials suggested that regardless of male diet, eggs were afforded better protection when females had been raised on milkweed. However, many eggs were at least partially consumed. This suggests that although chemical defence of eggs does not guarantee protection to eggs on an individual basis, they may increase the probability that some eggs in a clutch are left intact thereby potentially conferring a fitness advantage to more offspring than if eggs are left unprotected. Based on HPLC analysis we found that maternal contribution of cardenolides was significantly greater than paternal contribution of cardenolides to the eggs, supporting the results of our predation trials that a maternal diet of milkweed makes eggs more distasteful than a paternal diet of milkweed.     

Studies on D-alanine in the Heteroptera*

Four milkweed associated insects were assayed for free D-alanine by gas-liquid chromatographic methods. Oncopeltus fasciatus and Lygaeus kalmii, reared aseptically on milkweed seeds, showed high titres of free D-alanine. Labidomera clivicollis showed traces of the D-isomer whereas none could be detected in Tetraopes tetrophthalmus. O.fasciatus reared aseptically on sunflower seed showed high titres of D-alanine. No D-alanine could be detected in milkweed seed hydrolysates. Several museum specimens of the orders Hemiptera and Homoptera were assayed and all showed high titres of D-alanine.     

Intraspecific variation in responses to aposematic prey in a jumping spider (Phidippus regius)

Aposematic signals often allow chemically defended prey to avoid attack from generalist predators, including jumping spiders. However, not all individual predators in a population behave in the same way. Here, in laboratory trials, we document that most individual Phidippus regius jumping spiders attack and reject chemically defended milkweed bugs (Oncopeltus fasciatus), immediately releasing them unharmed. However, a small number of individuals within the population kill and completely consume these presumably toxic prey items. This phenomenon was infrequent with only 14% of our sample (17/122) consuming the milkweed bugs over the course of the study. Individuals that killed and consumed bugs often did so repeatedly; specifically, individuals that consumed a bug in their first test were more likely to kill a bug in their second test and also tended to consume them again. We explored what might drive some (but not all) individuals to consume these bugs and found that neither sex, sexual maturity, body size, laboratory housing type, nor being wild-caught or being laboratory-reared, predicted milkweed bug consumption. Consuming bugs had no negative effects on spider mass or body condition; contrary to expectations, individuals that consumed milkweed bugs actually gained more body mass and increased in body condition. We discuss potential behavioural and physiological variation between individuals that may drive these rare behaviours and the implications for the evolution of prey defences.     

Identification of the adipokinetic hormone of the large milkweed bug Oncopeltus fasciatus

Abstract The adipokinetic hormone (AKH) of the large milkweed bug Oncopeltus fasciatus is isolated from an acidified methanolic extract of 200 corpora cardiaca, purified by single step reversed phase high‐performance liquid chromatography (HPLC) and N‐terminally deblocked using pyroglutamate aminopeptidase. The sequence is identified by Edman degradation and matrix assisted laser desorption/ionization‐time of flight mass spectroscopy as pGlu‐Leu‐Asn‐Phe‐Ser‐Pro‐Asn‐Trp amide. This structure is confirmed by chemical synthesis and coelution of native and synthetic peptide on HPLC. The AKH of O. fasciatus is identical to Tenmo‐HrTH, a member of the adipokinetic/red pigment‐concentrating hormone peptide family that had been isolated earlier from several tenebrionid beetles. Tenmo‐HrTH causes a significant rise in the concentration of haemolymph lipids when injected into adult male and female O. fasciatus, but displays no hyperglycaemic activity. There is no indication of the presence of other AKHs in O. fasciatus. The large milkweed bug represents the first member of the seed bugs (Lygaeidae) for which the endogenous AKH has been identified.     

Climate Change May Alter Breeding Ground Distributions of Eastern Migratory Monarchs (Danaus plexippus) via Range Expansion of Asclepias Host Plants

Climate change can profoundly alter species’ distributions due to changes in temperature, precipitation, or seasonality. Migratory monarch butterflies (Danaus plexippus) may be particularly susceptible to climate-driven changes in host plant abundance or reduced overwintering habitat. For example, climate change may significantly reduce the availability of overwintering habitat by restricting the amount of area with suitable microclimate conditions. However, potential effects of climate change on monarch northward migrations remain largely unknown, particularly with respect to their milkweed (Asclepias spp.) host plants. Given that monarchs largely depend on the genus Asclepias as larval host plants, the effects of climate change on monarch northward migrations will most likely be mediated by climate change effects on Asclepias. Here, I used MaxEnt species distribution modeling to assess potential changes in Asclepias and monarch distributions under moderate and severe climate change scenarios. First, Asclepias distributions were projected to extend northward throughout much of Canada despite considerable variability in the environmental drivers of each individual species. Second, Asclepias distributions were an important predictor of current monarch distributions, indicating that monarchs may be constrained as much by the availability of Asclepias host plants as environmental variables per se. Accordingly, modeling future distributions of monarchs, and indeed any tightly coupled plant-insect system, should incorporate the effects of climate change on host plant distributions. Finally, MaxEnt predictions of Asclepias and monarch distributions were remarkably consistent among general circulation models. Nearly all models predicted that the current monarch summer breeding range will become slightly less suitable for Asclepias and monarchs in the future. Asclepias, and consequently monarchs, should therefore undergo expanded northern range limits in summer months while encountering reduced habitat suitability throughout the northern migration.     

The effect of different host plants on the reproduction and longevity of Nysius natalensis

Nysius natalensis Evans (Hemiptera: Orsillidae) is a pest of sunflower in South Africa. Adults invade sunflower fields from their weedy hosts which occur inside crop fields and on surrounding headlands. The host plant suitability for survival and reproduction as well as the effect of within‐generation host switching was studied on different wild host plants and sunflower. Life history parameters used to assess host plant suitability were F₁ adult survival, pre‐oviposition period, fecundity, and longevity. Nymphs and adults were provided with stems and seeds of five host plants, viz., Amaranthus hybridus L. (Amaranthaceae), Portulaca oleracea L. (Portulacaceae), Chenopodium album L. (Chenopodiaceae), Conyza albida Spreng. (Asteraceae), and sunflower, Helianthus annuus L. (Asteraceae). Nymphs were reared on crushed seed of the five plant species. After completion of the nymphal stage, emerging adults of each host plant group were provided with seed of a different host plant species for food. Adults did not survive long on stems only and very few eggs were laid. Seeds of the host plant species were shown to be an essential source of nutrients for N. natalensis reproduction, whereas the vegetative plant parts were unsuitable. Nymphal food and host‐plant switching between the nymphal and adult stages significantly affected the pre‐oviposition period. Nymphal and adult food source also affected female longevity. The number of eggs laid was not influenced by nymphal food, but was influenced by adult food and the switch between nymphal and adult food. The comparative attractiveness of sunflower and wild host plants for oviposition was also investigated and showed that females preferred to lay eggs on wild host plants, compared with sunflower. These results may explain why N. natalensis will lay their eggs on sunflower after weeds in the vicinity are controlled, or senesce toward the end of the growing season.