Laboratory performance of two polyphagous invasive weevils on the predominant woody plant species of a northern hardwood community

A complex of invasive weevils that consume roots as larvae and foliage as adults have become established in northern hardwood forests in North America. We evaluated adults of the two most prominent species, Phyllobius oblongus and Polydrusus sericeus, for longevity, foliage consumption, and egg production on several putative hosts commonly found in this ecosystem. Adult pairs were monitored in no-choice laboratory assays for the duration of their lifespans on basswood, Tilia americana, ironwood, Ostrya virginiana, sugar maple, Acer saccharum, raspberry, Rubus spp., or leatherwood, Dirca palustris. Overall, P. sericeus lived more than twice as long as P. oblongus and lived longer on all hosts. P. sericeus consumed more total leaf area than P. oblongus on basswood, ironwood, and raspberry, but P. oblongus had a higher leaf consumption rate on sugar maple. Basswood was a very good host for P. sericeus. Leatherwood was not a suitable host for either weevil species. The higher longevity and fecundity of P. sericeus than P. oblongus did not agree with that expected from population data, in that the latter species is substantially more abundant. This likely reflects P. oblongus ' superior performance on sugar maple, the dominant flora in the study area. These data provide a basis for estimating the broader impacts of adult weevil feeding.     

Prior host feeding experience influences ovipositional but not feeding preference in a polyphagous insect herbivore

Black vine weevils, Otiorhynchus sulcatus (Fabricius) (Coleoptera: Curculionidae), are globally‐distributed polyphagous pests of many horticultural crops. We investigated how adult weevils were affected by host switching and, in particular, how host plant species nutritional and defensive chemistry affected subsequent host plant species selection and oviposition. Adults were fed one of three host plant species, blackcurrant [Ribes nigrum L. (Grossulariaceae)], raspberry [Rubus idaeus L. (Rosaceae)], or strawberry [Fragaria x ananassa Duchesne (Rosaceae)], throughout their pre‐reproductive periods and then subjected to behavioral choice assays with these plants. Foliar chemistry differed significantly among the three host plant species. Compared to raspberry and strawberry foliage, blackcurrant foliage was 13% lower in nitrogen, 3% higher in carbon, and 28% higher in phenolic compounds. Initial host plant species had a significant effect on weevil mortality, with more weevils dying when previously fed blackcurrant (12%) than strawberry (3%) or raspberry (0%) regardless of subsequent host. Initial host plant species also affected oviposition, with weevils laying only ca. two eggs per week when previously fed blackcurrant, compared to those on raspberry or strawberry (ca. 11 and 15 eggs per week, respectively). When given a choice, weevils discriminated among host plant species and tended to oviposit on plants on which they had previously fed, even when the plant was nutritionally inferior for egg production and adult survival. In contrast, feeding behavior was only affected by the current host plant species. Feeding and oviposition were related to leaf chemistry only in blackcurrant, as leaf consumption was negatively correlated with foliar carbon and zinc concentrations, and positively correlated with foliar phosphorus and potassium concentrations.     

Diversity of birch sawfly responses to seasonally atypical diets

Most insect herbivores are specialised on a particular plant taxon. To have a better understanding of host shift functions and consequences for insect herbivores, it is essential to gather more information on the effects of variation in host quality on specialists across species and environments. We examined the effects of seasonally atypical food on mortality, developmental time, and final body mass of six sawfly species (Hymenoptera: Symphyta) feeding on the foliage of mountain birch (Betula pubescens ssp. czerepanovii), whose pooled larval feeding periods form a gradient and cover the growing season. Insect phenology was manipulated so that the larvae of early-season species would feed on atypically mature leaves and mid- or late-season species would feed on atypically young leaves of their major host plant. Mortality increased dramatically for all species when the larval feeding schedule was advanced or delayed. This indicates a high degree of specialisation not only on a particular host but also to its phenological phases. The main cause of mortality on novel food was a rejection of the diet by the young larvae and their subsequent starvation. An interesting observation was that late-season species showed this response on nutritious young foliage. The effects of seasonally atypical diets on larval development and growth were species-specific and milder than the effects on mortality. Interestingly, for those individuals that accepted it, atypical food seemed to be most beneficial for species appearing at both ends of the seasonal gradient, which might be related to a wider exposure to variable food quality in natural conditions compared with other species. The diversity of responses to atypical food among closely related herbivore species with overlapping feeding periods on the same host plant is the most crucial finding of this study.     

Aerial dispersal and host plant selection by neonate Thyridopteryx ephemeraeformis (Lepidoptera: Psychidae)

Abstract.  1. Neonate evergreen bagworms, Thyridopteryx ephemeraeformis (Haworth) (Lepidoptera: Psychidae), disperse by dropping on a strand of silk, termed silking , and ballooning on the wind. Larvae construct silken bags with fragments of plant foliage. This species is highly polyphagous, feeding on more than 125 species of woody plants of 45 families. The larvae commonly infests juniper ( Juniperus spp.) and arborvitae ( Thuja spp.), but rarely feed on deciduous hosts such as maples. The hypothesis is proposed that polyphagy in T. ephemeraeformis is maintained by variation among larvae in dispersal behaviour, and time constraints on the opportunity to disperse, but patterns of host species preference result from a predisposition for larvae to settle on arborvitae and juniper but disperse from other hosts.
2. Consistent with that hypothesis, laboratory experiments revealed: (a) starved larvae varied in their tendency to disperse from paper leaf models; (b) starved larvae readily silked only during their first day; (c) larvae became increasingly sedentary the longer they were exposed to plant foliage; (d) when provided with several opportunities to silk, larvae became sedentary after exposure to arborvitae foliage, but repeatedly silked after exposure to maple ( Acer species) foliage or paper; and (e) larvae were less inclined to silk from foliage of arborvitae than from maple.
3. Field experiments supported the hypothesis by demonstrating that: (a) neonates tended to disperse from maple leaves while larvae older than 1 day tended to settle and remain; and (b) neonates were less likely to disperse from arborvitae and juniper trees than from maples.     

Variable host phenology does not pose a barrier to invasive weevils in a northern hardwood forest

• 1 A suite of invasive weevils has established in hardwood forests of the North American Great Lakes Region. We quantified patterns of host availability and the capacity of adults to succeed in a system with high host variability both within and between seasons in Michigan, U.S.A.
• 2 We quantified phenological development of foliage on three host species [sugar maple, Acer saccharum Marshall; ironwood, Ostrya virginiana (Mill.) K. Koch; and raspberry, Rubus spp.]. We estimated adult abundance using emergence traps and sweep net sampling over 3 years, and compared field host associations with laboratory choice assays.
• 3 Host plant phenology varied among species, between years, and in their interactions. The four most common weevils, Phyllobius oblongus (L.), Polydrusus sericeus (Schaller), Barypeithes pellucidus (Boheman) and Sciaphilus asperatus (Bonsdorff), emerged in early to mid‐June, in approximately that order. After emergence, each species showed evidence of host preference based on their abundances on foliage. Overall, P. oblongus and B. pellucidus were most prevalent on sugar maple, P. sericeus was most prevalent on ironwood, and S. asperatus was relatively evenly distributed. Laboratory choice tests with P. oblongus and P. sericeus confirmed these preferences.
• 4 These four invasive species comprised over 99% of all 12 845 weevils obtained, suggesting displacement of native species. The optimal sampling methods varied among weevil species.
• 5 These invasive weevils contend with the highly variable conditions of their environment, and also potential phenological asynchrony, via relatively late emergence, even at the cost of lower host quality. Annual variation is greater for numbers of adults than larvae, suggesting that mortality of late instars or pupae is particularly important.

     

SEASONAL AND INDIVIDUAL VARIATION IN LEAF QUALITY OF TWO NORTHERN HARDWOODS TREE SPECIES

Seasonal trends in five traits of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula allegheniensis Britt.) leaves thought to influence feeding by herbivores were measured from 17 May through 19 September, 1979. Total nitrogen and water contents declined and toughness increased through the growth season. These seasonal changes were more pronounced in sugar maple than in yellow birch. Total polyphenol contents and tanning coefficients of leaf extracts from both species reached a season high by the end of May and changed very little after that date; these patterns differ from those reported by several other investigators. Sugar maple leaf extracts exhibited much higher tanning coefficients than did those of yellow birch, a finding which is consistent with current plant defense theory. Significant differences in total polyphenol content and tanning coefficients were found between individual trees in yellow birch, but not sugar maple. The relationship between successional status, leaf quality traits, and variability in these traits in forest trees is discussed.     

Plant phenology-mediated indirect effects: The gall midge opens the phenological window wider for a leaf beetle

We examined whether larvae of the gall midge Rabdophaga rigidae (Diptera: Cecidomyiidae) can modify the seasonal dynamics of the density of a leaf beetle, Plagiodera versicolora (Coleoptera: Chrysomelidae), by modifying the leaf flushing phenology of its host willow species, Salix serissaefolia and Salix eriocarpa (Salicaceae). To test this, we conducted field observations and a laboratory experiment. The field observations demonstrated that the leaf flushing phenology of the willows and the seasonal dynamics of the beetle density differed between shoots with stem galls and shoots without them. On galled shoots of both willow species, secondary shoot growth and secondary leaf production were promoted; consequently, leaf production showed a bimodal pattern and leaf production periods were 1 to 2 months longer than on non‐galled shoots. The adult beetle density on galled shoots was thus enhanced late in the season, and was found to change seasonally, synchronizing with the production of new leaves on the host willow species. From the results of our laboratory experiment, we attributed this synchrony between adult beetle density and willow leaf flush to beetles’ preference to eat new leaves rather than old. Indeed, beetles consumed five times more of the young leaves when they were fed both young and old leaves. These results indicate that stem galls indirectly enhance the adult beetle density by enhancing food quality and quantity late in the beetle‐feeding season. We therefore conclude that midge galls widen the phenological window for leaf beetles by extending the willows’ leaf flush periods.     

Early spring defoliation,secondary leaf flush,and leafminer outbreaks on American holly

Summary Adults of Phytomyza ilicicola (Diptera: Agromyzidae), a univoltine specialist leafminer, emerge in close synchrony with leaf flush of American holly and feed on and oviposit in soft, partially expanded leaves. Early spring defoliation, such as commonly results from freezing injury to young shoots, is followed several weeks later by a second flush of young leaves from lateral buds. We simulated this phenomenon by manually defoliating whole small trees and individual shoots of large trees to test the hypothesis that freezing injury can encourage leafminer outbreaks by inducing an abundance of soft, protein rich young leaves late in the adult activity period, when availability of vulnerable leaves becomes limited. Defoliation of small trees one or two weeks after bud break resulted in six- to 13-fold increases in the incidence of feeding punctures and larval mines on second flush leaves as compared with densities on original young leaves of control trees. Similarly, we induced significant increases in feeding punctures and larval mines on second flush leaves of individual defoliated shoots, although leaves that did not open until after the flight period escaped this injury. These observations underscore the capability of adult female P. ilicicola to locate and exploit a small number of phenologically available leaves among many hundreds of older leaves on the same tree. By altering the phenology of leaf flush, certain kinds of environmental stress may predispose perennial plants to outbreaks of early season folivores that restrict their feeding or oviposition to very young leaves.     

Variation in the susceptibility of the forest tent caterpillar (Lepidoptera: Lasiocampidae) to Bacillus thuringiensis variety kurstaki HD-1: effect of the host plant.

Host-mediated effect on the efficacy of Bacillus thuringiensis Berliner against larvae of the forest tent caterpillar. Malacosoma disstria Hübner, was investigated under controlled conditions. Host plants used in this study were quaking aspen, Populus tremuloides Michx., a preferred host, and sugar maple, Acer saccharum Marsh., a secondary host. Larvae were reared in the laboratory on leaves of these hosts, and upon reaching the third, fourth, and fifth instar, they were fed leaves treated with one of a range of concentrations of B. thuringiensis variety kurstaki HD-1 suspensions. Larvae were tested on the host on which they were feeding before the 4-d bioassays. The estimated LC50s were 100-fold greater on quaking aspen than on sugar maple. Also, there was a decrease in efficacy over the whole ranges of concentrations with larval age on both hosts. LC50s varied approximately two-fold between third and fifth instar. These results indicate that host-mediated effects on B. thuringiensis efficacy warrant more interest. In particular, they strongly indicate that the host plant modifies the interaction between B. thuringiensis and a target insect, and offer the opportunity to investigate the mechanism(s) that may be involved in the enhancement of B. thuringiensis toxicity.     

14C transfer between the spring ephemeral Erythronium americanum and sugar maple saplings via arbuscular mycorrhizal fungi in natural stands

We investigated in the field the carbon (C) transfer between sugar maple (Acer saccharum) saplings and the spring ephemeral Erythronium americanum via the mycelium of arbuscular mycorrhizal (AM) fungi. Sugar maple saplings and E. americanum plants were planted together in pots placed in the ground of a maple forest in 1999. Ectomycorrhizal yellow birches (Betula alleghaniensis) were added as control plants. In spring 2000, during leaf expansion of sugar maple saplings, the leaves of E. americanum were labelled with ¹⁴CO₂. Seven days after labelling, radioactivity was detected in leaves, stem and roots of sugar maples. Specific radioactivity in sugar maples was 13-fold higher than in yellow birches revealing the occurrence of a direct transfer of ¹⁴C between the AM plants. The quantity of ¹⁴C transferred to sugar maple saplings was negatively correlated with the percentage of ¹⁴C allocated to the storage organ of E. americanum. A second labelling was performed in autumn 2000 on sugar maple leaves during annual growth of E. americanum roots. Radioactivity was detected in 7 of 22 E. americanum root systems and absent in yellow birches. These results suggest that AM fungi connecting different understorey species can act as reciprocal C transfer bridges between plant species in relation with the phenology of the plants involved.     

Progressive restriction of host plant exploitation along a climatic gradient: the willow psyllid Cacopsylla groenlandica in Greenland

1. Host plant selection by the endemic willow psyllid Cacopsylla groenlandica was studied at eight sites in three locations along a N–S climatic gradient in west Greenland.
2. C. groenlandica oviposited and developed on four willow species: Salix glauca, S. arctophila, S. uva-ursi and S. herbacea. Development took place on female catkins of all species and on leaf shoots of S. glauca .
3. At sites in southern Greenland, C. groenlandica fully utilized all four willow species. However, there was progressive reduction of host plant species and tissues exploited as latitude increased, such that towards the northern limit of its range the psyllid became highly specialized, feeding only on female catkins of S. glauca , despite the presence of alternative hosts. Where plant growth was tightly constrained within a restricted growing season, the demands of synchronizing with more than one species may have become too great, resulting in specialization on a single host.
4. The probable mechanisms producing specialization were between-species differences in host plant phenology, linked to variation in the growing season, set against the partially non-overlapping geographical distribution patterns of the host willows.
5. A small degree of phenological asynchrony between host plants can be advantageous for the insect. Under favourable conditions the reproductive season can be extended by using a series of host plants with staggered phenologies. Some 'early' hosts also create predator-free space. However, as asynchrony increases, the advantages disappear as the insect becomes unable to exploit the full range of host options.
6. The importance of host plant synchrony in the life histories of arctic insects is discussed.     

Abundance and Leaf Damage Caused by Gall‐Inducing Insects in a Mexican Tropical Dry Forest1

We evaluated the temporal and spatial patterns of abundance and the amount of damage caused by gall‐inducing insects (GII) in deciduous and riparian habitats in a seasonal tropical dry forest in Mexico. Plants occurring in these habitats differ in their phenology and moisture availability. Deciduous habitats are seasonal and xeric, while riparian habitats are aseasonal and mesic. We found 37 GII species and each one was associated with a specific plant species. In total, 19 species (51.3%) were present in deciduous habitats, 13 species (35.2%) in riparian habitats, and only 5 species (13.5%) occurred in both. Abundance and leaf damage by GII were greater in deciduous than in riparian habitats during the wet season. For each GII species that occurred in both habitats, host plant species supported greater abundance and leaf damage by GII in deciduous habitats during the wet season. These results indicate a greater association of GII species with host plants in deciduous than in riparian habitats during the wet season. In riparian habitats, 11 plant species (61.1%) had greater density of GII in the dry than in the wet season. Similarly, leaf damage by GII was significantly greater in the dry than in the wet season in riparian habitats for 12 plant species (66.7%). Dry forest plants of riparian habitats presented two peaks of leaf‐flushing: GII colonized leaves produced in the first peak at the beginning of the wet season, and accumulated or recolonized leaves in the second peak at the beginning of the dry season. The levels of leaf damage by GII detected in this study in the rainy season were considerably higher than those obtained for folivorous insects in other neotropical forests, suggesting that this GII guild might have an important impact on their host plant species in this tropical community.     

Intra-plant host selection, oviposition preference and larval survival of Chrysophtharta agricola (Chapuis) (Coleoptera: Chrysomelidae: Paropsini) between foliage types of a heterophyllous host

Abstract 1 Paropsine chrysomelid beetles defoliate commercial eucalypt plantations in Australia. Adults and larvae feed on the same host, with the larval food source determined by the oviposition choice of females. Most eucalypt species are heterophyllous, with their foliage undergoing distinct morphological and chemical changes between adult and juvenile growth. 2 The intra‐plant foliage feeding and oviposition preference adults and the larval development of Chrysophtharta agricola were examined using adult and juvenile foliage of a heterophyllous plantation species, Eucalyptus nitens. The foliage types differ in chemistry, toughness, waxiness and timing of production. 3 In the field, feeding damage caused by adult beetles was 15% more frequent on adult foliage than on juvenile foliage; however, egg batches were three times more common on juvenile than on adult foliage. 4 Oviposition preference for juvenile foliage over adult foliage was confirmed in choice trials in the laboratory, with adult fecundity and longevity not significantly different between foliage types. 5 Larval survival, development time and subsequent pupal weight were also unaffected by foliage type, suggesting that neither foliage type is nutritionally superior for adults or for larvae. However, adult foliage was significantly thicker than juvenile foliage and this may prove a physical constraint to larval establishment. Biotic and abiotic factors (including interactions with natural enemies, competition, microclimate and mate location) that may affect patterns of host plant utilization are discussed.     

Biology and preliminary host range assessment of two potential kudzu biological control agents

Two insect species from China, Gonioctena tredecimmaculata (Jacoby) (Coleoptera: Chrysomelidae) and Ornatalcides (Mesalcidodes) trifidus (Pascoe) (Coleoptera: Curculionidae), were studied in quarantine in the United States as potential biological control agents for kudzu, Pueraria montana variety lobata (Willd.) Maesen and S. Almeida. Adults of G. tredecimmaculata were ovoviviparous and reproduced throughout the summer, producing offspring that had an obligate adult diapause. In no-choice tests, adult and larval G. tredecimmaculata rejected most of the plant species tested, but consumed foliage and completed their life cycle on soybean (Glycine max L. Merr.) and on a native woodland plant, hog-peanut (Amphicarpaea bracteata L. Fernald), which are in the same subtribe as kudzu (Glycininae). Insects showed similar responses to field- and greenhouse-grown soybean and kudzu foliage, despite measurable differences in leaf traits: field-grown foliage of both plants had greater leaf toughness, higher total carbon content, higher trichome density, and lower water content than greenhouse foliage. O. trifidus adults also rejected most of the plants tested but fed on and severely damaged potted soybean and hog-peanut plants in addition to kudzu. Further tests in China are needed to determine whether these species will accept nontarget host plants under open-field conditions.     

Differences in the success of sugar maple and red maple seedlings on acid soils are influenced by nutrient dynamics and light environment

Variation in tolerance to nutrient limitations may contribute to the differential success of sugar maple ( Acer saccharum Marsh.) and red maple ( Acer rubrum L.) on acid soils. The objectives of this study were to examine these relationships as influenced by light environment and test whether sensitivity to nutrient stress is mediated by oxidative stress. First-year sugar maple and red seedlings were grown on forest soil cores contrasting in nutrient availability under high or low light intensity. Foliar nutrition, photosynthesis, growth and antioxidant enzyme activity were assessed. Photosynthesis and growth of sugar maple were significantly lower on nutrient-poor soils and were correlated with leaf nutrient status with Ca and P having the strongest influence. For red maple, only chlorophyll content showed sensitivity to the nutrient-poor soils. High light exacerbated the negative effects of nutrient imbalances on photosynthesis and growth in sugar maple. Antioxidant enzyme activity in sugar maple was highest in seedlings growing on nutrient-poor soils and was inversely correlated with photosynthesis, Ca, P, and Mg concentrations. These results suggest that: (1) sugar maple is more sensitive to nutrient stresses associated with low pH soils than red maple; (2) high light increases sugar maple sensitivity to nutrient stress; (3) the negative effects of nutrient imbalances on sugar maple may be mediated by oxidative stress.     

Timing is everything? Phenological synchrony and population variability in leaf‐chewing herbivores of Quercus

Abstract.  1. Specialization on ephemeral resources (e.g. new leaves) should produce large annual variation in herbivore population size when the timing of availability of those resources is unpredictable. Despite considerable evidence for impacts of synchrony with budburst on survival of larval Lepidoptera, previous studies of adult Geometridae and Noctuidae found no correlations between insect phenology and population variability.
2. We surveyed larval Lepidoptera feeding on Quercus alba and Q. velutina in Missouri from 1993 to 2003 and examined population variability, measured as the coefficient of variation of population density (CV), in a subset of abundant species. We compared CV values among species whose larvae feed only in spring, early summer, mid-summer, late summer, or all season. We predicted that univoltine species whose larvae eclose and complete development in spring during leaf expansion would have higher variability than species feeding later in the season, having multiple generations, or having longer development times.
3. As predicted and consistent with hypotheses, spring-feeding species had CV values 32% higher than species feeding in summer months. Coefficients of variation were also 34% higher in leaf-rolling and mining guilds compared with free-feeders, suggesting that mobile species may compensate for asynchrony with budburst by dispersing to higher quality plants or plant parts. Multivoltine species, however, did not differ from univoltine species in population variability.
4. Our results suggest that asynchrony with plant phenology and factors that might exacerbate it, such as climate change, will have the largest impacts on the dynamics of spring-feeding Lepidoptera, particularly species with limited mobility.     

Feeding patterns of monophagous,oligophagous, and polyphagous insect herbivores: The effect of resource abundance and plant chemistry

Summary Leaf tissue preferences of monophagous, oligophagous, and polyphagous insect herbivores were determined using young and mature leaf tissue abundances and herbivore feeding observations. Larvae of monophagous and oligophagous herbivores preferred young leaf tissues while, overall, larvae of polyphagous species preferred mature leaves of their various host plants. Even though a species is often polyphagous over its geographical range, larvae from local populations may be very specialized in their diet. When this occurs these specialized larvae prefer the more nutritious and perhaps more toxic young leaves of some of their host plants. Resource abundance and plant chemistry are discussed as major factors influencing herbivore feeding patterns.     

Larval food affects oviposition preference,female fecundity and offspring survival in Yponomeuta evonymellus

1. Yponomeuta evonymellus is a monophagous moth that feeds on Prunus padus which is native to Europe. In recent years, larval feeding and egg clusters have also been observed on non‐native Prunus serotina plants; however, survival of larvae on this new host is very low. 2. The objective of the present study was to determine how the feeding of larvae on each of the two host plants impacts oviposition, offspring survival and fecundity in Y. evonymellus. Our hypothesis was that, under controlled conditions, females will lay eggs on the host on which they fed as larvae. We also hypothesised that the lower survival of young larvae feeding on P. serotina was due to the smaller buds and leaves present in this species, relative to those of P. padus. 3. A dual‐choice experiment conducted under laboratory conditions demonstrated that females preferentially chose to oviposit on the plant species on which they fed as larvae. In the experiment, potential fecundity and offspring survival were significantly higher on P. padus than on P. serotina. The reduced performance of Y. evonymellus on P. serotina was correlated with a smaller bud mass and volume, lower leaf mass and surface area, and difficulty in constructing a protective tent against unfavourable weather conditions. 4. In summary, the identity of the host plant species during larval feeding determines adult oviposition preference for that host species. The survival of larvae on P. serotina growing in the nature is low, but for phenology‐related reasons.     

Experience-based food consumption by larvae of Pieris rapae: addiction to glucosinolates?

When newly hatched larvae of P. rapae were transferred to cowpea foliage, they readily accepted this non-host as food, whereas later instars that had fed on cabbage rejected cowpea. However, when cowpea leaf discs were treated with aqueous extracts of cabbage foliage, they were accepted by cabbage-reared larvae. Experiments were conducted to determine whether larvae reared on one host plant would be stimulated to feed by extracts of other hosts. Larvae reared on Brassica juncea, Cleome spinosa, Tropaeolum majus, Sinapis alba, Alliaria petiolata, Barbarea vulgaris and cabbage (Brassica oleracea) were offered extracts of each of the other host plants on cowpea discs in choice assays. Larvae were generally stimulated to feed by extracts of all the alternate hosts, but quantitative differences in consumption occurred. In most cases, levels of discrimination between treatment and control cowpea discs showed no significant preference for extracts of the previously experienced plant. Since the test plants (and their extracts) contain glucosinolates of widely different structures, a general addiction to glucosinolates was suggested. A single glucosinolate, sinigrin, was sufficient to elicit feeding by cabbage-reared larvae. The time required for individual neonates to become addicted to glucosinolates as they fed on cabbage, as measured by refusal of cowpea, varied from 6 to 30 hours. Bioassays of cowpea extracts failed to show any deterrent activity and, therefore, supported the conclusion that addiction to glucosinolates is responsible for the fixation of P. rapae larvae on their host plants.     

Pathogen invasion triggers an evolutionary trap for an endangered checkerspot butterfly dependent on an exotic host plant

Exotic pathogen invasions can change host eco-evolutionary interactions and possibly create an evolutionary trap when the pathogen generates mismatches between developmental phenology and reproductive cues. Taylor’s checkerspot butterfly (Euphydryas editha taylori), is an endangered species of western North America with 80 % of the extant populations dependent on an exotic host, Plantago lanceolata. Survey of occupied, recently extinct, and unsuccessful butterfly reintroduction sites spanning 4° of latitude revealed widespread disease on P. lanceolata caused by Pyrenopeziza plantaginis. This fungal pathogen, new to North America, reduces the standing crop of P. lanceolata foliage throughout the winter post-diapause larval feeding period. However, disease is absent when adult butterflies and pre-diapause larvae are active. Diseased plants were frequent in Taylor’s checkerspot populations with a history of persistence, but >90 % of the host plants in these populations had initiated new leaves within the first few weeks of post-diapause larval feeding. Conversely, host plants in recently extinct and unsuccessfully reintroduced populations were severely diseased, >66 % mean foliage necrosis/plant, and <23 % had initiated new leaves. Feeding choice trials with 25 larvae indicated that new leaves were strongly and consistently preferred by post-diapause larvae over all other available leaf types, both diseased and non-diseased. Because the influence of disease on post-diapause larval food resources is developmentally disassociated from oviposition, P. plantaginis invasion appears to have triggered an evolutionary trap for Plantago-dependent populations of Taylor’s checkerspot.