Host plant nutritional quality affects the performance of the parasitoid Diadegma insulare

The well documented biochemical profile of Brassicaceae, oligophagy of the herbivore Plutella xylostella (L.) (Lepidoptera: Plutellidae), and host specialization of the parasitoid Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) provide an ideal system for investigating tritrophic interactions mediated by nutritional quality of plants. We evaluated the bottom-up effects of five soil fertility regimes on nutritional quality of canola (Brassica napus L.) and then on several fitness correlates of female and male D. insulare as mediated through P. xylostella. Variation in soil fertility influenced the nutritional quality of host plants and this in turn affected the performance of D. insulare. In general, D. insulare performed best on plants grown with 3.0 g fertilizer pot⁻¹; these plants had 2.06-, 3.77-, and 1.02-fold more nitrogen, phosphorous and potassium, respectively than ones grown without any added fertilizer. P. xylostella escape from D. insulare was highest (32%) on plants grown at 1.0 g fertilizer, and this could be attributed to both physical and physiological defense mechanisms mediated by host plant nutritional quality. Plant stress and plant vigor are competing paradigms pertaining to the performance of herbivorous insects on their host plants. These hypotheses were originally proposed to predict responses of herbivores, but may also explain the effects of plant quality on koinobiont parasitoids, such as D. insulare.     

Diadegma insulare development is altered by Plutella xylostella reared on water‐stressed host plants

Natural enemies of herbivores function in a multitrophic context, and their performance is directly or indirectly influenced by herbivores and their host plants. Very little is known about tritrophic interactions between host plants, pests and their parasitoids, particularly when the host plants are under any stress. Herbivores and their natural enemies’ response to plants under stress are diverse and variable. Therefore, in this study we investigated how diamondback moth, Plutella xylostella (L.), reared on water‐stressed host plants (Brassica napus L. and Sinapis alba L.) influenced the development of its larval parasitoid, Diadegma insulare (Cresson). No significant differences were observed in development of P. xylostella when reared on water‐stressed host plants. However, all results indicated that water stress had a strong effect on developmental parameters of D. insulare. Development of D. insulare was delayed when the parasitoid fed on P. xylostella, reared on stressed host plants. Egg to adult development of D. insulare was faster on non‐stressed B. napus than non‐stressed S. alba followed by stressed B. napus and S. alba. Female parasitoids were heavier on non‐stressed host plants than stressed counterparts. Furthermore, the parasitoid lived significantly longer on stressed B. napus. However, body size was not affected by water treatment. Most host plant parameters measured were significantly lower for water‐stressed than non‐stressed treatments. Results suggest that development of this important and effective P. xylostella parasitoid was influenced by both water stress and host plant species.     

Effect of Bt broccoli and resistant genotype of <Emphasis Type="Italic">Plutella xylostella</Emphasis> (Lepidoptera: Plutellidae) on development and host acceptance of the parasitoid <Emphasis Type="Italic">Diadegma insulare</Emphasis> (Hymenoptera: Ichneumonidae)

The ecological implications on biological control of insecticidal transgenic plants, which produce crystal (Cry) proteins from the soil bacterium Bacillus thuringiensis (Bt), remain a contentious issue and affect risk assessment decisions. In this study, we used a unique system of resistant insects, Bt plants and a parasitoid to critically evaluate this issue. The effects of broccoli type (normal or expressing Cry1Ac protein) and insect genotype (susceptible or Cry1Ac-resistant) of Plutella xylostella L. (Lepidoptera: Plutellidae) were examined for their effects on the development and host foraging behavior of the parasitoid, Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) over two generations. Parasitism rate and development of D. insulare were not significantly different when different genotypes (Bt-resistant or susceptible) of insect host larvae fed on non-Bt broccoli plants. D. insulare could not discriminate between resistant and susceptible genotypes of P. xylostella, nor between Bt and normal broccoli plants with different genotypes of P. xylostella feeding on them. No D. insulare could emerge from Bt broccoli-fed susceptible and heterozygous P. xylostella larvae because these larvae were unable to survive on Bt broccoli. The parasitism rate, developmental period, pupal and adult weights of D. insulare that had developed on Bt broccoli-fed Cry1Ac-resistant P. xylostella larvae were not significantly different from those that developed on non-Bt broccoli-fed larvae. Female D. insulare emerged from Cry1Ac-resistant P. xylostella that fed on Bt plants could successfully parasitize P. xylostella larvae. The life parameters of the subsequent generation of D. insulare from P. xylostella reared on Bt broccoli were not significantly different from those from non-Bt broccoli. The Cry1Ac protein was detected in P. xylostella and in D. insulare when hosts fed on Bt broccoli. These results are the first to indicate that Cry1Ac did not harm the development or host acceptance of an important endoparasitoid after two generations of exposure. We suggest that using other Bt crops and resistant insect species would likely lead to similar conclusions about the safety of the presently used Bt proteins on parasitoids.     

Natural enemies ofPlutella xylostella (Lep.: Plutellidae) on crucifers in Honduras

Three primary parasitoids in three genera were reared fromPlutella xylostella (L.) larvae and pupae collected in various crucifer producing regions of Honduras. The ichneumonidDiadegma insulare (Cresson) was by far the most abundant species. TwoSpilochalcis species, facultative hyperparasitoids attackingP. xylostella andD. insulare, were encountered as well as eleven species in nine genera of obligate hyperparasitoids attackingD. insulare. Three vespid predators are noted as predating on larvae.      

Does a Specialist Parasitoid Adapt to its Host on a New Host Plant?

The host plant expansion of a diamondback moth, Plutella xylostella (L.) (DBM) strain to snowpea (Pisum sativum L.) raised the question whether a specialist parasitoid Diadegma semiclausum (DS) could be conditioned to locate and parasitize its host on the new host plant. In a specialist parasitoid a behavioural change towards a plant outside the normal host plant range of its host due to developmental experience is not expected. The responsive behaviour, parasitism rates and fitness of three subsequent DS generations were investigated on the snowpea-strain of DBM. After three generations of DS on the pea 62.5% of females chose an DBM-infested pea plant over DBM infested cabbage. Only 16.4% of cabbage-reared DS was attracted to infested pea. Rearing of the parasitoid in host larvae on peas significantly increased the number of larvae parasitized on this host plant in the first generation; however, there was no further increase in generations 2 and 3. Larval mortality was similar for all parasitoid/DBM combinations on both host plants, but significantly higher mortality occurred in parasitoid pupae from peas. Development time of the parasitoid was slightly prolonged on the pea strain of DBM. The number of females produced by parasitoids reared on the pea strain of DBM was significantly reduced as compared to D. semiclausum reared on the cabbage strain on both host strains. Results show that DS has the potential to change its responsive behaviour in order to locate its host on a new host plant. According to the current view, a specialist parasitoid is not expected to change its reaction to a plant outside the normal host plant range of its host. Within 3 generations, responsive behaviour towards snowpea could be increased. However, fitness trade-offs, especially an extreme shift in sex ratio to males reduced reproductive success.     

Herbivore‐induced plants do not affect oviposition but do affect fitness of subsequent herbivores on canola

The diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), is oligophagous on plants in the Brassicaceae, and is considered one of the most significant insect pests of canola (Brassica napus L.), a major oilseed crop grown in the Prairie Provinces of Canada. The bertha armyworm (BAW), Mamestra configurata Walker (Lepidoptera: Noctuidae), is a generalist herbivore that preferentially feeds on canola plants. In the canola growing season in the Prairie Provinces of Canada, DBM feeding occurs prior to BAW herbivory. In this study, we test the effect of DBM herbivory on subsequent host use by BAW. Oviposition by female BAW was not influenced by previous DBM‐herbivory or mechanical damage of canola plants. Bertha armyworm larvae were deterred from feeding on canola previously damaged by DBM and larvae developed into lighter pupae when reared on DBM‐damaged plants. Bertha armyworm pupae that developed from larvae fed on mechanically damaged plants had similar pupal weights to those fed undamaged plants. Adult BAW reared on canola with previous DBM feeding damage had marginally smaller wings than moths reared on canola treated differently. The combination of these results suggests that herbivory by the brassicaceous specialist, DBM, negatively influences host use and larval performance by the generalist, BAW, on canola.     

Leaf nutrient levels and the spatio-temporal distributions of <Emphasis Type="Italic">Plutella xylostella</Emphasis> and its larval parasitoids <Emphasis Type="Italic">Diadegma insulare</Emphasis> and <Emphasis Type="Italic">Microplitis plutellae</Emphasis> in canola

Seasonal distribution patterns of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), and its principal parasitoids Diadegma insulare (Cresson) (Hymenoptera: Ichneumonidae) and Microplitis plutellae (Muesebeck) (Hymenoptera: Braconidae) were investigated over three site-years in commercial fields of canola (Brassica napus L.) in southern Alberta, Canada. The sampling of P. xylostella, D. insulare, and M. plutellae from points arranged in grid patterns, together with the mapping and analysis of their spatial distributions over time, generated a detailed picture of the pattern of crop infestation by the herbivore and its parasitoids. Plutella xylostella exhibited significant aggregations on different scales most often when its host plants were in early flowering. Diadegma insulare adults exhibited significant aggregated distributions during early flowering and distributions subsequently became more uniform as the wasps moved into the crop later in the season. However, M. plutellae distributions were aggregated in mid flowering in only one site-year. The close spatial associations between densities of D. insulare and P. xylostella indicated that host abundance was the main determinant of parasitoid distribution patterns. Spatial distributions of nutrient contents in leaf tissue and their spatial associations with the herbivore and parasitoids were also investigated. Significant spatial associations existed between certain nutrients (e.g. nitrogen, sulfur, and potassium) and P. xylostella distributions. Sulfur exhibited a positive effect on the distributions of D. insulare but not of M. plutellae. We observed similar relationships between nutrients and the distribution of P. xylostella parasitoids as for nutrients and P. xylostella, but these relationships lacked consistency and may be the results of the spatial associations between the parasitoids and their hosts. Aggregated distributions of adults and larvae of P. xylostella hold promise for spatially targeted insecticidal applications as a means for reducing the environmental impact of insecticides on nontarget and beneficial species in canola agroecosystems.     

Do Larvae of Bertha armyworm,Mamestra configurata (Walker) (Lepidoptera: Noctuidae), express induced feeding responses?

The Bertha armyworm, Mamestra configurata (Walker) (Lepidoptera: Noctuidae), is a polyphagous Nearctic insect known to feed on over 40 different host plant species, and can be a pest of canola (Brassica napus L. and Brassica rapa L.), flax (Linum usitatissimum L.), and alfalfa (Medicago sativa L.) in the Northern Great Plains. Bertha armyworm is known to sometimes switch hosts through between‐field movements, prompting this study to investigate its adherence to a particular host plant species after completing a period of initial development on it. In a laboratory study, larvae were reared to either their fourth or sixth instars on either intact or excised leaf tissue of one of seven host species, and were then allowed to select feeding hosts from various choices that included canola, B. napus and B. rapa (Brassicaceae), Canada thistle, Cirsium arvense (L.) Scop. (Compositae), flax, L. usitatissimum (Linaceae), field pea and alfalfa, Pisum sativum L. and M. sativa (Leguminosae), and lamb’s quarters, Chenopodium album L. (Chenopodiaceae). In general, Bertha armyworm larvae showed little propensity to feed on the host plant species on which they had been reared in their early life stages. Exceptions occurred for larvae reared on intact tissue of B. rapa and P. sativum, where larvae were subsequently observed feeding on these hosts significantly more frequently than on other host plants. When larvae of Bertha armyworm were reared through early developmental stages on intact plant tissue of a single host, B. rapa was frequently the choice for subsequent feeding. Movements of Bertha armyworm larvae between fields therefore appear to result from larvae that have exploited food resources in one area and are dispersing to regions of improved host plant availability.     

Interaction of glucosinolate content of Arabidopsis thaliana mutant lines and feeding and oviposition by generalist and specialist lepidopterans

The diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), is an insect specialized on glucosinolate-containing Brassicaceae that uses glucosinolates in host-plant recognition. We used wild-type and mutants of Arabidopsis thaliana (L.) Heynh. (Brassicaceae) to investigate the interaction between plant glucosinolate and myrosinase content and herbivory by larvae of the generalist Helicoverpa armigera Hübner (Lepidoptera: Noctuidae) and the specialist P. xylostella. We also measured glucosinolate changes as a result of herbivory by these larvae to investigate whether herbivory and glucosinolate induction had an effect on oviposition preference by P. xylostella. Feeding by H. armigera and P. xylostella larvae was 2.1 and 2.5 times less, respectively, on apk1 apk2 plants (with almost no aliphatic glucosinolates) than on wild-type plants. However, there were no differences in feeding by H. armigera and P. xylostella larvae on wild-type, gsm1 (different concentrations of aliphatic glucosinolates compared to wild-type plants), and tgg1 tgg2 plants (lacking major myrosinases). Glucosinolate induction (up to twofold) as a result of herbivory occurred in some cases, depending on both the plant line and the herbivore. For H. armigera, induction, when observed, was noted mostly for indolic glucosinolates, while for P. xylostella, induction was observed in both aliphatic and indolic glucosinolates, but not in all plant lines. For H. armigera, glucosinolate induction, when observed, resulted in an increase of glucosinolate content, while for P. xylostella, induction resulted in both a decrease and an increase in glucosinolate content. Two-choice tests with wild-type and mutant plants were conducted with larvae and ovipositing moths. There were no significant differences in preference of larvae and ovipositing moths between wild-type and gsm1 mutants and between wild-type and tgg1 tgg2 mutants. However, both larvae and ovipositing moths preferred wild-type over apk1 apk2 mutants. Two-choice oviposition tests were also conducted with P. xylostella moths comparing undamaged plants to plants being attacked by larvae of either P. xylostella or H. armigera. Oviposition preference by P. xylostella was unaffected as a result of larval plant damage, even in the cases where herbivory resulted in glucosinolate induction.     

Insect Attraction versus Plant Defense: Young Leaves High in Glucosinolates Stimulate Oviposition by a Specialist Herbivore despite Poor Larval Survival due to High Saponin Content

Glucosinolates are plant secondary metabolites used in plant defense. For insects specialized on Brassicaceae, such as the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), glucosinolates act as “fingerprints” that are essential in host plant recognition. Some plants in the genus Barbarea (Brassicaceae) contain, besides glucosinolates, saponins that act as feeding deterrents for P. xylostella larvae, preventing their survival on the plant. Two-choice oviposition tests were conducted to study the preference of P. xylostella among Barbarea leaves of different size within the same plant. P. xylostella laid more eggs per leaf area on younger leaves compared to older ones. Higher concentrations of glucosinolates and saponins were found in younger leaves than in older ones. In 4-week-old plants, saponins were present in true leaves, while cotyledons contained little or no saponins. When analyzing the whole foliage of the plant, the content of glucosinolates and saponins also varied significantly in comparisons among plants that were 4, 8, and 12 weeks old. In Barbarea plants and leaves of different ages, there was a positive correlation between glucosinolate and saponin levels. This research shows that, in Barbarea plants, ontogenetical changes in glucosinolate and saponin content affect both attraction and resistance to P. xylostella. Co-occurrence of a high content of glucosinolates and saponins in the Barbarea leaves that are most valuable for the plant, but are also the most attractive to P. xylostella, provides protection against this specialist herbivore, which oviposition behavior on Barbarea seems to be an evolutionary mistake.     

Parasitism and developmental parameters of the parasitoid <Emphasis Type="Italic">Diadegma</Emphasis><Emphasis Type="Italic">majale</Emphasis> (Gravenhorst) in control of <Emphasis Type="Italic">Plutella xylostella</Emphasis> (L.) on selected cultivars of canola

The diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae), is one of the most economically significant pests of canola, Brassica napus L., in Ardabil region, Iran. Use of host plant resistance integrated with biocontrol agents such as Diadegma majale (Gravenhorst) (Hymenoptera: Ichneumonidae) is an essential component of integrated management of P. xylostella. In this study, we investigated the parasitism by D. majale on six selected cultivars of canola under field conditions and preference and performance of the parasitoid on P. xylostella larvae under laboratory conditions. In field experiments, the highest larval density of P. xylostella was observed on Zarfam during 2008 and 2009. Larval densities were not significantly different among Opera, Hyola 401, Okapi, and Option 500 and Elite in 2008, but the lowest larval density was observed on Opera in 2009. No significant differences were observed among the rate of parasitized larvae on tested cultivars in 2008, while in 2009 the parasitism rate was significantly higher on Opera than on Zarfam. In free-choice situations, the percentage of parasitized larvae was significantly highest on Opera (88.7%) and lowest on Zarfam (62.95%). Developmental time from egg to adult, body mass, length of forewings and hindwings, length of hind femur and hind tibia of D. majale females reared on larvae of P. xylostella fed on Opera did not differ from other cultivars. Our results suggest that cultivation of Opera integrated with D. majale could provide effective and sustainable management of P. xylostella in the region.     

Movement of transgenic plant-expressed Bt Cry1Ac proteins through high trophic levels

The movement of Bacillus thuringiensis (Berliner) (Bt) Cry1Ac endotoxin through high trophic levels was assessed to help elucidate the effects of Bt toxin on non‐target insects. The diamondback moth (Plutella xylostella L., Lepidoptera: Plutellidae), the parasitic wasp (Cotesia vestalis Haliday, Hymenoptera: Braconidae) and the predatory green lacewing Chrysoperla carnea (Stephen) (Neuroptera: Chrysopidae) were used as a model system in this laboratory study. Bt‐resistant P. xylostella larvae fed Cry1Ac‐expressing transgenic oilseed rape (OSR, Brassica napus L., Cruciferae), before and after parasitization by C. vestalis, consumed Cry1Ac with the ingested plant material but only a proportion of Cry1Ac consumed was recovered from the bodies and faeces of P. xylostella larvae. Cry1Ac was not detected in newly emerged parasitoid larvae. In contrast, Cry1Ac was detected in C. carnea larvae fed on resistant P. xylostella larvae reared on Bt OSR. However, no Cry1Ac could be detected in C. carnea larvae when the lacewings were transferred to P. xylostella larvae reared on conventional OSR and tested 24–48 h. The metabolizing ability of Cry1Ac is discussed for the larvae of P. xylostella and C. carnea.     

Influence of plant sulphur nutrition on oviposition and larval performance of the diamondback moth

We measured adult oviposition preference, larval growth, and feeding behaviour of the crucifer specialist Plutella xylostella (L.) (Lepidoptera: Plutellidae) on plants of Brassica napus (L.) cv. Express (Brassicaceae), grown under three different sulphur regimes. The nutrient solutions used were the following: one sulphur‐free (S₀), one normal sulphur (S_n, normal field concentration), and one sulphur‐rich (S₊, double concentration of S_n). Females laid more eggs on S_n than on S₀ plants, while only a slight, non‐significant difference was observed between S_n and S₊ plants. Moreover, the development time from hatching to emergence was significantly shorter, and adults were heavier on S_n than on S₀ plants. Comparing these same two parameters from S_n and S₊ plants, we found a shorter development time on plants rich in sulphur, although this trend was not statistically significant. Larval feeding preferences were tested in a dual choice assay using leaf discs. A significantly higher number of larvae preferred leaf discs of S_n plants than those of S₀ plants. Furthermore, the larvae preferred S₊ to S_n discs. An optimal supply of sulphur to oilseed rape is necessary for a good seed harvest, and it also plays an important role in acceptance by P. xylostella of the host plant. Maintaining higher levels of sulphur in the plant nutrient solution benefits insect performance, both at the adult and larval stage.     

Trade‐offs in oviposition choice? Food‐dependent performance and defence against predators of a herbivorous sawfly

The sawfly Athalia rosae L. (Hymenoptera: Tenthredinidae) is a feeding specialist on plant species of the Brassicaceae, which are characterised by secondary metabolites, called glucosinolates. The larvae can take up the respective glucosinolates of their hosts and concentrate them in their haemolymph to protect themselves against predators. Oviposition preferences of naïve females were tested for three species, Sinapis alba L., Brassica nigra (L.) Koch, and Barbarea stricta Andrz., and were related to larval performance patterns. Larvae were reared on either one of these plants and it was investigated how host‐plant quality influences both the developmental times and growth of larvae (bottom‐up) and the defence efficiency against predators (top‐down). Innately, almost all adult females avoided B. stricta for oviposition and clearly preferred B. nigra over S. alba. On average, larvae developed best on B. nigra. Female larvae reached similar final body masses on all host‐plant species, but males reared on S. alba were slightly lighter. The developmental time of larvae reared on B. stricta was significantly longer than on the other two plants. However, larvae reared on B. stricta were best protected against the predatory wasp Polistes dominulus Christ (Hymenoptera: Vespidae). The wasps rejected these larvae most often, while they attacked larvae reared on S. alba most frequently. Thus, larvae feeding on B. stricta theoretically run a higher risk of predation due to a prolonged developmental time, but in practice they are better protected against predators. Overall, oviposition preferences of A. rosae seem to be more influenced by bottom‐up effects on larval performance than by top‐down effects.     

Interactions between the diamondback moth, Plutella xylostella L. and glasshouse and outdoor-grown cabbage cultivars

Interactions between Brassica cultivars (cvs) and adult and larval stages of Plutella xylostella L. were examined. In six-way choice oviposition experiments, glasshouse-grown Chinese cabbage cv. Tip Top was preferred to five outdoor-grown temperate cabbage cultivars but was similar to four out of five tropical cultivars. Bionomic studies with neonate larvae on temperate cabbage cultivars showed that mean larval duration was longer and percentage survival less on older compared with younger plants but that plant age or cultivar had little effect on pupal weight. The fecundity of P. xylostella reared on outdoor-grown cabbage cultivars varied ten-fold between the poorest and the best cultivar but plant age had little effect on P. xylostella reared on Chinese cabbage. In a glasshouse experiment, survival of P. xylostella on Chinese cabbage was significantly greater compared with field-grown glossy-leaved, normal bloom green cabbage and red cabbage. In bioassays with neonate larvae, when the leaves were placed vertically instead of horizontally a significant level of intrinsic resistance was revealed for glasshouse and field-grown glossy-leaved cabbage but not for Chinese cabbage. Observed differences in host status to P. xylostella are discussed in relation to the potential for partial plant resistance in control programmes.     

Importance of primary metabolites in canola in mediating interactions between a specialist leaf-feeding insect and its specialist solitary endoparasitoid

The role of primary plant chemistry on trophic interactions is not well studied. We examined the effect of primary plant metabolites, focusing on nitrogen, on several biological indices of second and third trophic level insects in a model tritrophic system, consisting of two strains of the crucifer, Brassica napus (canola) (SLM₀₄₆ and RGS₀₀₃), the specialist insect herbivore Plutella xylostella (L.) (Lepidoptera: Plutellidae), and its specialist koinobiont larval-pupal parasitoid Diadegma semiclausum (Hellén) (Hymenoptera: Ichneumonidae). In particular, we measured relative growth rate of the herbivore in relation to an index for plant quality (nitrogen content of leaf tissues), developmental time of the herbivore (sum of second, third, and fourth larval instars durations), and intrinsic rate of increase (r _m ) of the herbivore and the parasitoid. Tritrophic studies were conducted on development, survivorship curve analysis, reproductive potential, life history, parasitism, and several other fitness correlates of the parasitoid. The life table parameters of D. semiclausum were determined under laboratory conditions. The intrinsic rate of increase (r _m ) of the parasitoid was significantly higher on RGS₀₀₃ than SLM₀₄₆. In this tritrophic model, the results indicated that the bottom-up direct effect on the herbivore population growth rate was marginally as strong as the direct effect of top-down force due to the parasitoid population growth rate; but it was higher than its indirect counterpoint mediated with the parasitoid population growth rate. Consequently, D. semiclausum performed better on RGS₀₀₃, which was the most inferior host to P. xylostella in comparison with another plant cultivar and had the lowest content of nitrogen in its leaves.     

Innate and acquired components of oligophagy in the herbivorous lepidopteran,Manduca sexta

Innate and acquired aspects of oligophagy were investigated in the tobacco hornworm, Manduca sexta L. (Lepidoptera: Sphingidae), a species normally restricted to members of the family Solanaceae. Larvae were reared in the laboratory on solanaceous species tobacco (Nicotiana tabacum L.), strawberry groundcherry (Physalis pruinosa L.), and potato (Solanum tuberosum L.), as well as the non‐solanaceous foods cowpea [Vigna sinensis (L.) (Fabaceae)], rapeseed [Brassica napus L. (Brassicaceae)], and artificial diet. Feeding assays were conducted using the above plants plus corkscrew vine [Vigna caracalla (L.) Verdc. (Fabaceae)], lettuce [Lactuca sativa L. (Asteraceae)], and moist glass‐fiber filter paper. Larval feeding was characterized using two camera‐monitored assays: (1) a quantitative no‐choice disc test that determines consumption over time, delays in initiation of feeding, and % of larvae rejecting the food, and (2) a novel no‐choice emigration test that measures how soon a larva abandons a whole plant. Experimental results from both assays revealed that larvae displayed high levels of acceptance of Solanaceae regardless of whether they had been reared on solanaceous or non‐solanaceous food. We conclude that solanaceous oligophagy in M. sexta is primarily innate and does not require (but may be strengthened by) previous feeding experience on Solanaceae. In contrast, larvae tested on non‐solanaceous plants or moist filter paper showed large variation in both acceptance of foods and emigration times that were strongly dependent on the food on which they had been reared (analogous to the ‘induction of preference’ of earlier literature). Two types of induction were identified: an increase in acceptability of a plant of (1) the same species as the one on which the larva was reared, or (2) a related species. These discoveries both challenge and expand our current understanding of oligophagy in the tobacco hornworm.     

Larval survival,host plant preferences and developmental responses of the diamondback moth Plutella xylostella (Lepidoptera: Plutellidae) on wild brassicaceous species

The diamondback moth Plutella xylostella (L.) (Lepidoptera: Plutellidae) is an important pest of cultivated brassicaceous crops worldwide. The host plant preferences, developmental biology and survival and longevity of P. xylostella are relatively well understood on commercial crop species; however, its relationship with brassicaceous weeds is poorly known. Sinapis arvensis L., Erysimum cheiranthoides L. and Capsella bursa‐pastoris (L.) Medicus are among the most common brassicaceous weeds worldwide and can serve as important bridge hosts of P. xylostella. In this study, preference and performance of P. xylostella were compared on these weed species. In free‐choice situations, females deposited 5.5 and 18.8 times more eggs on S. arvensis than on E. cheiranthoides and C. bursa‐pastoris, respectively. Survival from neonate to pupa and from pupa to adult was highest on S. arvensis and E. cheiranthoides and lowest on C. bursa‐pastoris. Development was fastest, foliage consumption was greatest, pupae and silk cocoons were heaviest, adult body masses and longevities were highest and forewings were largest for both females and males when reared as larvae on S. arvensis. Realized fecundity of new generation adults was highest for individuals reared on S. arvensis compared to those reared on E. cheiranthoides or C. bursa‐pastoris. Relative growth rates of pupae and adults were highest on S. arvensis, suggesting that this plant species is a high‐quality host for P. xylostella compared with other species tested. Potential impacts of these wild brassicaceous species on P. xylostella populations are discussed.     

Cross habituation to feeding deterrents and acceptance of a marginal host plant byPieris rapae larvae

Sensitivity of the cabbage butterfly,Pieris rapae L. to feeding deterrents was compared for larvae reared on different food sources under laboratory conditions. Since cabbage-reared larvae normally reject nasturtium,Tropaeolum majus L., the effects of previous exposure to allelochemicals on larval acceptance or rejection of this plant were also examined. When compared with cabbage-reared larvae, nasturtium-reared larvae were less sensitive to feeding deterrents including cymarin, erysimoside and 2-O-β-d-glucosyl cucurbitacin E. Nasturtium-reared larvae were insensitive to chlorogenic acid, which was deterrent to cabbage-reared larvae. Feeding by larvae reared on a wheat germ diet was not deterred by these compounds. The results indicate that dietary experience can extensively affect larval sensitivity to feeding deterrents and that cross habituation of larvae to deterrents occurs in response to certain chemical constituents of nasturtium and wheat germ diet. Digitoxin, however, proved to be an exception. Larvae reared on either nasturtium or wheat germ diet were as sensitive to digitoxin as those reared on cabbage. Previous results have shown that rejectionof nasturtium by cabbage-reared larvae is due to the presence of strong feeding deterrents in this plant. However, more than 50% of 2nd instar larvae reared from neonate on cabbage leaves treated with strophanthidin, cymarin, erysimoside, digitoxigenin and digitoxin accepted nasturtium as a food source. 2-O-β-d-glucosyl cucurbitacin E, 2-O-β-d-glucosyl cucurbitacin I and rutin were also active in causing larvae to feed on nasturtium. Thus dietary exposure to unrelated plant chemicals can profoundly affect insect acceptance of a plant that contains feeding deterrents.