Host Plant Quality Factors That Influence the Growth and Development of Oxyops vitiosa, a Biological Control Agent of Melaleuca quinquenervia

The Australian weevil Oxyops vitiosa was released in 1997 in Florida as a biological control agent of Melaleuca quinquenervia. The larvae of this agent are flush-feeders, found only on the growing tips of their host. Knowledge of this restriction to feeding on the growing tips and other nutritional requirements may assist in the establishment and dispersal of this species. Therefore, O. vitiosa survival was assessed when neonates were fed M. quinquenervia leaves from branches that had dormant buds or emerging bud leaves. Additionally, the influence of leaf quality from different sites and within sites was determined by the feeding of neonates emerging bud leaves collected at three sites and from three leaf qualities (poor, intermediate, and high). Within-site leaf qualities were described in the field by leaf color and in the laboratory by percentage dry mass and nitrogen. Larval survival was lowest when fed leaves from branches that had dormant buds. Associated with this low survival were high leaf toughness and percentage dry mass. When larvae were fed emerging bud leaves, most of the variation in larval survival and performance was attributed to differences in within-site plant quality. Generally, the highest-quality leaves had relatively low percentage dry mass and high percentage nitrogen. Larval survival generally decreased when fed the poor-quality leaves, and in one site, the intermediate-quality leaves. Larvae required less time to develop to adults when fed the high-quality leaves. Development time increased in females but not in males when the larvae were fed the poor-quality leaves. Adult biomass of both females and males generally increased when the larvae were fed the high-quality leaves from two of the three sites. The results indicate that the larvae of O. vitiosa are restricted to feeding on flush foliage with low toughness. Additionally, variations in foliar percentage dry mass and nitrogen influence larval survival and performance. This knowledge benefited the development of mass-production nursery sites and the selection of suitable release sites, which facilitated the establishment of this biological control agent.     

Egg size, first instar behaviour and the ecology of Lepidoptera

There is striking variation in egg size among Lepidoptera. Part of the explanation could be a link between egg size and larval feeding ecology.
The relationship between absolute egg size and aspects of feeding ecology for different Lepidoptera families from different temperate regions is examined. Species that overwinter in the egg stage have larger eggs. There are significant differences in egg size with respect to feeding specificity but different families show different patterns. Woody plant feeders have larger eggs than herb feeders. There is little effect of proximity of the egg to the plant part that is eaten.
Patterns in the behaviour and survival of newly hatched larvae of 42 spp. of British Lepidoptera and their relationship to egg and larval size and to food plant characteristics are examined. Patterns in egg size with respect to feeding ecology are similar to those described above. There is a strong correlation between egg size and the size of newly hatched larvae. Newly hatched larvae survived for a mean of 1–20 days without food. Survival is not correlated with larval weight. Grass feeders survive longer than herb and woody plant feeders; the species surviving the longest feeds on lichens. Newly hatched larvae moved at a mean speed of 0.7-267.8 cm h^-1. Speed is not correlated with larval weight or survival time. Grass feeders move faster than woody plant feeders which, in turn, move faster than herb feeders. Woody plant feeders tend to move upwards, grass feeders downwards and herb feeders both upwards and downwards. The proportion of larvae silking is negatively correlated with larval weight.
The strong links between egg size and larval feeding ecology and between feeding ecology and larval behaviour are discussed. It is surprising that larval body size does not appear to constrain the speed of movement, nor tolerance to starvation.     

Evidence for a negative fitness-density relationship between parent density and offspring quality for two Xenopsylla spp. parasitic on desert mammals

We tested the hypothesis of negative fitness-density relationships and predicted that an increase in the density of parent fleas would result in lower survival rates and longer development time of pre-imagoes as well as shorter time to death from starvation of newly emerged imagoes. These predictions were experimentally tested on Xenopsylla conformis Wagner and Xenopsylla ramesis Rothschild feeding on two rodent species, Meriones crassus Sundevall or Dipodillus dasyurus Wagner. Survival of larvae and pupae, but not eggs, was negatively affected by parent density. An increase in parent density led to a decrease in the number of imagoes of the next generation. Eggs of both species developed faster when the parents were at low densities on either host. The same was true for larval X. ramesis, but not larval X. conformis. The negative effects of parent density on the duration of pupal development were evident in X. conformis, parents of which fed on both hosts, and X. ramesis from parents fed on M. crassus, whereas X. ramesis from parents fed on D. dasyurus developed faster at low densities. A negative effect of density on the development of offspring from egg to imago in X. conformis was manifested mainly when parent fleas fed on D. dasyurus, whereas the negative effect of density on offspring development in X. ramesis was manifested mainly when parent fleas fed on M. crassus. Although there was no general effect of parent density on the resistance of imago offspring to starvation, male X. ramesis from parents fed at the highest density survived starvation for significantly shorter times compared with those from parents fed at lower densities. Manifestation of the negative effect of parent density on offspring quality appears to vary with flea species and may be affected by host species.     

Grandparental Effects on Fertility Vary by Lineage in the United Kingdom

Grandparental presence is known to correlate with the number of grandchildren born, and this effect may vary according to grandparental sex and lineage. However, existing studies of grandparental effects on fertility mostly concern traditional subsistence societies, while evidence from contemporary developed societies is both scarce and mixed. Here, we explore how grandparents affect the transition to second and subsequent children in the contemporary United Kingdom. The longitudinal Millennium Cohort Study (n = 10,295 families) was used to study the association between grandparental investment and parents’ probability of having a new child within 4.5 years. Results show that contact with paternal grandparents is associated with higher probability of parents having a second child. In contrast, contact with maternal grandparents is associated with lower probability of having a third or subsequent child. Kin may have opposite effects on fertility even in contemporary societies, which may explain the lack of consistent effects of grandparental investment on fertility in previous studies.     

Onion maggot feeding and development on heterogeneous sections of the onion bulb

When given a choice of feeding sites in laboratory arenas, Delia antiqua (Meigen) larvae exhibited a 3: 1 bias for internal vs outer sections of onion bulbs and distributed themselves non-randomly among identical preferred onion sections. Larval clustering did not appear to be correlated with increased fitness in the laboratory; larval development was identical across the range of densities from 1 to 50 larvae per onion. Larvae feeding on preferred internal sections developed 14% faster, were 38% heavier upon pupation, and were two times more likely to survive to the adult stage than larvae feeding on outer sections. D. antiqua larval food preference was therefore positively correlated with increased fitness.     

Adaptive transgenerational plasticity in an annual plant: grandparental and parental drought stress enhance performance of seedlings in dry soil

Stressful parental (usually maternal) environments can dramatically influence expression of traits in offspring, in some cases resulting in phenotypes that are adaptive to the inducing stress. The ecological and evolutionary impact of such transgenerational plasticity depends on both its persistence across generations and its adaptive value. Few studies have examined both aspects of transgenerational plasticity within a given system. Here we report the results of a growth-chamber study of adaptive transgenerational plasticity across two generations, using the widespread annual plant Polygonum persicaria as a naturally evolved model system. We grew five inbred Polygonum genetic lines in controlled dry vs. moist soil environments for two generations in a fully factorial design, producing replicate individuals of each genetic line with all permutations of grandparental and parental environment. We then measured the effects of these two-generational stress histories on traits critical for functioning in dry soil, in a third (grandchild) generation of seedling offspring raised in the dry treatment. Both grandparental and parental moisture environment significantly influenced seedling development: seedlings of drought-stressed grandparents or parents produced longer root systems that extended deeper and faster into dry soil compared with seedlings of the same genetic lines whose grandparents and/or parents had been amply watered. Offspring of stressed individuals also grew to a greater biomass than offspring of nonstressed parents and grandparents. Importantly, the effects of drought were cumulative over the course of two generations: when both grandparents and parents were drought-stressed, offspring had the greatest provisioning, germinated earliest, and developed into the largest seedlings with the most extensive root systems. Along with these functionally appropriate developmental effects, seedlings produced after two previous drought-stressed generations had significantly greater survivorship in very dry soil than did seedlings with no history of drought. These findings show that plastic responses to naturalistic resource stresses experienced by grandparents and parents can "preadapt" offspring for functioning under the same stresses in ways that measurably influence realized fitness. Possible implications of these environmentally-induced, inherited adaptations are discussed with respect to ecological distribution, persistence under novel stresses, and evolution in natural populations.     

Studying parents and grandparents to assess genetic contributions to early-onset disease

Suppose DNA is available from affected individuals, their parents, and their grandparents. Particularly for early-onset diseases, maternally mediated genetic effects can play a role, because the mother determines the prenatal environment. The proposed maximum-likelihood approach for the detection of apparent transmission distortion treats the triad consisting of the affected individual and his or her two parents as the outcome, conditioning on grandparental mating types. Under a null model in which the allele under study does not confer susceptibility, either through linkage or directly, and when there are no maternally mediated genetic effects, conditional probabilities for specific triads are easily derived. A log-linear model permits a likelihood-ratio test (LRT) and allows the estimation of relative penetrances. The proposed approach is robust against genetic population stratification. Missing-data methods permit the inclusion of incomplete families, even if the missing person is the affected grandchild, as is the case when an induced abortion has followed the detection of a malformation. When screening multiple markers, one can begin by genotyping only the grandparents and the affected grandchildren. LRTs based on conditioning on grandparental mating types (i.e., ignoring the parents) have asymptotic relative efficiencies that are typically >150% (per family), compared with tests based on parents. A test for asymmetry in the number of copies carried by maternal versus paternal grandparents yields an LRT specific to maternal effects. One can then genotype the parents for only the genes that passed the initial screen. Conditioning on both the grandparents' and the affected grandchild's genotypes, a third log-linear model captures the remaining information, in an independent LRT for maternal effects.     

Evaluation of Napier grass (Pennisetum purpureum) varieties for use as trap plants for the management of African stemborer (Busseola fusca) in a push–pull strategy

We evaluated eight Napier grass [Pennisetum purpureum Schumach (Poaceae)] varieties, used in various parts of eastern Africa as fodder, for their potential role as trap plants in the management of the African stemborer, Busseola fusca Füller (Lepidoptera: Noctuidae) through a push–pull strategy. Oviposition preference, larval orientation, settling, arrest and dispersal, feeding, mortality and survival, and development were determined for each of these varieties under laboratory and screen house conditions. Two‐choice tests showed that only two of the varieties tested (cv. Bana and cv. Uganda Hairless) were preferentially chosen by gravid female moths for oviposition over a susceptible maize variety, cv. Western Hybrid 502. Larval preference was, however, highly variable. Larval feeding by first instars on the maize leaves was more intense and significantly more than on leaves of all the Napier grass varieties evaluated. Food consumed and amounts assimilated by the third instars over a 24‐h period were not different among larvae fed on stems of maize and those fed on stems of the various Napier grass varieties. Larval survival was significantly lower on all the Napier grass varieties (below 3%) than on maize (about 44%). Similarly, larval development was about 2–3 weeks longer on majority of the Napier grass varieties. It was concluded that cv. Bana had potential for use as a trap plant in the management of B. fusca because it was more preferred by the moths for oviposition, equally preferred as maize by the larvae for orientation, settling, and arrest, and allowed minimal survival of the larvae. It can thus be used with such ‘push’ plants as Desmodium spp. (Fabaceae) in a ‘push–pull’ strategy, but the effectiveness of such a strategy would strictly depend on proper establishment and management of these companion plants.     

Maternal diet and larval diet influence survival skills of larval red drum Sciaenops ocellatus

Larval red drum Sciaenops ocellatus survival, turning rate, routine swimming speed, escape response latency and escape response distance were significantly correlated with essential fatty‐acid (EFA) concentrations in eggs. Of the five traits that varied with egg EFA content, two (escape response latency and routine swimming speed) were significantly different when larvae were fed enriched diets compared with the low fatty‐acid diet, indicating that the larval diet can compensate for some imbalances in egg composition. Turning rate during routine swimming and escape response distance, however, did not change when larvae predicted to have low performance (based on egg composition) were fed an enriched diet, indicating that these effects of egg composition may be irreversible. Escape response distances and survival rates of larvae predicted to perform well (based on egg composition) and fed highly enriched diets were lower than expected, suggesting that high levels of EFA intake can be detrimental. Altogether, these results suggest that both maternal diet, which is responsible for egg EFA composition, and larval diet may play a role in larval survivorship and recruitment.     

Effect of nitrogen on resistance of sweet potato to sweetpotato weevil (Coleoptera: Curculionidae) and on storage root chemistry.

The effects of nitrogen fertilizer on sweet potato, Ipomoea batatas (L.) Poir., resistance to the sweetpotato weevil, Cylas formicarius elegantulus (Summers), was studied. Adult weevil feeding and oviposition preference, larval survival, and pupal weight were used as measures of sweet potato resistance. Sweet potato resin glycosides and caffeic acid concentrations in the periderm tissue of storage roots also were measured. Sweet potato genotypes (Beauregard, Excel, W-244, W-250, and Sumor) with varying levels of resistance to sweetpotato weevil were grown in the field under three nitrogen regimes (0, 45, and 135 kg N/ha). Harvested storage roots were evaluated in the laboratory for feeding and oviposition activity of sweetpotato weevil female adults under no-choice and choice test conditions. Larval survival rate and pupal weight were determined by rearing the insects individually on storage root sections. Nitrogen level had a significant effect on the number of eggs deposited, but not on the number of feeding punctures. Sweetpotato weevils laid fewer eggs on plants with the highest level of nitrogen. Nitrogen levels did not significantly affect larval survival and pupal weight. Genotype had a significant effect on feeding, oviposition, and larval survival. Beauregard had higher levels of feeding, oviposition, and larval survival compared with the other genotypes. No interaction effects between nitrogen and genotype were significant. Resin glycosides and caffeic acid concentrations were significantly different among genotypes and between years. Nitrogen levels significantly affected the concentrations of caffeic acid in 1997.     

The effect of invertebrate and vertebrate predators on the foraging movements of Ischnura elegans larvae (Odonata: Zygoptera)

SUMMARY.

• 1 The foraging movements of late instar Ischnura elegans larvae were monitored in laboratory experiments to study the effects of predators on larval feeding behaviour.
• 2 Ischnura larvae are sit-atid-wait, or ambush, foragers, moving occasionally between perches in search of profitable feeding sites. Larval foraging movements, monitored at different densities of Daphnia prey, increased significantly when prey were absent.
• 3 In experiments without prey, larval movement was inhibited by the presence of fish predators, as well as by invertebrate predators (Notonecta glauca), but not by closely related, non-predatory invertebrates (Corixa punctata) or physical disturbance of the water (intermittent air bubbles).
• 4 Further experiments varied Ischnura hunger levels (0–8 days without food) and illumination (light or dark) with and without notonectid predators. Hunger had no consistent effect on penultimate instar behaviour but final instar foraging activity was significantly modified: movements increased after 4 days starvation and decreased again after 8 days. This response was suppressed by the presence of predators. Both larval instars moved significantly less often in the light, even when predators were absent.
• 5 These phenotypically flexible predator-avoidance responses are likely to decrease the risk of predation by both visual and tactile predators. However, predators clearly have an important influence on the feeding niche of Ischnura larvae, and may decrease the overall feeding efficiency, growth rate, and survival of larvae by constraining their movement in search of profitable feeding sites.

     

The contribution of ancestry,chance, and past and ongoing selection to adaptive evolution

The relative contributions of ancestry, chance, and past and ongoing election to variation in one adaptive (larval feeding rate) and one seemingly nonadaptive (pupation height) trait were determined in populations ofDrosophila melanogaster adapting to either low or high larval densities in the laboratory. Larval feeding rates increased rapidly in response to high density, and the effects of ancestry, past selection and chance were ameliorated by ongoing selection within 15–20 generations. Similarly, in populations previously kept at high larval density, and then switched to low larval density, the decline of larval feeding rate to ancestral levels was rapid (15-20 generations) and complete, providing support for a previously stated hypothesis regarding the costs of faster feeding inDrosophila larvae. Variation among individuals was the major contributor to variation in pupation height, a trait that would superficially appear to be nonadaptive in the environmental context of the populations used in this study because it did not diverge between sets of populations kept at low versus high larval density for many generations. However, the degree of divergence among populations (FST) for pupation height was significantly less than expected for a selectively neutral trait, and we integrate results from previous studies to suggest that the variation for pupation height among populations is constrained by stabilizing selection, with a flat, plateau-like fitness function that, consequently, allows for substantial phenotypic variation within populations. Our results support the view that the genetic imprints of history (ancestry and past selection) in outbreeding sexual populations are typically likely to be transient in the face of ongoing selection and recombination. The results also illustrate the heuristic point that different forms of selection-for example directional versus stabilizing selection—acting on a trait in different populations may often not be due to differently shaped fitness functions, but rather due to differences in how the fitness function maps onto the actual distribution of phenotypes in a given population. We discuss these results in the light of previous work on reverse evolution, and the role of ancestry, chance, and past and ongoing selection in adaptive evolution.     

Cannibalism and Virus Production in Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) Larvae Fed with Two Leaf Substrates Inoculated with Baculovirus spodoptera

Cannibalism in the fall armyworm, Spodoptera frugiperda (J.E. Smith) (Lepidoptera: Noctuidae) (FAW), is a limiting factor in a baculovirus production system. To detect the impact of cannibalism, a two-step bioassay was conducted with different larval ages of FAW fed on two food sources (corn and castor bean leaves) contaminated with the S. frugiperda multiple-embedded nucleopolyhedrovirus. In a first bioassay, the food source affected the cannibalism, being higher for all larval ages tested (5-, 6- and 7-day-old larvae) in larvae fed on corn than on those fed on castor bean leaves. Larval mortality, weight equivalent and larval equivalents (LEs) per hectare decreased as the larval age increased. Larval weight, occlusion bodies (OBs)/larva and total OBs increased when the larval age increased. In a second bioassay, in which only 6- and 7-day-old larvae were used because of the performance in the first bioassay, the cannibalism rates were affected by the interaction between food sources and time of feeding (48 and 72 h), reaching the highest values for 6- and 7-day-old larvae fed on corn leaves for 72 h. Mortality of the FAW was affected by the interaction between food sources, larval age and time of feeding. The lowest mortalities were on 7-day-old larvae when they were fed on castor bean leaves for 48 and 72 h. Larval weight, OBs/larva, total OBs and LEs were affected by the interaction between food sources and larval age. A significant correlation was observed between larval weight and OBs/larva that fed on both food sources, suggesting that larval weight can be used to achieve a concentration to be sprayed in 1 ha.     

A preliminary report on the development, growth and survival of laboratory reared larvae of the grey mullet, Mugil cephalus L.

Larvae from artificially bred grey mullet were reared in the laboratory and survival rates of 0.2 %, 5 % and 5 % achieved in three of six trials. Food consisted of wild zooplankton and Anemia nauplii. Feeding began on the fifth day, when the yolk sac was depleted, and intensified on the ninth day. The rate of yolk absorption and feeding intensity were reflected in the growth curve. Larval survival was not affected by withholding food from the larvae till the seventh day from hatching. Two critical periods associated with high larval mortality were apparent on the 2nd–3rd and 8th–11th days after hatching. These were preceded by an increase in specific gravity of larvae followed by passive sinking to the bottom of the rearing tank. Larval length increased from 2.63 mm at hatching to 17.69 mm at the end of the 42-day larval period. The larvae survived on benthic diatoms therefter. Maximum survival rates were achieved at 22°C. (Oceanic Institute Contribution No. 101).     

Performance of pine looper Bupalus piniarius larvae under population build-up conditions

In the first year of an outbreak, Bupalus piniarius larvae, encounter intense crowding. In the later stages of larval development, they are forced to feed on the non-preferred current-year needles of Scots pine or even on alternative hosts. It was hypothesized that larval feeding on a non-preferred resource (current-year needles) will negatively affect B. piniarius performance. It was also hypothesized that larval mutual interference (crowding without competition for food) will have negative additive effects. These hypotheses were tested in laboratory and field experiments. Fourth instar larvae were reared singly and under crowded conditions in cohorts of ten. Larvae in both situations were reared on control branches (containing both mature and current-year shoots) and branches containing only current-year shoots. Crowded larvae were reared also on Norway spruce, an alternative host. Crowding and feeding on a non-preferred resource had opposite effects on B. piniarius larval performance. Crowding in the late larval instars enhanced larval performance while absence of the preferred resource had negative effects. Larval growth rate was higher and development time was shorter for larvae exposed to crowded conditions than for solitary larvae. There was, however, no difference between the groups in final pupal weights or survival. Survival was 25% lower for larvae feeding on non-preferred current-year needles and pupal weights 9% lower, compared with results for larvae feeding on mature needles. Larvae feeding on Norway spruce suffered greatly extended development time, 82% lower survival, and resulted in 60% lighter pupae compared with conspecifics on Scots pine. It was concluded that not only quantity but also quality of the available food resource is critical for B. piniarius development.     

Larval food source promotes cyclic seasonal variation in polyandry in the moth Lobesia botrana

In Mediterranean vineyards the European grapevine moth, Lobesiabotrana, usually completes three non-overlapping larval generationsper year. Larvae feed on inflorescences and unripe or ripe grapes.Previous work has shown that the larval food source has a hugeeffect on adult size and fitness. We investigated if larvalfood also affects the propensity of females to mate polyandrously.Levels of polyandry were monitored with baited traps in a vineyardfor two years and then compared with data from laboratory trialsusing vine-reared individuals. In the field, polyandry levelsfollowed a cyclic annual pattern associated with the larvalfood source. Large females that fed on ripe grapes exhibitedsignificantly higher levels of polyandry than smaller femalesthat fed on inflorescences. Females that fed on unripe grapesshowed intermediate levels of remating. Polyandry also tendedto increase later in each flight. Laboratory trials confirmeda direct effect of larval feeding on levels of polyandry. Overall,these results showed that the seasonal variation in polyandryexhibited by L. botrana in the field occurs, at least in part,because of food-derived changes regulating female size. Resultsalso suggested that larval feeding could shape sexual size dimorphismand polyandry levels. Larval nutrition should therefore be consideredas an important ecological factor by those studying the evolutionarysignificance of polyandry and sperm competition in the Lepidoptera.     

Life history of Spodoptera exigua (Lepidoptera: Noctuidae) on various host plants

The incubation period of Spodoptera exigua (Hübner) was not influenced by the host plant, whereas larval development time and pupal period were affected. Larval development time was longest on shallot and lady's finger, followed by cabbage and long bean. Larvae did not develop beyond the first instar when fed on chilli. The pupal period was longer on lady's finger than on cabbage, shallot and long bean. Overall, adult longevity was not influenced by the host plant but there was a difference between female and male longevity among the host plants. Survival of S. exigua was affected by the host plant at the larval stage. The number of larval instars varied between 5 and 8 within and between the studied host plants. Long bean was found to be the most suitable host plant and provide the best food quality for S. exigua compared to the other host plants, as it allowed faster development, fewer larval instars and a higher survival rate.     

Expression of the oligopeptide transporter, PepT1, in larval Atlantic cod (Gadus morhua)

The intestinal absorption of di- and tri-peptides generally occurs via the oligopeptide transporter, PepT1. This study evaluates the expression of PepT1 in larval Atlantic cod (Gadus morhua) during the three weeks following the onset of exogenous feeding. Larval Atlantic cod were fed either wild captured zooplankton or enriched rotifers. cDNA was prepared from whole cod larvae preceding first feeding and at 1000 each Tuesday and Thursday for the following three weeks. Spatial and temporal expression patterns of PepT1 mRNA were compared between fish consuming the two prey types using in situ hybridization and quantitative real-time PCR. Results indicated that PepT1 mRNA was expressed prior to the onset of exogenous feeding. In addition, PepT1 was expressed throughout the digestive system except the esophagus and sphincter regions. Expression slightly increased following first-feeding and continued to increase throughout the study for larvae feeding on both prey types. When comparing PepT1 expression in larvae larger than 0.15-mg dry mass with expression levels in larvae prior to feeding, no differences were detected for larvae fed rotifers, but the larvae fed zooplankton had significantly greater PepT1 expression at the larger size. In addition, PepT1 expression in the zooplankton fed larvae larger than 0.15-mg dry mass had significantly greater expression than rotifer fed larvae of a similar weight. Switching prey types did not affect PepT1 expression. These results indicate that Atlantic cod PepT1 expression was slightly different relative to dietary treatment during the three weeks following first-feeding. In addition, PepT1 may play an important role in the larval nutrition since it is widely expressed in the digestive tract.     

Preference and performance linkage of a leaf-mining moth on different Salicaceae species

The relationship between oviposition prefer-ence and offspring performance of a leaf-mining moth (Paraleucoptera sinuella) on four Salicaceae species was investigated in 1997 and 1998. We observed the egg distribution pattern on different plant species in the field and carried out oviposition experiments in the laboratory to determine the preference of ovipositing females. We also examined larval survival, pupal mass, and developmental time to compare larval performance on each plant species. Egg density in the field differed significantly among plant species. However, egg density was not correlated exactly with demonstrated oviposition preference. No larvae could develop on two Salix species. This finding indicated that larval survival is the most critical index of larval performance. Larval performance on each plant species was correlated well with oviposition preference that was revealed by a no-choice experiment in the laboratory. However, this correlation was not found in the field. These results indicate that the preference–performance linkage that was observed under laboratory conditions, was not always maintained in the field. Received: September 25, 2000 / Accepted: April 27, 2001     

The effect of season and group size on survivorship and larval growth in Plagiodera versicolora

Abstract.

• 1 The larvae of the multivoltine imported willow leaf beetle, Plagiodera versicolora (Laicharting), are found in larval aggregations during the first half of the larval stage. An experimental study was conducted to test for seasonal (brood) and group size effects on larval fitness. For each of the three broods, egg group sizes of 1, 4, 8, 16 , 24 and 32 were established in the field and monitored for the first 6 days post-hatching.
• 2 There was no effect of group size on larval weight, but there was a significant effect of group size on survival. Individuals from group size 16 had the highest survival, which coincides with the mean natural cluster size of 15.3 eggs.
• 3 There were no significant interactions between brood and group size.
• 4 The effect of larval weight on adult fitness was determined in the laboratory and the field. Larval weight was correlated with egg production in the laboratory but not in the field.