Larval dispersal from potential hosts within a population of a generalist herbivore, Choristoneura rosaceana

The dispersal behavior of Choristoneura rosaceana (Harris) (Lepidoptera: Tortricidae) first instar larvae was studied in the laboratory. The objectives were to investigate the proximal factors influencing larval dispersal and to establish whether a correspondence exists between larval host acceptance and performance. A dispersal bioassay was validated by demonstrating the presence of a positive correlation between larval host acceptance in the laboratory and in the field. Larval age and family origin, as well as host species attributes were shown to influence larval dispersal rates. Seasonal changes in host plants slightly changed the rank order of larval host acceptance. Leaf texture and the availability of refuges on host plants seemed to be important factors influencing the rate of larval dispersal. Plant odor appeared to be used by the larvae to locate leaves. Nitrogen content of plant species corresponded to larval dispersal rates, but the cause of this association is unclear. Larval dispersal did not match host suitabilities as measured by larval performance. The relationship between host preference and suitability in the obliquebanded leafroller is discussed in an ecological and evolutionary perspective.     

Ubiquitous water-soluble molecules in aquatic plant exudates determine specific insect attraction

Plants produce semio-chemicals that directly influence insect attraction and/or repulsion. Generally, this attraction is closely associated with herbivory and has been studied mainly under atmospheric conditions. On the other hand, the relationship between aquatic plants and insects has been little studied. To determine whether the roots of aquatic macrophytes release attractive chemical mixtures into the water, we studied the behaviour of mosquito larvae using olfactory experiments with root exudates. After testing the attraction on Culex and Aedes mosquito larvae, we chose to work with Coquillettidia species, which have a complex behaviour in nature and need to be attached to plant roots in order to obtain oxygen. This relationship is non-destructive and can be described as commensal behaviour. Commonly found compounds seemed to be involved in insect attraction since root exudates from different plants were all attractive. Moreover, chemical analysis allowed us to identify a certain number of commonly found, highly water-soluble, low-molecular-weight compounds, several of which (glycerol, uracil, thymine, uridine, thymidine) were able to induce attraction when tested individually but at concentrations substantially higher than those found in nature. However, our principal findings demonstrated that these compounds appeared to act synergistically, since a mixture of these five compounds attracted larvae at natural concentrations (0.7 nM glycerol, <0.5 nM uracil, 0.6 nM thymine, 2.8 nM uridine, 86 nM thymidine), much lower than those found for each compound tested individually. These results provide strong evidence that a mixture of polyols (glycerol), pyrimidines (uracil, thymine), and nucleosides (uridine, thymidine) functions as an efficient attractive signal in nature for Coquillettidia larvae. We therefore show for the first time, that such commonly found compounds may play an important role in plant-insect relationships in aquatic eco-systems.     

Diagnostic Characters of Mosquito Thoracic Pleurites: Geographic Variability of the Number of Setae in Coquillettidia richiardii (Ficalbi) and Culex modestus Ficalbi (Diptera,Culicidae)

Entomological Review - Geographic variability of the thoracic chaetome in females of Coquillettidia richiardii (Ficalbi, 1889) and Culex modestus Ficalbi, 1890 was revealed for the first time. We...     

Colonization of Coquillettidia linealis (Skuse) with reference to other Coquillettidia and Mansonia species

The immature stages of Coquillettidia and Mansonia mosquitoes are cryptic and spend the duration of their development attached to the tissues of subsurface aquatic plants. This obligatory association makes them difficult to collect and has precluded detailed investigation of the biology of Coquillettidia linealis, a species of significant pest and vector status in Australia, as well as other species in the genus Coquillettidia in Australia and elsewhere. This paper describes the first successful establishment and maintenance of a colony of Cq. linealis. Blood‐seeking wild adults were collected and induced to oviposit under laboratory conditions, where methods were developed to rear larval populations to adult maturity in a colony that was maintained through eight successive generations. Colonies of Coquillettidia xanthogaster and Coquillettidia sp. nr crassipes were also established and reared through at least six generations and five generations, respectively, while two species of Mansonia, Mansonia uniformis and Mansonia septempunctata, were also reared successfully for six and two generations, respectively.     

Host plant genotype influences survival of hybrids between Eurosta solidaginis host races

Extrinsic, host-associated environmental factors may influence postmating isolation between herbivorous insect populations and represent a fundamentally ecological cause of speciation. We investigated this issue in experiments on hybrids between the host races of Eurosta solidaginis, a fly that induces galls on the goldenrods Solidago altissima and S. gigantea. To do so, we measured the performance of parental host races and their hybrids on five genotypes of S. gigantea and nine genotypes of S. altissima to test hypotheses about how variation in plant genotype affects performance (i.e., fitness) and potentially influences gene flow between these host races. We found that rates of gall induction and of survival to adult emergence by hybrid larvae were significantly lower than those of both parental host races on both host species, adding support to the hypothesis that there is partial postmating isolation between the host races. Hybrid flies significantly varied in their performance across plant genotypes of both host species. A significant interaction between the effects of plant genotype and mating treatment (parental vs. hybrid crosses) on larval performance indicated that the relative suitability of particular plant genotypes differed between the parental host races and their hybrids. These patterns illustrate a poor correspondence between optimal parental and hybrid environments, consistent with the hypothesis that these host races are partially isolated due to extrinsic (ecological) factors. Based on these findings, we discuss the possibility that plant genotypes in which hybrid performance is high can facilitate hybridization and gene flow between partially reproductively isolated populations of herbivorous insects, thus affecting the dynamics of ecological speciation.     

Cockchafer Larvae Smell Host Root Scents in Soil

In many insect species olfaction is a key sensory modality. However, examination of the chemical ecology of insects has focussed up to now on insects living above ground. Evidence for behavioral responses to chemical cues in the soil other than CO₂ is scarce and the role played by olfaction in the process of finding host roots below ground is not yet understood. The question of whether soil-dwelling beetle larvae can smell their host plant roots has been under debate, but proof is as yet lacking that olfactory perception of volatile compounds released by damaged host plants, as is known for insects living above ground, occurs. Here we show that soil-dwelling larvae of Melolontha hippocastani are well equipped for olfactory perception and respond electrophysiologically and behaviorally to volatiles released by damaged host-plant roots. An olfactory apparatus consisting of pore plates at the antennae and about 70 glomeruli as primary olfactory processing units indicates a highly developed olfactory system. Damage induced host plant volatiles released by oak roots such as eucalyptol and anisol are detected by larval antennae down to 5 ppbv in soil air and elicit directed movement of the larvae in natural soil towards the odor source. Our results demonstrate that plant-root volatiles are likely to be perceived by the larval olfactory system and to guide soil-dwelling white grubs through the dark below ground to their host plants. Thus, to find below-ground host plants cockchafer larvae employ mechanisms that are similar to those employed by the adult beetles flying above ground, despite strikingly different physicochemical conditions in the soil.     

Performance of Danaini larvae is affected by both exotic host plants and abiotic conditions

The consequences of the introduction of invasive plants for the diet of herbivorous insects have been little explored in nature where, potentially, abiotic and biotic factors operate. In this study, we examined the larval performance of two Neotropical Danaini butterflies when using either a native or an exotic Apocynaceae species as host plant in both field and laboratory experiments. Hosts greatly differ in their amount of latex exudation and other physicochemical traits, as well as in the amount of evolutionary time they have interacted with herbivores. First, herbivore performance on the hosts was investigated under laboratory conditions. Larvae of both Danaini species took more time to develop on the exotic host; larval survivorship did not vary between hosts. Second, first instar survivorship on both hosts was evaluated in two field sites, one site per host. To do so, in both sites half of the larvae were bagged (protected against both abiotic and biotic factors) while the remainder were nonbagged (exposed). The interaction between larval exposure with the use of the exotic host reduced larval survival. We concluded that the combined effects of host plant traits and abiotic factors reduced survival of herbivores in field conditions. Therefore, the performance of herbivores when using hosts of different origins should be considered together with the multiple ecological factors found in natural environments, as these factors can modify the result of plant–herbivore interactions.     

Group feeding as a strategy for exploiting food resources in the burnet moth Pryeria sinica

Summary Pryeria sinica (Zygaenidae) larvae feed on young and growing leaves of Euonymus japonicus in groups. The larvae often defoliate their host plant. Hence, the larvae are occasionally subject to serious shortage in food resources. We hypothesize that larval aggregation is an adaptation for the economical utilization of limited food resources. To test this hypothesis, the patterns of resource utilization were studied on larvae settled on shoots of host plant in various group sizes. The amount of food resources in a shoot was affected by the following three factors; (1) shoot growth, (2) food consumption of larvae, and (3) inhibition of growth or degeneration of shoot by larval activity on it. These factors were measured and the efficiency in resource utilization was compared among groups of varied sizes. The loss in resources caused by the third factor was found to be decreased with group size, i.e., the larger the group size, the larvae utilized the food resources more economically. The advantages of group feeding in survival and reproduction of this species were discussed using a simple graphical model.     

Adult beetles compensate for poor larval food conditions

Life history traits of herbivores are highly influenced by the quality of their hosts, i.e., the composition of primary and secondary plant metabolites. In holometabolous insects, larvae and adults may face different host plants, which differ in quality. It has been hypothesised that adult fitness is either highest when larval and adult environmental conditions match (environmental matching) or it may be mainly determined by optimal larval conditions (silver spoon effect). Alternatively, the adult stage may be most decisive for the actual fitness, independent of larval food exposure, due to adult compensation ability. To determine the influence of constant versus changing larval and adult host plant experiences on growth performance, fitness and feeding preferences, we carried out a match–mismatch experiment using the mustard leaf beetle, Phaedon cochleariae. Larvae and adults were either constantly reared on watercress (natural host) or cabbage (crop plant) or were switched after metamorphosis to the other host. Growth, reproductive traits and feeding preferences were determined repeatedly over lifetime and host plant quality traits analysed. Differences in the host quality led to differences in the development time and female reproduction. Egg numbers were significantly influenced by the host plant species experienced by the adults. Thus, adults were able to compensate for poor larval conditions. Likewise, the current host experience was most decisive for feeding preferences; in adult beetles a feeding preference was shaped regardless of the larval host plant. Larvae or adults reared on the more nutritious host, cabbage, showed a higher preference for this host. Hence, beetles most likely develop a preference when gaining a direct positive feedback in terms of an improved performance, whereby the current experience matters the most. Highly nutritious crop plants may be, in consequence, all the more exploited by potential pests that may show a high plasticity in reproduction and feeding preferences.     

Oriented responses of carrot fly larvae, Psila rosae, to plant odours, carbon dioxide and carrot root volatiles

ABSTRACT. The responses of carrot fly larvae, Psila rosae , to host plant volatiles were studied. Two criteria for an oriented response were used: one based on whether an individual reached within 1.5 cm of the odour source and one involving analysis of the track patterns of larvae within 3 cm of the odour source. All larval instars were attracted to both host and non-host plant material, though to a lesser degree by the latter; they were also attracted to CO₂, but not to N₂. A steam volatile fraction of carrot roots, and methyl eugenol, a volatile component of this fraction, also attracted the larvae, differentially at different concentrations. The possible roles of both CO₂ and the host-specific secondary plant volatiles in the host plant finding behaviour of soil inhabiting arthropods are discussed.     

Spatial patterns of Trybliographa rapae parasitism of Delia radicum larvae in oilseed rape and cauliflower

Trybliographa rapae (Westwood) is an important parasitoid of Delia radicum (L.). Parasitism of D. radicum larvae by T. rapae in relation to host density on canola (oilseed rape) and cauliflower roots was examined at 10 field sites in Germany and Switzerland. For roots with host larvae, the proportion of roots with one or more parasitized hosts increased with increasing host density. However, for these infested roots, the parasitism of individual larvae was not consistently related to host density. When considering only roots on which there were parasitized larvae and the opportunity for multiple attacks, the proportion of larvae that were parasitized decreased with increasing host density in the field locations, and in a cage study under controlled conditions. A model of patch‐finding and number of attacks by female parasitoids suggests that patch‐finding is density‐dependent, but that low attack rate and interference effects limit numbers of attacks to three or less per visit to a host patch; the reduced number of attacks per visit leads to the inverse relationship of larval parasitism with host density in the host patches visited. The interplay of the density‐dependent and inversely density‐dependent processes appears to be responsible for the inconsistency of density dependence of overall larval parasitism in this and previous studies. In the laboratory, adult female T. rapae parasitized hosts at ≤4 cm deep in soil, but not at 6 cm deep. From the depth distribution of larval feeding sites in the field, we infer that between 4% and 20% of Delia larvae may be in a physical refuge from T. rapae parasitism, which may have a stabilizing influence on the host–parasitoid interaction.     

Dynamic interaction between a leaf beetle, Galerucella nipponensis, and an aquatic plant, Trapa japonica. II. Dispersal behavior of larvae

The dispersal behavior of the larvae of a leaf beetle, Galerucella nipponensis, was investigated to analyze the dynamic interaction between the leaf beetle and its aquatic host plant, Trapa japonica. The hatchlings dispersed from the hatched leaf to other leaves in a rosette within 12 h. Some larvae even dispersed on the water surface. As a result, the hatchlings distributed uniformly within the rosette. This larval dispersal appeared to be adaptive behavior responding to a rapid turnover of T. japonica leaves. The female adults preferred young or middle-aged leaves to old leaves as oviposition sites. Received: June 14, 2001 / Accepted: December 17, 2001     

Where to from here? The mechanisms enabling the movement of first instar caterpillars on whole plants using Helicoverpa armigera (Hübner)

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) is an economically-important, polyphagous herbivore in Old World countries. The distribution of larvae within various host plants has been described, but few studies have tried to determine the behavioural mechanisms by which the given distributions arose. Our aim was to determine the mechanisms which enable larval movement on pea plants, starting with first instars. Observations and bioassays determined larval movement in response to light and angled surfaces, as well as the effect of feeding and plant volatiles on these responses. The majority (68–72%) of 1st instars were positively phototactic towards blue, green and white light and 42% towards UV light. In the dark, larvae showed negative geotaxis. The angle of their substrate also had a kinetic effect on larvae; the steeper the angle from horizontal the more larvae moved under all conditions. Phenylacetaldehyde (a flower volatile) suppressed larval movement except at 90°. (Z)-3-Hexenyl acetate (a green leaf volatile) reversed the direction of movement at the flattest angle. Feeding lessened the probability of moving. We suggest that phototaxis and geotaxis are behaviours common to larval lepidopterans (caterpillars), and that these basic behaviours are modulated by environmental, larval, and plant factors to give observed distributions. Using a multinomial model approach, we created a flow chart to qualitatively and quantitatively represent the decision-making process of first instar H. armigera in response to the factors influencing movement.     

On the fauna and ecology of mosquitoes (Diptera: Culicidae) in forest-steppe and steppe regions of Western Siberia

Fauna and ecology of the mosquitoes from the southern forest-steppe (Chany Lake) and the northern steppe (Karasuksky lakes) have been investigated during 2004-2006. Species composition, dominant species, biotopical and seasonal distribution are examined. Detailed data on the biotopical and seasonal distribution, sex ratio and terms of development for the dominant species Coquillettidia richiardii (Ficalbi, 1889) are reported.     

Lack of evidence for local adaptation to individual plant clones or site by a mobile specialist herbivore

Several studies have documented local adaptation by sedentary insects to individual phenotypes of their host plants. Here, I examined whether a similar phenomenon could be found in a mobile, specialized insect, the sumac flea beetle. Previous work has shown that sumac individuals differ in their suitability as hosts for these beetles and that differences have both an environmental and a genetic basis. Using beetle populations collected as eggs from eight different sumac clones along an east-west transect, a reciprocal transfer experiment was conducted to determine whether there was any evidence for local adaptation by beetles to individual plant clones or to site. Variables examined were larval survivorship past first instar, development time, weight at pupation and patterns of predation by enemies. While no evidence for local adaptation was found, there were significant effects of plant clone on which larvae developed, origin of the larval population and the interaction of these effects on larval performance. For larval weight at pupation, there was also some indication that trade-offs may exist in ability of larvae to use different host plant clones. In addition, there were significant environmental effects on several measures of larval performance. Predation rates differed by plant clone, but not by site or with respect to origin of larvae. While no evidence for local adaptation was found in this study, prerequisites for finding such patterns may exist in this system. Received: 23 May 1996 / Accepted: 26 September 1996     

EVOLUTIONARY LOSS OF LARVAL FEEDING: DEVELOPMENT,FORM AND FUNCTION IN A FACULTATIVELY FEEDING LARVA,BRISASTER LATIFRONS

Species with large eggs and nonfeeding larvae have evolved many times from ancestors with smaller eggs and feeding larvae in numerous groups of aquatic invertebrates and amphibians. This change in reproductive allocation and larval form is often accompanied by dramatic changes in development. Little is known of this transformation because the intermediate form (a facultatively feeding larva) is rare. Knowledge of facultatively feeding larvae may help explain the conditions under which nonfeeding larvae evolve. Two hypotheses concerning the evolutionary loss of larval feeding are as follows: (1) large eggs evolve before modifications in larval development, and (2) the intermediate form (facultatively feeding larva) is evolutionarily short-lived. I show that larvae of a heart urchin, Brisaster latifrons, are capable of feeding but do not require food to complete larval development. Food for larvae appears to have little effect on larval growth and development. The development, form, and suspension feeding mechanism of these larvae are similar to those of obligate-feeding larvae of other echinoids. Feeding rates of Brisaster larvae are similar to cooccurring, obligate-feeding echinoid larvae but are low relative to the large size of Brisaster larvae. The comparison shows that in Brisaster large egg size, independence from larval food, and relatively low feeding rate have evolved before the heterochronies and modified developmental mechanisms common in nonfeeding echinoid larvae. If it is general, the result suggests that hypotheses concerning the origin of nonfeeding larval development should be based on ecological factors that affect natural selection for large eggs, rather than on the evolution of heterochronies and developmental novelties in particular clades. I also discuss alternative hypotheses concerning the evolutionary persistence of facultative larval feeding as a reproductive strategy. These hypotheses could be tested against a phylogenetic hypothesis.     

The distribution and incidence of larval trematodes in the freshwater fauna of the Wentloog level, South Wales

The Wentloog level, in South Wales, is a low-lying coastal plain dissected by a network of drainage channels. The latter contain a rich aquatic invertebrate fauna which harbour a wide range of larval digenetic trematodes. Few of the host species examined were not infected with larval trematodes. Miracidia together with their associated sporocysts, rediae and cercariae were more host specific than metacercariae but many of the miracidia which developed in pulmonate molluscs could do so in more than one genus. Metacercarial host specificity is less rigid, and usually species are only restricted to members of one phylum although several were found which could develop in two or more phyla. Most larval stages showed some host preference when they occurred in several host species and this was attributed to ecological or physiological factors or to mechanical barriers. Seasonal variation of infection rate occurs in all larval stages: infection with developing cercariae showed peaks during June and August, but metacercarial infection rates varied according to the host. In molluscs there may be two peaks of infection with metacercariae (as in the cercarial stages), but for many arthropods infections show a gradual increase through the year. The factors mainly responsible for these seasonal changes are considered to be the life-cycle of the intermediate hosts and the environmental temperature, the latter acting on the rate of development of the trematodes. Rainfall, and patterns of egg production of adult trematodes in the definitive hosts are also important.     

Oviposition,larval preference,and larval performance in two polyphagous species: does the larva know best?

It is expected that females preferentially oviposit on plant hosts that allow for optimal larval performance. However, this expectation contradicts empirical evidence where adults do not always choose the best host for their descendants. Recent evidence suggests that females’ host selection depends on the number of potential hosts. Females from oligophagous species seem to be able to choose an appropriate host in terms of larval performance, whereas in polyphagous species, adult oviposition preference is not related with larval performance. This suggests that larvae in polyphagous species could be taking a more active role in host selection than their mothers. Here, we evaluated the oviposition preference and the larval preference and performance of two polyphagous species of economic importance, Copitarsia decolora (Guenée) (Lepidoptera: Noctuidae: Cuculliinae) and Peridroma saucia (Hübner) (Lepidoptera: Noctuidae: Noctuinae), on eight species of cultivated plants. In laboratory and greenhouse choice assays, we tested adult preference for oviposition and larval preference at 1 and 24 h. Larval performance was measured in terms of survival to adulthood, length of larval period, and pupal weight. We found that both adult females and larvae actively choose their hosts and that the larval preference toward the hosts is related to the females’ preference in both herbivore species. However, the females and larvae did not preferentially select the host with the best larval performance, indicating that larval performance is not related to female or larval preference and that other selective pressures are influencing the choice of the host plant in these two species.     

Plant quality or quantity? Host exploitation strategies in three Chrysomelidae species associated with Asteraceae host plants

Phytophagous insects which feed on the leaves of herbaceous host plants have to adapt their life histories to the fact that protein nitrogen is usually highest in growing tissues in spring. We monitored field populations of larvae and adults of three chrysomelid species (Galeruca tanaceti (L.) (main host Achillea millefolium (L.) Yarrow), Cassida rubiginosa (Mueller) (main host Cirsium arvense (L.) Scop.) and Oreina luctuosa (Suffrian) (host Centaurea scabiosa (L.)) together with the amount of protein nitrogen of their food resources and host plant biomass. As expected, the development of host quality, measured as concentration of protein nitrogen, and host plant biomass showed inverse trends during the season. The euryphagous G. tanaceti attacks Achillea early and profits from high nitrogen concentrations in the leaves. Occasional overexploitations of local populations of Achillea are compensated by the capacity to move to other host species. In C. rubiginosa, a species with a host range restricted to the Cardueae, the main larval feeding activity is postponed to a period when the nitrogen content of the host leaves had dropped to 50% of its initial value, but when host plant biomass had increased by 30%. In the monophagous O. luctuosa the larval development is synchronized with a still later phase of host phenology, at which the nitrogen content is below 50% but plant biomass has reached its maximum. There seem to be selection factors, which oppose the use of high quality food in spring and which force the latter two species to postpone their larval development to a later time in the year. This could be caused by numerous factors like, for example, mean daytime temperature. Later in the season the larvae have to cope with the low quality of their host plants. They have, however, the advantage of large quantities of food available.A laboratory study with adults and mature larvae of O. luctuosa shows that this species can overcome low levels of protein nitrogen either by selecting younger leaves with higher nitrogen concentrations or by increasing the daily food consumption rate (RCR) on leaves with a low level of nitrogen and by a prolongation of the feeding period. In this way the larvae compensate the effect of lower daily growth rates (RGR) and a lower food conversion index (ECI) on poor food quality: Regardless of the level of protein nitrogen there was no statistically significant difference in total gain of weight during the third-instar feeding period and in the weight at the end of the third larval stage. The three investigated chrysomelids show that there exists a broad spectrum of adaptations to overcome the dilemma of variable food quality.     

How important are aquatic predators to mosquito larval populations in natural wetlands? A case study from Carolina bays in Georgia

Predation is believed to be an important natural control on larval mosquito populations. However, empirical evidence for predator impacts is lacking, especially from natural wetlands (swamps and marshes). Over a 2-year period, we sampled larval mosquito populations and naturally co-occurring predator assemblages (aquatic invertebrates, fishes) from ten depressional wetlands (Carolina bays) located on a wildlife management area in east central Georgia. We collected a diversity of mosquito larvae and predators (odonates, bugs, beetles, flies, and fishes) from the wetlands, with predator numbers substantially exceeding mosquito larval numbers. However, using a community ecology approach with multivariate ordination and correlation techniques, we found no compelling evidence that these predators were controlling mosquito larval distributions (i.e. significant negative statistical associations were not detected). Those mosquitoes that successfully breed in Carolina bay wetlands (Culiseta melanura, Coquillettidia perturbans, Anopheles crucians) appear well adapted to co-exist with a plethora of naturally occurring predators.