Selection of healthy and nuclear polyhedrosis virus infectedAnticarsia gemmatalis [Lep.: Noctuidae] as prey by nymphalNabis roseipennis [Hemiptera: Nabidae] in laboratory and on soybean

Nabis roseipennis Reuter nymphs demonstrated a preference for nuclear polyhedrosis virus (NPV) — infected over healthyAnticarsia gemmatalis Hübner larvae when offered a choice of larval prey in Petri dishes and on soybean. In Petri dishes, small (second-third instar) and large (fifth-sixth instar) nymphs attacked a significantly greater number of diseased than healthy larvae at all larval instars tested (first-fifth instars) and exposure periods (2, 5 and 24 h), except that at 2 h the number of 1^st and 3^rd instar larvae attacked by large nymphs did not differ significantly (P≤0.05). Nabis roseipennis caged with larvae on individual soybean plants in the greenhouse resulted in a generally low percentage of attack by small and large nymphs after 2 days, ranging from 5.6 to 36.7%. As in the Petri dishes, the nabids showed a significant preference for diseased larvae over healthy larvae attacked for all nabid and larval sizes on soybean, with the percentage of diseased larvae attacked ranging from 28.0 to 65.4% (P≤0.05). This preference for diseased larvae on soybean as well as in Petri dishes demonstrates that the preference was not due to the close proximity in which the host and prey were found in the Petri dishes. The preference for diseased larvae may be due to a reduction in a defensive response in late stages of disease. This material is based upon work supported in part by USDA Grant No. 83-CRCR-1-1212.     

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.     

The role of food plant and pathogen-induced behaviour in the persistence of a nucleopolyhedrovirus

Insect baculoviruses can survive between epidemics as infectious particles external to the host. Many pathogens persist in reservoirs, i.e., microhabitats where survival is enhanced, for example due to protection from the degrading effects of UV irradiation. However, the probability of infecting new susceptible hosts is usually reduced. Persistence of pathogens and their movement in and out of reservoirs is an important, albeit little understood, aspect of insect pathogen ecology. This study investigated interactions between the behaviour of infected insect hosts, virus distribution and plant species on the persistence of the winter moth (Operophtera brumata) nucleopolyhedrovirus. Habitat influenced the persistence of infectious baculovirus in the field: virus on Sitka spruce (Picea sitchensis) and oak (Quercus robur) in forested areas retained more infectivity than virus on heather (Calluna vulgaris) in an unshaded habitat. Plant species per se did not directly affect the persistence of virus on the foliage of potted seedlings. Virally infected insects had altered behaviour and moved down plants relative to control insects, whereas in other systems larvae show height-seeking behaviour. Consequently, the majority of virus particles were distributed on plant stems. In two experiments (one using winter moth NPV and one Mamestra brassicae NPV) virus persisted better on plant stems relative to foliage. Neonate larvae were shown to be able to acquire infections from tree stems contaminated with a low level of virus. These data suggest that plant stems may be important reservoirs for between-year persistence of this pathogen. The observed virus-induced changes in host behaviour in winter moth could enhance the viral persistence by increasing the deposition of occlusion bodies in these reservoirs.     

Crown structure and leaf area of the understorey species Prunus serotina

A detailed biometrical study of the exotic understorey invader Prunus serotina (Ehrh.) was carried out in a mixed coniferous forest stand in northern Belgium. Based on detailed destructive measurements of eight selected model trees, allometric relations of tree height, crown projected area, woody and leaf dry mass and leaf area on tree diameter at breast height (DBH) were derived. The scaling-up procedure from the tree to the stand level was done using the frequency distribution of DBH obtained at the selected experimental plot. The vertical and radial distributions of the tree foliage were estimated by the “cloud” technique. The vertical profile of leaf area showed a bimodal distribution pattern with maxima at heights of 4 and 6 m above the ground. The leaf area index (LAI) of the understorey Prunus serotina as estimated by the described up-scaling procedure (5.1) was significantly higher than the LAI (2.6) as measured by a plant canopy analyser and was also significantly higher than the LAI of the overstorey species Scots pine (1.5–3.0). The LAI of a neighbouring Rhododendron understorey reached only 1.25. This study emphasises the importance of an exotic understorey species in the total leaf area of mixed coniferous forests which might have important implications for the energy and mass exchanges of the entire forest.     

Host–parasitoid interaction as affected by interkingdom competition

Although still underrepresented in ecological research, competitive interactions between distantly related organisms (so-called “interkingdom competition”) are expected to be widespread in various ecosystems, with yet unknown consequences for, e.g. trophic interactions. In the model host–parasitoid system Drosophila melanogaster–Asobara tabida, toxic filamentous fungi have been shown to be serious competitors that critically affect the density-dependent survival of host Drosophila larvae. This study investigates the extent to which the competing mould Aspergillus niger affects key properties of the well-studied Drosophila–parasitoid system and how the host–parasitoid interaction influences the microbial competitor. In contrast to slightly positive density-dependent host mortality under mould-free conditions, competing A. niger mediated a strong Allee effect for parasitised larvae, i.e. mortality decreased with increasing larval density. It was found that the common toxic fungal metabolite kojic acid is not responsible for higher death rates in parasitised larvae. Single parasitised Drosophila larvae were less harmful to fungal reproduction than unparasitised larvae, but this effect vanished with an increase in larval density. As predicted from the negative effect of fungi on host survival and thus on parasitoid fitness at low larval densities, A. tabida females spent less time foraging in fungus-infested patches. Interestingly, even though high host larval densities increased host survival, parasitoids still reduced their search efforts in fungus-infested patches, indicating a benefit for host larvae from feeding in the presence of noxious mould. Thus, this experimental study provides evidence of the potentially important role of interkingdom competition in determining trophic interactions in saprophagous animal communities and the dynamics of both host–parasitoid and microbial populations.     

Long-distance dispersal helps germinating mahogany seedlings escape defoliation by a specialist caterpillar

Herbivores and pathogens with acute host specificity may promote high tree diversity in tropical forests by causing distance- and density-dependent mortality of seedlings, but evidence is scarce. Although Lepidoptera larvae are the most abundant and host-specific guild of herbivores in these forests, their impact upon seedling distributions remains largely unknown. A firm test of the mechanism underpinning the Janzen–Connell hypothesis is difficult, even for a single tree species, because it requires more than just manipulating seeds and seedlings and recording their fates. Experimental tests require: (1) an insect herbivore that is identified and highly specialised, (2) linkage to an in situ measure (or prevention) of herbivory, and (3) evaluation and confirmation among many conspecific adult trees across years. Here we present experimental evidence for a spatially explicit interaction between newly germinating seedlings of a Neotropical emergent tree, big-leaf mahogany (Swietenia macrophylla, Meliaceae), and caterpillars of a noctuid moth (Steniscadia poliophaea). In the understory of a southeastern Amazon forest, the proportion of attacks, leaf area lost, and seedling mortality due to this specialised herbivore peaked near Swietenia trees, but declined significantly with increasing distance from mature fruiting trees, as predicted by the Janzen–Connell hypothesis. We conclude that long-distance dispersal events (>50 m) provided an early survival advantage for Swietenia seedlings, and propose that the role of larval Lepidoptera as Janzen–Connell vectors may be underappreciated in tropical forests.     

Leaf age effects of elevated CO2-grown white oak leaves on spring-feeding lepidopterans

Folivorous insect responses to elevated CO₂-grown tree species may be complicated by phytochemical changes as leaves age. For example, young expanding leaves in tree species may be less affected by enriched CO₂-alterations in leaf phytochemistry than older mature leaves due to shorter exposure times to elevated CO₂ atmospheres. This, in turn, could result in different effects on early vs. late instar larvae of herbivorous insects. To address this, seedlings of white oak (Quercus alba L.), grown in open-top chambers under ambient and elevated CO₂, were fed to two important early spring feeding herbivores; gypsy moth (Lymantria dispar L.), and forest tent caterpillar (Malacosoma disstria Hübner). Young, expanding leaves were presented to early instar larvae, and older fully expanded or mature leaves to late instar larvae. Young leaves had significantly lower leaf nitrogen content and significantly higher total nonstructural carbohydrate:nitrogen ratio as plant CO₂ concentration rose, while nonstructural carbohydrates and total carbon-based phenolics were unaffected by plant CO₂ treatment. These phytochemical changes contributed to a significant reduction in the growth rate of early instar gypsy moth larvae, while growth rates of forest tent caterpillar were unaffected. The differences in insect responses were attributed to an increase in the nitrogen utilization efficiency (NUE) of early instar forest tent caterpillar larvae feeding on elevated CO₂-grown leaves, while early instar gypsy moth larval NUE remained unchanged among the treatments. Later instar larvae of both insect species experienced larger reductions in foliage quality on elevated CO₂-grown leaves than earlier instars, as the carbohydrate:nitrogen ratio of leaves substantially increased. Despite this, neither insect species exhibited changes in growth or consumption rates between CO₂ treatments in the later instar. An increase in NUE was apparently responsible for offsetting reduced foliar nitrogen for the late instar larvae of both species.     

Developmental morphology of the Indian cyprinid fish Barilius canarensis

Embryonic and larval development of an Indian cyprinid fish, Barilius canarensis, is described from laboratory-reared specimens. The eggs, measuring 2.1–2.4 mm in diameter, were demersal, almost spherical in shape, transparent and unpigmented, with a pale yellow yolk without oil globules. Hatching occurred 39–45 h after fertilization at 26.8°–27.4°C. The newly hatched larvae, measuring 4.8–5.1 mm in body length (BL) with 22 + 17 = 39 myomeres, were characterized by melanophores already deposited on the eyes. The eggs of B. canarensis resembled those of the related danionin species Candidia barbatus, Opsariichthys uncirostris uncirostris, Zacco platypus, Z. sieboldii, and Z. temminckii. Although the larvae of B. canarensis were also similar to those of the foregoing species in general morphology, they differed in having a straight notochord tip and pigmentation on the eyes at hatching and the almost entire absence of melanophores on the ventral body surface from the yolk sac to postflexion larval stages. Conversely, melanophores occurred on the anterior abdominal and pericardial cavities from the preflexion to postflexion larval stages.     

Understorey gaps influence regeneration dynamics in subtropical coastal dune forest

Dominant understorey species influence forest dynamics by preventing tree regeneration at the seedling stage. We examined factors driving the spatial distribution of the monocarpic species Isoglossa woodii, a dominant understorey herb in coastal dune forests, and the effect that its cover has on forest regeneration. We used line transects to quantify the area of the forest understorey with I. woodii cover and with gaps in the cover. Paired experimental plots were established in semi-permanent understorey gaps with I. woodii naturally absent and in adjacent areas with I. woodii present to compare plant community composition, soil, and light availability between the two habitats. Isoglossa woodii was widespread, covering 65–95% of the understorey, while gaps covered the remaining 5–35% of the area. The spatial distribution of this species was strongly related to tree canopy structure, with I.␣woodii excluded from sites with dense tree cover. Seedling establishment was inhibited by low light availability (<1% of PAR) beneath I.␣woodii. When present, I. woodii reduced the density and species richness of tree seedlings. The tree seedling community beneath I. woodii represented a subset of the seedling community in gaps. Some species that were found in gaps did not occur beneath I. woodii at all. There were no significant differences between the sapling and canopy tree communities in areas with I. woodii gaps and cover. In the coastal dune forest system, seedling survival under I. woodii is dependent on a species’ shade tolerance, its ability to grow quickly during I. woodii dieback, and/or the capacity to regenerate by re-sprouting and multi-stemming. We propose a general conceptual model of forest regeneration dynamics in which the abundant understorey species, I. woodii, limits local tree seedling establishment and survival but gaps in the understorey maintain tree species diversity on a landscape scale.     

Secondary transmission of a nuclear polyhedrosis virus ofNeodiprion taedae linearis [Hym.: Diprionidae] between larval colonies on loblolly pine

Transmission of a nuclear polyhedrosis virus (NtNPV) of the loblolly pine sawfly,Neodiprion taedae linearis Ross was examined in larval populations on loblolly pine. NtNPV infection of larvae in one colony resulted in transmission to other colonies in the tree. Mortality in untreated colonies was highest when the treated and untreated colonies were located in the upper canopy and when small larvae were treated with virus (42.6–61.9%). Levels of mortality from NtNPV in the untreated colonies were low (2.8–25.0%) when the treated colony was located low in the canopy. Larvae from untreated colonies in the tree were usually large at death. Mortality in untreated larval colonies collected from nearby trees was much less (0.0–27.7%) than that in the trees containing the treated colony. Published with the approval of the Director, Arkansas Agricultural Experiment Station. Use of a trade name does not imply endorsement or guarantee of the product or the exclusion of other products of similar nature.     

Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees

The effects of elevated atmospheric CO₂ and increased wet N deposition on leaf quality and insect herbivory were evaluated in nine model ecosystems composed of 7-year-old spruce trees (Picea abies) and three understorey species established on natural forest soil. Each model ecosystem was grown in a simulated montane climate, and was exposed to one of three CO₂ concentrations (280, 420, and 560 μl l⁻¹), and to one of three levels of N deposition (0, 30, and 90 kg ha⁻¹ year⁻¹) for 3 years. In the 3rd year of the experiment second to third instars of the nun moth (Lymantria monacha) were allowed to feed directly on current-year needles of top canopy branches of each tree for 12 days. Specific leaf area (SLA), water content, and N concentration decreased in needles exposed to elevated CO₂, whereas the concentrations of starch, condensed tannins, and total phenolics increased. Increased N deposition had no significant effect on SLA, and water content, but the concentrations of starch, condensed tannins, and total phenolics decreased, and sugar and N concentrations increased. Despite higher relative consumption rates (RCRs) larvae consumed 33% less N per unit larval biomass and per day at the two high CO₂ treatments, compared to those feeding on 280 μl l⁻¹-needles, but they maintained similar N accumulation rates due to increased N utilization efficiencies (NUE). However, over the 12-day experimental period larvae gained less N overall and reached a 35% lower biomass in the two high-CO₂ treatments compared to those at 280 μl l⁻¹. The effects of increased N deposition on needle quality and insect performance were generally opposite to those of CO₂ enrichment, but were lower in magnitude. We conclude that altered needle quality in response to elevated CO₂ will impair the growth and development of L. monacha larvae. Increasing N deposition may mitigate these effects, which could lead to altered insect herbivore distributions depending on regional patterns of N deposition. Received: 8 June 1998 / Accepted: 27 October 1998     

The Foraging Behavior of <Emphasis Type="Italic">Lysiphlebus fabarum</Emphasis> (Marshall), a Thelytokous Parasitoid of the Black Bean Aphid in Iran

A series of laboratory experiments examined the foraging behavior of a thelytokous strain of Lysiphlebus fabarum (Marshall), a strongly proovigenic parasitoid of Aphis fabae Scopoli, in Iran. Females use chemical camoflage to forage undisturbed in ant-tended aphid colonies and solicit honeydew from aphids in the manner of ants. Rates of oviposition are very low (∼ 1.2 eggs / h) despite many aphid encounters and persistent ovipositor probing which appears to prime aphids for subsequent honeydew solicitation. Starved females spent 3.6 times longer in host patches (leaf disks with 15 2nd–3rd instar A. fabae) than did females sated on honey, and 40% of this time was spent soliciting honeydew. Five d-old females spent longer in patches than did one d-old females, and parasitized three times as many aphids. A 24 h pre-trial foraging experience did not reduce mean egg load significantly compared to a one h experience, but was sufficient to reduce patch residence time and number of aphids parasitized. Wasps reared under short day conditions (L:D = 10:14) were more pessimistic foragers (remained longer in patches and parasitized more aphids) than females raised under long days (L:D = 16:8). Wasps that encountered aphids previously parasitized by conspecifics began to superparasitize and remained longer in patches than females that encountered only unparasitized aphids. Encounters with other females had no effect on foraging behavior, possibly because cuticular camoflage interferes with conspecific recognition. The exceptionally low oviposition rate of this wasp may reflect a life history in which individual fitness has evolved to be strongly dependent on continued ant attendance throughout the period of progeny development.     

Surplus Killing by Predatory Larvae of <Emphasis Type="Italic">Corethrella appendiculata</Emphasis>: Prepupal Timing and Site-Specific Attack on Mosquito Prey

Surplus or ‘wasteful’ killing of uneaten prey has been documented in the fourth larval instar of various species of the mosquito genus Toxorhynchites that occur in treeholes and other phytotelmata. Here we document surplus killing by the predatory midge Corethrella appendiculata, which in Florida cohabits treeholes and artificial containers with larvae of Toxorhynchites rutilus. Provided with a surfeit of larval mosquito prey, surplus killing was observed only in the fourth instar of C. appendiculata, peaking in intensity in the final 24 h prior to pupation, as observed for Toxorhynchites spp. Attack sites identified from videotaped encounters with mosquito prey were divided among head, thorax, abdomen, and siphon. Consumed mosquito larvae (n = 70) were attacked primarily on the head (46%) or siphon (34%), but surplus-killed prey (n = 30) were attacked predominantly on the thorax (83%). Despite its independent evolution among different insect species in aquatic container habitats, the functional significance of prepupal surplus killing remains unclear.     

Larval and juvenile development of <Emphasis Type="Italic">Lestidiops sphyraenopsis</Emphasis> Hubbs, 1916 (Aulopiformes: Paralepididae) in the western North Pacific,with a reexamination of the holotype of <Emphasis Type="Italic">Stemonosudis molesta</Emphasis> (Marshall, 1955)

The early life stages of Lestidiops sphyraenopsis (Paralepididae) are described on the basis of 14 specimens [7.8 mm in notochord length (NL)–88.6 mm in standard length (SL)] collected from the western North Pacific, and the holotype of Stemonosudis molesta is reexamined. Larval L. sphyraenopsis occurred in the Kuroshio waters, and juveniles were taken in the Kuroshio–Oyashio transition waters. Diagnostic characters of larval and juvenile L. sphyraenopsis are 96–101 myomeres; 27–31 anal fin rays; 4–9 peritoneal pigment sections in larvae (7.8 mm NL–27.3 mm SL); dorsal and anal pigment patches present; and anus located anterior to a vertical through dorsal fin origin. Stemonosudis molesta, known only from the holotype from the South Pacific, is similar to immature specimens of L. sphyraenopsis, but can be clearly distinguished from the latter by having higher vertebral counts (105 vs. 96–101) and by morphometric and pigment differences. Consequently, S. molesta is a valid species, and the distribution of L. sphyraenopsis is restricted to the North Pacific.     

Size compensation in moth larvae: attention to larval instars

Environmental perturbations such as starvation and poor diet often prevent animals from attaining their optimal sizes. When the perturbation has a transient character, compensatory responses are expected in terms of faster growth or a prolonged developmental period. In the case of insect larvae, details of such responses are insufficiently known at the proximate level. Attention to responses at the level of particular larval instars should promote an understanding of insect developmental plasticity also in a more general context. To provide an instar‐specific analysis of compensatory growth, larvae of the moth Orgyia antiqua (L.) are reared on inferior diet during one larval instar. Responses in growth parameters are recorded in the course of the manipulated instars, as well as at the level of the entire larval period. The negative relationship between development time and size in response to the inferior food quality, typical of the entire larval periods, is also observed within the manipulated instars taken separately. The manipulated larvae remain smaller than the larvae of the control group (significant in males only), even by the end of the subsequent instar during which all individuals are provided with superior host. In males, close to full size compensation by the time of pupation is achieved only by means of adding an extra larval instar. The inability of larvae to fully compensate during one and even two instars is considered as an indication of the presence of constraints on the within‐instar growth pattern. An alternative, adaptational explanation for the incomplete compensation could be based on the cost of prolonged development period. Given the ecological context of the species' life history, such an explanation appears less likely.     

Developmental morphology of the cyprinid fish Inlecypris auropurpureus

Embryonic, larval, and juvenile development of a Myanmarese cyprinid fish, Inlecypris auropurpureus, is described from laboratory-reared specimens. The eggs, measuring 0.9–1.0 mm in diameter, were demersal, almost spherical in shape, transparent and unpigmented, with a pale yellow yolk without oil globules. Hatching occurred 49–56 h after fertilization at 26.2°–27.3°C. The newly hatched larvae, measuring 2.9–3.1 mm in body length (BL) with 17 + 19–20 = 36–37 myomeres, had melanophores on the head and body. A cement organ on the forehead for adhering to objects during the yolk sac and early preflexion larval stages was distinctive. The yolk was completely absorbed at 3.6–4.0 mm BL. Notochord flexion was initiated at 5.1–5.6 mm BL and finished at 7.1 mm BL. Aggregate numbers of all fin rays were completed at 14 mm BL. Squamation was initiated midlaterally on the anterior trunk at 14 mm BL and completed at 27 mm BL. Although the eggs of I. auropurpureus resembled those of the closely related species Chela dadiburjori, Danio rerio, and Devario malabaricus, they differed from those of Danio rerio and Devario malabaricus in having a narrower perivitelline space. The larvae and juveniles of I. auropurpureus were also similar to those of C. dadiburjori, Danio rerio, and Devario malabaricus in general morphology, but they differed from the latter three species in having a series of dark blotches laterally on the body in the juvenile stage. Moreover, I. auropurpureus differed from C. dadiburjori in having more myomeres and a near-single row of melanophores on the body along the dorsal midline from the yolk-sac to early postflexion larval stages, from Danio rerio in having a cement organ on the forehead during the yolk-sac and early preflexion larvae, and a single melanophore on the lower eye margin in the early yolk-sac larvae, and from Devario malabaricus in having a single melanophore on the lower eye margin in the early yolk-sac larvae. The presence of a cement organ on the forehead indicates a close relationship among the genera Inlecypris, Chela, and Devario.     

Carotenoid-based colour polyphenism in a moth species: search for fitness correlates

Carotenoid‐based integumental coloration is often associated with individual performance in various animals. This is because the limited amount of the pigment has to be allocated to different vital functions. However, most of the evidence for the carotenoid‐based trade‐off comes from vertebrate studies, and it is unclear if this principle can be applied to insects. This possibility was investigated in Orgyia antiqua L. (Lepidoptera: Lymantriidae). The larvae of this species are polyphenic in their coloration, varying from a highly conspicuous combination of yellow hair tufts on black background to cryptic appearance with brown hair tufts. The conspicuous larvae are aposematic, advertising their aversive hairiness. The maintenance of different colour morphs in O. antiqua requires explanation, as an aposematic signal is expected to evolve towards monomorphism. Chromatographic analysis showed that the yellow coloration of the hair is based on the carotenoid pigment lutein (α‐carotene‐3,3’‐diol). The colour of hair tufts was dependent on their carotenoid content. This justifies an expectation of carotenoid‐based physiological trade‐offs between aposematic coloration and individual performance. To test this hypothesis, we monitored life histories of differently coloured larvae reared on various host plants, recording their body sizes, growth rates, and mortalities in each instar. There was a significant but relatively low heritability of tuft coloration, which allowed us to expect environmental effects. We found no phenotypic associations between hair tuft colour and performance indices in O. antiqua larvae, neither did the quality of host plant affect the frequency of colour morphs. However, the frequency of colour morphs differed between larval instars. Our results suggest that carotenoid‐mediated physiological trade‐offs are not involved in the maintenance of colour morphs in O. antiqua larvae, and factors other than individual condition should be responsible for the observed variability.     

Host tree influences on the dispersal of first instar gypsy moths, Lymantria dispar (L.)

Abstract. 1. In laboratory tests, first instar gypsy moths attempted dispersal more frequency when exposed to less acceptable foliage.
2. First instars from small eggs attempted dispersal less frequently than larvae from large eggs when exposed to foliage from highly acceptable or marginally acceptable hosts. Dispersal rates of larvae from medium sized eggs were intermediate.
3. These results (1–2) confirm and expand upon the findings of Capinera & Barbosa (1976).
4. In the field, data on the relative densities of larvae on different host species support the conclusion that the frequency of dispersal attempts is inversely related to host acceptability.
5. The implications of these findings for the population dynamics of the gypsy moth are discussed.     

Impact of an introduced ant on native rain forest invertebrates: Pheidole megacephala in monsoonal Australia

Pheidole megacephala is an exotic ant species that has severely affected native invertebrate biodiversity throughout the tropics. Its impacts have been documented extensively in relatively depauperate invertebrate communities, but not in species-rich habitats such as tropical rain forests. Here we describe the local distribution of P. megacephala and its impacts on native invertebrate assemblages in and around a rain forest patch at Howard Springs, in Australia's monsoonal tropics. P. megacephala was found to be confined to a single area of approximately 25 ha, with its distribution centered on drainage lines and the rain forest. Significant but weak correlations were found between its abundance and vegetative canopy cover (positive) and distance from the rain forest (negative). In the most heavily infested area within the rain forest, the abundance of P. megacephala was 37–110 times that of total native ant abundance found within uninfested plots, as measured by pitfall traps. The abundance and richness of native ants and other invertebrates were significantly reduced in litter samples, pitfall catches and foliage beats where P. megacephala was present, inversely relative to the abundance of P. megacephala. Only two individuals of a single native ant species were found within the most infested plot, with native ant richness being reduced to about half in the least infested plot. The most persistent functional groups of native ants in infested plots were Cryptic species, which forage primarily within soil and leaf litter, and Opportunists, which exhibit highly generalised foraging behaviour. The highest abundance of P. megacephala corresponded with a 42–85% decrease in the abundance of other native invertebrates. Insect larvae were totally absent from foliage beats collected at the most heavily infested plot. P. megacephala was found overall to be expanding its range, averaging 12 m range expansion in the dry season and contracting 7 m in the wet season. It is able to spread into surrounding savanna habitats by occupying relatively sheltered microsites, such as beneath logs and at the bases of trees. However, it is unlikely to attain high population densities in open savanna habitats because of its relative intolerance of desiccation, and the prevalence of behaviourally dominant native ant species. Howard Springs is currently the only rain forest patch in monsoonal Australia known to be infested by P. megacephala, but clearly this ant is a serious potential threat to the region's rain forest invertebrate fauna. Received: 19 August 1998 / Accepted: 12 May 1999     

Influence of larval experience on preference of a subterranean insect Delia antiqua on Allium hosts

Numerous studies have reported that larval experience can affect subsequent host plants selection and future oviposition decisions of many different species, but the investigation of pre‐imaginal experiences on host preference of adults has rarely been tested for soil‐dwelling insects. In this study, we present evidence that larval feeding experience can affect adult host preference in the onion maggot, Delia antiqua. By rearing D. antiqua on different host plants, we were able to examine the role of the natal host of different generations and the effect of larval density on host‐choice behaviour. We also performed bioassays by means of switched host treatment to evaluate the host‐selection principle. Choice bioassays among the three host species demonstrated that D. antiqua females preferred to oviposit on their natal host in each generation and host‐switching treatments. Additionally, increasing larval density could intensify this ovipositional preference on the natal host. The overall results showed that host preference of female D. antiqua is determined by larval experience and density. These findings also add support for the controversial Hopkins’ host‐selection principle.