Oviposition site preference and larval mortality in a leaf-mining moth

Abstract. 1. The univoltine leaf-mining moth, Lithocolletis quercus Ams., is endemic to Israel, where it spends its 10.5 month larval period feeding only in the leaves of Quercus calliprinos Webb.
2. We compared patterns of egg deposition and sources of larval mortality to test whether oviposition patterns and site preferences confer an enhanced likelihood of larval survival.
3. Dominant sources of larval mortality were premature leaf abscission and death from unknown causes, whereas predation, parasitism and intraspecific interference accounted for relatively little larval mortality.
4. Eggs, and thus mines, were aggregated among leaves of host trees even though premature leaf abscission was positively correlated with density of mines per leaf. Interference competition among larvae was the only other density-dependent mortality factor.
5. Oviposition patterns within leaves mitigated the probability of death from larval interference, and probably also from early leaf abscission.
6. Despite these density-dependent mortality factors, overall probability of larval survival to pupation was independent of initial density of mines on a leaf.
7. The long larval period allows synchrony between oviposition flights and times of predictable resource availability.     

Regulation of damseifly populations: the effects of larval density on larval urvival, development rate and size in the field

SUMMARY. 1. The popuhttion density of Coenagrion pttella larvae was monitored in five populations, and of Ischntira elegans in two populations, between October 1982 and May 1983.
2. There was no measurable mortality of larvae over winter and no larval growth until April. Larvae in high density populations were smaller than those in low density populations and were more likely to have a semi- voltine life history.
3. The population density of C. ptiella was also monitored (more frequently) in two populations with differenl initial densities between July and November 1983. In the high density population there was a constant rate of larval mortality, while in the low density population there was no detectable larval mortality, indicating that larval mortality may be density dependent. Larvae in the high density population were again smaller, and more likely to be semi-voitine, than those in the low density population.
4. The role of density dependent larval growth, development and mortality in the regulation of damseifly populations is discussed.     

Field experiments on the relationship between drift and henthic densities of aquatic insects in tropical streams (Ivory Coast). III Trichoptera

SUMMARY. 1. Based on in situ gutter trials we related the drift of caddis flies to their benthic densities and to various abiotic factors in streams in the Ivory Coast (West Africa). Members of the families Hydropsychidae, Philopotamidae. Hydroptilidae and Leptoceridae were considered in detail.
2. The drift of larvae peaked at night in both early and late larval instars.
3. Drift of a larval group (a certain instar, species or higher taxon) was more often related to the benthic density of other larval groups than to its own benthic density.
4. Self-regulation of an upper benthic density of a larval group by emigration through drift was not statistically evident.
5. There was no straightforward relationship between drift and abiotic factors.
6. Drift rates differed between taxa as well as between larval instars (size groups) within a taxon. Newly hatched larvae had very high drift rates, whereas the last larval instar usually had the lowest drift rate.
7. We related these results to the violently fluctuating discharge of the streams in the study area and the consequent variability of space for lotic insects.
8. Drift estimates, made at the same time as a monitoring programme on possible side-effects of insecticides (Onchocerciasis Control Programme), failed to reflect benthic densities except in the night drift of Hydropsychidae.     

Avoidance of occupied hosts by the Hessian fly: oviposition behaviour and consequences for larval survival

Abstract 1. Unoccupied wheat plants and wheat plants occupied by conspecific eggs or larvae were presented to ovipositing female Hessian flies in choice tests.
2. The presence of conspecific eggs on the leaf surfaces of wheat plants did not appear to have any effect on the responses of ovipositing Hessian fly females.
3. The presence of conspecific larvae at the base and nodes of wheat plants for 1, 6, or 11 days had significant effects on Hessian fly oviposition. Eggs oviposited on plants were inversely proportional to larval densities and days of larval occupation.
4. Feeding by Hessian fly larvae is associated with several changes in wheat plants. One of these changes, the growth arrestment of the plant, was measured by recording the heights of plants used in oviposition tests. Plant heights were inversely proportional to both larval densities and days of occupation. Plant heights were directly proportional to eggs oviposited on plants.
5. The consequences of adult female avoidance of plants occupied by conspecific larvae were investigated by allowing females to oviposit on unoccupied plants and 1-day, 6-day, and 11-day larval occupied plants, then scoring at the end of the first larval instar the survival of the offspring that resulted from this oviposition.
6. Survival during the first larval instar was 88% for the offspring of females that oviposited on unoccupied plants, decreasing to 82, 31, and 4% on the 1-day, 6-day, and 11-day occupied plant treatments. On these four plant treatments, a positive correlation was found between larval performance (i.e. survival) and the preferences of ovipositing females.
7. On the four plant treatments, relationships between first-instar larval density and first-instar larval survival varied significantly. On unoccupied plants, survival was inversely proportional to density. On plants oviposited on at 6 days of larval occupation, survival was directly proportional to density.     

The Lepidoptera fauna associated with Calluna vulgaris: effects of plant architecture on abundance and diversity

Abstract .1. The effects of plant architecture on the Lepidoptera assemblage associated with heather Calluna vulgaris were studied at four locations in northern England and southern Scotland in 1992 and 1993. The study areas were Calluna vulgaris – Vaccinium myrtillus heaths, where management by rotational burning had created a mosaic of stands of different-aged Calluna . Lepidoptera were sampled in the larval stage.
2. Larval abundance was considered in relation to Calluna age, height, cover, green shoot density, and flower density. For most study areas and species, multiple regression selected Calluna height as the variable that explained most variation in larval density, although the explanatory variables were intercorrelated.
3. After compensating for the effects of different study locations, the slope of the regression between the logarithm of larval density and the logarithm of Calluna height was common to the macrolepidoptera, microlepidoptera, geometrid, and noctuid larval groups.
4. There was a significant progressive increase in larval diversity with increase in Calluna height, due to the presence of uncommon moth species in the samples from taller Calluna, and a change in the contribution of common species to the community in different height zones.
5. The significance of these results is discussed in relation to other heathland invertebrate studies and management practices.     

Life-history responses of larviparous Boettcherisca formosensis (Diptera: Sarcophagidae) to larval competition for food, including comparisons with oviparous Hemipyrellia ligurriens (Calliphoridae)

Abstract. 1 The effect of larval rearing density on life-history parameters of Boettcherisca formosensis Kirner & Lopes (Sarcophagidae) was investigated. Increases in rearing density resulted in lowered larval survivorship, shortened larval development time and production of smaller, shorter-lived adults with reduced fecundity.
2. B. formosensis is larviparous. Average brood size was 17.5±1.0 (mean±M) larvae, which was much less than the average number of mature larvae inside gravid females. Females apparently produced a series of small broods, distributing their offspring over a number of carcasses.
3. Compared with the oviparous species Hemipyrellia ligurriens (Wiedemann) (Calliphoridae), B. formosensis adults were larger and longer-lived, with a longer larval development time but shorter larval feeding period. However, females had a shorter pre-reproductive period, were less fecund, and had a lower life time reproductive investment.
4. B. formosensis had lower relative performance (measured by the composite index of performance, r') than H. ligurriens over the larval rearing density range, and was more sensitive to increases in density. Although the r' values suggest that the sarcophagid may be a competitively inferior species, other features which are not included in the index (such as larvipary, short larval feeding period and spreading of offspring from a single brood among carcasses) may be of significant adaptive value to B. formosensis.     

Zooplankton responses to predation by larval bluegill: an enclosure experiment

SUMMARY 1. Larval fish are gape-limited predators that forage on prey of specific sizes, and thus may be expected to differentially affect members of a zooplankton community, possibly altering the size-structure or species composition.
2. I used an enclosure experiment to look at the effect of predation by larval bluegill on the dynamics of two zooplankton communities, one dominated by large-bodied individuals and the other by small-bodied individuals. Enclosures containing these zooplankton received a zero, low, medium, or high density of larval bluegill predators.
3. Increasing larval density had a negative effect on zooplankton abundance and abundance declined similarly in the large-bodied and small-bodied communities.
4. Zooplankton size-structure, as estimated by the length of the average zooplankton, increased and then decreased during the experiment, decreasing faster at higher larval fish densities. When zooplankton size-structure was estimated as the length of the average cladoceran, size-structure declined in the large-bodied but not in the small-bodied community and the greatest decline in size-structure was seen in the medium and high larval density treatments.
5. Ordination of each community using multidimensional scaling (MDS) indicated that the trajectory of change in species composition differed between the presence and absence of larval fish. In both communities, the degree of response by individual taxa depended on the density of bluegill larvae. This effect on zooplankton abundance, size-structure and community composition suggests that larval fish may make an important contribution to zooplankton dynamics in many lakes and ponds.     

Microclimate and tree community linked to differences in lepidopteran larval communities between forest fragments and continuous forest

Aim  We aim to assess the impact of forest fragmentation on lepidopteran larval community and study the associations of microclimate and tree community with lepidopteran assemblage.
Location  Kibale National Park, Uganda.
Methods  We investigated the effects of forest fragmentation on leaf herbivory, density of lepidopteran caterpillars, species richness and diversity as well as the composition of herbivorous lepidopteran larval community. Microclimate, size of the fragment, distance to the continuous forest, and tree diversity were studied as possible explanatory factors. We sampled 10 Neoboutonia macrocalyx Pax. (Euphorbiaceae) trees in each fragment during dry and rainy season, total of four times, in a year to cover the seasonal variation.
Results  The rates of herbivory, total larval density and species richness were significantly lower in the forest fragments than in the continuous forest but species diversity expressed as Fisher's alpha did not differ. The dominance structure and community composition of the larval communities in the fragments was different from that of the continuous forest. None of the differences we observed were related to the fragment area or distance to the continuous forest. Instead, we found an indication of association between the herbivore and the tree communities. The fragments had significantly lower humidity during most of the day and higher temperature during the afternoons (14–17 h), which might partially explain the differences in lepidopteran larval communities.
Main conclusions  Decreased larval density and species richness as well as differences in the community composition and structure all highlight the importance of large continuous forest areas for maintaining larval biodiversity.     

Egg hatching inhibition: field evidence for population regulation in a treehole mosquito

ABSTRACT. 1. We investigated the influence of larval density on egg hatching in the treehole mosquito, Aedes triseriatus Say (Diptera: Culicidae) by submerging laboratory-produced egg batches in natural habitats stocked with fixed densities of larvae.
2. We found a significant decrease in hatching rate as larval density increased. The results suggest that inhibited eggs refrain from hatching into environments in which competitive effects of large larvae are potentially severe, and in which cannibalism may occur.
3. The amount of time the egg batches were exposed to experimental conditions (between 2 and 16 days) had no significant influence on hatch rate.     

Variations in the life-history parameters of Hemipyrellia ligurriens (Diptera: Calliphoridae) in response to larval competition for food

Abstract. 1. Larval rearing densities of Hemipyrellia ligurriens (Wiedemann) (Diptera: Calliphoridae) in standardized carrion were manipulated in order to investigate changes in life-history parameters in response to larval competition for food.
2. Competition was of the typical scramble type. Survivorship remained high at densities up to 32 larvae g liver^-1 but decreased rapidly as larval density increased further.
3. Emergent adults were undersized with reduced fecundity and longevity. Variations in adult body size apparently reduced the effects of competition on larval mortality.
4. Females of dry weight corresponding to only 10.4% of the potential maximum emerged at the highest rearing densities of 128 larvae g liver-1. However, these females had a nearly four-fold increase in reproductive investment (per unit weight) when compared to the largest individuals.
5. The duration of larval development declined when competition was intense (i.e. at high larval densities).
6. The short adult life of H.ligurriens, combined with the unpredictability of larval habitat availability, may reduce the value of long-range dispersal so that females 'do better' by maintaining reproductive investment despite a concomitant decline in dispersal ability.     

Density-dependent melanism in sub-arctic populations of winter moth larvae (Operophtera brumata)

Abstract.  1. The aim of this 4-year observational study was to test for the presence of direct and delayed density-dependent larval melanism in the geometrid moth species Operophtera brumata (winter moth) in northern Norway.
2. Data from many populations with a wide range of population densities in time and space facilitated statistical analyses that could separate the effects of current and past density. The data also included different phases of the 10-year population cycle of this species so that eventual non-linear density effects due to population phase could be detected.
3. The results showed that the prevalence of melanism had a strong positive, linear relation to population density within years, whereas there was no evidence for a delayed effect from the year before or dependency on the phase of the population cycle.
4. In combination, these results limit the range of possible explanations of larval melanism in this outbreaking species. The possible reasons why winter moth larvae might benefit from crowding-induced melanism are discussed.     

Intraspecific variation in the strength of density dependence in aphid populations

Abstract.  1. Experimental evidence is presented for positive, negative, and no density dependence from 32 independent density manipulations of milkweed aphids ( Aphis nerii ) in laboratory and field experiments. This substantial variation in intraspecific density dependence is associated with temperature and host-plant species.
2. It is reported that as population growth rate increases, density dependence becomes more strongly negative, suggesting that the monotonic definition of density dependence used in many common population models is appropriate for these aphids, and that population growth rate and carrying capacity are not directly proportional.
3. For populations that conform to these assumptions, population growth rate may be widely applicable as a predictor of the strength of density dependence.     

Multistage density dependence in an amphibian

Density dependence is the major process keeping the sizes of natural populations within bounds. In organisms with complex life cycles, the stage at which density dependence occurs and whether it occurs in one or several life stages have important consequences for the dynamics of their populations. I manipulated density of pool frogs ( Rana lessonae) during the aquatic larval and the terrestrial juvenile stages and examined the effect on growth and survival until 1 year of age. High larval density, but not high juvenile density, led to smaller size at this age. Both larval and juvenile density led to reduced growth during the early juvenile stage, but the effect of the larval density appeared stronger than the effect of juvenile density. No density dependence in survival could be found. My results suggest that density dependence in both the larval and the terrestrial juvenile stage may play important roles in the regulation and dynamics of amphibian populations.     

Thermal ecology of western tent caterpillars Malacosoma californicum pluviale and infection by nucleopolyhedrovirus

Abstract 1. Western tent caterpillars hatch in the early spring when temperatures are cool and variable. They compensate for sub-optimal air temperatures by basking in the sun.
2. Tent caterpillars have cyclic population dynamics and infection by nucleopolyhedrovirus (NPV) often occurs in populations at high density.
3. To determine whether climatic variation might influence viral infection, the environmental determinants of larval body temperature and the effects of temperature on growth and development rates and larval susceptibility to NPV were examined.
4. In the field, larval body temperature was determined by ambient temperature, irradiance, and larval stage. The relationship between larval body temperature and ambient temperature was curvilinear, a property consistent with, but not necessarily limited to, behaviourally thermoregulating organisms.
5. Larvae were reared at seven temperatures between 18 and 36 °C. Larval growth and development increased linearly with temperature to 30 °C, increased at a lower rate to 33 °C, then decreased to 36 °C. Pupal weights were highest for larvae reared between 27 and 30 °C.
6. The pathogenicity (LD₅₀) of NPV was not influenced by temperature, but the time to death of infected larvae declined asymptotically as temperature increased.
7. Taking into account larval growth, the theoretical yield of the virus increased significantly between 18 and 21 °C then decreased slightly as temperatures increased to 36 °C.
8. Control and infected larvae showed no difference in temperature preference on a thermal gradient. The modes of temperature preference were similar to those for optimal growth and asymptotic body temperatures measured in the field on sunny days.
9. Warmer temperatures attained by basking may increase the number of infection cycles in sunny springs but do not protect larvae from viral infection.     

Host-specific variation in Ieaf miner population dynamics: effects on density, natural enemies and behaviour of Stilbosis quadricustatella (Lepidoptera: Cosmopterigidae)

Abstract. 1. We compared high and low density populations of a leaf miner ( Stilbosis quadricustatella (Cham.)) on two host oaks to ascertain mechanisms influencing abundance. High density miner populations occurred on sand live oak, Quercus geminata (Fagaceae), found in homogeneous stands at inland and coastal sites. Quercus nigra , water oak, a patchily distributed inland species, supported a low density leaf miner population.
2. Average foliar nitrogen of Q.geminata was significantly lower than that of Q.nigra , and lad mining period on Q.geminata was correspondingly longer than on Q.nigra .
3. The average leaf area of Q.nigra was significantly greater than that of Q.geminata .
4. Parasitism was significantly greater in Q.geminata miner populations and predation was significantly higher in Q.nigra populations. However, parasitism and predation rates were roughly reciprocal so that overall larval mortality did not differ significantly between hosts.
5. In a laboratory experiment, pupal overwintering survivorship did not differ significantly between moist and dry treatments of the sand and loam soil types that typify Qgeminata and Q.nipra habitats, respectively.
6. Within-leaf miner density on Q.geminata significantly affected larval survivorship, parasitism and predation. Leaf area and within-leaf miner density were positively correlated.
7. We detected no host-patch area or isolation effect on miner densities on coastal Qgeminata and there was no significant gradient of local coastal conditions affecting larval survivorship or natural enemies.
8. Coastal leaf miner densities were significantly higher on oak patch edges than in interiors.     

Environmental variation and behaviour: resource availability during ontogeny and feeding performance in salamander larvae (Ambystoma texanum)

1. The effects of resource availability during ontogeny on subsequent feeding performance were investigated in larvae of the small-mouthed salamander ( Ambystoma texanum ).
2. Salamander larvae were reared individually in either high or low prey density treatments for 7 weeks prior to intermediate prey density foraging trials. Larvae from the low prey density treatment were on average 35% smaller in body size than individuals from the high prey density treatment.
3. Resource availability during development influenced larval feeding rates and altered the relationship between body size and three feeding performance measures (attack rates, capture success and feeding rates). Feeding rates in predation trials were also positively correlated with growth rate early in the larval period (until the end of week 5).
4. These results suggest that the environment to which developing organisms are exposed can have significant effects on subsequent behaviour, and that small-mouthed salamander larvae may show state-dependent changes in feeding behaviour in response to differences in long-term feeding history. Additionally, differences in feeding performance may influence the probability of survival to the adult stage for organisms that utilize ephemeral habitats.     

Environmental variation and behaviour: resource availability during ontogeny and feeding performance in salamander larvae (Ambystoma texanum)

1. The effects of resource availability during ontogeny on subsequent feeding performance were investigated in larvae of the small-mouthed salamander ( Ambystoma texanum ).
2. Salamander larvae were reared individually in either high or low prey density treatments for 7 weeks prior to intermediate prey density foraging trials. Larvae from the low prey density treatment were on average 35% smaller in body size than individuals from the high prey density treatment.
3. Resource availability during development influenced larval feeding rates and altered the relationship between body size and three feeding performance measures (attack rates, capture success and feeding rates). Feeding rates in predation trials were also positively correlated with growth rate early in the larval period (until the end of week 5).
4. These results suggest that the environment to which developing organisms are exposed can have significant effects on subsequent behaviour, and that small-mouthed salamander larvae may show state-dependent changes in feeding behaviour in response to differences in long-term feeding history. Additionally, differences in feeding performance may influence the probability of survival to the adult stage for organisms that utilize ephemeral habitats.     

Integrated control of apple pests in New Zealand 10. Population dynamics of codling moth in Nelson

Abstract

Sampling methods are described for estimating the population density, mortality, and natality of a univoltine population of codling moth attacking mature apple trees (cv. ‘Delicious’) at Nelson, New Zealand. These methods were used to construct life tables for the species over eight generations (1967–68 to 1974–75) on trees variously sprayed and not sprayed with ryania in an integrated control programme. Bait traps provided a sensitive measure of seasonal adult population density. Analysis of the life tables shows that migration of adults was the main key factor and that overwintering larval mortality (particularly that due to bird predation), fecundity, and ryania also made a major contribution to variation in generation mortality. In the absence of ryania the resident population usually increased between generations, whereas it usually decreased when ryania sprays were applied. The density dependence of overwintering larval mortality was due to bird predation, and the inverse density dependence of larval mortality from ryania was due to changes in the site of fruit entry with larval population density. Fecundity was density independent, and inconclusive evidence was obtained on the density dependence of migration. The wide variation in fecundity is attributed primarily to weather conditions. The impact on control strategy of the above key factors, density dependence, and total natural mortality is discussed. Ryania is found to be uneconomic, whereas the granulosis virus of codling moth and male removal with pheromone traps show promise as future control methods. The need to eliminate reservoirs of codling moth close to orchards under integrated pest control is emphasised. Regulation of codling moth populations at Nelson on neglected, unsprayed trees appears to result from intraspecific competition for fruits and cocooning sites, and weakly density-dependent mortality of mature larvae when seeking cocooning sites and while overwintering in their cocoons. Variation in fecundity also cohtributes to fluctuations in abundance of the species. In contrast, at low density in an integrated control programme no intraspecific competition was evident; migration, winter mortality, and fecundity were the main determinants of abundance. This illustrates the need to study pest populations at densities similar to those tolerable commercially.     

Oviposition of the dung beetle Aphodius ater in relation to the abundance of yellow dungfly larvae (Scatophaga stercoraria)

1. The dung beetle Aphodius ater and the yellow dungfly Scatophaga stercoraria are temporally co-occurring species in sheep dung, which they use for reproduction and nutrition ( A. ater ) or for reproduction only ( S. stercoraria ) during the spring in northern Germany. Scatophaga stercoraria uses fresh sheep dung pellets a few hours old for oviposition, whereas A. ater lays eggs into 2–10-day-old pellets. In the present study, the egg laying behaviour of A. ater in sheep dung in relation to the presence of larvae of S. stercoraria was investigated experimentally.
2. Choice experiments, based on examining the egg laying behaviour of beetles in 2- and 4-day-old pellets with and without high and low densities of fly larvae, showed the following. In 2-day-old pellets, the beetles did not distinguish between pellets without fly larvae or with fly larvae at low larval density but avoided laying eggs into pellets with a high larval density. In 4-day-old pellets, the beetles always preferred to lay their eggs into pellets without fly larvae, regardless of larval density.
3. The influence of different densities of larvae of S. stercoraria on dung depletion was examined by measuring the dry weight, organic matter content and organic nitrogen content of the remaining dung after larval development. The presence of the larvae led to a reduction in all three parameters.
4. The beetles' behaviour of laying eggs into older pellets, and their awareness of the presence of high densities of fly larvae, enables them to avoid egg laying into pellets that will have been depleted by fly larvae before the beetle larvae have finished their development.