Pupation Success Research Articles

Effect of Rhazya stricta, Calotropis procera, and Francoeuria crispa on Larvae and Eggs of Culex pipiens

ABSTRACT Toxicity of the aqueous extracts of Rhazya stricta Decne, Calotropis procera (Ait.) Cass., and Francoeuria crispa (Forssk.) R.Br. leaves against larvae of Culex pipiens L. mosquitoes was investigated by incorporating the extracts into egg and larval rearing media. Acute LC50 were 270 and 322 ppm for R. stricta and C. procera, respectively. The chronic toxicities for R. stricta at concentrations 0.02 and 0.04 percent, reached 70 and 100 percent larval mortality, respectively, and 91.1 and 100 percent at concentrations of 0.05 and 0.075 percent, respectively, for C. procera. Both materials conferred significantly reduced larval development and thus, consequently reducing pupation and adult emergence. Only 10 and 33.3 percent of the larvae reared in media containing 0.02 percent of R. stricta water extract and 0.025 percent of C. procera completed development to the pupa stage. None of the larvae in R. stricta extract reached adulthood and only 20 percent of the larvae did in the C. procera extract. The highest concentration of F. crispa tested (0.25%) caused only 55.3 percent mortality after 10 days, leading to 34.3 and 21.2 percent successful pupation and adult emergence, respectively. No mosquito eggs were laid in media containing at concentrations ≥ 0.05 percent aqueous extract from any of the three plants. Application of R. stricta and C. procera extracts applied to mosquito larval breeding sites may well provide an environmentally safe method for control of mosquito populations.

Hormesis and stage specific toxicity induced by cadmium in an insect model, the queen blowfly, Phormia regina Meig.

Hormesis is an adaptive response, commonly characterized by a biphasic dose–response that can be either directly induced, or the result of compensatory biological processes following an initial disruption in homeostasis [Calabrese and Baldwin, Hum. Exp. Toxicol., 21 (2002), 91]. Low and environmentally relevant levels of dietary cadmium significantly enhanced the pupation rate of blowfly larvae, while higher doses inhibited pupation success. However, dietary cadmium at all exposure levels adversely affected the emergence of the adult fly from the pupal case. Such findings represent the first report of a heavy metal displaying a hormetic-like biphasic response for pupation success, while at the same time displaying stage-specific toxicity at a later developmental period. These conclusions are based on substantial experimentation of over 1750 blowflies, in seven replicate experiments, involving 10 concentrations per experiment. These findings indicate the need to assess the impact of environmental stressors over a broad range of potential exposures as well as throughout the entire life cycle.

Threshold weight for starvation-triggered metamorphosis in the yellow-spotted longicorn beetle, Psacothea hilaris (Coleoptera: Cerambycidae)

The development of 4th stadium Psacothea hilaris larvae under various feeding and starving regimens was investigated to determine whether there is a threshold weight for metamorphosis. In larvae fed ad libitum, 56% of individuals spent a mean of 13 d in the 4th stadium and 18 d in the 5th stadium before pupation, whereas the rest remained in the 4th stadium for 24 d and pupated. When starved upon ecdysis to the 4th stadium, no larvae molted to the 5th stadium and most eventually died without pupation. In contrast, when larvae were fed for 1 d and then starved, 47% prematurely metamorphosed into small pupae after a mean of 18 d in the 4th stadium. Pupation success from the 4th stadium increased as the feeding period was extended, reaching 97% in the larvae that had fed for four days. All these pupae gave rise to small but morphologically normal adults. The relationships between the weight changes of experimental larvae during starvation and their pupation success suggested the threshold weight for metamorphosis to be 180 mg. The adaptive significance of premature pupation in starved P. hilaris is discussed as a life history strategy under unpredictable food conditions for larval growth.

Biology of Centistes delusorius, a parasitoid of adult apple blossom weevil

Abstract Natural control of apple blossom weevil, Anthonomus pomorum (L.), deserves attention, as the pest is regaining importance with the declining use of non‐selective pesticides in apple and pear orchards. In this study the biology of Centistes delusorius (Förster), a specific parasitoid of adult apple blossom weevil, is investigated. The parasitoid hibernates as young larva in an adult weevil, and juvenile development is resumed in early spring. The fully grown parasitoid larvae leave their hosts during full bloom at the end of April and early May, to pupate. The adults emerging in May oviposit into the newly emerged weevils, which initially feed on apple leaves. Centistes delusorius was detected in six out of 15 host‐weevil infested orchards, but was only common in two with larger apple trees standing in grass. There, parasitism levels of around 30% were usual in hosts taken from treebands in winter. The delicate larva is vulnerable, and the thin cocoon provides little protection against either desiccation or drowning on a weedless orchard floor. Observations indicate that successful pupation of C. delusorius demands stable humid conditions and some shelter, such as that found in grass or woodland soils. Parasitoid females, provided with honey, lived for a mean of 6.3 ± 2.1 days under outdoor conditions in June. Their life span was similar whether they had access to and oviposited in hosts, or not. The species is pro‐ovigenic, and potential fecundity is about 40 eggs. Oviposition usually takes a few seconds. Parasitized female hosts do not reproduce. Up to 95% of the parasitoid eggs laid in May develop into a second generation, the adults of which appear in July, when the host has entered aestivation. Older (British) records of C. delusorius outside orchards suggest that some parasitized hosts, like the healthy ones, leave the orchard prior to aestivo‐hibernation, so that the latter do not escape parasitoid attack in July. A trapping sample in late June, when most non‐parasitized weevils have gone into aestivo‐hibernation, is probably the most efficient method to detect parasitized weevils. The (near‐)absence of C. delusorius in many orchards is probably due not only to pesticide side‐effects, or scarcity of its host, but also to the absence of suitable pupation sites for the wasp.

Longevity and Reproductive Success of Aethina tumida (Coleoptera: Nitidulidae) Fed Different Natural Diets

The longevity and reproductive success of newly emerged, unfed adult Aethina tumida Murray assigned different diets (control = unfed; honey-pollen; honey; pollen; empty brood comb; bee brood; fresh Kei apples; and rotten Kei apples) were determined. Longevity in honey-fed small hive beetle adults (average maximum: 167 d) was significantly higher than on other diets. Small hive beetles fed empty brood comb lived significantly longer (average maximum: 49.8 d) than unfed beetles (average maximum: 9.6 d). Small hive beetle offspring were produced on honey-pollen, pollen, bee brood, fresh Kei apples, and rotten Kei apples but not on honey alone, empty brood comb, or in control treatments. The highest reproductive success occurred in pollen fed adults (1773.8 ± 294.4 larvae per three mating pairs of adults). The data also show that A. tumida can reproduce on fruits alone, indicating that they are facultative parasites. The pupation success and sex ratio of small hive beetle offspring were also analyzed. Larvae fed pollen, honey-pollen, or brood had significantly higher pupation success rates of 0.64, 0.73, and 0.65 respectively than on the other diets. Sex ratios of emerging adults fed diets of pollen or brood as larvae were significantly skewed toward females. Because small hive beetle longevity and overall reproductive success was highest on foodstuffs located in honey bee colonies, A. tumida are efficient at causing large-scale damage to colonies of honey bees resulting in economic injury for the beekeeper. Practical considerations for the control of A. tumida are briefly discussed.

Longevity and reproductive success of Aethina tumida (Coleoptera: Nitidulidae) fed different natural diets.

The longevity and reproductive success of newly emerged, unfed adult Aethina tumida Murray assigned different diets (control = unfed; honey-pollen; honey; pollen; empty brood comb; bee brood; fresh Kei apples; and rotten Kei apples) were determined. Longevity in honey-fed small hive beetle adults (average maximum: 167 d) was significantly higher than on other diets. Small hive beetles fed empty brood comb lived significantly longer (average maximum: 49.8 d) than unfed beetles (average maximum: 9.6 d). Small hive beetle offspring were produced on honey-pollen, pollen, bee brood, fresh Kei apples, and rotten Kei apples but not on honey alone, empty brood comb, or in control treatments. The highest reproductive success occurred in pollen fed adults (1773.8 +/- 294.4 larvae per three mating pairs of adults). The data also show that A. tumida can reproduce on fruits alone, indicating that they are facultative parasites. The pupation success and sex ratio of small hive beetle offspring were also analyzed. Larvae fed pollen, honey-pollen, or brood had significantly higher pupation success rates of 0.64, 0.73, and 0.65 respectively than on the other diets. Sex ratios of emerging adults fed diets of pollen or brood as larvae were significantly skewed toward females. Because small hive beetle longevity and overall reproductive success was highest on foodstuffs located in honey bee colonies, A. tumida are efficient at causing large-scale damage to colonies of honey bees resulting in economic injury for the beekeeper. Practical considerations for the control of A. tumida are briefly discussed.

Priority effects of bamboo‐stump mosquito larvae: influences of water exchange and leaf litter input

Abstract 1. A series of laboratory experiments was conducted to examine the influences of leaf litter input and flushing of medium on the priority effects of the two bamboo‐stump breeding mosquitoes, Aedes albopictus and Tripteroides bambusa, using 150‐ml microcosms. Larvae of either species were introduced to microcosms on day 0 (early cohort), at different densities, and day 14 (late cohort). The effects of the early cohort on survival, pupation, and biomass yield of the late cohort were compared among various combinations of the two species and among different treatments (water exchange, leaf addition, both of them, neither of them) on days 14 and 44.2. Survivorship and pupation success of the late A. albopictus cohort were affected negatively by the presence of either species of the early cohort to a greater extent than those of the late T. bambusa cohort.3. Water exchange reduced mortality of the late A. albopictus cohort over a short term in the presence of the early cohort of either species, indicating that a toxic substance was involved in the inhibitory priority effects. The addition of leaf litter enhanced survivorship and pupation of the late A. albopictus cohort, whether or not water was exchanged. The late T. bambusa cohort showed high survival rate with all treatments.4. The results indicate that leaf‐litter input moderates the inhibitory priority effects on A. albopictus larvae, not only by supplying food resources but also by alleviating the toxicity of accumulated dissolved substances.

Effects of the polydnavirus of Cotesia congregata on the immune system and development of non-habitual hosts of the parasitoid

Polydnaviruses (PDV) are obligate mutualistic symbionts found in association with some groups of parasitic Hymenoptera. In these groups, they suppress the immune response of the parasitoid’s host and are required for successful parasitoid reproduction. Several PDV effects have been described in different experimental systems, but no clear picture of PDV mode of immunosuppression has emerged. No study to date has directly tested if PDV modes of action are evolutionarily conserved or divergent among parasitoid taxa within the Ichneumonoidea. We hypothesize the divergence in PDV mode of immunosuppression can be detected by identifying points of divergence in the immune response of different host species to PDV from one parasitoid species. This study tests the effects of purified PDV from Cotesia congregata on the immune response of three larval lepidopteran species that naturally are hosts of parasitoid species that differ in taxonomic relatedness to C. congregata. Here we demonstrate that despite associations with distantly related parasitoids (Ichneumonidae and Braconidae), Manduca sexta and Heliothis virescens showed similar patterns of increased glucose dehydrogenase (GLD) activity, suppressed cellular encapsulation in vitro, and increased time to pupation. In contrast, Lymantria dispar showed no response to C. congregata PDV across any of the parameters measured, even though it has an evolutionary association with several parasitoids closely related to C. congregata and within the Microgastrinae. The PDV immunosuppression in H. virescens and M. sexta does not correlate with host molecular phylogeny either. The suborganismal effects shown in M. sexta and H. virescens translated into significantly reduced pupation success in M. sexta only. Results demonstrate that while some PDV modes of immunosuppression in hosts may be divergent, others may be conserved across broad host groups.

Food availability controls the onset of metamorphosis in the dung beetle Onthophagus taurus (Coleoptera: Scarabaeidae)

SummaryIn nature, larvae of the dung beetle Onthophagus taurus (Schreber 1759) are confronted with significant variation in the availability of food without the option of locating new resources. Here we explore how variation in feeding conditions during the final larval instar affects larval growth and the timing of pupation. We found that larvae respond to food deprivation with a reduction in the length of the instar and premature pupation, leading to the early eclosion of a small adult. To achieve pupation, larvae required access to food for at least the first 5 days of the final instar (= 30% of mean third‐instar duration in control individuals), and had to exceed a weight of 0.08 g (= 58% of mean peak weight in control individuals). Larvae that were allowed to feed longer exhibited higher pupation success, but increased larval weight at the time of food deprivation did not result in increased pupation success except for larvae weighing > 0.14 g. Larvae responded to food deprivation by initiating and undergoing the same sequence of developmental events, requiring the same amount of time, as ad libitum‐fed larvae once those had reached their natural peak weight. Our results reveal a striking degree of flexibility in the dynamics and timing of larval development in O. taurus. They also suggest that premature exhaustion of a larva's food supply can serve as a cue for the initiation of metamorphosis. Premature metamorphosis in response to food deprivation has been documented in amphibians, but this is, to the best of our knowledge, the first time such a behaviour has been documented for a holometabolous insect. We discuss our findings in the context of the natural history and behavioural ecology of onthophagine beetles.

Enhanced development in nature of larval Anopheles arabiensis mosquitoes feeding on maize pollen.

To determine whether pollen produced by maize (Zea m. mays) may contribute to the development of larval Anopheles gambiae complex mosquitoes, the main African vectors of malaria, we correlated duration of larval development, pupation success, and size of the resulting adults with degree of access to this potential nutriment. Maize pollen is abundant during the wet season on the surface of water near maize plantings in a malaria-endemic region of Ethiopia, and larval Anopheles arabiensis readily ingest these particles in nature. Larvae develop to the pupal stage more rapidly, more frequently, and produce larger adults where maize pollen is abundant than do those that have little access to this food. The force of transmission of malaria in sub-Saharan Africa might be reduced if maize plantings were excluded from the immediate vicinity of homes or, perhaps, if pollen of such maize were to express entomotoxins.

Effects of the Timing of Entomopoxvirus Administration to the Smaller Tea Tortrix,Adoxophyessp. (Lepidoptera: Tortricidae) on the Survival of the Endoparasitoid,Ascogaster reticulatus(Hymenoptera: Braconidae)

Abstract Infection of larvae of the smaller tea tortrix,Adoxophyessp., by an entomopoxvirus (AsEPV) adversely affected the development and survival of the endoparasitoid,Ascogasterreticulatus.Percentages of emergence of parasitoid larvae that developed in AsEPV-infected hosts were significantly lower than those of parasitoid larvae that developed in noninfected hosts. The percentages of emergence ofA. reticulatuswere significantly greater for parasitoids developing in the host larvae that were exposed to AsEPV in third and fourth instars than for the parasitoids developing in the host larvae that were exposed to AsEPV in first and second instars. Successful pupation and eclosion of parasitoids significantly increased as duration between parasitization and administration of AsEPV increased. No parasitoid larvae developing from hosts that were inoculated with AsEPV in the first or second instars pupated and emerged as adults, whereas 13.8 and 54.0% of parasitoids developing from the hosts that were exposed to AsEPV in the third and fourth instars, respectively, pupated. The development of parasitoids in AsEPV-infected hosts was delayed. This developmental retardation was more severe in parasitoid larvae developing in the early-infected hosts than in those developing in the late-infected hosts. The average times required for death due to AsEPV infection were significantly shorter in parasitized hosts than in nonparasitized hosts. Significant numbers of parasitized and infected larvae exhibited apolysis in preparation for the final larval molting. This abnormal metamorphosis appeared to be a possible reason for the earlier death of the parasitized and infected hosts.

Can the tortoise beat the hare? A possible mechanism for the coexistence of competing mosquitoes in bamboo groves

AbstractWe studied interspecific competition between the larvae of the two mosquitoes Aedes albopictus and Tripteroides bambusa, which are predominantly found in water‐filled bamboo stumps in northern Kyushu, south‐western Japan, using microcosms with dead bamboo leaves in the laboratory. We compared short‐term competition between single cohorts of the two species and long‐term competition involving four cohorts of each species, which were introduced at 6‐day intervals. In the single cohort experiment, A. albopictus grew faster than T. bambusa. However, in the multiple cohort experiment, although the first cohort of A. albopictus grew faster and began to pupate earlier than that of T. bambusa, molting rates of later cohorts of A. albopictus, that were introduced on the 12th and the 18th day, were lower than those of T. bambusa. The survival rate of A. albopictus became lower than that of T. bambusa after the 18th day. The cumulative number of the pupated T. bambusa individuals exceeded that of A. albopictus on the 96th day. The final pupation success was higher in T. bambusa than in A. albopictus, especially when additional leaves were supplied on the 48th and the 96th days. The reversed outcomes between short‐ and long‐term interspecific competition and the variation in the lifespans of small aquatic sites may contribute to the coexistence of the two mosquito species in bamboo groves.

DoMicrocystis aeruginosa toxins accumulate in the food web: a laboratory study

The accumulation ofMicrocystis aeruginosa hepatotoxins (microcystin-LR) in the phantom midge larvaChaoborus was studied in a 16 d laboratory experiment. In the cyanobacteria treatment,Chaoborus larvae were fed withDaphnia pulex juveniles which had been feeding for two days on a mixture ofScenedesmus obtusiusculus and toxicMicrocystis aeruginosa. In the control treatment theChaoborus larvae were offeredD. pulex raised onScenedesmus only. An HPLC analysis failed to detect any cyanobacteria toxin in theChaoborus larvae, indicating that the toxin was metabolized or excreted byD. pulex andChaoborus. There was a statistically significant increase in mortality of larvae in the cyanobacteria treatment, but no difference between treatments in pupation success ofChaoborus was observed.

Rearing Culicoides variipennis sonorensis (Diptera: Ceratopogonidae) on agar and nematodes.

Culicoides variipennis sonorensis Wirth & Jones was reared from egg to adult in petri dishes on a 0.5 or 1% agar substrate with bacterial-feeding nematodes (Pelodera, Panagrellus) as food. All larval stages except the first instar were observed attacking and feeding on nematodes. Developmental rate on agar was approximately 25% slower than that of larvae reared in pans of water with bacteria, fungi, and algae as food. Pupation success was approximately 75% from dishes (3.5 or 9 cm diameter) with 30 and 100 eggs, respectively. The technique has significant advantages for maintainance of small laboratory colonies and allows continuous observation of developing immatures for experimental studies.

Temperature effects on pupation-height response in four Drosophila species group triads

A comparison of pupation-height responses among four Drosophila species group triads ( virilis, repleta, melanogaster and willistoni) was carried out in the laboratory at temperatures ranging from 4.5 to 38°C and 100% r.h. The effects of temperature on pupation-height response appear complex in the species groups examined. Two-way analyses of variance (species × temperature) reveal that significant effects differ for each triad. At temperatures where at least 50% pupation success was observed, mean pupation heights are low for all species (less than 5 mm). At high-temperature extremes, all species show little or no upward movement. At low-temperature extremes, maximal height is observed in some species within the repleta and willistoni groups, and the entire melanogaster triad. However, other species within the repleta and willistoni groups, and the entrie virilis triad show no upward movement at all under the same low-temperature conditions. A species' pupation-height response at one temperature extreme cannot be used to predict its response at the other temperature extreme. Temperature effects on pupation height in one species do not necessarily reflect the effects on other species within the group. Temperature effects for one species group cannot be used to predict responses for other groups from different phylogenetic and ecological backgrounds.

Physiological Basis of Ratio‐Dependent Predator‐Prey Theory: The Metabolic Pool Model as a Paradigm

Ratio—dependent models of predators and prey are approximations of the biology of resource acquisition and allocation and their consequences for population birth and death rates. A demand—driven functional response model that has a physiological basis in mass (energy) dynamics is reviewed (i.e., the metabolic pool model), and its obvious links to the logistic model are outlined. To demonstrate the utility of this approach, a distributed maturation time age—structure model of the dynamics of A. J. Nicholson's classic laboratory population data on the sheep blow fly (Lucilia cuprina Weidman) is developed. The model provides sufficient information on the dynamics of the intermediate life stages to show that the blow fly oscillations were due to the effects of larval competition for food on size, fecundity, and pupation success. These results agree with Nicholson's conclusions. The advantage of this model, in contrast to prior models, is that the dynamics emerge by considering the processes of resource acquisition and allocation as they affect growth, reproduction, and survival. No explicit time delays, which automatically lead to oscillations, were included. Lastly, the notions of the metabolic pool model are found in Nicholson's original model for equilibrium population density. The metabolic pool paradigm in an age—structure setting is used to model the tri—trophic dynamics of Acyrthosiphon aphids in an alfalfa ecosystem. The model explains the role of the various natural enemies in the regulation of the aphids.

Resource limitation, habitat segregation, and species interactions of british tree-hole mosquitoes in nature.

The insect fauna of water-filled tree holes in southern Britain consists primarily of the mosquitoes Aedes geniculatus, Anopheles plumbeus, Culex torrentium, and a benthic detritivorous fauna that includes primarily the scirtid beetle Prionocyphon serricornis and the chironomid midge Metriocnemus martinii. Culex torrentium has been documented only relatively recently in tree holes but all three species of mosquitoes partition the resource in space and time. When mosquito larvae were forced to coexist in natural tree holes at limiting densities and at higher than natural levels of interspecific encounter, there was no evidence that Aedes geniculatus or Anopheles plumbeus affected pupation success, pupal weight, or development time of the other or that either Aedes geniculatus or C. torrentium affected the survivorship, pupation success, pupal weight, and biomass yield of the other. When A. geniculatus at limiting densities were forced in natural tree holes to live without or to coexist with natural or twice natural densities of P. serricornis and M. martinii, the presence, absence, or superabundance of the benthic insects did not affect pupation success or pupal weight of A. geniculatus; development time of A. geniculatus was faster when a superabundance of the benthic fauna was present. Effects of the benthic fauna on A. geniculatus are slight and the only significant interaction is facilitative, not competitive. The pattern of habitat segregation among treehole mosquitoes in southern Britain is characteristic of their respective genera and we propose that this pattern is more likely (but not certain) to have arisen through a process of independent evolution than through competitively driven niche shifts among already coexisting species.

Fitness and habitat segregation of British tree-hole mosquitoes

Abstract. 1 Populations of three mosquitoes, Aedes geniculutus (Olivier), Anopheles plumbeus Stephens and Culex torrentium Martini, were found in three types of tree holes: rot holes penetrating the bark of the host tree and exposed, or deeper, more cryptic, bark-lined buttress holes. Aedes geniculatus occupied all three tree-hole types but reached their greatest abundance in exposed buttress holes; A.plumbeus predominated in rot holes; C.torrentium occupied buttress holes exclusively. 2 After a 1 month period of winter freezing, larvae of A.plumbeus survived better than larvae of A. genicularus. Freeze resistance of A.plumbeus increased from cryptic buttress to rot holes. Freeze resistance of A.geniculatus declined from exposed buttress to cryptic buttress to rot holes and was markedly higher in more protected than in more exposed macrohabitats. 3 Among manipulated larval cohorts of A.geniculutus and A.plumbeus, survivorship, pupation success, pupal weight, and/or biomass yield did not differ among tree-hole types or was the reverse of freeze-related survivorship. Despite seasonal differences in fitness correlates, annual fitness may be similar among tree-hole types for both species. Manipulations carried out at a single census or during a single season are therefore likely to produce misleading information about fitness variation among sub-habitats. 4 Both species should exploit a broad range of tree-hole types but A.plumbeus occupies more restricted types of tree holes than does A.geniculutus. The restricted habitat usage by A.plumbeus does not parallel their fitness among tree-hole types in southern Britain but does resemble that of other tree-hole anophelenes. We therefore propose that habitat selection by A.plumbeus is more likely due to their phylogenetically determined physiological tolerances and behaviour than to competition from coexisting A.geniculatus.

Mechanisms of interference competition in the western tree‐hole mosquito, Aedes sierrensis

Abstract. Mechanisms of interference competition affecting Aedes sierrensis (Ludlow) were investigated in laboratory microcosms using reconstituted, natural tree‐hole detritus as food and substrate. Pupation success, larval development time, and pupal weight were all affected by larval density but not by surface area:volume ratio of the microcosm. Mature fourth instars affected pupation success, pupal weight, and development time of developing cohorts separated from them by a 2 μm pore membrane impermeable to bacteria, indicating that chemical competition is proportional to density of at least older instars. Cannibalism does not occur, regardless of the presence or absence of food or physical complexity. Fourth instars inhibited each other's feeding at densities of 64 larvae/ml or greater. Feeding inhibition due to physical contact (encounter competition) was abated but not eliminated by increasing physical complexity of the microcosm by the addition of leaves. Levels of detritus and larval density both affect weight of day‐1 fourth instars. Resistance to encounter competition is proportional to fourth instar weight and weight‐specific resistance is correlated with rank weight within a developing cohort. At densities around the population equilibrium in nature, encounter competition should be taking place, especially in tree holes with few leaves or other large litter.

Density-dependent natural selection does not increase efficiency

Summary Populations of Drosophila melanogaster kept at high population density (K-selected) for 125 generations have higher larval viability than populations kept at low densities (r-selected) when both are raised under crowded conditions. In addition K-selected adults that emerge from crowded cultures are larger than their r-selected counterparts. These differences cannot be explained by differences in efficiency of food use. The minimum food required for successful pupation is actually greater in the K-selected populations. I conjecture that there may be a trade-off between minimum food requirements and competitive ability, which has changed substantially in the K-selected populations. The possibility that K-selected larvae can dig more more deeply and gain access to unused food is examined and rejected as a possible explanation of the viability differences. Evidence is provided supporting the hypothesis that the differences in viability may be due to an increased tendency of the K-selected larvae to pupate off the surface of the medium.