Low Egg Loads Research Articles

Host patch quality increases parasitoid locomotor activity despite risk of egg limitation

Abstract Theory predicts that egg load and foraging activity of insects should be positively related. Species that emerge with a low initial egg load and mature additional eggs as they age may limit initial foraging activity to avoid egg limitation. Similarly, females, which deplete their egg supply through oviposition, should decrease activity immediately afterwards and then increase activity as new eggs are matured. The authors used activity monitors to measure daily locomotor activity separately from exposure to host cues with the stink bug egg parasitoid, Trissolcus japonicus. The authors measured activity and egg load as newly emerged wasps aged and tested whether egg depletion from oviposition would reduce activity. Parasitoid activity increased with age in parallel with egg maturation, with both egg load and activity reaching their maximum about 6–7 days after emergence. Newly emerged wasps began activity later in the day and had lower activity intensity compared with older individuals. When given host experience, females that had laid many offspring, and had thus depleted their egg loads, unexpectedly increased their activity for up to 3 days. The present study results challenge the generally accepted positive relationship between foraging activity and egg load, suggesting that this relationship may be secondary to host patch quality. The authors provide evidence that host patch quality can influence not only within‐patch but also between‐patch foraging activity. Plastic responses in activity patterns may allow parasitoids to better exploit rich host environments while minimising foraging costs and decreasing likelihood of egg limitation in low‐quality environments.

The transgenerational consequences of maternal parasitism for aphid life history and suitability for subsequent parasitism.

Aphids parasitized in later instars can give birth to several nymphs before their reproduction is curtailed by the developing parasitoid. We examined the life histories of Aphis fabae Scopoli born to mothers parasitized by Lysiphlebus fabarum Marshall, and their suitability as subsequent hosts, to test the 'fecundity compensation' hypothesis. Maternal parasitism negatively impacted life history parameters, resulting in reduced estimates of population increase (rm, R0, and λ), and increased generation time (GT) and doubling time (DT). These impacts were greater when the larva developing in the mother turned out to be female rather than male, and greater still when mothers were superparasitized. Maternal parasitism produced aphids with shorter hind tibia (HTL), at birth and at maturity, but their developmental time was unaffected. Although female L. fabarum readily accepted such aphids for oviposition, rates of mummification and wasp emergence were lower, and more so when the maternal parasitoid was female. The resulting parasitoids took longer to develop than progeny from control wasps, had shorter HTLs, lower egg loads, smaller eggs, and produced fewer mummies with lower rates of adult emergence, all differences that were more pronounced when the maternal parasitoid was female. The progeny of these wasps exhibited similar impairments to these biological parameters as their parents, demonstrating that the negative impacts of development in maternally parasitized hosts extended for at least two generations. Thus, our results do not support fecundity compensation, but suggest that any benefits of post-parasitism reproduction will be offset by reduced fitness in both aphid progeny and the parasitoids that develop in them.

Epidemiological and ultrasonographic investigation of bovine fascioliasis in smallholder production system in Eastern Nile Delta of Egypt

Regular updating of our knowledge on the epidemiological determinants of bovine fascioliasis is necessary to increase the awareness of the disease’s significance and subsequently, improve the control measures. The objectives of this study were (1) to estimate the prevalence of bovine fascioliasis, and identify the association of epidemiological characteristics under traditional householders’ production systems, (2) to describe the association between the clinical picture, Fasciola spp. egg count and hepatobiliary ultrasonography findings. In total, 270 faecal samples were examined microscopically for the presence or absence of Fasciola spp. egg, using the sedimentation-flotation method. Copro-positive animals were subjected to ultrasonographic examination. Overall prevalence of copro-positive animals was 27.4% (22.4–33.0%, 95% CI). The final multivariate analysis showed that there was a significant association between fascioliasis and animal species (P < 0.03), and administration of anthelmintic (P < 0.0001). Cattle have a less chance of being positive to Fasciola spp. by 0.55 (95% CI: 0.30 – 0.99) compared to water buffaloes. Administration of anthelmintic to animals on a regular basis decreased the risk of copro-positivity to Fasciola spp by 0.17 (95% CI: 0.07 – 0.36) compared to animals received anthelmintic on an irregular basis. Infected animals having different Fasciola spp. egg burden revealed different clinical symptoms associated with hepatobiliary changes on ultrasonographic examination ranged from normal hepatic parenchyma and bile system in low faecal egg load to hyperechogenic hepatic parenchyma, hyperechogenic with distal shadowing bile duct, and distended gallbladder in high faecal egg load of Fasciola spp.In conclusion, the prevalence of bovine fascioliasis is high under the traditional household’s production system. Regular administration of anthelmintic significantly reduces the animal’s chance of being copro-positive to Fasciola spp. Ultrasound poses a valuable prognostic technique for assessment of bovine fascioliasis.

An asexual strain of Lysiphlebus fabarum (Hymenoptera: Braconidae) gains fitness in superparasitised hosts, but a sexual strain pays costs

1. The fitness impacts of two levels of superparasitism were compared in a sexual and an asexual strain of Lysiphlebus fabarum (Marshall) developing in black bean aphids.2. Asexual females were larger than sexuals, had higher eggs loads, and better adult emergence in two of three treatments, but parasitised fewer available aphids in 24 h (80% fewer when foraging alone).3. Superparasitism resulted in wasps that were larger than those emerging from singly parasitised aphids, and asexual females had larger ovaries, without delayed development. In contrast, sexual wasps took about 2 days longer to develop in superparasitised hosts, and females had smaller ovaries with lower egg loads.4. The growth of host aphids bearing asexual larvae was not reduced relative to healthy aphids, except when heavily superparasitised, whereas sexual larvae reduced aphid growth in all treatments.5. Elimination of supernumerary larvae was virtually complete at 4.5 days after parasitism by sexual females, but no elimination occurred among asexual larvae. The evolution of diminished aggression in asexual larvae may be facilitated by greater genetic similarity, without necessarily leading to gregarious development, assuming the extended life of supernumeraries somehow improves survivor fitness.6. Sons of virgin sexual females developed faster than sons of mated females, implicating parental effects that slowed development of the latter.7. It is concluded that asexual females of this strain use superparasitism to improve host quality, and thus the fitness of their daughters, whereas females of the sexual strain prioritise offspring number.

Biology of the Introduced Biocontrol AgentMicroctonus hyperodae(Hymenoptera: Braconidae) and Its HostListronotus bonariensis(Coleoptera: Curculionidae) in Northern New Zealand

The South American weevil Listronotus bonariensis (Kuschel) is an important pest of pastures in New Zealand. As a component of management strategies for this pest, the South American parasitoid Microctonus hyperodae Loan (Hymenoptera: Braconidae) was released in northern New Zealand during 1991 as a biological control agent. Over the subsequent 5-6 yr, the reproductive biology of M. hyperodae and its relationship to, and effects on, the reproductive phenology and fitness of L. bonariensis were studied at three sites. M. hyperodae was first recovered in the field in December 1991. Subsequently, the incidence of parasitism in L. bonariensis increased to seasonal maxima of 75-90%. There was variable synchrony between parasitoid generations and the two generations of its host, leading to marked seasonal variation in rates of parasitism and parasitoid abundance. Despite marked inter-year variation, abundance of host adult and egg populations declined in the presence of parasitoids. Parasitized host females had lower ovarian maturity scores, had lower egg loads, and exhibited less investment in wing muscle development than females that had escaped parasitism. There was almost complete elimination of egg maturation in parasitized females and these hosts contributed little to population recruitment. Rate of buildup and seasonal maxima in parasitism, frequency of superparasitism, adult abundances, and wing muscle development in adult L. bonariensis varied among the three sites in a manner that was only partially related to climate differences across the 1.83° gradient of latitude. Site effects were weak to absent in measures of reproductive condition in L. bonariensis females.

A Schistosoma haematobium-Specific Real-Time PCR for Diagnosis of Urogenital Schistosomiasis in Serum Samples of International Travelers and Migrants

BackgroundDiagnosis of urogenital schistosomiasis by microscopy and serological tests may be elusive in travelers due to low egg load and the absence of seroconversion upon arrival. There is need for a more sensitive diagnostic test. Therefore, we developed a real-time PCR targeting the Schistosoma haematobium-specific Dra1 sequence.Methodology/Principal FindingsThe PCR was evaluated on urine (n = 111), stool (n = 84) and serum samples (n = 135), and one biopsy from travelers and migrants with confirmed or suspected schistosomiasis. PCR revealed a positive result in 7/7 urine samples, 11/11 stool samples and 1/1 biopsy containing S. haematobium eggs as demonstrated by microscopy and in 22/23 serum samples from patients with a parasitological confirmed S. haematobium infection. S. haematobium DNA was additionally detected by PCR in 7 urine, 3 stool and 5 serum samples of patients suspected of having schistosomiasis without egg excretion in urine and feces. None of these suspected patients demonstrated other parasitic infections except one with Blastocystis hominis and Entamoeba cyst in a fecal sample. The PCR was negative in all stool samples containing S. mansoni eggs (n = 21) and in all serum samples of patients with a microscopically confirmed S. mansoni (n = 22), Ascaris lumbricoides (n = 1), Ancylostomidae (n = 1), Strongyloides stercoralis (n = 1) or Trichuris trichuria infection (n = 1). The PCR demonstrated a high specificity, reproducibility and analytical sensitivity (0.5 eggs per gram of feces).Conclusion/SignificanceThe real-time PCR targeting the Dra1 sequence for S. haematobium-specific detection in urine, feces, and particularly serum, is a promising tool to confirm the diagnosis, also during the acute phase of urogenital schistosomiasis.

Is parasitoid acceptance of different host species dynamic?

Summary Parasitoid acceptance of host individuals that differ in suitability is dynamic, varying with physiological state and experience. Female parasitoids with high egg loads and low life expectancy (i.e. time limited) are more willing to accept hosts with low suitability for progeny development than females with low egg loads and high life expectancy (i.e. egg limited). However, studies of dynamic acceptance behaviour in parasitoids have only considered high‐ vs. low‐suitability host individuals within the same host species. Here, we report the first results on whether acceptance of individuals from different host species that vary in suitability is also dynamic, using two aphid parasitoid species in the genus Aphelinus (Hymenoptera: Aphelinidae). Stressors (starvation and age) and experience that increase time limitation did not affect the acceptance of low‐suitability host species by two Aphelinus species with reciprocal specificities. Oviposition in a high‐suitability host species increased with egg load, but did not vary with egg load for females exposed to low‐suitability host species. This is an unexpected pattern since it results in proportionally higher acceptance of low‐suitability hosts with decreasing egg load. The stability of behavioural host specificity in these parasitoids under very stressful conditions might be explained by (i) frequent transient egg limitation, (ii) higher fitness from egg resorption than oviposition or (iii) neural constraints on host recognition. If neural constraints restrict the ability of these specialists to change their behaviour under stress, we predict that generalist species of Aphelinus should show greater dynamism in acceptance of low‐suitability hosts. We are testing this prediction using Aphelinus species with very broad host ranges.

Dynamics of Aggregation Formation in Japanese Beetles, Popillia japonica

For most insect aggregations to form, they need to be started by an initial individual (the pioneer) and joined by later individuals (the joiners). Pioneers and joiners may differ with regard to characteristics such as sex and body size. We carried out three field experiments to examine the characteristics of Japanese beetles, Popillia japonica, pioneering and joining aggregations on host plants. Individual beetles were captured as they arrived on uninhabited grape plants, as well as plants designed to simulate aggregations with model beetles and feeding damage. For all experiments and treatments, the beetles arriving were significantly female-biased. Females pioneering later in the day had higher egg loads than those arriving earlier, and the results of two experiments suggested that females arriving at existing aggregations tend to have lower egg loads than females pioneering elsewhere. Male beetles found on uninhabited plants were smaller and arrived earlier in the day than males in the aggregation area of the experiment. Overall, these results indicate that female Japanese beetles may be the initiators of aggregations (i.e. the pioneers) with males joining later in the process, and suggest that females with fewer eggs and males with larger body sizes are more likely to join aggregations. We use these patterns to hypothesize on the different functions of aggregations for male and female Japanese beetles.

Impacts of belowground herbivory on oviposition decisions in two congeneric butterfly species

AbstractRoot‐feeding insects can affect the performance of aboveground insect herbivores when they are forced to feed on the same host plant. Here we explored whether the oviposition behaviour of two closely related herbivorous species (cabbage butterflies; Lepidoptera: Pieridae) is influenced by root‐feeding insects, when they are given the chance to choose between host plants with and without root herbivores. Considering that egg load is an important physiological factor influencing the foraging behaviour of insects, we also examined whether root‐feeding insects differentially influence oviposition preference in butterflies with low and high egg loads. Oviposition preference in both butterfly species with low and high egg loads was monitored using host plants with and without root herbivores. To ascertain the status of butterfly age with low and high egg loads, the oviducts of a separate group of butterflies was dissected to record the number of immature and mature eggs in butterflies of various ages.Pieris brassicaeL. butterflies with low egg loads preferred plants without root herbivores over plants with root herbivores, and laid more egg clutches on the leaves of plants that were not attacked by root herbivores. Butterflies with comparatively high egg loads also selected a larger proportion of plants without root herbivores, but laid a similar number of egg clutches on the plant shoots independent of the presence or absence of root herbivores belowground. Independent of the age and egg load,Pieris rapaeL. butterflies selected a larger proportion of plants not attacked by root herbivores to lay eggs, but the number of eggs laid was similar in plants with and without root herbivores. This study shows that belowground insects can influence behavioural decisions of aboveground insect herbivores. Interestingly, the strength of these interactions depends on the physiological state of the insects which is probably correlated with their perception of environmental quality.

Ovigeny in Selected Generalist Predators

Ovigeny refers to egg production and develop ment in adult female insects. Extremes in ovigeny yield a dichotomy: species where adults emerge with a fixed complement of mature eggs are pro ovigenic; whereas, species that display continued egg development during adulthood are (Heimpel & Rosenheim 1998). Over 98% of parasitoid species surveyed emerged with at least some immature eggs (Jervis et al. 2001). A synovi genic insect may experience short-term egg deple tion, though lifetime fecundity limitation should be rare (Carbone & Rivera 2003). Egg limitation has been demonstrated in the field, affecting host parasitoid population dynamics, possibly result ing in inverse density-dependent parasitism (Heimpel & Rosenheim 1998). Jervis et al. (2001) suggested that synovigenic species tend to be longer-lived, and that the early reproductive cost in pro-ovigenic species incurred a biological cost. Most research in ovigeny has been performed on parasitoids (mostly Hymenopterans), possibly because of the direct relationship between egg load and parasitism rates, and hence reproduc tion. However, ovigeny is a potentially unifying concept in entomology and should be applicable to all insect species (Jervis et al. 2001). Previous studies of ovigeny in insects other than parasi toids include those of Boggs (1997) in Lepidop tera. Further, Cyzenis albicans Fallen (Diptera: Tachinidae) and hoverflies in general (Diptera: Syrphidae) were determined to be synovigenic by Hassell (1968) and Gilbert (pers. comm., 1990), respectively. Legaspi & Legaspi (2004) found evi dence of strong ovigeny in Podisus maculiventris (Say) (Heteroptera: Pentatomidae) by comparing egg loads at different times during female adult hood, against lifetime fecundity in the laboratory. Limited evidence of ovigeny in P. maculiventris was also found in the field (Legaspi & Legaspi 2005). In this study, ovigeny was examined in 4 com mon generalist predators: P. maculiventris, Orius insidiosus (Say) (Hemiptera: Anthocoridae), Geo coris punctipes (Say) (Heteroptera: Geocoridae), and Delphastus catalinae (Horn) (Cole?ptera: Coccinellidae). Dissection and feeding methods for P. maculiventris are described in Legaspi & Legaspi (2004). For the O. insidiosus, G. punctipes and D. catalinae, 40 newly-emerged females of each species were selected for study and placed individually in 10-cm Petri dishes with a damp cotton ball and lined with filter paper. The Petri dishes were secured with a rubber band. A male was placed with each female throughout the ex periment except on day zero. Each Petri dish com prised 1 replicate. The 40 females were assigned to one of 4 dissection times (10 replicates each). Prey was provided ad libitum as: Bemisia tabaci (Gennadius) (Homoptera: Aleyrodidae) for D. cat alinae and G. punctipes; and Ephestia kuehniella Zeller eggs and pollen. Oviposition substrates for the predators were stringbean (Phaseolus vul garis L.) for O. insidiosus and tomato (Lycopersi con esculentum Mill. syn. Solanum esculentum L.) leaf cuttings for D. catalinae and G. punctipes. Four dissection times were determined based on the expected lifespan of each species. At the be ginning of the experiment (dissection time = 0), 10 females were dissected, and the numbers of ma ture and immature eggs were recorded. At each of the remaining 3 dissection times, 10 females were dissected and examined. At each designated dis section date, total numbers of eggs laid were re corded. Numbers of mature and immature eggs were counted upon dissection. Results of dissections in P. maculiventris (Fig. 1A) showed that the numbers of immature eggs increased with the predator age, whereas the numbers of mature eggs declined (Legaspi & Le gaspi 2005) providing a clear indication of contin uous egg production during the adult stage. In the other predators that we studied, egg loads at time zero were always zero and tended to increase with time, except in D. catalinae (Fig. IB). However, the numbers of eggs laid all increased with time. Total numbers of eggs laid were higher than total egg count recorded in dissections. In O. insidio sus, 71 eggs (7.7%) were counted in dissections, versus 857 laid (92.3%). Results were similar for D. catalinae: 7 eggs (5.8%) found in dissections and 113 laid (94.2%); and G. punctipes: 189 (13.8%) versus 1183 (86.2%). These findings sug gested that egg production occurred during the adult stage in all these predators and that all predators studied were synovigenic to various de grees. In the case of D. catalinae, low egg load was likely a consequence of oviposition. Despite the fact that most studies on ovigeny have been done on parasitoids, it is not surprising that synovig eny is also common in other groups, including predators. Egg load dissections underestimate re productive potential of various species and may not be good criteria in evaluating predators as po tential biological control agents.

Seasonality and life histories of two endemic New Zealand carabid beetles (Coleoptera: Carabidae): Mecodema oconnori Broun and Megadromus capito (White)

Seasonal activity and life histories of two large endemic carabid beetles, Mecodema oconnori Broun and Megadromus capito (White), were investigated in two lowland forest remnants in the Horowhenua region, North Island, New Zealand. Seasonal activity was estimated from manual searches of surface refuges, plus pitfall trapping from October to March. Adults of both species were present throughout the year, although M. oconnori was much more abundant in spring. Megadromus capito larvae were caught from October to March, and the three instars overlapped temporally, implying a long period of larval emergence and development. Adult females were dissected to investigate their reproductive phenology. Both species carried eggs for at least 6 months of the year. M. oconnori females contained very few eggs, with a maximum of three eggs each and a mean of 1.6 eggs per female. Mg. capito females had up to 28 eggs each and a mean of 10.5. Both species appear to have low egg loads, a relatively long period of reproductive activity and long life spans, but longer‐term studies on the pre‐adult stages and reproductive activity of adults are required to complete understanding of their life cycles.

FITNESS CONSEQUENCES OF CHOOSY OVIPOSITION FOR A TIME-LIMITED BUTTERFLY

For the majority of insects, a female's choice of oviposition site(s) greatly influences both the success of individual offspring and her own total fitness. Theory predicts that females most strongly limited by egg number will employ greater oviposition site discrimination than those predominately subject to time limitation. The reproductive success of the butterfly Pieris virginiensis at our Connecticut, USA, field site is strongly time constrained on two fronts. First, during their three-week flight season, only 60% of days and 28% of daytime hours were suitable for flight. Second, larval survival is impacted by the rapid senescence of their spring ephemeral host plant Dentaria diphylla, with eggs laid during the first half of the flight season having approximately three times the survival chance of those laid later. Yet, on average, females choose to oviposit on only half the plants they closely inspect and fly over most ramets without any inspection. Our experiments demonstrate that the preferred host ramets confer an approximate two-fold survival advantage. Females are not choosing plants that senesce later, despite the advantage that such plants would confer. We use empirical data on female behavior and larval performance to parameterize a simulation model. Model results suggest that, despite the notable time limitation in this system, the observed level of female oviposition site preference not only increases individual larval survival, but also total female fitness. Low egg loads in this species may contribute to selection for strong host plant discrimination.

Host range in solitary versus gregarious parasitoids: a laboratory experiment

AbstractBehavioural interactions among relatives may have consequences for many other traits. We tested the hypothesis that solitary parasitoids (displaying siblicidal behaviour in their larvae) have narrower host ranges than gregarious parasitoids (with tolerant larvae). In laboratory experiments, we compared parasitization success in two sister species of braconid wasp [Aphaereta genevensis (Fischer), solitary, and Aphaereta pallipes (Say), gregarious (Hymenoptera: Braconidae: Alysiini)] on eight Drosophila species or strains. Host species or strain was the most important factor affecting parasitization success, and some of this variation was accountable to host physiological defences. Although two hosts were more suitable for the solitary species, and one more suitable for the gregarious species, these differences were small, and there was no consistent difference across all hosts. Wasp body size was positively correlated with parasitization success in both wasp species. This may be because body size increases oviposition success, or the motivation to oviposit. In A. pallipes parasitization success peaked after 3–4 days, but later in A. genevensis. This is likely due to low life expectancy and high egg loads increasing oviposition tendency in young A. pallipes, and egg limitation decreasing oviposition tendency in old A. pallipes. These data suggest that interactions among wasp larvae do not greatly affect the size of the fundamental niche examined here. However, they show the potential for life history traits, which differ between the species as a likely consequence of larval interactions, to affect the extent of the realized niche.

Oviposition decisions in the seed beetle, Callosobruchus maculatus (Coleoptera: Bruchidae): effects of seed size on superparasitism

The seed parasite Callosobruchus maculatus generally disperses eggs uniformly among seeds. We used cowpeas ( Vigna unguiculata ) of two class sizes (large and small) to test predictions based on a simple resource threshold model that females will distribute eggs among seeds in a manner that maximizes the amount of resources allocated to each offspring. When females were presented with multiple seeds of the same size, they tended to distribute their eggs relatively uniformly among seeds ( I ⩽0.27 where I is the variance/mean ratio, and I =1 reflects a random Poisson distribution). However, when seeds varied in size females distributed their eggs in a manner that maximized the amount of resources per offspring; females distributed eggs as predicted by seed differences in mass rather than as predicted by seed differences in surface area. Therefore, females must evaluate the relative quantity of resources available inside of a seed more accurately than if they compared the ratio of surface areas between seeds of varying size. Instead, females must either use cues other than surface area when estimating seed mass, or must have the ability to extrapolate non-linearly from surface area to seed mass. Females with higher egg loads (4-d-old females) laid more eggs when presented with seeds, but did not distribute their eggs less uniformly, than females with lower egg loads (1-d-old females), indicating that high egg load does not reduce female sensitivity to seed size and the presence of conspecific eggs.

Facilitation in Ovipositional Behavior of Bactrocera tryoni Flies

Mature female Queensland fruit flies, Bactrocera tryoni Froggatt, released into canopies of field-caged trees containing host fruit, did not discriminate between fruit having or lacking two resident conspecific females when selecting fruit on which to alight. When released individually onto a fruit, mature females showed no greater or lesser propensity to initiate boring with the ovipositor into fruit having or lacking a resident female engaged in pre-ovipositional or post-ovipositional behavior. However, boring propensity of introduced females was significantly enhanced or facilitated when released onto fruit on which a conspecific female was in the act of ovipositing. Introduced females were significantly more prone to initiate boring into high-ranking fruit (nectarines) than low-ranking fruit (lemons) and to do so when having a high rather than a low egg load. Neither fruit type nor egg load of introduced females had a significant effect on the occurrence of encounters between introduced and resident females, on the occurrence of contests, or on the propensity of introduced females to initiate boring in the presence of a resident female. Among introduced females, there was a consistent trend to initiate boring earlier in the presence than in the absence of a resident female. We discuss circumstances, likely to be narrow, under which boring propensity of an intruding female might be facilitated in the presence of a resident female.

The effects of physiological factors and host plant experience on the ovipositional activity of the sweet potato whitefly, Bemisia tabaci

AbstractBoth egg load and levels of vitellogenin and vitellin increased when Bemisia tabaci Gennadius (Homoptera: Aleyrodidae), were deprived of host plants. These increases resulted in significant increases in rates of oviposition. There was no difference in total fecundity between whiteflies temporarily deprived of hosts and a control treatment that had continuous access to hosts. The results of previous exposure to a preferred host, melon (Cucumis melo), and a non‐preferred host, cotton (Gossypium hirsutum), were also examined. B. tabaci deposited significantly more eggs during a 6‐h period on melon than on cotton, regardless of host species they were previously confined to. In addition, whiteflies previously exposed to cotton, deposited more eggs than females previously exposed to melon. This higher rate of oviposition resulted in a low egg load as well as lower levels of vitellogenin and vitellin. Conversely, a low rate of oviposition led to high egg load and high levels of vitellogenin and vitellin. Individuals from the same treatment exhibited variation in egg load, which was negatively and significantly correlated with previous oviposition activity. We conclude that the effects of previous host plant exposure on oviposition were, in part, due to behavioral differences that are not related to the reproductive physiology.

Evaluation of Odor Lures for Use with Red Sticky Spheres To Trap Apple Maggot (Diptera: Tephritidae)

Synthetic odor lures were evaluated in association with red sticky sphere traps for their attractiveness to apple maggot flies, Rhagoletis pomonella (Walsh). Spheres baited with either butyl hexanoate (fruit odor) alone, ammonium carbonate (food odor) alone, or both were tested in an artificial orchard and several commercial orchards. In artificial orchard assays with released flies of high or low egg load, the combination of butyl hexanoate and ammonium carbonate was more attractive than either odor alone or unbaited spheres. Spheres baited with butyl hexanoate or ammonium carbonate alone were more attractive than unbaited spheres. This pattern was consistent for both high egg load, protein-fed flies, and low egg load, protein-starved flies. In commercial orchards, butyl hexanoate-baited spheres captured significantly more flies than ammonium carbonate-baited or unbaited spheres, which did not differ in attractiveness. Butyl hexanoate was as attractive as the combination of butyl hexanoate and ammonium carbonate. Ratios of flies captured on butyl hexanoate baited spheres to lies captured on unbaited spheres in commercial orchards declined slightly from 5.8:1 to 4.4:1 over the 10-wk trapping season. Dissections of captured females suggested that there was no difference between the odor types in terms of egg load and sexual maturity of attracted flies. Throughout the season, most (>90%) of the captured flies were sexually mature and of high egg load (>17 eggs per female). Implications of these findings for control of R. pomonella in orchards using baited red spheres are discussed.

Egg Limitation, Host Quality, and Dynamic Behavior by a Parasitoid in the Field

Patterns of host selection and host use by insects with parasitic lifestyles are predicted to be fundamentally dynamic, responding to environmental conditions and physiological states. A particularly robust prediction is that the number of mature eggs that a female carries (her “egg load”) should influence behavior. As egg load decreases, females are predicted to exhibit increasingly selective host use patterns. We tested the hypothesis that egg load and host size influence the behavior of the parasitoid wasp Aphytis aonidiae attacking the armored scale insect Quadraspidiotus perniciosus. Aphytis females can use hosts either for oviposition or for adult “host feeding,” which is the consumption of host material by adult parasitoids. Theory predicts that host feeding should be (i) favored by low egg loads and (ii) more prevalent on lower—quality (smaller) hosts. We conducted observations of individual females foraging freely in the field to determine whether hosts were used for oviposition or for host feeding. As predicted by theory, the likelihood that a host was used for oviposition (as opposed to host feeding) increased with both parasitoid egg load and host size. Thus, parasitoids exhibited higher host—selectivity at lower egg loads. Since egg load and a parasitoid's experience with hosts were not experimentally separated, the egg load effect could in principle have been either direct or indirect and operating through a correlation with experience. In either case, this result constitutes the first demonstration of a link between physiological state and oviposition behavior by a parasitoid in the field. Parasitoid age, as measured using a wing wear index, did not influence behavior. A laboratory study revealed that the relationship between a host's size and the size and initial egg load of the parasitoid developing on that host was positive, but that it followed a pattern of diminishing returns. No effect of host size on behavior could be detected over the range of host sizes that were within the asymptotic region of this relationship.

Patterns of Egg Load in the Walnut Fly Rhagoletis juglandis (Diptera: Tephritidae) in Nature and Their Possible Significance for Distribution of Sexes

In a field study of the tephritid fly Rhagoletis juglandis Cresson on its host Juglans major (Torrey) Heller, the density of both sexes on host foliage remained more or less constant throughout the season. The density of each sex on fruit, by contrast, increased steadily over the course of several weeks. The density of males on fruit increased much more sharply than that of females. Associated with the temporal increase in relative fly density on fruit was an increase in the mean number of eggs in a female's ovaries (i.e., egg load). This pattern in egg load was not caused by an increase in the mean egg load of individuals carrying eggs (which remained more or less constant over the season), but rather by an increase in the number of individuals that carried any eggs at all (i.e., the number of reproductively mature individuals). Late in the season, mean egg load of females in the foliage was lower than that of females on fruit for each of 2 yr, but the difference was not statistically significant. Within a given location (fruit versus foliage), egg load was associated with female activity. Females attempting to oviposit on fruit and females found in mating pairs on foliage had high mean egg loads, whereas females feeding on foliage and females dragging their ovipositor (indicative of recent clutch deposition) on fruit had low mean egg loads. We discuss how results for this species conform to a generally held scenario for the distribution of frugivorous tephritid flies in time and space.

Nocturnal activity and resource utilization in the aphid hyperparasitoid, Dendrocerus carpenteri

Abstract. Females of the aphid hyperparasitoid Dendrocerus carpenteri (Curtis) search successfully for hosts during both day and night. Oviposition numbers per host patch did not differ significantly between day and night. D.carpenteri females also displayed a nocturnal flight activity, showing that they are not only capable of searching on a given host plant but also of dispersing between host plants. Nocturnal oviposition activity was mainly influenced by egg load. Females with a high egg load laid more eggs at night than females with a comparatively low egg load. Thus, D.carpenteri females may use nocturnal foraging to compensate for the lack of oviposition opportunities during day. D.carpenteri females which foraged continuously for hosts both day and night (= for 24 h per day) benefitted from an 1.4‐fold increase in lifetime reproductive success when compared to females which foraged only by day (= for 16 h per day). The benefit of night foraging for this species is a significantly increased reproductive success.