Egg Load and Body Size of Lab-cultured Cotesia marginiventris

The egg load of lab-cultured Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae), a solitary koinobiont endoparasitoid of noctuid caterpillars, was determined in this study. Information on egg load may provide clues to more efficient in vivo rearing of C. marginiventris. I tested the hypothesis that egg load, defined as the number of mature oöcytes (i.e., fully chorionated eggs) found in adult females, was related to body size. Cotesia marginiventris females possessed two ovaries and two ovarioles per ovary; mature eggs were found in ovaries and oviducts. Newly-emerged females held an average of 149 mature eggs. Immature eggs were slightly visible in the distal portions of the ovarioles; they were not counted. Egg load was marginally related to body size (i.e., hind tibia length). The results of this study suggest that (1) body size can sometimes predict egg load or potential fecundity of lab-cultured C. marginiventris and (2) an efficient rearing system that exploits the potential fecundity of C. marginiventris might involve using young females and allowing them to oviposit in new hosts, each day, for up to a week.     

Egg load of lab-cultured <Emphasis Type="Italic">Anaphes iole</Emphasis> and effects of mate presence and exposure time on load depletion

Anaphes iole Girault (Hymenoptera: Mymaridae) is a solitary egg parasitoid of Lygus bugs (Heteroptera: Miridae) in North America. This research considered factors that might impact the egg load of lab-cultured A .iole females, reared from Lygus hesperus Knight egg patches. The following hypotheses were tested: (1) egg load was related to body size and not affected by female age, and (2) egg load depletion was not affected by mate presence and time (in days) that females were exposed to host patches. Initial egg load averaged 48 mature eggs and no immature eggs were detected in the ovarioles of dissected females. Egg load was neither related to body size (hind tibia or forewing length) nor affected significantly by age (0, 1 or 2days old honey-fed females). Mate presence (females with or without males) and exposure time (1, 3 or 5days on the same host patch) had no effect on egg load depletion. Females usually depleted most of their egg load within 24h. From 86 to 92% of females contained less than six mature eggs and no immature eggs after 1, 3 or 5days of exposure to host patches. The results of this study suggest that A. iole females are certainly pro-ovigenic and initial egg load does not correlate with body size or age. Since mated and unmated females deplete most of their egg load in 24h, time-efficient production of progeny may result when ovipositing parasitoids are exposed to suitable hosts for just a few days.The United States Government has the right to retain a non-exclusive, royalty-free license in and to any copyright of this article. This article reports the results of research only. Mention of a commercial or proprietary product does not constitute an endorsement of the product by the United States Department of Agriculture.     

Are Lab-cultured <Emphasis Type="Italic">Anaphes iole</Emphasis> Females Strictly Proovigenic?

Anaphes iole Girault (Hymenoptera: Mymaridae) is a solitary egg parasitoid of Lygus bugs (Heteroptera: Miridae) in North America. Circumstantial evidence suggests that A. iole females are strictly proovigenic. This study was designed to determine if honey-fed A. iole females could in fact mature additional eggs if exposed to hosts for a few days then removed from hosts and held at 25 °C for 0, 3, or 6 days. Contrary to expectation, honey-fed A. iole females matured considerably more eggs when deprived of hosts for 3 or 6 days rather than 0 days. This research suggests that A. iole females are not strictly proovigenic. However, they do have proovigenic tendencies, since most females emerge with at least 71% of their potential fecundity.     

Influence of host age and host deprivation on longevity, egg load dynamics and oviposition rate in Microplitis rufiventris

Adult size, longevity, egg load dynamics and oviposition ofMicroplitis rufiventris Kok. which began their development in the first, second, third （preferred hosts） or fourth （non-preferred hosts） instar larvae of Spodoptera littoralis （Boisd.） were studied. The parasitoid size was largely determined by the initial host size at parasitism. Non-ovipositing females derived from older hosts lived for longer periods than those derived from younger ones. However, the ovipositing females, irrespective of their size, lived for almost the same periods. At emergence, the oviducts of adult females contain a significant amount of mature eggs available for oviposition for a few hours on eclosion day. Egg load increases during the early phase of adult life. The amount of additional mature eggs and rate of egg maturation per hour was greater for wasps derived from preferred hosts compared with those in females derived from non-preferred hosts. The pattern of egg production in M. rufiventris females depended on the availability of hosts for parasitization. Host-deprived females depleted the egg complement with aging; the longer the host deprivation, the lower the oviduct egg load. Marked reduction in both realized or potential fecundity of host-deprived females was observed following host availability. Host privation for more than 3 days induced a marked deficit fecundity pattern through the female＇ s life. The realized fecundity was determined by the interaction among host availability, the number of eggs that are matured over the female＇ s life span, oviposition rate and host size from which the female was derived. These results suggest that： （i） M. rufiventris wasp is a weak synovigenic species; （ii） the maturation of additional eggs is inhibited once the maximum oviduct egg load is reached; （iii） the egg load of the newly emerged female is significantly less than the realized fecundity; and （iv） because M. rufiventris females oviposit fewer eggs when they begin depleting their egg supply at 3 days, augmentative releases will require release immediately following emergence to ensure the highest parasitization rate in the field.     

Egg resorption behaviour by the solitary egg parasitoid Anaphes nitens under natural conditions

We investigated under outdoor conditions and inside a climate chamber: (i) whether Anaphes nitens Girault (Hymenoptera: Mymaridae), an egg parasitoid of the Eucalyptus snout beetle, Gonipterus scutellatus Gyllenhal (Coleoptera: Curculionidae), experienced egg resorption, and (ii) how various treatments (location, food, and/or host supply) and body size influenced egg load, egg resorption, fecundity, and longevity. One‐day‐old females were mated and randomly assigned to five groups: (A) honey + hosts, (B) water + hosts, (C) only honey, (D) only water, and (E) control females killed at emergence. We compared the egg load of the newly emerged females, which represent the control group (egg load = ovarian eggs present at emergence), with the lifetime egg load (i.e., ovarian eggs + emerged offspring + not emerged offspring) of the females with various host and diet treatments, by dissection of the ovaries to find evidence of egg resorption. All groups reared outdoors had fewer eggs than the control, while indoors there was no significant difference. Outdoors, starved but host‐provided wasps (B) experienced the highest reduction of the lifetime egg load (51%). Groups without access to food (B + D) resorbed more eggs than groups provided with honey (A + C). Females with honey and hosts (A) had the highest lifetime fecundity, but those with water and hosts (B) showed a higher daily realized fecundity. Host‐deprived females with access to food (C) attained the longest lifespan. Our results suggest that under stress conditions, such as low temperature and food shortage, A. nitens females practice egg resorption, probably to save energy.     

Determinants of egg load in the soybean aphid parasitoid Binodoxys communis

We used a series of laboratory studies to investigate factors contributing to variability in egg load of the parasitoid Binodoxys communis (Gahan) (Hymenoptera: Braconidae), a biological control agent of the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae). The change in egg load in newly emerged females over time was determined in response to three treatments: post‐eclosion temperature, sugar meals, and host density. Binodoxys communis females emerge with an average egg load of approximately 40 mature eggs that increases to approximately 200 eggs within 24 h of emergence. The egg maturation rate over this time period is higher when females are held at 26 °C than at 18 °C. And although the egg load of sugar‐fed females was slightly higher than that of starved females, this difference was not statistically significant. Binodoxys communis females that were held with 150 hosts for 8 h laid more eggs than those that were held with 30 hosts, and they also matured more eggs over the subsequent 16 h than those held with 30 hosts or no hosts at all. However, we detected no difference in the egg maturation pattern between B. communis females held with the low host density and that of control females held with no hosts at all. Thus, we conclude that enhanced egg maturation in the higher host‐density treatment is more likely explained by a rapid replenishing of partially depleted ovaries than a host‐induced stimulus of egg maturation per se. Taken together, these results suggest a strategy of maintaining a high egg load and thus avoiding or mitigating the negative effects of egg limitation.     

Overwintering of reproductively mature females of a pro‐ovigenic parasitic wasp,Leptopilina heterotoma (Hymenoptera: Figitidae)

Leptopilina heterotoma is a Drosophila parasitoid mainly occurring in temperate regions, and females of this species are pro‐ovigenic (i.e. all or nearly all of their lifetime eggs are already mature at emergence). Here I investigated how L. heterotoma overwinters in Sapporo, Japan, a cool temperate region, by outdoor and laboratory experiments. Females of this species had mature eggs at emergence as expected, and they did not resorb eggs even if they were exposed to cold or starvation. Nevertheless, females were able to overwinter, and post‐overwintering females retained parasitization capacity. In this species, thus, adult overwintering is not associated with reproductive diapause. However, females grown in late autumn or at low temperatures (e.g. 5°C) had a fewer number of mature eggs and higher winter survival. At low temperatures, female larvae could save energy by reducing egg production and allocate the saved energy to expenditure for overwintering. In contrast to females, males and pre‐adult individuals were not able to overwinter.     

Egg maturation dynamics of the parasitoid Microplitis rufiventris: starvation speeds maturation in early life

The number of mature eggs carried by a female parasitoid at any given moment (egg load) is a fitness‐related parameter affecting reproductive potential and impacting upon host population dynamics. Microplitis rufiventris Kokujev (Hymenoptera: Braconidae) is a solitary koinobiont endoparasitoid wasp of several noctuid pests, including Spodoptera littoralis. The number of mature eggs carried by females at emergence is approximately 50. The rate of egg maturation is strongly affected both by feeding status and access to host larvae. In early adult life, egg maturation rates are lower for 6–72 h in fed wasps compared with food‐deprived wasps. When given access to hosts, honey‐fed wasps live for approximately 9 days with high lifetime fecundity (226 eggs). By contrast to early adult life, the total realized fecundity is positively affected by feeding status, where water‐fed and starved females have 140 and 107 eggs, respectively. Egg resorption is most pronounced in the later life of females. The results suggest, in addition to confirming the effect of honey‐feeding on total fecundity, that fecundity of starved wasps includes rapid egg maturation early in life, which potentially could improve the performance of the parasitoid as a biological control agent.     

Ovarian response to resource availability in female Rhagoletis indifferens

Fitness trade‐offs in behavioural ecology theory suggest that the costs associated with dispersal can impact reproductive output, but evidence of dispersal‐reproduction trade‐offs is often lacking. We present evidence of Rhagoletis indifferens Curran (Diptera: Tephritidae) developing its egg load in response to host availability [Prunus avium L. (Rosaceae)] and link this to our previous work on flight behaviour through the common methodology of these two studies. We replicated the study design from the flight mill experiment, such that females were maintained in one of six possible treatments combining two factors: social interactions (crowded vs. isolated) and resources (food = food only; leaf = food + leaf; and cherry = food + leaf + fruit), and then dissected the females to determine egg load. Egg counts varied significantly with ‘crowded’ females and those in the ‘cherry’ resource treatments producing the most mature eggs. Although the average mature egg count for females from the ‘leaf’ and ‘food’ resource treatments was similar, these two groups differed in the proportion of females that produced no mature eggs at all. No significant interaction terms between the factors tested were generated. We conclude that the effect of social interactions and resources on the maturation of eggs is additive in R. indifferens. These results support those of an earlier flight mill experiment and provide evidence that egg load may trade off with dispersal ability in R. indifferens.     

Sting frequency and progeny production of lab-cultured Cotesia marginiventris

A laboratory experiment was conducted to determine the impact of sting frequency on progeny production of Cotesia marginiventris (Cresson) (Hymenoptera: Braconidae), a solitary, koinobiont endoparasitoid of noctuid pests. In replicated trials, young, mated, host-deprived C. marginiventris females were exposed to three Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) early instar larvae, one at a time, in Petri dish arenas, and observed as they stung these hosts once, twice, or thrice. The average amount of time that elapsed between stinging a host ranged from a minimum of 3 min, when females were exposed to unstung hosts, to a maximum of 8–11 min when females were exposed to previously stung hosts. Sting frequency had no effect on the yield of C. marginiventris progeny; approximately 90% of hosts, in all treatment groups, yielded a parasitoid mature larva, which spun a cocoon. No hosts yielded more than one parasitoid. The percentage of adult female progeny tended to increase with sting frequency; hosts that were stung thrice yielded 60% female C. marginiventris adults. The production of female progeny may depend on the egg allocation pattern of C. marginiventris females as well as the competitive interactions between siblings developing inside a shared host. This study suggests that S. exigua larvae can sustain 2–3 stings without diminishing their ability to yield a C. marginiventris mature larva. Provisioning enclosures with enough hosts to exceed the daily (per capita) oviposition rate of C. marginiventris females might limit the repeated stinging of the same hosts.     

The effect of travel time on oviposition behavior and spatial egg aggregation: experiments with Drosophila

A higher degree of spatial egg aggregation is often observed in environments where resource patches are more sparsely distributed. This suggests a higher probability of species coexistence when resource distribution is sparse. However, it is still unclear how the degree of spatial egg aggregation increases. I propose a model to explain this phenomenon, which assumes that (i) egg load (the number of mature eggs in ovaries) increases in the travel period between resource patches and (ii) the retention of eggs in the ovaries is harmful (egg load pressure). With these assumptions, a female would lay accumulated eggs on arrival at a new resource patch, resulting in a higher degree of spatial egg aggregation. Laboratory experiments with three drosophilid species, Drosophila simulans Surtevant, Drosophila auraria Peng, and Drosophila immigrans Sturtevant, support the model. This study provides evidence that host availability affects the spatial egg aggregation via egg load.     

Dynamic egg maturation strategies in an aphid parasitoid

Abstract.  Females of the parasitoid Aphelinus albipodus both lay more eggs on younger stages of their aphid host, Aphis glycines , and contain more mature eggs in their ovaries when held with these younger host stages. This suggests that egg maturation is more rapid in the presence of younger host stages. Three factors explaining the difference in the egg loads of A. albipodus females exposed to various host stages are examined. These are (i) the number of host-feeding meals; (ii) the number of eggs laid; and (iii) the host stage utilized. Together, these factors explain 69% of the variance in A. albipodus egg load. The number of host-feeding meals taken by females is a strong predictor of egg load. More host meals are taken on young hosts, suggesting that host feeding contributes to the trend of faster egg maturation in the presence of younger hosts. Host stage has a strong impact on egg load even when the effect of host feeding is accounted for. There is no evidence for an effect of the oviposition rate on egg load. The results indicate that egg maturation by A. albipodus is dynamic; females mature eggs faster when in the presence of preferred hosts. It is hypothesized that this allows A. albipodus females to more closely match their reproductive effort to reproductive opportunities.     

Egg production of the marine,planktonic copepod,Calanus pacificus Brodsky: Laboratory observations

When acclimated to a continuous, superabundant food supply and constant temperature, Calanus pacificus Brodsky females produce eggs at a weight-specific rate ranging from 0.13 · day^?1 at 8°C to 0.21 · day^?1 at 15°C. Maximum weight-specific egg production rates do not change with seasonal changes in female body size. The relationship between egg production rate and food concentration is hyperbolic, with threshold and critical concentrations that are high relative to other species for which data are available. Food concentration and temperature influence spawning frequency (i.e., the time required for oocytes to mature) much more than the number of eggs in a single spawning event (i.e., clutch size). Clutch size is significantly related to female body size.     

The relationship between mating and oogenesis in monarch butterflies (Lepidoptera: Danainae)

We studied the relationship between the timing of mating and oogenesis in monarch butterflies (Danaus plexippus) to determine (1) the potential for male nutrient input into eggs and (2) whether mating stimulates egg development. Most females mated soon after they started maturing eggs. One and 2 days after mating, females contained the same number of mature oocytes as virgin females of the same age, while 3 days after mating they contained more mature oocytes than did virgins. These results confirm the potential for male-derived nutrients to augment oocyte production, but indicate that mating is not required for oocyte maturation to occur.     

Egg load influences search intensity,host selectivity,and clutch size inBattus philenor butterflies

Numerous studies have documented the influence of environmental factors such as host plant species and host quality on the oviposition behavior of female insects. This paper shows that an internal physiological factor, the number of mature eggs a female carries (egg load), correlates with host selectivity and clutch size in unmanipulated natural populations of the pipevine swallowtail butterfly, Battus philenor.In addition, search intensity and host selectivity differed among females whose egg loads were manipulated experimentally before they were released and followed in the field. Females with many eggs searched more intensely for hosts and were less selective when they encountered them.     

Field oviposition rates and egg load dynamics of pollen beetles (Meligethes aeneus Fab.) (Colepotera: Nitidulidae)

Abstract 1 Egg loads from field collected pollen beetles (Meligethes aeneus Fab., Coleoptera: Nitidulidae) were determined by dissecting beetles caught at the beginning and end of the putative daily oviposition period. Field collected beetles were offered Brassica napus (L.) plants in cages for 8 (morning and early afternoon), 16 (overnight), and 24 h to ascertain the number of eggs laid during these time periods. 2 Most eggs were laid in the morning and early afternoon. The proportion of gravid females was higher at the beginning of the oviposition period than at the end. Most females in the morning carried two eggs, whereas one egg was more common in the afternoon. 3 We hypothesized that the number of eggs laid during the oviposition period would be equivalent to the difference between egg loads at the beginning and end of oviposition. This was not the case; differences in egg loads were significantly lower than number of eggs laid. However, the number of eggs laid was equivalent to the egg load at the beginning of the oviposition period, suggesting that eggs available in the morning are laid during the following day. 4 Population estimates of daily oviposition rates, approximately 0.7 eggs per beetle and day, were close to estimates from laboratory studies when the proportion of gravid females was taken into consideration.     

EVOLUTION AND MANIPULATION OF PARASITOID EGG LOAD

In proovigenic parasitoids such as Leptopilina boulardi, the female emerges with a limited egg load and no further eggs are produced during its adult life. A female thus runs the risk of exhausting this limited supply of eggs before the end of her life. Given that the production of an egg is costly, what is the evolutionarily stable egg load at emergence? This question has attracted a lot of attention in the last decade. Here, we analyze a model that allows us to track both the evolution and the population dynamics of a solitary, proovigenic parasitoid. First, we show how host–parasitoid dynamics feedbacks on the evolution of parasitoid egg load. Second, we use this model to consider the situation in which the parasitoid can be infected by a virus that manipulates the oviposition behavior of the females. In particular, we model the effect of the LbFV virus in L. boulardi, a virus that is known to enhance its horizontal transmission by increasing superparasitism (i.e., the laying of eggs in a host already parasitized). Specifically, we model (1) the effect of the virus on parasitoid egg load strategies , and (2) the evolution of egg load manipulation by the virus. This analysis yields two alternative, yet not mutually exclusive, adaptive explanations for the observation that females infected by the virus harbor higher egg loads than uninfected females. Infected females could either respond plastically to the infection status, or be manipulated by the virus. Further experimental work is required to distinguish between these two hypotheses. In a broader context, we present a general theoretical framework that allows us to study the epidemiology, the evolution, the coevolution, and the evolution of manipulation of various reproductive strategies of parasitoids.     

Effects of a host plant,Brassica juncea, on calling behaviour and egg maturation in the diamondback moth,Plutella xylostella

The influence of the presence of seedlings of a host plant, Oriental mustard,Brassica juncea (L.) Coss., on the calling behaviour and egg maturation in the diamondback moth,Plutella xylostella (L.), was investigated. In the presence of plants, females began calling at a younger age, began calling earlier in the night, and spent more time calling. Females which were kept with plants until after their first night of calling and then were isolated from plants, subsequently called less, at a level not significantly different from females which had never been exposed to plants. Of all females, both with and without plants, which were dissected at the end of the first night of calling (n=150), 93% contained mature eggs. Heavier females were significantly more likely to have mature eggs at adult emergence, to begin calling at a younger age and to spend more time calling on their first night of calling. The number of developed eggs in calling females was not significantly different in the presence or absence of host plants on the first night of calling. However, by the second night, females in the presence of plants contained significantly more eggs, indicating that the presence of plants accelerated egg maturation, both before and after the onset of calling.     

Intraspecific variability in egg maturation patterns and associated life‐history trade‐offs in a polyembryonic parasitoid wasp

1. Life‐history theory predicts a trade‐off between the resources allocated to reproduction and those allocated to survival. Early maturation of eggs (pro‐ovigeny) is correlated with small body size and low adult longevity in interspecific comparisons among parasitoids, demonstrating this trade‐off. The handful of studies that have tested for similar correlations within species produced conflicting results. 2. Egg maturation patterns and related life‐history traits were studied in the polyembryonic parasitoid wasp, Copidosoma koehleri (Hymenoptera: Encyrtidae). Although the genus Copidosoma was previously reported to be fully pro‐ovigenic, mean egg loads of host‐deprived females almost doubled within their first 6 days of adulthood. 3. The initial egg‐loads of newly emerged females were determined and age‐specific realised fecundity curves were constructed for their clone‐mate twins. The females' initial egg loads increased with body size, but neither body size nor initial egg load was correlated with longevity and fecundity. 4. The variation in initial egg loads was lowest among clone‐mates, intermediate among non‐clone sisters and highest among non‐sister females. The within‐clone variability indicates environmental influences on egg maturation, while the between‐clone variation may be genetically based. 5. Ovaries of host‐deprived females contained fewer eggs at death (at ～29 days) than on day 6. Their egg loads at death were negatively correlated with life span, consistent with reduced egg production and/or egg resorption. Host deprivation prolonged the wasps' life span, suggesting a survival cost to egg maturation and oviposition. 6. It is concluded that adult fecundity and longevity were not traded off with pre‐adult egg maturation.     

Western corn rootworm (Diabrotica virgifera virgifera LeConte) (Coleoptera: Chrysomelidae) oviposition can be released by decapitation

Nervous system control of western corn rootworm (Diabrotica virgifera virgifera LeConte, WCR) (Coleoptera: Chrysomelidae) oviposition was studied using decapitation. Gravid females (n = 364) were decapitated with scissors and floated in water-filled Petri dishes. Oviposition by individuals was observed at 30 min intervals for 4 hr after decapitation; cumulative oviposition was tallied at 48 hr post-treatment. Overall, 82.7% of females laid eggs within 48 hr. Oviposition commenced quickly; 78.4% of females laid eggs during the observation period, 81.8% of these began egg laying within 30 min of decapitation. Egg-laying females deposited a total of 66.8 ± 2.1 eggs (mean ± SEM); this was 85.1% of the total mature egg load. Dissection revealed that 31.9% of n = 301 laying females and 14.3% of 63 non-laying females had a rupture of the common oviduct, manifest as an egg-filled hernia containing a mean of 8.17 ± 1.3 eggs (range: 1–83). Among n = 237 females that laid eggs during observations, females with hernias laid significantly fewer eggs (35.8 ± 4.2) than intact females (48.0 ± 2.7) during the 4-hr interval. There was no difference in the mean proportion of hatch for eggs collected from the same n = 10 females before (0.88 ± 0.03) or after decapitation (0.84 ± 0.04). Rapid oviposition following decapitation suggests that WCR egg laying is under constant descending neural inhibition; the motor programme controlling egg laying must reside posterior to the head. Decapitation can be used to quickly collect mature, fertilized WCR eggs.