Effects of constant and variable temperature extremes on sex ratio and progeny production by Aphytis melinus and A.lingnanensis (Hymenoptera: Aphelinidae)

Abstract. 1. Aphytis melinus DeBach and A.lingnanensis Compere were exposed to temperature extremes during development, mating, preoviposition, and opposition to assess the effect of these exposures on the progeny per female and F₁ sex ratio.
2. Both species showed a significant reduction in the expected proportion of F₁ females when the male parent was exposed to 32°C during its development.
3. The proportion of females was further reduced when mating and/or oviposition occurred at 32°C and these effects on sex ratio appeared cumulative.
4. Preovipositional exposure of the females of both species to 38°C, 2°C or -2°C for 1 5, 4 or 7 h in most cases significantly reduced the expected proportion of females.
5. In general our results showed that the sex ratios of A.melinus was less severely affected by these exposures than were those of A.lingnanensis.
6. It is suggested that this difference may be one factor explaining the ability of A.melinus to exclude A.lingnanensis from the climatically more extreme areas of southern California.     

Competition, resource partitioning and coexistence of an endoparasitoid Encarsia perniciosi and an ectoparasitoid Aphytis melinus of the California red scale

Abstract. 1. Laboratory experiments and field studies were conducted to explain the coexistence of an endoparasitoid, Encarsia perniciosi Tower, and an ectoparasitoid, Aphytis melinus DeBach, both of which were introduced into California to control the California red scale, Aonidiella aurantii (Mask.).
2. Encarsia parasitized all scale stages but it preferred first and second instar scales. This is in contrast to Aphytis melinus , in which previous studies have shown that it parasitizes second and third instar females and second instar males but prefers third instar female scales. Encarsia developed most rapidly when it parasitized an early second instar and slowest when it parasitized the mature female scale. However, on early second instar scales it was about 80% as fecund as a wasp that emerged from a mature female scale.
3. Second instar scales parasitized by Encarsia were accepted by Aphytis as readily as unparasitized scales.
4. Encarsia did not distinguish between unparasitized hosts and those previously parasitized by Aphytis.
5. Encarsia always outcompeted by Aphytis when both species parasitize the same host.
6. Encarsia prefers scale on stems whereas Aphytis prefers those on leaves and fruits. This, too, may be a result of interspecific competition with Aphytis.
7. The partitioning of the scale resource by the two species explains why they coexist in coastal southern California but it does not explain why Encarsia disappeared from citrus groves in the inland valleys coincident with the introduction of Aphytis melinus into southern California.     

Adult feeding and lifetime reproductive success in the parasitoid Aphytis melinus

The diet of adult females of the parasitoid Aphytis melinus DeBach (Hymenoptera: Aphelinidae) includes host insects and sugar-rich foods such as nectar and honeydew. We compared the contributions of host feeding to longevity and fecundity in A. melinus females in the presence and in the absence of honey meals. First, we assessed the longevity of females that were not allowed to oviposit. While the longevity of females fed honey was significantly increased by host feeding (median ages were 30.5 days for host-fed females and 17 days for females not allowed to host feed), the lifespan of parasitoids not fed honey did not exceed 3 days for any individual and there was no effect of host feeding on longevity in this group. In the second set of experiments, we assessed the fecundity and longevity of females allowed to oviposit. We conducted two experiments, one in which honey was continuously available, and one in which honey was not available. In both experiments, daily observations were made of females that were either allowed to host feed or manually prevented from host feeding. In the presence of honey, host feeding significantly increased both fecundity and longevity, and in the absence of honey, parasitoids died within 2 days and host feeding had no significant effect on either fecundity or longevity. The lifetime fecundity of females fed honey but not hosts exceeded the initial egg complement by 60% on average. Approximately one host per day was used for host feeding whether honey was supplied or not, and each host-feeding meal contributed approximately 3.9 eggs to the lifetime fecundity of honey-fed females. In the last experiment, we compared the rate of egg resorption over a 36-h period in A. melinus females that were deprived of hosts and either fed honey or starved. While no egg resorption was detected in honey-fed females over this time period, starved females resorbed approximately 9 eggs. Thus, the availability of a sugar-rich food interacts strongly with host feeding in influencing longevity and fecundity and has a strong direct effect on egg resorption.     

Competition between two parasites of the Florida red scale in Israel

ABSTRACT. 1. During the last decade, the endoparasitic wasp Pteroptrix smithi (Compere) has gradually replaced the ectoparasitic wasp Aphytis holoxanthus DeBach on the Florida red scale, Chrysomphalus aonidum (L.), on citrus in Israel. The studies reported herein were aimed at elucidating some of the mechanisms of this process.
2. Aphytis adults detected, and were attracted to hosts containing eggs of Pteroptrix , whereas adults of Pteroptrix did not detect hosts containing eggs of Aphytis. In this extrinsic aspect of host discrimination, Aphytis is superior to its competitor. Neither species responded to hosts containing heterospecific first-instar larvae, and both tended to avoid hosts containing more advanced developmental stages of the other species.
3. Aphytis outcompeted Pteroptrix in most cases of multiple parasitism, as observed under both controlled and field conditions.
4. Aphytis was superior in mixed-species cultures; however, its dominance was affected by the availability of adult food.
5. Aphytis adults were found to be more sensitive (×4.11) to the insecticide malathion as compared to adults of Pteroptrix. This finding provides a possible explanation to the displacement of Aphytis by Pteroptrix in the citrus groves along the coastal plain of Israel.     

Sex allocation and egg distribution of an autoparasitoid, Encarsia pergandiella (Hymenoptera: Aphelinidae)

Abstract. 1. Encarsia pergandiella Howard females develop as primary parasitoids on immature whiteflies, and males develop as secondary parasitoids on females of their own or a related species. The hypothesis that the sex ratio reflects the relative abundance of the two host types was tested in the laboratory using petri dish arenas with varying proportions of early fourth instar greenhouse whitefly (Trialeurodes vaporariorum (West.)) (primary hosts) and pupal female E.pergandiella (secondary hosts). Egg distribution was analysed with respect to sex ratio, super-parasitism and host discrimination.
2. The proportion of primary and secondary hosts parasitized in each treatment reflected the relative availability of each host type. Thus females presented with 75% primary hosts laid more female eggs than male. However, in all treatments, a greater proportion of secondary hosts were parasitized than would be expected from the proportion of secondary hosts available. This indicates that more male eggs were laid than expected.
3. More secondary hosts than primary hosts were superparasitized.
4. Host discrimination analysis using a new test statistic showed that females generally laid eggs at random with regard to previous parasitism of primary or secondary hosts. However, females in one treatment with 50% of each host type appeared to preferentially oviposit in secondary hosts which did not contain any eggs.     

Oviposition decisions in an endoparasitoid under self-superparasitism conditions

Superparasitism is a widespread phenomenon. Having accepted superparasitism, mated female parasitoids must decide on the sex of each egg they subsequently lay into the same host. Theory predicts that this decision is either based on host quality, when more male eggs are laid in hosts that are already parasitized because they are perceived to be of poorer quality; or more eggs are laid of the sex that is most likely to be a strong larval competitor, i.e. generally females.Anastatus disparis is a facultative endoparasitic egg parasitoid. We used ‘artificial’ hosts to explore outcomes of decision making by A. disparis during superparasitism under a manipulated absence of larval competition. When only one egg was laid it was always female. As the number of eggs laid increased, so more of them were male. This supports the theory that oviposition decisions are based on host quality; more male eggs were laid in hosts that were already parasitized and thus of poorer quality.In a second experiment, eggs were exposed to parasitoids for different periods of time. Half the eggs were dissected to determine the number of parasitoid eggs that had been laid. The remaining eggs were incubated and the number and sex of offspring that ultimately emerged, following larval competition, were recorded. Under superparasitism conditions fierce larval competition ensued; only one offspring survived and they were predominantly female.In conclusion, oviposition decisions by female A. disparis accepting self-superparasitism were made based on host quality.     

Effect of host instar on host discrimination of heterospecific‐parasitised hosts by sympatric parasitoids

1. Interspecific competition among hymenopteran parasitoids may shape their behavioural strategies for host resource exploitation. In order to reduce or prevent competition, many parasitoid species have evolved the ability to discriminate between unparasitised hosts and hosts parasitised by another parasitoid species (i.e. heterospecific host discrimination). However, discriminatory ability might be affected by host instar. 2. This study reports the first results on whether host instar can influence the use of heterospecific‐parasitised hosts by sympatric parasitoids of the genus Aphytis (Hymenoptera: Aphelinidae). 3. Aphytis melinus and Aphytis chrysomphali discriminated between unparasitised and heterospecific‐parasitised hosts when they found a third‐instar host (high quality), with a tendency to multi‐parasitise. However, this discrimination was not observed in the second instar (lower size). 4. The behavioural strategies adopted towards multi‐parasitise third‐instar hosts varied between both species. Aphytis chrysomphali reduced its clutch size in heterospecific‐parasitised hosts, whereas A. melinus tended to probe them for longer than healthy hosts. 5. Overall, our results highlight the importance of host instar in the study of intrinsic competition between parasitoids.     

REVISION OF THE GENUS APHYTIS (HYMENOPTERA: APHELINIDAE) WITH DESCRIPTIONS OF TWO NEW SPECIES FROM EGYPT

Abstract  The Egyptian species of the aphelinid genus Aphytis Howard are reviewed. Aphytis azai Abd-Rabou and Aphytis matruhi Abd-Rabou are described as new species from Egypt. Each species is briefly diagnosed and known information on hosts and distributed is given. A. azai sp. nov. is similar to A. melinus but different in the measurement of antennal segments. A. matruhi sp. nov. differs from A. lepidosaphes by the relative length of propodeum, metanotum and scutellum. A key to the Egyptian species of Aphytis is provided.     

Effects of winter temperatures on the sex ratios ofAphytis melinus [Hym.: Aphelinidae] in the San Joaquin Valley of California

Laboratory bioassays of overwintering field populations of the California red scale parasitoidAphytis melinus DeBach showed a shift in sex ratio of F₁ progeny after prolonged exposure to winter temperatures in Tulare County, California. In 7 of 8 tests, short-term winter field exposure (<75 days) did not result in a significant male bias in sex ratios of F₁ progeny when compared to sex ratios observed in laboratory cultures maintained at 27°C. Long term exposure (109–139 days), however, resulted in significantly (P<0.05) higher male bias in F₁ sex ratios in 5 of 6 tests. These results from field populations ofA. melinus agree with conclusions of low temperature laboratory tests onAphytis species (lingnanensis, melinus) by other workers. Mention of a proprietary or commercial product in this paper does not constitute an endorsement of this product by the University of California.     

Parasitoid size as a function of host sex: potential for different sex allocation strategies

Parasitoid females are known to preferentially allocate female eggs to hosts with the higher resource value, usually leading to oviposition of female eggs in larger hosts and male eggs in smaller hosts. For koinobiont parasitoids, if male and female hosts are of equal size at time of oviposition, but differ in size in later developmental stages, the sex of the host could be used to indicate future resource value. Using parasitoids of the braconid genus Asobara, which are larval parasitoids of Drosophila, it is shown that parasitoids emerging from female hosts are larger than those from male hosts. Given this difference in resource value, ovipositing females should preferentially allocate female eggs to female hosts. An alternative strategy would be to decrease the difference in resource value between male and female hosts by castrating male hosts. The primary sex ratio of A. tabida in their two main host species does not differ between male and female hosts. In contrast to A. tabida, A. citri is known to partially castrate male hosts, but this does not decrease the size difference between male and female hosts. As in A. tabida, there is no difference in sex allocation to male and female hosts in A. citri. Despite the clear difference between the resource value of male and female hosts, these parasitoid species do not seem to make optimal use of this difference. They may not be able to discriminate between host sexes or, alternatively, there is a presently unknown fitness disadvantage to ovipositing in female hosts.     

埃及Aphytis黄蚜小蜂属记述附二新种描述（膜翅目：蚜小蜂科）

对埃及的黄蚜小蜂属进行了综述,并描述了2个新种,即Aphytis azai Abd—Rabou和Aphytis matruhi Abd—Rabou。文章对产自埃及的17个种做了简要描述,还提供了寄主和分布信息,以及埃及黄蚜小蜂属的分种检索表。新种A．azai sp．nov．与A．melinus相似,但区别在于触角节的大小。新种A．matruhi sp．nov．区别于A．lepidosaphes之处在于间节、后胸腹板以及小盾片的相对长度不同。     

Oppositional responses to host density in Aphytis holoxanthus (Hymenoptera: Aphefinidae), an efficient gregarious parasite

Abstract. 1. Ovipositional responses of an efficient gregarious ectoparasite ( Aphytis holoxanthus ) to increasing densities of its host (the Florida red scale, Chrysomphalus aonidum , Homoptera: Diaspididae) were studied in a laboratory system.
2. The female parasite may lay one to several eggs at each encounter with a host.
3. As host density increased, the number of eggs laid by the female parasite during the experimental period increased at a decreasing rate. This type of functional response resulted in less parasitism at higher host densities.
4. As host density increased, the number of eggs laid per encounter decreased. This behavioural response partially compensated for the decrease in parasitism.     

Use of time in a decision‐making process by a parasitoid

1. Time perception is seldom studied in invertebrates, with the limited experimental evidence being insufficient to provide a comprehensive pattern of the capacity of invertebrates to measure time and use it in decision‐making processes. 2. In this study, it was hypothesized that insect parasitoids have evolved the capacity to measure time precisely and to use it to optimize foraging decisions related to host exploitation. To examine time perception in females of the gregarious egg parasitoid Trichogramma euproctidis, the present study used their ability to adjust their investment (number of eggs laid) in a host to the initial transit duration (interval between the first contact with the host and the following contact with the substrate). Females utilize this method to assess host egg size, as a large egg necessarily requires more time to evaluate than a small host. In this study, the initial transit duration for a given sized egg was artificially extended by suspending it. 3. For similar sized hosts, female T. euproctidis significantly increased both oviposition duration and progeny allocation following a longer initial transit duration. 4. These results demonstrate the intrinsic capacity of this parasitoid to measure time and to adjust their progeny investment accordingly. This is believed to be one of the few demonstrations of a retrospective measure of time in an invertebrate.     

Host discrimination modulates brood guarding behaviour and the adaptive superparasitism in the parasitoid wasp Trissolcus semistriatus (Hymenoptera: Scelionidae)

Because hosts utilized by parasitoids are vulnerable to further oviposition by conspecifics, host guarding benefits female wasps. The present study aims to test whether female adults regulate brood guarding behaviour by host discrimination in a solitary parasitoid Trissolcus semistriatus by presenting an intact or parasitized host egg mass to a female adult. Virgin females without oviposition experience have host discrimination ability, which enables them to adjust the number of eggs laid in the hosts. Mating experience increases superparasitism by female adults, whereas mated females achieve a higher discrimination ability as a result of oviposition experience and show a lower superparasitism rate. As expected, females exhibit brood guard after parasitizing an intact host egg mass, whereas those females visiting a previously parasitized host egg mass, do not. Because the survival of eggs in superparasitized hosts is relatively low, regulating brood guarding behaviour by host discrimination is adaptive for female wasps.     

Effect of host quality of Callosobruchus maculatus (Coleoptera: Bruchidae) on performance of the egg parasitoid Uscana lariophaga (Hymenoptera: Trichogrammatidae)

Development and reproductive success of the solitary egg parasitoid Uscana lariophaga Steffan were examined after development in eggs of the bruchid storage pest Callosobruchus maculatus Fabricius reared at either low or high densities on cowpea seeds and laid at day 1 and 4 of maternal life. Both bruchid larval competition and maternal age negatively affected egg size, but the latter more than the former. Uscana lariophaga reared in small hosts developed slower, were smaller and produced fewer eggs compared to parasitoids reared in large hosts. Fecundity of the parasitoid was heavily influenced by host egg size. This was reflected in the values for the intrinsic rate of increase of U. lariophaga, which differed for wasps that developed in host eggs laid by bruchid females of different age. Wasps allocated marginally more female offspring to larger hosts.     

Pre‐Patch Experience Affects the Egg Distribution Pattern in a Polyembryonic Parasitoid of Moth Egg Batches

According to foraging theory, female parasitoids should alter their host choice in response to cues that indicate a limitation of resources. We tested whether females of the polyembryonic parasitoid Ageniaspis fuscicollis (Hymenoptera: Encyrtidae), which attack egg batches of small ermine moths (Lepidoptera: Yponomeutidae), would alter their host acceptance pattern in response to different pre‐patch experience. We kept females of the parasitoid prior to a patch visit under different conditions, which should indicate different levels of competition for hosts. With increased competition as pre‐patch experience, females laid more eggs per host egg and self‐superparasitized more often, and the resultant egg distributions showed a trend from more regular distributions to increasingly Poisson and aggregated distributions. Consequently, females with a pre‐patch experience that would indicate low competition for hosts had the most even egg distributions. We conclude that pre‐patch experience of competitors may lead to a significant change of mutual interference patterns in egg‐laying A. fuscicollis wasps.     

Host selection by virgin and inseminated females of the parasitic wasp, Dinarmus basalis (Pteromalidae, Hymenoptera)

1. Fitness is related to reproduction and survival. There apparently exists a negative correlation between the numbers of male and female offspring. There also exists a trade-off between survival and reproduction. This paper investigates optimal decisions with the reproduction and survival trade-off in host selection by wasps.
2. Whereas inseminated female wasps could manipulate the sex of their offspring, virgin females produced only male offspring. I surveyed behavioural differences and the consequences of oviposition by inseminated and virgin females of a solitary parasitic wasp in host choice situations.
3. Two host types were available at the same time to both inseminated and virgin female wasps: one (a 17-day-old host in one bean) presenting difficulties for the laying of eggs, but more benefits for the offspring and the other (five 12- or 13-day-old hosts in one bean) easier for the female wasp for laying of eggs but less beneficial for the offspring.
4. Inseminated female wasps chose more 17-day-old hosts than 12-day-old hosts, but more 13-day-old hosts than 17-day-old hosts in each pair-wise choice. Virgin females chose the smaller hosts in both situations.
5. Virgin females, having greater longevity than inseminated females, laid larger numbers of eggs than the inseminated females during their lifetime by adopting an energy-saving host choice that had little effect on male offspring fitness.     

Fitness components of Aphytis melinus (Hymenoptera: Aphelinidae) reared in five California insectaries

Shipments of 50,000 commercially reared Aphytis melinus DeBach were obtained from each of the five insectaries that produce and sell this parasitoid to citrus growers in California for control of California red scale, Aonidiella aurantii (Maskell). Shipments were received from each insectary every 2 mo over a period of a year to assess variability in quality through time and between insectaries. As indices of quality, we assessed the percentage of live parasitoids (both sexes) 1, 3, 7, 14, and 28 d after receipt of the shipment, shipment sex ratio, and the size of female wasps. We found a fair amount of variation in the percentage of A. melinus that were alive on different sampling days. Despite the fact that all insectaries rear A. melinus in temperature controlled rooms and all of our studies were done at 22°C, wasp mortality occurred more quickly in the colder months of November, January through February, and March. Similar trends were observed with sex ratios; many of the insectaries had male-biased sex ratios in the colder months, especially January through February. Wasp size varied significantly for each of the insectaries throughout the year, with the summer months of July through August yielding significantly smaller females than other months. Collectively our results have important implications for biological control on citrus in California.     

Energy reserves of parasitoids depend on honeydew from non‐hosts

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Age-dependent clutch size in a koinobiont parasitoid

Abstract.  1. The Lack clutch size theory predicts how many eggs a female should lay to maximise her fitness gain per clutch. However, for parasitoids that lay multiple clutches it can overestimate optimal clutch size because it does not take into account the future reproductive success of the parasitoid.
2. From egg-limitation and time-limitation models, it is theoretically expected that (i) clutch size decreases with age if host encounter rate is constant, and (ii) clutch size should increase with host deprivation and hence with age in host-deprived individuals.
3. Clutch sizes produced by ageing females of the koinobiont gregarious parasitoid Microplitis tristis Nees (Hymenoptera: Braconidae) that were provided daily with hosts, and of females ageing with different periods of host deprivation were measured.
4. Contrary to expectations, during the first 2 weeks, clutch size did not change with the age of the female parasitoid, neither with nor without increasing host-deprivation time.
5. After the age of 2 weeks, clutch size decreased for parasitoids that parasitised hosts daily. The decrease was accompanied by a strong decrease in available eggs. However, a similar decrease occurred in host-deprived parasitoids that did not experience egg depletion, suggesting that egg limitation was not the only factor causing the decrease in clutch size.
6. For koinobiont parasitoids like M. tristis that have low natural host encounter rates and short oviposition times, the costs of reproduction due to egg limitation, time limitation, or other factors are relatively small, if the natural lifespan is relatively short.
7. Koinobiont parasitoid species that in natural situations experience little variation in host density and host quality might not have strongly evolved the ability to adjust clutch size.