Effect of precocene II on fatty acid metabolism in the pea aphid, Acyrthosiphon pisum, under cold stress

Pea aphids, Acyrthosiphon pisum (Harris), reared at 10 degrees C contain higher levels of fatty acids than those reared at 25 degrees C. This is primarily the result of an accumulation of triacylglycerols containing myristic acid. When aphids reared at 25 degrees C were transferred to 10 degrees C there was a gradual increase in triacylglycerol content that reached a maximum at 16 days post-transfer. Treatment of aphids with precocene II prior to transfer to 10 degrees C blocked the accumulation of fatty acids including myristic acid. A single application of 2 microg precocene II/aphid or two applications of 0.5 microg precocene II/ aphid administered on consecutive days resulted in aphids moved to 10 degrees C maintaining the same fatty acid profile as aphids maintained at 25 degrees C. Aphids that were treated with precocene II and maintained at 25 degrees C did not show changes in fatty acid profiles. Rearing aphids at 10 degrees C resulted in lower rates of reproduction and lower total numbers of progeny with longer longevity. Treatment with precocene II significantly decreased the total number of progeny produced at both temperatures. Precocene II did not reduce life span of aphids reared at 25 degrees C, however, the life span of treated aphids reared at 10 degrees C was decreased. The mechanism by which precocene II prevents the accumulation of myristic acid in aphids reared at 10 degrees C remains to be determined.     

Effects of beauvericin on Schizaphis graminum (Aphididae)

The effects of beauvericin, a toxic fungal metabolite common contaminant of maize and wheat, on aphid fitness were studied in three consecutive generations of females. Aphids were reared on wheat leaves inserted into a sandy substratum wetted with a solution of beauvericin. Ingestion of this solution through leaves did not significantly decrease the lifespan of females of all generations as compared to controls. However, the mean number of offspring from the third generation of treated females was significantly smaller than those in controls. Furthermore, treated second and third generation females produced a greater number of abortive embryos. Histological analysis revealed abundant DAPI and Feulgen positive material in the cytoplasm of some bacteriocytes of treated third generation females. This material was attributed to the endosymbionts of bacteriocytes. Tests by contact were also carried out and revealed a significantly lower survival of treated first instar aphids as compared to controls 18h after the start of the trial.     

A transgenerational interval timer inhibits unseasonal sexual morph production in damson-hop aphid, Phorodon humuli

Abstract.  The induction of sexual and parthenogenetic morphs of the damson-hop aphid, Phorodon humuli , on hops is controlled by daylength. The ability of P. humuli , to produce winged pre-sexual females (gynoparae) in the short-day conditions of spring is inhibited by an interval timer present in generations immediately after hatching of the overwintering egg. The inhibition expires after three generations when nymphs are born and reared in short days (LD 12 : 12 h), irrespective of whether their parents are reared in short or long days (LD 18 : 6 h). No gynoparae are produced by aphids maintained for 13 generations in long days. Two wingless aphids from 35 survive transfer from Prunus spinosa to hops. No winged females are produced during nine generations among their progeny maintained in long days on hops, but gynoparae, followed by males, are produced one generation after these aphids are transferred to short days.     

Host monitoring by aphid migrants: do gynoparae maximise offspring fitness?

Summary Gynoparae (autumn migrants) of the bird cherry-oat aphid,Rhopalosiphum padi (L.), were not found to land randomly on their primary host,Prunus padus L. Some trees, although within a few metres of heavily infested trees, were not colonized at all. This phenomenon occurs regularly from year to year.The reasons for this non-random landing pattern are examined in terms of the success of subsequent generations, special attention being paid to the reproductive rate of the oviparae and fundatrices and the success of the developing populations in the spring of the following year.Trees in Scotland and Finland were examined in this manner and the hypothesis developed that the gynoparae ofR. padi show maternal care in selecting hosts that favour their offspring's survival and reproduction.Aphids were reared under controlled conditions in the laboratory on twigs cut from trees of known previous infestation levels. Aphids on twigs from preferred trees had better survival rates and produced more offspring than those aphids on twigs from non-preferred trees.     

Production of sexual morphs by apterous virginoparae of Myzus persicae (Hemiptera: Aphididae) in relation to pre- and postnatal exposure to short day conditions

The production of males and females by apterae of a holocyclic clone of Myzus persicae (Sulzer) was studied in the laboratory by transferring apterous virginoparae from long day to short day conditions at different stages of their pre- and post-natal development. Prenatal exposure was also examined in nine additional clones coming from different regions of Greece. By increasing the number of short day cycles at 17 degrees C, the aphids switched to male production earlier and produced more males. The highest mean number of male progeny was observed in apterae that were transferred to short day conditions 13-14 days prenatally. Apterae that were transferred to short day conditions after the fourth nymphal instar produced only females. Apterous virginoparae were produced by aphids that were transferred to short day conditions 0-1 days prenatally or postnatally. Aphids, given short days prenatally or in the first instar, produced alate female progeny that were all gynoparae. The later the instar aphids were transferred to short day conditions the fewer the gynoparae produced among their female progeny. A reduction in the total number of progeny and embryos per ovariole was observed as aphids were transferred to short day conditions in earlier stages of their development. The increase in male production, which follows the increase in the duration of long night exposure, was also observed in the additional eight out of nine examined clones.     

Impact of transgenic cotton plants on a non-target pest, Aphis gossypii Glover

Abstract.  1. Transgenic crops have shown great promise for the control of target pest insects, but in some cases they can also influence non-target species. This study investigated the impact of Bt and Bt+CpTI transgenic cottons on the non-target cotton aphid, Aphis gossypii Glover, by comparing life-table parameters, feeding behaviour, and the fluctuating asymmetry of morphological traits of aphids reared on transgenic cotton and those on untransformed control plants.
2. Aphids on the Bt+CpTI cotton showed a shorter reproductive duration and maximum lifespan, lower survival rates and potential maximum fecundity, and an earlier occurrence of peak daily mortality in the first or second generation. However, the aphid population soon developed fitness and overcame the negative effect in the second or third generation. The aphids on the Bt cotton had significantly longer reproductive durations in the first generation, higher survival rates in the third generation, and apparently larger potential maximum fecundity in all three generations.
3. The percentages of accumulated duration of feeding waveforms E1 and E2 were significantly lower in aphids on the Bt+CpTI cotton than in those on the Bt or control cotton, whereas the frequencies of moving and finding feeding sites, and probe behaviour were significantly higher.
4. Fluctuating asymmetry in three morphological characters of aphids reared on transgenic and control cotton was detected. The fluctuating asymmetry value of the third segment of antenna in aphids on Bt+CpTI cotton was significantly higher than that of aphids on Bt or control cotton. Based on the fluctuating asymmetry value, the stress of cotton on the aphids could be ranked as Bt+CpTI cotton > Bt cotton > control cotton.     

Photoperiodic determination of gynoparae and males of damson-hop aphid Phorodon humuli

Abstract. Sexual morph production in Phorodon humuli is controlled by daylength. Wingless aphids reared from birth in short-day conditions (LD 12 : 12 h) and transferred when adult to long-day conditions (LD 18 : 6 h) produce only gynoparae and males until death some 7 weeks later, whereas those reared in long days and transferred to short days produce 20% wingless parthenogenetic females, 50% gynoparae and 30% of males in an overlapping sequence. No winged morph capable of re-infesting hop is produced. Less mature embryos are more sensitive to short days than older embryos because 100% of the former became gynoparae after 4 days of exposure of their mothers, and 59% when their mothers were exposed for the 4 days immediately preceding the birth of their offspring. Two days of exposure to short days switches 94% of young embryos from wingless to gynopara production when mature. The response to short days is irreversible. Wingless aphids reared from birth to adult in short days produce 30% fewer offspring than those reared to the same stage in long days and are male-biased, with 76–78% of their offspring being male.     

Relative suitability of crested wheatgrass and other perennial grass hosts for the Russian wheat aphid (Homoptera: Aphididae)

The Russian wheat aphid, Diuraphis noxia (Mordvilko) (Homoptera: Aphididae), reproduces parthenogenetically in North America and must survive year-round on host plants, including in late summer when small grains are not in cultivation. During this time, cool-season perennial wheatgrasses (Poaceae: Triticeae) contribute substantially to aphid survival, crested wheatgrass (Agropyron spp.) particularly. In greenhouse studies, the number of aphids per plant was measured after four infestation periods on unvernalized and vernalized wheatgrasses. Before placement on these test plant species, aphids were reared either on winter wheat or on the grass host species on which aphid progeny were counted. On vernalized plants, aphids reared on wheat resulted in more aphids per test plant than when the aphids were reared on wheatgrasses, but on unvernalized plants the number of aphids per test plant did not differ significantly regardless of rearing host. Aphids on crested wheatgrass were similar in number to the other grasses when plants were unvernalized. However, when plants were vernalized, crested wheatgrass supported significantly more aphids than some of the other hosts. Aphid numbers increased on all test species as infestation period lengthened, and plant growth was largely unaffected by aphid feeding. These results suggest if sufficient moisture is available during summer when small grains are not in cultivation, all host species observed are capable of sustaining aphids. Crested wheatgrass is an abundant and important host of the Russian wheat aphid in its northern range of the western United States, but other less prevalent wheatgrasses also may contribute to aphid survival during late summer when small grains are not in cultivation.     

Facultative bacterial endosymbionts benefit pea aphids Acyrthosiphon pisum under heat stress

Abstract 1. Natural populations of pea aphids in California contain at least two facultative bacterial secondary symbionts (pea aphid secondary symbiont, PASS, or pea aphid rickettsia, PAR) in a range of frequencies throughout the state.
2. Two pea aphid clones without either of these facultative associates failed to reproduce in the first 8 days after the final moult if they had been heat-stressed for a period of about 4 h at 39 °C as 1-day-old larvae in the laboratory.
3. Aphids infected artificially with PASS, however, were able to produce up to 48% of the normal complement of offspring produced by PASS-positive aphids that had not been heat-stressed. Clones infected artificially with PAR did not have the same advantage as those with PASS after heat stress.
4. In aphids without PASS or PAR, heat stress reduced the number of bacteriocytes (in which the obligate primary symbiont, Buchnera , resides) to 7% of non-heat-stressed aphids, while aphids with only PASS retained 70% of their bacteriocytes. Bacteriocytes in aphids with PAR but not PASS were reduced to 42% of controls.
5. When larvae were heat-stressed as older instars (5 days old), a similar pattern emerged, though the effect of heat stress was less extreme. Clones containing PASS produced the most offspring, three to 14 times as many as aphids without PASS or PAR. Aphids with PAR only, or PASS and PAR together, had reduced or no advantage over aphids without facultative symbionts.
6. Aphids of all clones that had been heat-stressed as later instars gave birth to a variable number of stillborn offspring. Aphids without facultative symbionts produced the most stillborn larvae.
7. Field studies showed a higher incidence of PASS in aphids collected in California in summer compared with aphids from the same sites collected 2–4 months earlier. The difference was significant in two of three widely dispersed locations.     

The influences of host plant and genetic resistance to Bacillus thuringiensis on trade-offs between offspring number and growth rate in cabbage loopers, Trichoplusia ni

Abstract.  1. Variation in progeny size and quality is common among insects and this variation can strongly influence individual fitness. Larger progeny typically survive better and develop faster under adverse conditions and may have higher fecundity. Due to resource limitations, however, trade-offs may arise between having fewer large offspring or more smaller ones.
2. For cabbage loopers, Trichoplusia ni , pepper leaves are a poorer larval host than cucumber or tomato leaves as indicated by survival, development rate, and body size. Moths reared on cucumber produced more slower growing offspring than those that had been reared on pepper, which produced fewer, faster growing progeny.
3. Traits conferring resistance to Bacillus thuringiensis ( Bt ) generally are associated with strong deleterious effects that may influence resource allocation and reproductive trade-offs between progeny size and number.
4. Unlike the host-plant related trade-off between progeny size and fecundity observed among susceptible control moths, Bt -resistant parents had both the lowest fecundity and smallest progeny size on all host plants. This finding suggests that the progeny size–number relationship is constrained in resistant individuals.     

Predator-induced morphological shift in the pea aphid

Aphids exhibit a polymorphism whereby individual aphids are either winged or unwinged. The winged dispersal morph is mainly responsible for the colonization of new plants and, in many species, is produced in response to adverse environmental conditions. Aphids are attacked by a wide range of specialized predators and predation has been shown to strongly influence the growth and persistence of aphid colonies. In two experiments, we reared two clones of pea aphid (Acyrthosiphon pisum) in the presence and absence of predatory ladybirds (Coccinella septempunctata or Adalia bipunctata). In both experiments, the presence of a predator enhanced the proportion of winged morphs among the offspring produced by the aphids. The aphid clones differed in their reaction to the presence of a ladybird, suggesting the presence of genetic variation for this trait. A treatment that simulated disturbance caused by predators did not enhance winged offspring production. The experiments indicate that aphids respond to the presence of a predator by producing the dispersal morph which can escape by flight to colonize other plants. In contrast to previous examples of predator-induced defence this shift in prey morphology does not lead to better protection against predator attack, but enables aphids to leave plants when mortality risks are high.     

Intraspecific variation in zucchini yellow mosaic virus transmission by Myzus persicae and the impact of aphid host plant

Three isofemale lines of Myzus persicae (Sulzer), two lines collected from and reared on a brassicaceous host, and one line collected from and reared on a malvaceous host, were evaluated for their efficiency of transmitting Zucchini yellow mosaic virus (family Potyviridae, genus Potyvirus, ZYMV). In the first experiment, the transmission efficiencies of two clones from Brassicaceae (B1 and B2) were 52.0 and 60.8%, respectively, and these transmissions were not significantly different. In a second experiment, the transmission efficiencies of the clone on Malvaceae (M1) and clone B2 were significantly different at 35.6 and 55.7%, respectively. Further experiments evaluated host-related mechanisms that may have contributed to the differential transmissions observed between clones M1 and B2. Studies on short-term feeding showed that aphids continuously reared on okra, Abelmoschus esculentus (L.) Moench (malvaceous host), and those that were reared on okra and allowed a 24-h preacquisition feeding period on mustard, Brassica juncea (L.) Czern (brassicaceous host), had significantly lower transmission than aphids continuously maintained on mustard. Aphids reared on mustard and allowed a 24-h preacquisition feeding period on okra had intermediate transmission efficiency. In long-term host association studies, we found that aphids reared on mustard had significantly higher transmission efficiency than those reared on okra, and aphids reared first on okra and then switched to mustard had a transmission efficiency that was intermediate and not significantly different from the other two treatments. Our study reveals the existence of intraspecific variation in the transmission of ZYMV by M. persicae, and it suggests that to accurately assess the transmission capability of ZYMV by this species, multiple clones should be examined. Furthermore, the host plant on which the aphid is reared as well as the host plant on which it feeds just before virus acquisition contribute to ZYMV transmission efficiency of M. persicae.     

Parasitoids induce production of the dispersal morph of the pea aphid, Acyrthosiphon pisum

In animals, inducible morphological defences against natural enemies mostly involve structures that are protective or make the individual invulnerable to future attack. In the majority of such examples, predators are the selecting agent while examples involving parasites are much less common. Aphids produce a winged dispersal morph under adverse conditions, such as crowding or poor plant quality. It has recently been demonstrated that pea aphids, Acyrthosiphon pisum, also produce winged offspring when exposed to predatory ladybirds, the first example of an enemy‐induced morphological change facilitating dispersal. We examined the response of A. pisum to another important natural enemy, the parasitoid Aphidius ervi, in two sets of experiments. In the first set of experiments, two aphid clones both produced the highest proportion of winged offspring when exposed as colonies on plants to parasitoid females. In all cases, aphids exposed to male parasitoids produced a higher mean proportion of winged offspring than controls, but not significantly so. Aphid disturbance by parasitoids was greatest in female treatments, much less in male treatments and least in controls, tending to match the pattern of winged offspring production. In a second set of experiments, directly parasitised aphids produced no greater proportion of winged offspring than unparasitised controls, thus being parasitised itself is not used by aphids for induction of the winged morph. The induction of wing development by parasitoids shows that host defences against parasites may also include an increased rate of dispersal away from infected habitats. While previous work has shown that parasitism suppresses wing development in parasitised individuals, our experiments are the first to demonstrate a more indirect influence of parasites on insect polyphenism. Because predators and parasites differ fundamentally in a variety of attributes, our finding suggests that the wing production in response to natural enemies is of general occurrence in A. pisum and, perhaps, in other aphids.     

Limited scope for maternal effects in aphid defence against parasitoids

Abstract.  1. A mother's environment frequently affects her offspring's phenotype. Such maternal effects may be adaptive, in particular with respect to pathogens or parasites, for example if maternal exposure increases offspring resistance.
2. In aphids, maternal effects are likely to occur as a result of their telescoping generations. This study investigated whether maternal effects influence the susceptibility of the peach-potato aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae), to its parasitoid Diaeretiella rapae (M'Intosh) (Hymenoptera: Braconidae: Aphidiinae).
3. In a first experiment, susceptibility was compared among offspring of aphid mothers that had either no contact to parasitoids, had contact but were not attacked, or were attacked but not mummified. Mothers from the last group had successfully resisted the parasitoid.
4. In a second experiment using two different clones, maternal and progeny environment were manipulated by rearing each generation either on a benign (radish) or a more stressful host plant (silver beet) before progeny exposure to parasitoids.
5. The first experiment revealed no significant effect of the maternal treatment on offspring susceptibility to parasitoids and thus no evidence for trans-generational defence. In the second experiment, maternal environment effects were also weak, yet with a trend towards less susceptible offspring of aphid mothers reared on the more stressful plant. However, there was a significant difference among clones and a strong clone × progeny host plant interaction, illustrating that the outcome of a parasitoid attack may be determined by a complex interplay of genetic and environmental factors.
6. Overall, the results suggest that there is limited scope for maternal effects in aphid defence against parasitoids.     

The influence of temperature on size as an indicator of host quality for the development of a solitary koinobiont parasitoid

The influence of aphid size on the host quality assessment and progeny performance of aphidiine parasitoids was examined using the mealy plum aphid parasitoid, Aphidius transcaspicus Telenga (Hymenoptera: Braconidae) and the black bean aphid, Aphis fabae Scopoli (Homoptera: Aphididae), as a readily acceptable alternate host. Aphid size in relation to stage of development was manipulated by rearing synchronous aphid cohorts at either 15 or 30 °C. At 15 °C, 2nd instar aphids were approximately the same size as 4th instar aphids reared at 30 °C. Cohorts of 30 aphids from each instar, reared at each temperature, were exposed to parasitism by a single parasitoid female for a period of 5 h. Overall susceptibility to parasitism did not vary between aphid cohorts, but the parasitoid response to aphid size differed significantly between rearing temperatures for both progeny sex ratio (parent female assessment of host quality) and larval growth and development (host suitability for parasitoid development). For aphids reared at 15 °C, the proportion of female progeny and emerging adult size for the parasitoid increased linearly with aphid size at the time of attack, while development time remained constant. In contrast, for aphids reared at 30 °C, the proportion of female progeny, emerging adult size, and the development time of the parasitoid all declined with aphid size at the time of attack. The contrasting responses of the parasitoid to host size for aphids reared at the two temperatures suggest that host quality is only indirectly related to aphid size among aphidiine parasitoids. The possible effects of higher temperatures on nutritional stress, obligate endosymbionts, and future growth potential of the aphids are discussed as explanations for the variation in host quality for parasitoid development.     

Development and reproductive potential of the cereal aphid Sitobion avenae on resistant wheat lines (Triticum monococcum)

The effect of wheat resistance in lines of Triticum monococcum L., on the reproductive performances of the cereal aphid (Sitobion avenae F.) was investigated. Aphids were reared from birth to adult moult either on resistant or susceptible wheat lines, and transferred as apterae to both host genotypes. The influence of these transfers on the subsequent adult weight, gonad status and reproductive performances was evaluated. Aphids transferred from resistant to susceptible plants proved able to compensate for their poor nymphal growth, mainly through additional embryo growth and an increase in the number of matured embryos within the first 10 days of their adult life. Most aphids transferred from susceptible to resistant plants died within the first week following the transfer. Their most advanced embryos matured and were born, but subsequent embryo growth was quickly reduced. The reproductive strategies adopted by S. avenae when facing plant resistance, and the hypothesis of a resistant mechanism based on a poor nutritional state of the resistant plants are discussed.     

Ovulation and the mechanism of follicle rupture

Summary The effects of penicillin and chlortetracyline HCl on the fine structure of the intracellular symbiotes of the pea aphid were studied in an attempt to remove the symbiote population. High penicillin concentrations, 1% and 0.1%, caused symbiote breakdown but were toxic and/or repellent to the aphids; at 0.1% specific effects were observed on the symbiotes' cell walls. After the use of 0.01% penicillin in the aphid diet, the symbiotes had abnormal cell walls and were abnormally dilated; however, symbiote division and transmission from one aphid generation to the next seemed unaffected and the aphids appeared normal. Aphids fed 0.1% chlortetracycline failed to reproduce. After 7 days, their symbiotes were found to break down at a high rate but aphid mitochondria were also adversely affected at this stage. Following 0.002% chlortetracycline, the aphids produced aposymbiotic progeny with apparently normal mitochondrial populations; these larvae failed to develop.Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, and by a research grant (GB 31904 X) from the National Science Foundation.     

A Dietary Test of Putative Deleterious Sterols for the Aphid Myzus persicae

The aphid Myzus persicae displays high mortality on tobacco plants bearing a transgene which results in the accumulation of the ketosteroids cholestan-3-one and cholest-4-en-3-one in the phloem sap. To test whether the ketosteroids are the basis of the plant resistance to the aphids, M. persicae were reared on chemically-defined diets with different steroid contents at 0.1–10 µg ml⁻¹. Relative to sterol-free diet and dietary supplements of the two ketosteroids and two phytosterols, dietary cholesterol significantly extended aphid lifespan and increased fecundity at one or more dietary concentrations tested. Median lifespan was 50% lower on the diet supplemented with cholest-4-en-3-one than on the cholesterol-supplemented diet. Aphid feeding rate did not vary significantly across the treatments, indicative of no anti-feedant effect of any sterol/steroid. Aphids reared on diets containing equal amounts of cholesterol and cholest-4-en-3-one showed fecundity equivalent to aphids on diets containing only cholesterol. Aphids were reared on diets that reproduced the relative steroid abundance in the phloem sap of the control and modified tobacco plants, and their performance on the two diet formulations was broadly equivalent. We conclude that, at the concentrations tested, plant ketosteroids support weaker aphid performance than cholesterol, but do not cause acute toxicity to the aphids. In plants, the ketosteroids may act synergistically with plant factors absent from artificial diets but are unlikely to be solely responsible for resistance of modified tobacco plants.     

The impact of Meu1-mediated resistance in tomato on longevity, fecundity and behavior of the potato aphid, Macrosiphum euphorbiae

The effect of the tomato resistance gene, Meu1, on feeding, longevity, fecundity and developmental rate of the pink biotype of the potato aphid, Macrosiphum euphorbiae (Thomas) (Hemiptera, Aphididae), was determined using nearly isogenic tomato (Lycopersicon esculentum Mill, Solanaceae) lines. Aphid mortality was significantly higher on resistant plants, with 60% of the aphids dying by the 4th day of exposure. By the 10th day, all the aphids on the resistant plants were dead whereas 100% of the aphids on susceptible plants were alive. Meu1-mediated resistance resulted in significantly decreased fecundity with a ten-fold decrease in the net fertility rate (4.5 and 45.7 progeny per aphid on resistant and susceptible tomato, respectively). A qualitative analysis showed that honeydew was produced by aphids on resistant and susceptible plants, suggesting that aphids initiate feeding on both plant types. However, significantly lower quantities of honeydew were present when aphids were caged on resistant plants. There were also significant differences in aphid location on resistant and susceptible leaves. Experiments evaluating behavior in less than 24 h showed that aphids left resistant leaves after relatively short exposure (3–6 h). Aphids transferred from resistant to susceptible tomato at intervals between 3 h and 24 h resumed feeding as evidenced by presence of honeydew. Although the mechanism by which Meu1-mediated resistance operates is not yet known, our data suggest that resistance factors act rapidly after initiation of feeding and that lower fecundity and longevity are related to reduction in aphid feeding.     

Parasitism byEphedrus cerasicola [Hym.: Aphidiidae] developing in different stages ofMyzus persicae [Hom.: Aphididae]

The parasitoidEphedrus cerasicola Stary oviposited in the 4 nymphal instars and in newly moulted apterous adults ofMyzus persicae (Sulzer). Development and reproduction of unparasitized and parasitized aphids at 21°C were compared. Unparasitized aphids developed to adults in 6.5 days and started to reproduce after 7 days. Longevity varied between 7 and 42 days. Net reproductive rate (R₀) was 40.7. In contrast to older nymphs, aphids parasitized in the 1 st instar almost never reached the adult stage before mummification. Aphids parasitized in 2nd, 3rd and 4th instar and as newly moulted adults produced respectively 0.07 %, 2 %, 23 % and 32 % of offspring produced by unparasitized aphids. Corresponding reproductive periods were 1, 1.4, 3 and 4 days. Host age at parasitization had a slight effect on the parasitoid's developmental rate and had no effect on egg or pupal survival, or on the sex ratio of the emerging parasitoids.