Heterospecific larval competition and host discrimination in two species of aphid parasitoids: Aphidius ervi and Aphidius smithi

Females of the solitary aphid parasitoids Aphidius ervi Haliday and A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae) discriminated between unparasitized pea aphids and those parasitized by the other species. Oviposition restraint varied with the attack sequence and the length of the interval between successive attacks. The tendency to reject a previously parasitized host increased with interval length; A. smithi females rarely oviposited in aphids that had been parasitized 30 h earlier by A. ervi. Early first-instar larvae of A. ervi physically attacked and killed older A. smithi larvae, and older A. ervi larvae killed younger A. smithi, possibly by physiological suppression. Neither species appeared to have a competitive advantage when their eggs hatched at the same time. The evolution of heterospecific host discrimination in A. ervi and A. smithi is discussed. It is suggested that avoidance of multiparasitism is adaptive for both parasitoid species: for A. smithi because it is the inferior larval competitor, and for A. ervi because immatures develop more slowly in multiparasitized than in initially unparasitized hosts.

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Compétition interspécifique et discrimination des hôtes chez deux parasitoïdes de pucerons: Aphidius ervi et A. smithi

Résumé Les femelles des parasitoïdes de pucerons: Aphidius ervi Haliday et A. smithi Sharma & Subba Rao (Hyméno. Aphidiidae) distinguent les pucerons du pois sains des parasités par d'autres espèces. La rétention de la ponte dépend de la séquence de l'attaque et du temps écoulé entre des attaques successives. La tendance au rejet d'un puceron précédemment parasité augmente avec l'importance du délai; A. smithi a rarement pondu dans des pucerons qui avaient été parasités 30 h avant par A. ervi. Les jeunes larves de premier stade de A. ervi ont attaqué physiquement et tué les larves plus âgées de A. smithi, et les larves plus âgées de A. ervi ont tué des larves plus jeunes de A. smithi par élimination physiologique. Aucune espèce ne semble avoir un avantage quand les oeufs ont éclos en même temps. L'évolution de la discrimination interspécifique de l'hôte chez A. ervi et A. smithi est discutée. On estime que la tendance à éviter le multiparasitisme est adaptative chez les 2 espèces: pour A. smithi parce qu'il est dominé dans la compétition larvaire et pour A. ervi parce que les larves se développent plus lentement dans un hôte multiparasité que dans un hôte initialement sain.

     

Decision to superparasitize based on larval survival: competition between aphid parasitoids Aphidius ervi and Aphidius smithi

Oviposition by a parasitoid wasp in an already parasitized host (= superparasitism) may result in larval competition and the loss of offspring. Consequently, in solitary species, the decision to superparasitize should be based on the probability of offspring survival. Females of Aphidius ervi Haliday and A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae), two parasitoids of the pea aphid, discriminated between conspecific-and heterospecific-parasitized hosts. Both species showed partial preference for aphids previously attacked by A. smithi, a result predicted from the fact that A. ervi is superior to A. smithi in larval competition under most conditions. Females of A. smithi selectively attacked conspecific-parasitized pea aphids when given a choice between these and self-parasitized hosts. Results indicated that females of both species responded to an external pheromone-like marker to discriminate between selfconspecific-and heterospecific-parasitized aphids. In addition, A. smithi may use internal cues to recognize, and avoid oviposition in, aphids previously parasitized by A. ervi.

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Résumé L'oviposition par une guêpe parasite dans un hôte qui a déjà été parasité (= superparasitisme) pourrait conduire à une compétition entre larves et à une perte de progéniture. Par conséquent, dans les espèces solitaires, la décision de superparasiter devrait être basée sur la probabilité de survie de la progéniture. Les femelles de Aphidius ervi Haliday et de A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae), deux parasites du puceron du pois, ont discriminé entre les hôtes parasités par des individus de la même espèce et ceux parasités par des individus d'une espèce différente. Les deux espèces de parasite ont montré une préférence partielle pour les pucerons préalablement parasités par A. smithi, résultat qui était prévisible du fait que, dans la majorité des cas, A. ervi est supérieur à A. smithi dans la compétition entre larves. Les femelles de A. smithi ont sélectivement attaqué les pucerons parasités par un individu de la même espèce lorsqu'il leur était donné de choisir parmi ceux-ci et ceux parasités par les même individu. Les résultats indiquaient que les femelles des deux espèces répondaient à une sorte de phéromone externe pour distinguer les pucerons parasités par le même individu de ceux parasités par un individu de la même espèce et ceux parasités d'une autre espèce. En plus, A. smithi pourrait utiliser des signaux internes pour reconnaître et éviter l'oviposition dans les pucerons préalablement parasités par A. ervi.

     

The use of visual cues in host evaluation by aphidiid wasps I. Comparison between threeAphidius parasitoids of the pea aphid

Host evaluation behaviour was examined in three species of aphid parasitoids,Aphidius ervi haliday,A. pisivorus Smith, andA. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae). Parasitoids were provided under laboratory conditions with three kinds of hosts representing two aphid species: (green) pea aphid,Acyrthosiphon pisum (Harris), and green and pink colour morphs of the alfalfa aphid,Macrosiphum creelii Davis. Females of all threeAphidius species distinguished between aphids on the basis of colour, movement, and host species. Patterns of host acceptance by parasitoids were species-specific. InA. ervi, host preference was the same in light and dark conditions: pea aphid>green alfalfa aphid≫pink alfalfa aphid. In contrast,A. pisivorus attacked and accepted pea aphid and green alfalfa aphid equally in the light and preferred both of these over pink alfalfa aphid; however, it made no distinction between pea aphid and pink alfalfa aphid in the dark. Females ofA. smithi attacked all three kinds of hosts (pea aphid>green alfalfa aphid≫pink alfalfa aphid) but apparently laid eggs only in pea aphid. The frequencies of attack and oviposition by all wasps were higher on ‘normal’ pea aphids than on those anaesthetized with CO₂. Host recognition is confirmed by chemical cues in the aphid cuticle that are detected during antennation, and host acceptance is dependent on an assessment of host quality during ovipositor probing.     

Histopathology of an aphid,Schoutedenia lutea,parasitized by a gall midge,Pseudendaphis sp

Pseudendaphis sp. (Barnes) [Diptera: Cecidomyiidae] is a larval endoparasitoid of the aphidSchoutedenia lutea (van der Goot) [Aphididae: Greenideinae]. A brief investigation of the biology ofPseudendaphis sp. revealed that parasitization by this species occurs only in viviparous instar III to adult life stages of the host aphid. Comparative histology of parasitized, unparasitized and starved aphids was used to evaluate proposed mechanisms of pathogenesis. Parasitization byPseudendaphis sp. was found to cause the destruction of host embryos and pathological changes in the host fat body, yet resulted in an increase in the number of host mycetocytes. There was no evidence of injury to host tissues through direct physical attack, a cytolytic enzyme or starvation-like factors. Remaining hypotheses for altered host histologies in parasitized aphids are selective nutrient depletion and production of a non-cytolytic bioactive substance by thePseudendaphis larvae.      

Patterns of host selection by four species of aphidiid (Hymenoptera) parasitoids: influence of host switching

Abstract.

• 1 We tested switching behaviour in four species of aphidiid parasitoids, using a two-aphid experimental system consisting of second-instar nymphs of pea aphid (Acyrthosiphon pisum (Harris)) and alfalfa aphid (Macrosiphum creelii Davis) feeding on broad beans in the laboratory.
• 2 Aphidius ervi Haliday, A.pisivorus Smith, A.smithi Sharma & Subba Rao, and Pram pequodorum Viereck showed an innate preference for pea aphid when both host species were provided in equal numbers.
• 3 Wasps encountered both aphid species equally but differed in their acceptance of alfalfa aphid. Females of A.pisivorus and P.pequodorum accepted alfalfa aphids when few pea aphids were available, but A. smithi always concentrated attacks on pea aphid. Aphidius ervi super-parasitized an increasing proportion of pea aphids as their availability declined.
• 4 Switching to the alfalfa aphid occurred in A.ervi and P.pequodorum (but not in A.pisivorus and A.smithi) under the condition of a 1:3 ratio of pea aphids:alfalfa aphids. Wasps did not switch when more pea aphids than alfalfa aphids were provided (3:1 ratio).
• 5 Alfalfa aphids were more likely than pea aphids to escape from parasitoid attack.
• 6 Switching to the most abundant host may not be adaptive in these four species of aphid parasitoids. A foraging wasp incurs a potentially higher cost in lost opportunity time when attacking (and failing to oviposit in) alfalfa aphids. In addition, alfalfa aphids may have lower host quality than pea aphids, a difference that could influence offspring fitness.

     

Performance of the parasitoid Aphidius colemani when reared on Myzus persicae on a fully defined artifical diet

Aphidius colemani Viereck(Hymenoptera: Aphidiidae) showed a developmenttime extended by 16% when reared on Myzus persicae Sulzer (Hemiptera: Aphididae) ofdifferent instars on artificial diet or onaphids on Brussels sprout plants. There wasalso a small but statistically significant 2%reduction in size (in hind tibia length) ofadult females. Rearing on diet had nosignificant effect on the number of eggsfemales contained at emergence. In a secondexperiment, aphids from Brussels sprouts ininstars 2–5 were compared with aphids from diettwo days older to give comparably sized aphidsfrom both substrates. Parasitisation by A.colemani gave the same direction of results,but showed that the effects in the firstexperiment were greater than predicted from thereduction in host size alone. However, in bothexperiments effects on parasitoids ofdiet-rearing the host were small, and culturingA. colemani on diet-reared aphids wassuccessful.     

Fecundity and oviposition ofEucelatoria bryani, a gregarious parasitoid ofHelicoverpa zea andHeliothis virescens

We examined longevity, fecundity, and oviposition strategies ofEucelatoria bryani Sabrosky (Diptera: Tachinidae), a gregarious endoparasitoid ofHelicoverpa zea (Boddie) andHeliothis virescens (F.) (Lepidoptera: Noctuidae). Longevity of adult femaleE. bryani was not related to body size. In contrast to longevity, largerE. bryani females had greater potential fecundity than smaller females, as determined by the number of embryonated eggs present in the common oviduct. However, female parasitoid size did not affect primary clutch size (number of eggs deposited in a host). Because embryos in eggs located in the ovisac were larger than those located elsewhere in the common oviduct, maximum primary clutch size may be physiologically limited by the number of fully mature eggs a female has available at one time.E. bryani females adjusted primary clutch size in response to host size, for bothH. zea andH. virescens. This adjustment appears to be adaptive because females did not overexploit hosts by depositing more larvae than a host could support. Adult emergence was not related to host size. Although host weight positively influencedE. bryani progeny weight, increases in progeny size with host size were counterbalanced by increases in primary clutch size with host size.     

Aspects of life-table studies and functional response of Lysiphlebia mirzai

The fecundity, reproductive rate, and survival of Lysiphlebia mirzai parasitising third instar nymphs of the cereal aphid Rhopalosiphum maidis were measured at six different host densities under constant laboratory conditions. The survival rate (l_x) of the female parasitoids was unaffected by host density, with an average adult life-span of 5–6 days at all densities. The age-specific fecundity rate (m_x) was host density-dependent. The value of m_x decreased rapidly from the first day of parasitisation. The number of hosts available determined the maximum possible number of mummies. At 200 hosts available per day, the average fecundity was 184.6 mummies/female; the maximum number of mummies yielded by any female was 200. The relationship between host density and the number of aphids parasitised per female was linear at 50 aphids/cage/day, but at higher host densities (100 aphids/cage/day) a significant curvilinear regression was observed. The intrinsic rate of natural increase (r_m) increased with increasing host density. Maximum value of r_m (0.262) was obtained at a host density of 200. The response of r_m to changes in host density and parasitoid sex ratio is shown. A typical type II functional response was observed for L. mirzai. The curve was described by a logistic curve, N_p=200/[1+exp(5.65–1.60 ln N_o)]. The search rate of the parasitoid was inverse host density-dependent. No significant variation in the sex ratio of F₁ offspring was observed at different initial host densities. Sex ratio values exceeded 0.5 at all host densities. The results evaluated the reproductive potential of L. mirzai as a promising biological control agent.     

3种寄主上桃蚜的选择性及形态分化

桃蚜是一种重要的农业害虫,寄主广泛,种下分化复杂。以采自黄土高原旱作区桃树、烟草、甘蓝上的桃蚜为研究对象,通过叶片选择法、传统比较形态测定法研究了3种寄主上桃蚜的选择性及形态分化。结果表明:在3种寄主同时存在的情况下,烟草上的桃蚜嗜食烟草,表现为63.5%的桃蚜选择烟草叶,13.8%选择甘蓝叶,8.2%选择桃叶,而甘蓝和桃树上的桃蚜对原寄主没有表现出明显的嗜好性;从形态指标来看,3种寄主上的桃蚜在体长、触角末节长度、后足腿节长度、触角与体长的比例方面存在显著差异(P0.05),说明这几个特征可以作为区分这3种寄主上桃蚜的依据。综合分析可以初步认为黄土高原旱作区烟草上的桃蚜可能形成了寄主专化型-烟草型。     

Influence of host distribution on foraging behaviour in the hyperparasitoid wasp, Dendrocerus carpenteri

The foraging behaviour of Dendrocerus carpenteriCurtis (Hymenoptera: Megaspilidae), an ectophagous hyperparasitoid of aphidiine wasps inside mummified aphids, was examined in the laboratory with an experimental system consisting of broad bean, Vicia fabaL, the pea aphid, Acyrthosiphon pisumHarris, and a primary parasitoid, Ephedrus californicusBaker. Pea aphids parasitised by E. californicusoften disperse from their feeding sites (or off host plants) before dying and mummifying. Response of female hyperparasitoids to host distribution was evaluated at two spatial scales. At the first scale, behaviour of hyperparasitoids was examined on individual plants with different densities of hosts. At the second scale, habitat complexity and host location were manipulated in large foraging cages containing several plants. I show that patterns of density-dependent hyperparasitism can result from the foraging behaviour of D. carpenteri. However, dispersal of parasitised aphids may not reduce the incidence of hyperparasitism if hyperparasitoids systematically search the habitat.     

Host plants affect predator fitness via the nutritional value of herbivore prey: Investigation of a plant-aphid-ladybeetle system

The interactions among host plants(Medicago sativa L., cv. `OKO8' and Vicia faba L., cv. `Windsor'), aphid prey(Acyrthosiphon pisum Harris, Homoptera:Aphididae), and Coccinella septempunctata L. (Coleoptera: Coccinellidae) preimaginal biology were evaluated. Interactions were measured over a range of limiting daily prey levels (1.2 mg–16.4 mg)from each host plant colony. Compared withA. pisum reared on V. faba, A. pisum reared on M. sativa storedsignificantly more fatty acids which resultedin a 1.17-fold increase in available caloriesfor developing C. septempunctata. Theincreased survival, decreased developmentaltimes, and larger size of C.septempunctata supplied with A. pisumreared on M. sativa clearly demonstratehost plant effects at the third trophic level. At low very limiting daily prey levels, A. pisum reared on M. sativa were moresuitable prey for C. septempunctatasurvival, development, and adult size thanA. pisum reared on V. faba. Coccinella septempunctata survival ratios(larval), developmental times, and adult sizeconverged (were not statistically different)between host plants at higher daily A.pisum levels. These convergence's supportthe hypothesis that there were quantitativedifferences in the nutritional value ofaphids, as influenced by differences in fattyacids and subsequent nutritional levels(calories), between aphids reared on separateplant hosts. The observed tritrophicinteractions appear to be modulated by thebiochemical response of A. pisum to hostplants.     

The effects of host plant on the coccinellid functional response: Is the conifer specialist Aphidecta obliterata (L.) (Coleoptera: Coccinellidae) better adapted to spruce than the generalist Adalia bipunctata (L.) (Coleoptera: Coccinellidae)?

Female Aphidecta obliterata consumed an average of 26.6 ± 5.8 Elatobium abietinum and males consumed an average of 19.9 ± 3.2 Elatobium abietinum, but there was no significant difference in the number consumed between the sexes. In Petri dish trials, the larval stages of A. obliterata and all stages of Adalia bipunctata appeared to exhibit a Type II response to prey density, although A. obliterata adults showed a more linear response to prey density. There was no significant difference between the functional responses of the 3rd instars of the two coccinellid species, but there was a significant difference between the responses shown by the 1st instars, with A. obliterata larvae consuming more than those of A. bipunctata, especially at low densities, suggesting that the two species respond differently to an increase in prey density.At low prey densities, adults and 4th instars of both species exhibited a similar response to an increase in prey density but at higher densities the 4th instars and adult stages of A. bipunctata showed higher attack rates when compared with the same stages of A. obliterata.Adult and 4th instar A. obliterata exhibited Type II functional responses on spruce sections. The 4th instar A. obliterata larva appeared to be a more effective predator than the adult of the species, and was more effective when compared with adult A. bipunctata at lower prey densities but A. bipunctata adults appeared to be a more effective predator at higher prey densities.The host plant affected the rate of consumption by adult A. obliterata as adults on Sitka spruce sections consumed significantly higher numbers of aphids than their counterparts on Norway spruce. This was most noticeable at densities above 16 aphids.The distributions of the two coccinellid species in the olfactometer were significantly affected by the presence of host plant material. Aphidecta obliterata adults were found in significantly higher numbers in the Sitka spruce chambers than the control chambers (those without plant material). Adalia bipunctata adults were found in significantly lower numbers in the Norway spruce chamber than the control chamber.Although A. bipunctata has a higher level of voracity than A. obliterata, the latter is more adapted to the spruce environment and the boom and bust population dynamics of E. abietinum.     

Recombinant and classically selected factors of potato plant resistance to the Colorado potato beetle, Leptinotarsa decemlineata, variously affect the potato aphid parasitoid Aphidius nigripes

Different forms of crop resistance developed against majorpotato pests such as the Colorado potato beetle (CPB), Leptinotarsadecemlineata, may be variously compatible with biological controlof secondary pests such as aphids. We compared the performance of theparasitoid Aphidius nigripes developing in the aphid hostMacrosiphum euphorbiae, on five potato lines, including atransgenic `Superior-BT' line expressing the CryIIIA toxin ofBacillus thuringiensis, which is specific to Coleoptera; andthe `NYL 235-4' line derived from Solanum berthaultii,characterized by relatively unspecific resistance to herbivores based onmoderately-high density of glandular trichomes. The other lines testedwere a `Kennebec-OCI' transgenic line expressing the protease inhibitorrice cystatin I (OCI), a potential resistance factor against coleopteranpests; and the commercial cultivars `Superior' and `Kennebec' used ascontrols. Parasitoid immature survival and adult size were reducedcompared to controls when the wasps developed on aphids fed the`Superior-BT' potato. In contrast, parasitoid size and fecundityincreased above control when the wasps developed on aphids fed the`Kennebec-OCI' potato. Parasitoids reached the adult stage faster andwere more fecund on `NYL 235-4' than control lines. The results indicatethat the different forms of potato resistance currently developed mainlyagainst the CPB had various unexpected effects on aphid parasitoidfitness. These effects on the parasitoid were complex but were generallyinterpretable in terms of host aphid quality variation among potatolines used as food by the aphids during parasitoiddevelopment.     

Body size variation in <Emphasis Type="Italic">Gryon gallardoi</Emphasis> related to age and size of the host

This study aimed to evaluate the influence of Spartocera dentiventris (Berg) (Hemiptera: Coreidae) egg quality (regarding age, size and superparasitism) on male and female body size of Gryon gallardoi Brèthes (Hymenoptera: Scelionidae). It was also analysed host size influence on offspring sex choice and female oviposition order (within egg groups). Groups of 12 eggs, 2, 3, 4, 5, 6, 7, 8, and 12 days old were individually exposed to a female parasitoid for 2 h, and the order of each egg parasitisation was recorded. Size of adult parasitoids (head width and tibia length of the second leg pair), and parasitised and superparasitised host eggs were measured. Sexual dimorphism related to size (SDs), was estimated through a model II linear regression. Females have a larger head width but a smaller tibia length. Parasitoid adult size is significantly related to host age and size. Within an egg group, females did not exhibit preference for any egg size category either to start oviposition or to oviposit male or female eggs. The body size of both sexes diminished with host aging. There was a positive response in both sexes to host size increasing. Estimated SDs diminished with host aging and increased with host volume augmentation. The results suggest that adult body size of G. gallardoi, as well as size differences between males and females, are strongly associated to host quality.     

Influence of initial egg density and host size on the development of the gregarious parasitoid <Emphasis Type="Italic">Bracon hebetor</Emphasis> on three different host species

Bracon hebetor (Say) (Hymenoptera : Braconidae) is a gregarious parasitoid that attacks a variety of important lepidopterous pests of stored product and in the field. In this study the effect of host species, size and larval competition on parasitoid size, survival and development were investigated. In laboratory studies, wasp eggs at a range of densities, were placed on larvae of different weight of three Lepidoptera host species namely Adoxophyes orana (Fischer von Röslerstamm, Tortricidae), Plodia interpunctella (Hubner, Pyralidae) and, Lobesia botrana(Dennis & Schiffermueller, Tortricidae). On A. orana survival of immature parasitoids was very low at all densities and different host weights. On L. botrana survival progressively reduced as egg density increased at both host weights examined for this host. Survival on P. interpunctella was significantly affected by egg density but not by host weight. Initial egg density had a significant effect on the size of emerging adults from each rearing host. Smaller adult parasitoids emerged as egg density per larva increased. Larval host weight of P. interpunctella and A. orana had a significant effect on the size of emerging adult parasitoids mainly at the higher egg densities used in these experiments. The above results of host quality on fitness of parasitoid are discussed.     

Morphological observations on the preimaginal stages ofAphelinus varipes (Hym., Aphelinidae) and the effects of this parasitoid on the aphidRhopalosiphum padi (Hom., Aphididae)

Three instar larvae could be observed forAphelinus varipes. In the second and third instars, the colon which forms the largest part of the hindgut was composed of secretory cells. There was a reduction in the size and number of embryos produced by parasitized aphids. Fat cells of parasitized aphids also degenerated sooner than those of unparasitized aphids. The number of mycetocytes remained higher in parasitizedRhopalosiphum padi than in non-parasitized aphids. During the last instar ofA. varipes the host aphid developed into a mummy in which black pigments were incorporated into the exocuticle and the integument of the aphid became sclerotized.     

Costs of secondary parasitism in the facultative hyperparasitoid Pachycrepoideus dubius: does host size matter?

Although hyperparasitism frequently occur in parasitic insects, many aspects of this strategy remain unknown. We investigated possible fitness costs of hyperparasitism as influenced by host size. Our study was conducted with the facultative hyperparasitoid Pachycrepoideus dubius Ashmead (Hymenoptera: Pteromalidae), which parasitizes host species differing greatly in size. We compared some fitness traits (level of successful parasitism, development time, sex ratio and offspring size) of P. dubius developing on large secondary/primary (Delia radicum L. (Diptera: Anthomyiidae)/Trybliographa rapae Westwood (Hymenoptera: Figitidae)) or small secondary/primary host species (Drosophila melanogaster L./Asobara tabida Nees (Hymenoptera: Braconidae)). In no-choice and choice experiments, P. dubius was able to develop on different stages of T. rapae (L2 (endophagous), L4 (ectophagous), and pupae) but that it preferred to parasitize unparasitized D. radicum pupae over pupae parasitized by T. rapae. Furthermore, in P. dubius, hyperparasitism was associated with fitness costs (lower level of successful parasitism, smaller adult size) and these costs were greater on the smallest host complex. We hypothesize that the size of D. melanogaster pupae parasitized by A. tabida may be close to the suboptimal host size for P. dubius beneath which the costs of hyperparasitism make this strategy nonadaptive. Hyperparasitism in terms of trade-offs between host quality and abundance of competitors is discussed.     

Physiological responses of Uroleucon jaceae (L.) to seasonal changes in the quality of its host plant Centaurea jacea L.: multilevel control of adaptations to the life cycle of the host

Summary Exploiters of short-lived plants have evolved strategies in response to physiological changes that occur during the development of their hosts. The ability to adapt to host quality changes is necessary particularly if the mobility of an animal is poor or risk-constrained. In the plant-aphid system Centaurea jaceae-Uroleucon jaceae, the responses of the aphid to the seasonal changes in its host plant grown in poor and good quality soil were investigated. Coarse- and fine-tuned physiological reactions were observed in discrete aphid generations which were reared on plants grown either in a growth chamber or in a greenhouse. The number of ovarioles and developmental time depended on extrinsic factors (length of photoperiod) not directly related to plant quality. The reproductive investment of the aphids was also independent of their total dry weight. However, within the gonadal system high correlations were found at the embryonic level (e.g. the number of sclerotized embryos or the length of the oldest embryos per ovariole were highly correlated with gonadal dry weight). Aphids which were living on the 4-leaf stage of high-quality host plants showed a significantly higher investment in their gonads than aphids feeding on senescenting hosts. Factor analysis corroborated that aphids reared on poor-quality hosts revealed no grouping of variables measured, whereas those which were reared on high-quality plants showed clustering in respect of somatic (tibia length, dry weight of soma) and gonadal (number of sclerotized and unsclerotized embryos, length of the oldest embryos, ovariole number, gonad dry weight) factors. Three different levels of adaptive response to plant quality are distinguished: individual response to host quality, maternal influences on offspring and response to changes not specific to habitat (photoperiod). These different levels of regulation are thought to enable the aphids to adapt to a host of a given nutritional quality and anticipate deteriorating habitat quality simultaneously. It is concluded that physiological constraints in aphids are only revealed when aphids are exposed to severe nutritional stress for several generations.     

Effect of temperature on development, growth and feeding of Coccinella septempunctata and Hippodamia convergens reared on the tobacco aphid, Myzus persicae nicotianae

Preimaginal development, mortality, aphid consumption rate, and size and weight upon reaching the adult stage of the aphidophagous coccinellids Hippodamia convergens Guérin-Méneville and Coccinella septempunctata L. collected from Karditsa, central Greece, were examined at four constant temperatures (14, 17, 20 and 23 °C) and L16:D8. The coccinellids fed on the tobacco aphid, Myzus persicae nicotianae Blackman. Egg, larval and pupal mortality was highest at 14 °C reaching 85.0, 73.8 and 29.4% in H. convergens and 49.3, 75.4 and 58.8% in C. septempunctata, respectively. Total preimaginal development ranged from 57.2 to 70.4 days at 14 °C, and to 16.9 and 22.1 days at 23 °C in H. convergens and C. septempunctata, respectively. Heavier and larger adults of H. convergens were obtained at 17 and 20 ° C. In C. septempunctata temperature did not affect adult weight while the lowest size was observed at 14 and 17 °C. Day-degrees requirements for preimaginal development in H. corvengens were 212.9 above a developmental threshold of 11.0 °C. The corresponding values for C. septempunctata were 281.5 and 10.7 °C. In H. convergens total and daily aphid consumption ranged from 46.8 aphids at 14 °C to 85.0 aphids at 23 °C and from 1.5 aphids at 14 °C to 9.2 aphids at 23 °C, respectively. The corresponding values for C. septempunctata were 112.0 and 2.7 at 14 °C and 157.7 and 12.4 at 23 °C. The results show the high potential of both predators as biological control agents against the tobacco aphid. The knowledge obtained could be essential for their appropriate use and for the improvement of mass rearing systems.     

Hymenopterous parasitoids of bark beetles [Scolytidae] in Israel: Relationships between host and parasitoid size,and sex ratio

Information on parasitoids of bark beetles infesting conifers and broadleaves is presented to show the relations between host and parasitoid size, and parasitoid sex ratios. Sticky traps were employed to determine the sex ratios of parasitoid species operating on beetle-infested material. A wide range in body length was found for most species. Females ofCerocephala eccoptogasteri Masi,Cheiropachus quadrum Febr.,Eurytoma morio Boheman,Heydenia pretiosa Forster,Metacolus unifasciatus Forster,Rhaphitelus maculatus Walker andRoptrocerus xylophagorum (Ratz.) were usually significantly longer than their males, whereas the males of all fourDendrosoter spp. were significantly longer than the females. Body lengths of the males and females were almost identical inEntedon ergias Walker andEcphylus caudatus Rusch. The sex ratio varied markedly within most species; the mean ratio for 5 of them differed significantly from 1∶1. The sex ratio ofM. unifasciatus andE. morio was significantly related to host size. The sex-ratio of parasitoids trapped on sticky traps varied with time in relation to the phenology of the host beetle. Females were more numerous during the larvae development while males were more abundant between the pupal stage and emergence of the host. Male courtship behaviour ofDendrosoter caenopachoids Rusch.,D. protuberans (Nees),M. unifasciatus andR. xylophagorum is described. The effect of host size, the diversity in sex ratios, and possible interaction between parasitoid size and sex-ratio are discussed. Contribution from the Agricultural Research Organization, Bet Dagan, Israel. N° 1328-E, 1984 series.