Inheritance of resistance to lettuce root aphid in the lettuce cultivars 'Avoncrisp' and 'Lakeland

The inheritance of resistance in two lettuce cultivars to lettuce root aphid, Pemphigus bursarius, was studied in a series of laboratory and field experiments at Wellesbourne between 1989 and 1992. A source of total resistance in the cv. ‘Avoncrisp’ which is linked to the downy mildew resistance gene Dm6, was shown to be governed by a single dominant gene. There were no maternal effects evident in the inheritance of this resistance. The basis of the high level of resistance which exists in the cv. ‘Lakeland’ (formerly known as ‘Jubilee’) was also shown to be controlled by the same dominant gene. The linkage between Dm6 and root aphid resistance was broken in ‘Lakeland’ as this cultivar does not possess the Dm6 gene. The linkage was presumably broken when the original cross between the parents of cv. ‘Lakeland’, ‘Calmar’ and ‘Avoncrisp’ was made. Under laboratory conditions small numbers of aphids commence development on cv. ‘Lakeland’ but colonies fail to develop and under field conditions the resistance provides adequate Protectión against the pest. The resistance in both ‘Avoncrisp’ and ‘Lakeland’ was effective against a population of lettuce root aphid collected from an endive crop in southern France as well as being effective against the Wellesbourne population of this aphid.     

Host-associated populations in the lettuce root aphid, Pemphigus bursarius (L.)

Pemphigus bursarius is a host-alternating aphid in which annual rounds of sexual reproduction on its primary host, Populus nigra, are interspersed with parthenogenesis on a range of secondary hosts. Evidence was sought for the existence of genetically distinct populations, associated with different secondary hosts, in P. bursarius. Microsatellite markers revealed that genetically distinct populations were present on three different secondary host species. Microsatellites were also used, in conjunction with mitochondrial DNA sequence variation, to investigate the relationships between aphids on Populus, following sexual reproduction, and those on the secondary hosts. Evidence was found for a distinct, cyclically parthenogenetic population that exploited Lactuca sativa as its secondary host. In contrast, populations associated with Matricaria inodora appeared to be largely composed of obligate parthenogens or may even have been another species of Pemphigus. Populations on Lapsana communis appeared to be a mixture of cyclical and obligate parthenogens and were more genetically heterogeneous than those on other secondary hosts, possibly due to founder effects. Experiments to measure the performance of P. bursarius clones on different secondary hosts were inconclusive, failing to demonstrate either the presence or absence of adaptations to secondary hosts.     

Induction of winged sexuparae in root-inhabiting colonies of the aphid Pemphigus betae

Abstract. Growth chamber experiments were conducted to determine what environmental factors induce the development of sexuparae in root-feeding colonies of the aphid Pemphigus betae Doane (Aphididae: Pemphiginae). Experimental colonies were derived from clones that had been reproducing for at least 15 months under constant laboratory conditions. At constant 20–23^oC, sexuparae were almost never produced regardless of the level of crowding or photoperiod. In contrast, every culture of several clones produced sexuparae when subjected to temperatures alternating between 20 and 15^oC during the light and dark phases respectively. This fluctuating temperature regime resulted in sexupara production regardless of photoperiod. All cultures also produced some sexuparae when cultures were kept in constant darkness at temperatures that were either alternated between 20 and 15^oC or kept constant at 15^oC. Constant 17^oC resulted in sexupara production in most cultures. At 17^oC, experimental manipulation of densities within cultures showed that crowding promotes sexupara production. Thus, within the range of conditions occurring at the time of sexupara induction in the field, photoperiod has no effect on sexupara production, temperature below a threshold between 17 and 20–23^oC triggers sexupara production, and crowding promotes sexupara production but only if temperature is low enough.     

Population genetic structure of the lettuce root aphid,Pemphigus bursarius (L.), in relation to geographic distance,gene flow and host plant usage

Microsatellite markers were used to examine the population structure of Pemphigus bursarius, a cyclically parthenogenetic aphid. Substantial allele frequency differences were observed between populations on the primary host plant (collected shortly after sexual reproduction) separated by distances as low as 14 km. This suggested that migratory movements occur over relatively short distances in this species. However, the degree of allele frequency divergence between populations was not correlated with their geographical separation, indicating that isolation by distance was not the sole cause of spatial genetic structuring. Significant excesses of homozygotes were observed in several populations. Substantial allele frequency differences were also found between aphids on the primary host and those sampled from a secondary host plant after several parthenogenetic generations at the same location in two successive years. This could have been due to the existence of obligately parthenogenetic lineages living on the secondary host or genetically divergent populations confined to different secondary host plant species but sharing a common primary host.     

Parturition and alate morph determination in the aphid Megoura viciae

In reproducing apterae of Megoura viciae, parturition is often completely arrested during periods of isolation from the host plant. In contrast, surgical removal of the rostrum (including the stylets), amputation of the extremities of the legs, or decapitation, all stimulate parturition away from the plant. These operations also induce alata-producing aphids to revert immediately to the production of apterae, but have no detectable effect on aptera-producers. Carbon dioxide or ether anaesthesia and nitrogen narcosis have a similar action on this maternally controlled response. Although the rostrum and tibio-tarsus bear sensilla whose removal might well be involved in inducing parturition, the influence on morph change is probably indirect and is to a great extent associated with the delay in the resumption of parturition. The effect can be reproduced by isolating individual aphids away from the food plant. The morph change cannot, however, be attributed to starvation since it also occurs when the genital pore of an actively feeding aphid is temporarily occluded. The change in physiology appears to be associated with the retention of embryos at a time when there is no sensory input from crowding.

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Résumé Chez les Megoura viciae aptères, la parturition est souvent complètement arrêtée pendant les périodes de séparation de la plante hôte. Par opposition, l'amputation chirurgicale du rostre (y compris les stylets), de l'extrémité des pattes (tarse et une partie du tibia), ou la décapitation, stimulent toutes la parturition en l'absence de la plante. Les pucerons induits à produire des ailés (élevage antérieur en groupe) retournent, après ces opérations, immédiatement vers la production d'aptères. Le dioxyde de carbone ou l'anesthésie à l'éther et la narcose à l'azote ont une action semblable sur ce déterminisme maternel. Bien que l'élimination des sensilla portées par le rostre et par l'ensemble tarse-tibia puisse être déterminante dans l'induction de la parturition, l'action sur le changement de type semble être principalement associée au retard consécutif dans la reprise de la parturition. Les anesthésiants qui, eux aussi, retardent l'apparition ou la reprise de la parturition, ont probablement une action indirecte du même type.Les pucerons groupés, isolés de la plante hôte pendant plus de 24 h, ont aussi tendance à retourner immédiatement à la production d'aptères. Ce changement de type ne peut, cependant, être attribué au jeûne puisqu'il se produit aussi quand, chez un puceron s'alimentant activement, le pore génital est momentanément bouché. Le changement physiologique semble être associé à la rétention des embryons à un moment où il n'y a pas l'influence sensorielle du groupement.Aucun de ces traitements, à l'exception du groupement, n'induit des pucerons antérieurement isolés à devenir des producteurs d'ailés.

     

Defensive Behavior in Primary- and Secondary-Host Generations of the Soldier-Producing Aphid, Pemphigus bursarius (Hemiptera: Aphididae)

The genus Pemphigus comprises several species that produce soldiers (defensive morphs) in galls on the primary host. At the moment, it is unclear if host-alternating species also produce defenders on their secondary host. We therefore examined how P. bursarius morphs of the secondary host generations respond to predators to test whether they show defensive behavior. We further examined how this response compares with the antipredator behavior of soldiers in P. bursarius and P. spyrothecae occurring on the primary host. We performed two manipulative experiments using two predatory species to quantify the behavior of the different morphs in response to predators. In both experiments, secondary host morphs of P. bursarius showed no attacking behavior and antipredator behavior in these morphs was limited to escaping natural enemies by walking away. In contrast, the first instars of the primary host generations in both Pemphigus species showed attacking behavior and were capable of killing the predators.     

Latitudinal variation in life-cycle characteristics of Potamogeton pectinatus L.: vegetative growth and asexual reproduction

Across latitudinal gradients, environmental conditions that influenceplant growth and reproduction largely change. Here we study clonal variation inlife-cycle characteristics of the cosmopolitan water plantPotamogetonpectinatus L. across a broad latitudinal range.Two consecutive experiments were performed under standardised laboratoryconditions (photoperiod, irradiance and temperature). In the first experimentweinvestigated asexual reproduction among fifteen clones, obtained from latitudesranging from 24 to 68° N. After 90 days of growth, high-latitude clonesproduced more but smaller tubers, while the aboveground biomass was lower ascompared to the clones obtained from low latitudes.In a second experiment we studied inherent differences in early growth,morphology and photosynthesis for eleven clones (obtained from the samelatitudinal range as in experiment 1). We found high among clonal variation formost measured variables, but the number of latitude-correlated traits waslimited. The only trait that correlated with latitude was the number of leavesper plant, which increased in clones from higher latitudes.Our results agree with the hypothesis of a latitude-correlated switch inlife-cycle strategy for this species. For northern clones this results in ashort life-cycle, with an early and high investment in tuber biomass, while forlow latitude clones the length of the life-cycle is prolonged, with a delayedreproduction and increased total plant biomass.     

Gyne development in the Argentine ant Iridomyrmex humilis: role of overwintering and queen control

Abstract A possible stimulatory effect of overwintering on gyne development in Iridomyrmex humilis (Mayr) was investigated. Although gyne-potent larvae are present in the nest throughout the year, small queenless colony fragments composed of freshly overwintered ants (collected in late winter or early spring) produced 3–8 times more gynes than comparable fragments made up of non-freshly overwintered ants (collected at other times of the year). Apparently, this stimulatory effect of overwintering acts on both the developmental potential of larvae and the tendency of workers to rear sexually competent larvae as gynes; queenless colony fragments in which either the brood or workers were freshly overwintered produced more gynes than fragments composed of non-freshly overwintered workers or brood but fewer than fragments containing both brood and workers freshly overwintered. This increased sexualization potential of larvae due to overwintering is enough to overcome weak pheromonal inhibition of queens associated with low queen number; a single queen in a large freshly overwintered colony fragment is insufficient to inhibit gyne development, whereas ten queens are totally inhibitory. In non-freshly overwintered colony fragments one queen is completely inhibitory. Variability in egg developmental potential according to queen age does not appear to play a major role in the seasonal production of gynes, because at least some eggs of very young queens (less than 3 weeks old) are capable of gyne development. In the field this stimulatory effect of overwintering is superimposed on a seasonal fluctuation in the combined strength of pheromonal queen control. In southern France, gynes are produced only in spring where they arise primarily from overwintered larvae just after a sharp drop in queen number, and presumably the total level of inhibitory queen pheromone, due to the massive execution of queens by workers.     

Host plant selection by migrant Rhopalosiphum padi in autumn and the occurrence of an intermediate morph

Alate female Rhopalosiphum padi (L.) were trapped alive in suction traps in autumn, 1986–1988. Alate exules and gynoparae were identified by the morph of their offspring. Gynoparae largely replaced alate exules in September indicating that most of the population were holocyclic even in southern England where the primary host, Prunus padus L., is scarce. However, a few alate exules occurred throughout the autumn indicating that a proportion of the population is anholocyclic. A morph intermediate between alate exules and gynoparae occurred at low frequency throughout the autumn. Alate exules and gynoparae preferred barley and P. padus, respectively, on which to settle and larviposit; gynoparae were less selective than exules possibly due to an incomplete physiological switch from a preference for Gramineae to P. padus on which the sexual cycle is completed. Intermediate individuals preferred to settle on P. padus, but their virginoparous and oviparous nymphs were produced on barley and P. padus respectively. Intermediate morphs maintained under continuous short day conditions (L12 : D12) produced mainly apterous virginoparae with males occurring at the end of their reproductive live; intermediates occurred occasionally amongst the apterae. It is proposed that intermediates are a programmed component of the morph structure and have the potential to contribute to both the holocyclic and anholocyclic portions of the population.

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Résumé Des femelles de R. padi L. ont été capturées vivantes dans des pièges a succion en 1986–88. Les ailés exules et les gynopares ont été identifiés par le morphe de leurs descendants. Les gynopares ont remplacé généralement les ailés exules en septembre, ce qui montre que la majorité de la population est holocyclique même dans le sud de l'Angleterre où l'hôte primaire, Prunus padus L., est rare. Cependant, quelques ailés exules se maintiennent pendant l'automne montrant qu'une partie de la population est anholocyclique. Au cours de l'automne, on observe un morphe intermédiaire entre les ailés exules et les gynopares. Les exules ailés et les gynopares préfèrent respectivement l'orge et P. padus, sur lesquels ils s'installent et produisent des larves; les gynopares étaient moins sélectifs que les exules, peut-être par suite d'une rupture physiologique incomplète de leur préférence pour l'orge sur laquelle ils se développent rendant moins impérative leur attraction par P. padus sur lequel leur cycle sexuel est complet. Les individus intermédiaires préfèrent s'établir sur P. padus, mais leur larves virginipares et ovipares sont produites respectivement sur oreg et sur P. padus. Les morphes intermédiaires maintenus en jours courts (L12/N12) ont donné surtout des virginipares aptères les mâles apparaissant à la fin de leur vie; des intermédiaires sont apparus occasionnellement parmi les aptères. Les intermédiaires seraient une composante programmée de la structure polymorphe et contribueraient à la fois à l'apparition des fractions holocyclique et anholocyclique de la population.

     

Hormonal control of seasonal morph determination in the swallowtail butterfly, Papilio xuthus L. (Lepidoptera: Papilionidae)

Seasonal morphs (spring and sumer forms) of Papilio xuthus L. are determined coincidentally with diapause and non-diapause in pupae by larval exposure to short days and long days respectively. The neuroendocrine principle underlying seasonal-morph determination was studied using surgical operations in P. xuthus.When recipient 0-day old or chilled diapause pupae were joined to donor 0-day old non-diapause pupae, the recipients developed into summer or intermediate morphs. When the same kinds of recipients used above were joined to 0-day old or chilled diapause pupae, there were no significant effects on the adult morph. In contrast, recipient non-diapause pupae all developed into summer morphs, regardless of groups of the type of donors.Furthermore, when the brain of 5th-instar larvae, pharate pupae or pupae, predetermined to be diapause, was transplanted into the abdomen of 0-day old, 30-day old or chilled diapause pupae, the recipients developed into summer or intermediate morphs.The results indicate that the brain of non-diapause pupae secretes a humoral factor producing the summer morph. In non-diapause pupae, the factor may be secreted at about the stage of larval-pupal ecdysis coincidentally with that of prothoracicotropic hormone.     

Sex determination and sex ratio in the dioecious shrub Salix viminalis L.

 Various ecological factors (e.g. herbivory, difference between males and females in colonising ability) have been invoked to explain female-biased sex ratios in populations of willow species. It was implicitly assumed that genetic factors would lead to a balanced sex ratio in the absence of ecological disturbances. In an experiment carried out in a homogeneous environment and in the absence of herbivores the progeny sex ratio of 13 crosses of basket willow (Salix viminalis L.) was observed to range from extreme female bias to extreme male bias. The observed sex ratio cannot be explained by the presence of sex chromosomes without assuming that additional loci are also involved in the sex determination. Alternatively, the sex ratios in this study can be explained by a sex determination mechanism governed by multiple independent loci. Received: 1 February 1996 / Accepted: 14 June 1996     

Costs and benefits of hydroxamic acids-related resistance in winter wheat against the bird cherry-oat aphid, Rhopalosiphum padi L.

Estimations of infestation by the bird cherry-oat aphid (Rhopalosiphum padi) as well as measurements of grain yield in 26 Hungarian winter wheat cultivars under field conditions were correlated with the concentration of hydroxamic acids (Hx) in seedlings of those cultivars. The significant inverse relationship between infestation ratings and Hx levels in wheat showed that Hx, despite their decreased accumulation at later plant phenological stages, may be able to confer resistance against aphid infestation in the field. Since no significant relationship was found between grain yield and Hx levels in plants it is suggested that Hx accumulation does not impose a cost to the plant in terms of yield. These findings support earlier claims stressing the potential of Hx as breeding targets for aphid resistance in wheat.     

Geographic and clonal variation in the milkweed-oleander aphid,Aphis nerii (Homoptera: Aphididae), for winged morph production,life history,and morphology in relation to host plant permanence

Summary Populations of the milkweed-oleander aphid,Aphis nerii, were sampled in California, Iowa and Puerto Rico. Among these localities the aphid's host plants differ greatly in permanence. I compared populations for migratory potential, measured as the proportion of winged offspring produced in response to being crowded, and for life history and morphometric traits of the subsequent adult winged aphids. I predicted a negative correlation between degree of host plant permanence and migratory potential. As predicted, aphids from Iowa, where migration on to temporary hosts must occur each year, produce a greater proportion of winged offspring (37.7%) than those from California (25.7%) or Puerto Rico (31.6%) where hosts are more permanent. However, hosts in Puerto Rico appear to be more permanent than those in California, yet the difference between populations for migratory potential was opposite to that predicted. Within California the prediction again held: aphids collected from the most impermanent sites produce the greatest proportion of winged offspring. There were no population differences for any life history or morphometric traits of winged aphids that are important contributors to fitness or migratory ability such as time to reproductive maturity, fecundity or wing length. Nor did any traits covary with migratory potential. Thus, there does not appear to be an association of life history and morphology with migratory potential that could enhance the colonizing ability of migrant aphids. I was unable to detect population differentiation for life history and morphology even though there is ample genetic variation within populations on which selection could act and an absence of constraints arising from genetic correlations that could prevent appropriate evolution of traits within populations. The exploitation of temporary host plants therefore occurs by an increase in the number of colonists produced and not by change in life history or morphology of those colonists.     

Freeze-protection of overwintering monarch butterflies in Mexico: critical role of the forest as a blanket and an umbrella

Abstract. 1. At their high-altitude overwintering sites in Mexico, monarch butterflies frequently are subjected to sub-zero°C temperatures during December-March. Although monarchs have moderate supercooling ability, two ecological factors strongly influence their capacity to resist freezing: wetting and exposure to the clear night sky. 2. As shown in Fig. 2, 50% of a population of butterflies with water on their body surfaces freeze at warmer sub-zero temperatures (-4.2°C) compared to butterflies with no water on their bodies (-7.7°C). 100% mortality occurs, respectively, at ?7.7°C and ?15°C. 3. Comparative measurements of rainfall within a large overwintering colony in Mexico indicated that the intact canopy acts as an umbrella that reduces butterfly wetting during winter storms. 4. Variable experimental exposure of butterflies to the clear night sky indicated that openings in the forest canopy increases radiational cooling and causes monarch body temperatures to drop as much as 4°C below ambient air temperature. Monarchs under dense cover had body temperatures approximately the same as the ambient air temperature, but more exposed individuals had body temperatures below ambient in direct proportion to the degree of exposure. Consequently, forest thinning increases the probability that the butterflies will freeze to death. 5. Whereas both wetting and exposure are increased by disturbance of the forest canopy, the interaction of these two factors exacerbates freezing mortality during winter storms: 50% of dry and unexposed butterflies froze at ?8°C, whereas wetted and fully exposed butterflies froze at only ?0.5°C. 6. Butterflies inside and on the bottom of the fir bough clusters are better protected from wetting than those on the outside. This supports the hypothesis that the structure of the butterfly clusters has evolved through individual selection to avoid wetting. 7. The data strongly reinforce previous evidence that forest thinning should be totally prevented within and adjacent to the overwintering sites in order to minimize both wetting and exposure of the butterflies that synergistically increase winter mortality at the overwintering sites in Mexico.     

Induction and floral determination in the terminal bud of Nicotiana tabacum L. cv. Maryland Mammoth,a short-day plant

Floral determination in the terminal bud of the short-day plant Nicotiana tabacum cv. Maryland Mammoth has been investigated. Plants grown continuously in short days flowered after producing 31.4±1.6 (SD) nodes while plants grown continuously in long days did not flower and produced 172.5±9.5 nodes after one year. At various ages, expressed as number of leaves that were at least 1.0 cm in length above the most basal 10-cm leaf, one of three treatments was performed on plants grown from seed in short days: 1) whole plants were shifted from short days to long days, 2) the terminal bud was removed and then rooted and grown in long days, and 3) the terminal bud was removed and then rooted and grown in short days. Whole plants flowered only when shifted from short days to long days at age 15 or later. Only rooted terminal buds from plants at age 15 or older produced plants that flowered when grown in long days. Only terminal buds from plants at age 15 or older that were rooted and grown in short days produced the same number of nodes as they would have produced in their original locations while buds from younger plants produced more nodes than they would have in their original locations. Thus, determination for floral development in the terminal bud, as assayed by rooting, is simultaneous with the commitment to flowering as assayed by shifting whole plants to non-inductive conditions.Abbreviations LD long day(s) - SD short day(s) - DN dayneutral     

Host range testing and life history of the defoliator Hymenomima nr. memor: an unsuitable biological control agent for Schinus terebinthifolia in the U.S.A.

Host range of larvae of Hymenomima nr. memor (Lepidoptera: Geometridae) was examined in quarantine to evaluate its suitability as a biological control of Brazilian peppertree, Schinus terebinthifolia. Brazilian peppertree, S. terebinthifolia is an environmental and agricultural weed from South America that had invaded many subtropical and tropical areas of the world including Florida and Hawaii, USA. Laboratory life history and quarantine host range studies of H. memor were conducted with no-choice feeding tests. These tests included eight species of the Anacardiaceae and one species of Sapindaceae. Larvae of H. memor had five to six instars with each head capsule width increasing by 1.68-X. Development time from neonate to adult was 46.7?±?2.2 days. In host range tests, neonates completed development to the adult stage on all non-target species, except Toxicodendron radicans. Moreover, developmental times were delayed and pupal weights were reduced for larvae fed Spondias purpurea leaves. Due to the broad host range exhibited by H. memor larvae, this species will not be considered as a biological control agent of S. terebinthifolia in the continental U.S.A.     

In vivo nuclear-magnetic-resonance analysis of polyamine and alkaloid metabolism in transformed root cultures of Datura stramonium L.: evidence for the involvement of putrescine in phytohormone-induced de-differentiation

The utilisation and accumulation of ¹⁵N-labeled metabolites by a ¹⁵N-labeled transformed root culture of Daturastramonium L. was investigated by in vivo ¹⁵N-nuclear-magnetic-resonance (NMR) spectroscopy. After resuspension in spent growth medium, the pools of [¹⁵N]glutamate and [¹⁵N]glutamine were rapidly depleted and there was an increase in the ¹⁵N-NMR signals from conjugated putrescines and hyoscyamine. The signal from the conjugated putrescines passed through a maximum 2 d after the roots were resuspended, and it was concluded that putrescine could be stored as putrescine conjugates prior to its utilisation in other pathways. The transient accumulation of ¹⁵N-label in the hydroxy-cinnamoylputrescines was reduced when the de-differentiation of the root cultures into a suspension culture was initiated by exposure to a medium containing α-napthaleneacetic acid and kinetin. This led to the hypothesis that phytohormone-induced de-differentiation of the root cultures required the presence of free polyamines, and this was tested using two potent inhibitors of putrescine biosynthesis, dl-α-difluoromethylarginine and dl-α-difluoromethylornithine. In-vivo ¹⁵N-NMR spectra of roots grown in ¹⁵N-enriched medium supplemented with these inhibitors showed that the ¹⁵N-labelling of the conjugated polyamines and hyoscyamine was markedly reduced. dl-α-difluoromethylarginine also prevented the phytohormone induced de-differentiation of the root cultures, and this effect could be reversed by the supply of exogenous putrescine. Thus the supply of putrescine appears to play a crucial role in mediating the phytohormone induced de-differentiation of the root culture. Received: 13 September 1997 / Accepted: 12 November 1997     

Introduction, Rearing, and Host Range of Aerenicopsis championi Bates (Coleoptera: Cerambycidae) for the Biological Control of Lantana camara L. in Australia

The biology and host range of the cerambycid beetle Aerenicopsis championi Bates, a potential biological control agent for the weed Lantana camara L., were studied. A. championi is a univoltine species associated with L. camara, L. urticifolia, and L. hirsuta in Mexico and Central America. In Mexico, adult emergence occurred in May and June at the start of the rainy season. Larvae fed within the stems over a 9- to 12-month period and caused damage to the plant. The insect was imported into Australia, where a procedure for rearing it in the laboratory was developed. Host-range tests indicated that adults oviposited and larvae commenced development in L. camara and L. montevidensis but not in any of 57 other species tested. A CLIMEX model indicated that most areas infested with lantana in Australia would have a favorable climate for A. championi. Permission to release this insect in Australia was obtained and three small releases were made in southern Queensland in February 1995.