How well can relative specialistRhagoletis flies learn to discriminate fruit for oviposition?

We show that blueberry maggot females [Rhagoletis mendax(Curran)], apple maggot females [Rhagoletis pomonella(Walsh)], and walnut husk fly females [Rhagoletis suavis(Loew)], all relative specialists in range of fruit species attacked in nature, are able to learn to discriminate between types of fruit in which they oviposit. It appears, however, that these relative specialists express less capacity to learn fruit characters than relative generalist tephritids. This difference in expression of learning ability may be associated in part with differences in assignments species are asked to learn. Apparent differences in learning capability between relative specialist and relative generalist tephritids may therefore depend as much upon differences in the physical and chemical nature of host fruit as upon species differences in the adaptive value of learning.     

The chemosensory basis for behavioral divergence involved in sympatric host shifts. I. Characterizing olfactory receptor neuron classes responding to key host volatiles

The recent shift of Rhagoletis pomonella from its native host hawthorn to introduced, domestic apple has been implicated as an example of sympatric speciation. Recent studies suggest that host volatile preference might play a fundamental role in host shifts and subsequent speciation in this group. Single sensillum electrophysiology was used to test a proposed hypothesis that differences in R. pomonella olfactory preference are due to changes in the number or odor specificity of olfactory receptor neurons. Individuals were analyzed from apple, hawthorn, and flowering dogwood-origin populations, as well as from the blueberry maggot, Rhagoletis mendax Curran (an outgroup). Eleven compounds were selected as biologically relevant stimuli from previous electroantennographic/behavioral studies of the three R. pomonella populations to host fruit volatiles. Cluster analysis of 99 neuron responses showed that cells from all tested populations could be grouped into the same five classes, ranging from those responding to one or two volatiles to those responding to several host volatiles. Topographical mapping also indicated that antennal neuron locations did not differ by class or fly taxa. Our results do not support the hypothesis that differences in host preference among Rhagoletis populations are a result of alterations in the number or class of receptor neurons responding to host volatiles.     

Rhagoletis sibling species and host races differ in host odor recognition

Electronantennograms (EAG) were recorded from the apple and hawthorn host race of the apple maggot fly, Rhagoletis pomonella (Walsh), and from the blueberry maggot fly, R. mendax (Curran) (Diptera: Tephritidae), in response to host fruit extracts and nine volatile host fruit odor compounds at six concentrations. Mean relative EAG response to apple odor is the same in both species, but in respect to blueberry odor, it is significantly stronger in R. mendax than in both host races of R. pomonella (P<0.05), indicating that antennal sensitivity is selectively adapted to species specific host fruit odors. Differences in antennal response to several host fruit odor compounds were found between both species as well as between the host races. This indicates differences in antennal receptor cell types and/or numbers between species and host races. The flies had no prior host fruit experience which indicates that the measured differences are genetically based. Because Rhagoletis fruit flies are highly host specific parasites which meet and mate on their respective host plants, the results suggest that antennal sensitivity plays an important role in host shifts and speciation in this genus.

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Zusammenfassung Elektroantennogramme (EAG) der Apfel- und der Weissdorn-Wirtsrasse der Apfelfruchtfliege, Rhagoletis pomonella, und der Blaubeerenfruchtfliege, Rhagoletis mendax (Diptera: Tephritidae), wurden aufgezeichnet als Reizantworten auf Wirtsfruchtextrakte und auf neun flüchtige Duftkomponenten ihrer Wirtsfrüchte in sechs Konzentrationen. Die Sensitivität der Antennen bezüglich Apfelduft ist in beiden Arten gleich, aber die Reizantwort von R. mendax auf den Duft ihrer spezifischen Wirtsfrucht, Blaubeeren, ist signifikant stärker als diejenige beider Wirtsrassen von R. pomonella (P<0.05), was darauf hinweist, dass die antennale Sensitivität möglicherwiese an den artspezifischen Wirtsfruchtduft adaptiert ist. Unterschiede in der antennalen Reizantwort auf mehrere Duftkomponenten der Wirtsfrüchte konnten sowohl zwischen den Arten als auch Wirtsrassen gefunden werden. Dies deutet auf Unterschiede in antennalen Rezeptorzelltypen und/oder Rezeptorzellzahl zwischen Arten und Wirtsrassen. Die Unterschiede wurden von Individuen aufgezeichnet, die keine vorherige Erfahrung mit den Wirtsfrüchten hatten und sind daher genetischen Ursprungs. Fruchtfliegen der Gattung Rhagoletis sind stark wirtsspezifische Pflanzenparasiten, die ihre Wirtspflanze als Treffpunkt und Paarungsort benützen. Diese Resultate deuten daher darauf hin, dass antennale Sensitivität eine wichtige Rolle in Wirtswechseln und Speziation in diesem Genus spielt.

     

The effect of prior adult experience on components of habitat preference in the apple maggot fly (Rhagoletis pomonella)

Summary Numerous authors have suggested that genetic subdivision within a population in a heterogeneous environment is more likely if individuals tend, through prior experience, to breed in the same habitat in which they developed. Under semi-field conditions we demonstrate that prior adult experience alters habitat preference in the apple maggot fly, Rhagoletis pomonella (Tephritidae), a frugivorous parasitic insect thought to have undergone sympatric divergence in host use in historical times. Females exposed to a particular host fruit species — apple (Malus pumila) or hawthorn (Crataegus mollis) — in a field cage oviposited at a higher rate in test fruit of that species than did inexperienced females or females exposed to the other species. Females exposed to a particular host fruit species also tended to remain longer in test trees harboring fruit of that species than did inexperienced females or females exposed to the other species. Prior adult experience thus alters two components of habitat preference in the apple maggot fly: oviposition preference and habitat fidelity. We discuss how these effects of experience on habitat preference should increase the likelihood that individuals mate assortatively and may further increase the likelihood that apple maggot populations become genetically subdivided.     

Prior experience affects the visual ability ofRhagoletis pomonella flies (Diptera: Tephritidae) to find host fruit

Females of the apple maggot fly,Rhagoletis pomonella, were allowed for 3 days to alight upon and oviposit in green or red 18- to 20-mm hawthorn host fruit (Crategus mollis) or green or red 45- to 55-mm apple host fruit (Malus pumila) hung from branches of potted host trees in field enclosures. Subsequently, when females were released individually on potted host trees harboring fruit of one of these types, their ability to find fruit of unfamiliar size proved unaffected by prior experience with fruit but their ability to find fruit of unfamiliar color was significantly affected. Specifically, females exposed to red hawthorns or red apples were less able to find green hawthorns or green apples than were females experienced with either of the latter fruit types. Fruit odor was found to have no effect on female ability to find familiar compared with unfamiliar green fruit. In contrast, a difference in size (or surface chemistry) between familiar and unfamiliar fruit but not a difference in fruit color had a significant negative influence on the propensity of alighting females to bore into unfamiliar fruit. Three bouts of experience with alighting upon and ovipositing into fruit over a period of about 1 h had no detectable effect on female ability to find unfamiliar fruit but did reduce propensity to bore into unfamiliar fruit. Our findings are discussed in relation to insect ability to learn visual and chemical stimuli of resources and insect propensity to form host races. We also discuss the potential impact of our findings on nonpesticidal, behavioral methods of managingR. pomonella in commercial apple orchards.     

Apple maggot fly response to red sphere traps in relation to fly age and experience

Responses of the apple maggot fly,Rhagoletis pomonella (Walsh), to 8 cm red sticky sphere traps, baited with the synthetic fruit odor butyl hexanoate, were investigated in field-caged apple trees containing green or red Gravenstein apples. Trap capture rate and the probability of oviposition in apples before capture generally increased with female age and number of mature eggs in ovaries. Two days of pre-test exposure of mature females to red hawthorns, green Red Delicious apples or green Gravenstein apples had no significant effect on the likelihood of a fly finding a red sphere. However, before capture on a sphere or departing a tree, hawthorn-exposed females found significantly fewer apples and laid significantly fewer eggs than females exposed to Gravenstein apples. Variation in duration of pre-test exposure (1–4 days) of flies to Gravenstein apples had no detectable influence on female response to apples or to a red sphere in a test tree. The relevance of these findings to effectiveness of sphere traps, forR. pomonella control in commercial orchards is discussed.     

Prior experience influences the fruit residence of male apple maggot flies,Rhagoletis pomonella

Male apple maggot flies spend considerable time residing on individual host fruit as territories on which they force-copulate arriving females in search of oviposition sites. Here, we present evidence from investigations in nature and the laboratory that shows the propensity of males to reside on a hawthorn or apple fruit as a territory is significantly modifiable through prior experience with fruit and, hence, involves learning. Previous studies revealed that after a female apple maggot fly, Rhagoletis pomonella, arrived on a host hawthorn or apple fruit, its propensity to accept or reject that fruit for egg-laying was similarly modifiable through prior fruit-exposure experience and also involved learning. We discuss how host fruit learning in males and females, in concert with genetic-based differences in host fruit residence and acceptance behavior between populations of flies originating from hawthorn and apple, could give rise to a reduction in gene flow between populations of flies on these two host types.     

Behavioral differences between hawthorn-origin and apple-originRhagoletis pomonella flies in patches of host trees

We compared patterns of patch residence and oviposition ofRhagoletis pomonella (Diptera: Tephritidae) females that originated as larvae from hawthorn or apple trees and were released into patches of host trees containing hawthorn or apple fruit in an open field. There were no detectable differences between hawthorn-origin females in patches of hawthorns and apple-origin females in patches of apples in numbers of females observed on food, fruit or foliage or in numbers of eggs laid in fruit during the course of the 7-h experimental period. Apple-origin females in patches of hawthorns behaved similarly to hawthorn-origin females in patches of hawthorns. In contrast, hawthorn-origin females in patches of apples differed significantly from apple-origin females in patches of apples. The former were observed on fruit only 40% as often and laid only 20% as many eggs before departing a patch. Our findings support the hypothesis of G. L. Bush thatR. pomonella flies of hawthorn and apple origin represent distinct host races.     

Behavioral evidence for host races in Rhagoletis pomonella flies

Summary One of the most controversial putative cases of host race formation in insects is that of the apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae). A principal cause of the controversy is lack of relevant data. In laboratory and field enclosure experiments, we compared the host acceptance behavior of sympatric populations of flies originating from naturally infested hawthorn (the native host) and apple (an introduced host) in Amherst, Massachusetts or East Lansing, Michigan. In general, hawthorn fruit were accepted for ovipositional attempts nearly equally by apple and hawthorn origin females, whereas apples were accepted much more often by apple than hawthorn origin females. Similarly, males of apple and hawthorn origin exhibited about equal duration of residence on hawthorn fruits as sites at which to acquire potential mates, while males of apple origin tended to reside substantially longer than males of hawthorn origin on apples. Irrespective of fly origin, both sexes always responded more positively to hawthorn fruit than to apples. Because all flies assayed were naive (ruling out effects of prior host experience of adults) and because tests revealed no influence of pre-imaginal fruit exposure on pattern of host fruit acceptance by females, the combined evidence suggests the phenotypic differences we observed in host response pattern between hawthorn and apple origin flies may have an underlying genetic basis. Further tests showed that while larval progeny of flies of each origin survived better in naturally growing hawthorn fruit than in naturally growing apples, there was no differential effect of fly origin on larval survival ability in either host. We discuss our findings in relation to restriction in gene flow between sympatric populations of R. pomonella and in relation to current models of host shifts in insects.     

Host fruit chemical stimuli eliciting distinct ovipositional responses from sibling species of Rhagoletis fruit flies

Laboratory experiments tested whether two economically-important sibling species of tephritid fruit flies have evolved distinct egg-laying responses to chemical stimuli on the fruits of their respective hostplants. The egg-laying preferences displayed by apple maggot flies, R. pomonella, and blueberry maggot flies, R. mendax, on artificial fruits treated with apple and blueberry extract paralleled their egg-laying responses to whole apples and blueberries. R. pomonella flies laid more eggs than R. mendax flies in artificial fruits treated with extract from ripe McIntosh apples, and vice versa for artificial fruits treated with extract from ripe Bluehaven blueberries. Furthermore, both species laid more eggs in artificial fruits treated with extract from their respective host fruits than control artificial fruits which were not treated with fruit extract. Prior electroantennogram recordings from R. mendax and R. pomonella flies exposed to volatiles from pentane extracts of apples and blueberries indicate that the antennal sensitivity of both species is selectively tuned to their respective host fruit odors. This differentiation in their olfactory responses to fruit odors could be important in mediating their distinct ovipositional responses to blueberry and apple fruits. Extract from unripe McIntosh apples also elicited egg laying by R. pomonella flies, however, artificial fruits treated with unripe apple extract received 1.9 times fewer eggs than those treated with ripe apple extract. Moreover, the numbers of R. pomonella ovipositor punctures and eggs placed in wax artificial fruits were increased when the artificial fruits were treated with a blend of 7 identified apple esters. Black coloration on these artificial fruits and the presence of apple esters had a synergistic effect on the egg-laying behavior of R. pomonella flies, which caused them to lay substantially more eggs per black fruit than white fruit treated with the same concentration of apple esters. In summary, our results indicate that the egg-laying responses of R. pomonella flies are mediated by the integration of information from fruit chemical and visual cues, and that R. mendax and R. pomonella flies have evolved divergent egg-laying responses to chemical stimuli on the fruits of their respective hostplants. These findings are discussed in the context of other studies on plant compounds which influence the ovipositional behavior of phytophagous Diptera.

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Stimuli chimiques des pommes et des myrtilles induisant la ponte des espèces jumelles, Rhagoletis pomonella et R. mendax

Résumé Des fruits artificiels en cire traités avec des extraits de fruits ont provoqué chez les espèces jumelles de R. mendax (Curran) et R. pomonella (Walsh) des réactions de ponte différentes suivant les stimulations chimiques par les fruits. Le comportement de ponte sur des fruits artificiels traités avec des extraits au pentane des myrtilles mûres (Vaccinium corymbosum L.) et de pommes mûres (Malus pumila Miller = Pyrus malus L.), est le même que sur des fruits naturels, ce qui montre que la réponse aux stimulations chimiques provenant du fruit constitue un aspect important de la reconnaissance de l'hôte. R. pomonella pond plus d'ufs que R. mendax sur les fruits artificiels traités à l'extrait de pommes mûres; c'est l'inverse pour les fruits traités aux extraits de myrtille. Les fruits artificiels traités avec des pommes ou des myrtilles provoquent la ponte de R. pomonella, tandis que les myrtilles mûres seules provoquent la ponte de R. mendax. Les extraits de pommes vertes stimulent la ponte de R. pomonella mais elle est alors 2 fois plus faible qu'avec des extraits de pommes mûres. Un mélange de 7 esters identifiés dans l'extrait de pomme induit aussi la ponte de R. pomonella. Le nombre de piqûres de tarièresfli dans les fruits artificiels en cire et le nombre d'ufs par fruit ont été augmentés par addition d'esters de pommes à des fruits blancs ou noirs. La couleur des fruits artificiels influence aussi la réaction de ponte de R. pomonella; la fréquence des piqûres de tarière contenant un uf et le nombre d'ufs par fruit étaient significativement plus élevés sur les fruits noirs que sur les fruits blancs traités avec la même concentration d'esters de pomme. Les fruits artificiels noirs traités avec la concentration la plus stimulante d'esters de pommes ont reçu 2, 3 fois plus d'ufs que les fruits blancs avec les mêmes concentrations en esters. Ces résultats montrent que les esters de pomme et la couleur noire stimulent synergiquement la ponte de R. pomonella sur des fruits artificiels.

     

The chemosensory basis for behavioral divergence involved in sympatric host shifts II: olfactory receptor neuron sensitivity and temporal firing pattern to individual key host volatiles

The Rhagoletis species complex has been a key player in the sympatric speciation debate for much of the last 50 years. Studies indicate that differences in olfactory preference for host fruit volatiles could be important in reproductively isolating flies infesting each type of fruit via premating barriers to gene flow. Single sensillum electrophysiology was used to compare the response characteristics of olfactory receptor neurons from apple, hawthorn, and flowering dogwood-origin populations of R. pomonella, as well as from the blueberry maggot, R. mendax (an outgroup). Eleven volatiles were selected as stimuli from behavioral/electroantennographic studies of the three R. pomonella host populations. Previously, we reported that differences in preference for host fruit volatile blends are not a function of alterations in the general class of receptor neurons tuned to key host volatiles. In the present study, population comparisons involving dose–response trials with the key volatiles revealed significant variability in olfactory receptor neuron sensitivity and temporal firing pattern both within and among Rhagoletis populations. It is concluded that such variability in peripheral sensitivity and temporal firing pattern could influence host preference and contribute to host fidelity and sympatric host shifts in the Rhagoletis complex.     

Genetic differentiation in the viability of sibling species of Rhagoletis fruit flies on host plants, and the influence of reduced hybrid viability on reproductive isolation

Prior theoretical studies have analyzed host-associated factors restricting gene flow between phytophagous insect species, and simulated evolutionary changes in components of fitness contributing to the adaptation of species to distinct host plants. However, there is relatively little empirical information on these topics. In this paper we report data on two host-associated sibling species of tephritid fruit flies, Rhagoletis mendax (Curran) and R. pomonella (Walsh) which infest ericaceous and rosaceous host plants, respectively. To test the hypothesis that these species have evolved viability differences on alternate hosts which can contribute to the restriction of gene flow between them, we measured the larval-to-adult viability of R. mendax, R. pomonella and F₁ interspecific hybrid progeny reared on naturally-growing, highbush blueberry (Vaccinium corymbosum L.) and apple (Malus pumila Miller = Pyrus malus L.) plants in the field. Our results indicate that genetic changes associated with the adaptation of these species to distinct host plants could also cause reduced fitness of interspecific hybrids, and thereby restrict interspecific gene flow. Fewer interspecific hybrids survived to adulthood than either R. pomonella progeny reared in apples or R. mendax progeny reared in blueberries. These differences in the viability of progeny from hybrid versus conspecific crosses can substantially restrict gene flow between R. mendax and R. pomonella flies, and may be an important factor influencing their reproductive isolation. Genetic differences among hybrid and conspecific crosses were also detected for the lengths, widths and weights and weights of pupae reared from blueberries and apples. In a second experiment, the ovipositional preferences of R. mendax and R. pomonella flies were recorded on host plants inside a field cage. R. mendax flies had a strong ovipositional preference for blueberries over apples, whereas R. pomonella flies readily oviposited in both host fruits. Other studies have determined that R. pomonella flies also oviposit in the fruits of several other plants which are not hosts under field conditions; this behavior can promote host shifts to new plants by flies with suitable plant-finding and viability traits. Our results support the theoretical assumption that divergence in host-acceptance behaviors and viabilities on host plants are key aspects of evolutionary differentiation among closely-related taxa of phytophagous parasitic insects. We discuss our results in the context of other traits that can restrict gene flow between R. mendax and R. pomonella, and in relation to the findings of other studies on the evolution of host plant use.

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Résumé Bien que des études théoriques aient analysé tant l'influence des éléments de l'adaptabilité sur l'évolution de l'utilisation d'une plante par un insecte phytophage que les caractéristiques de l'hôte limitant le flux génique entre espèces voisines, peu de données concrètes concernent ces aspects.Nos résultats portent sur deux espèces jumelles de téphritides, R. mendax Curran et R. pomonella Walsh qui vivent respectivement sur éricacées et rosacées. Pour vérifier si des différences dans le comportement d'acceptation des hôtes et dans leur survie sur des hôtes alternatifs sont apparues chez ces espèces jumelles, nous avons observé dans la nature le comportement de ponte et la survie des 2 espèces et des F₁ hybrides interspécifiques sur Vaccinium corymbosum L. et Malus pumila Miller = Pyrus mali L. Le pourcentage de pondeuses, le nombre d'ufs pondus par femelle et la survie des larves ont été plus élevés pour R. mendax que pour R. pomonella sur V. corymbosum, tandis que le contraire était observé sur M. pumila. Des différences génétiques ont été observées pour la mortalité avant la nymphose et avant la mue imaginale lors des croisements hybrides et interspécifiques. Les descendants mâles et femelles de R. mendax ont mieux survécu sur V. corymbosum, tandis que la survie de la descendance des croisements entre R. pomonella était la même sur les deux plantes. Un peu plus d'hybrides ont atteint le stade adulte sur pomme que de descendants de croisements intraspécifiques, et la mortalité des descendants de femelles de R. pomonella sur V. corymbosum était plus élevée que celle des femelles de R. mendax. Les différences de survie et de comportement de ponte de R. mendax et de R. pomonella peuvent limiter le flux génique entre les 2 espèces. Des différences génétiques peuvent être observées quant à la taille et au poids des pupes de R. mendax et R. pomonella. Ces résultats confortent l'hypothèse que des divergences évolutives du comportement d'acceptation de l'hôte et de mortalité sur hôtes respectifs sont des facteurs clés séparant des taxa très voisins d'insectes parasites phytophages. La discussion intègre les autres aspects pouvant limiter le flux génique entre R. mendax et R. pomonella et les autres hypothèses sur l'évolution de l'exploitation des plantes hôtes.

     

Effects of pyriproxyfen on ovarian development and fecundity ofRhagoletis pomonella flies

Three-day old female apple maggot flies,Rhagoletis pomonella (Walsh), were topically exposed to different doses (0.1, 1, 10, and 100 μg per fly) of a juvenile hormone mimic, pyriproxyfen, in the laboratory. Pyriproxyfen had little lethal effect on females except at the extremely high dose of 100 μg. It also had no significant effect on egg viability of treated females. A non-lethal dose of 1 μg per fly did, however, enhance significantly the fecundity (egg production) as well as the ovarian development (number of eggs in ovaries and length of egg folicles) of treated flies. We conclude that pyriproxyfen could be a useful aid in exploring endocrine regulation of feeding and reproductive physiology behavior inR. pomonella, about which current knowledge is scant.     

Foraging Behavior and Patch Time Allocation by Fopius arisanus (Hymenoptera: Braconidae), an Egg-Larval Parasitoid of Tephritid Fruit Flies

This study quantitatively describes the host-searching behavior of Fopius arisanus (Sonan) (Hymenoptera: Braconidae), an important egg-larval parasitoid of tephritid fruit fly pests, on coffee berries infested with host eggs of the Mediterranean fruit fly, Ceratitis capitata (Wiedemann) (Diptera: Tephritidae). We also investigate the parasitoid's response to local variation in host patch quality. The temporal pattern of behavioral organisation was examined by constructing an ethogram. The parasitoid spent over 90% of its foraging time in detecting and locating hosts after arriving on a host-infested fruit, and displayed a relatively fixed behavioral pattern leading to oviposition. Patch residence time increased in the presence of host-associated cues, following successful ovipositions, and with increasing size of host clutches per fruit, but decreased with each successive visit to the same host patch and with increasing availability of alternative host patches. The parasitoid females discriminated against previously parasitized hosts and spent significantly less time and searching effort on patches previously exploited by herself or by conspecific females. The effective host-searching behavior, perfect host discrimination ability, and success-motivated searching strategy shown by F. arisanus ensured a thorough exploitation of host resources by this parasitoid.     

Influence of protein hydrolysate baits on oviposition in the apple maggot, Rhagoletis pomonella

Ovipositional responses of apple maggot (AM), Rhagoletis pomonella (Walsh), females were studied in the laboratory on apples (var: Golden Delicious) treated with different rates of four protein hydrolysate baits in choice and no-choice tests. Protein hydrolysate baits at rates of 0.5 and 1% had no significant effect, but oviposition was greatly reduced at higher rates of 5 and 10%. Apple maggot females exposed to apples treated with protein hydrolysate baits at a rate of 10% made 41–71% fewer punctures and laid 41–73% fewer eggs than in untreated control. No oviposition activity was shown on apples treated with 25 and 100% Nulure®. In no-choice tests the AM females laid 75–96% fewer eggs in apples treated with 10 and 25% Nulure compared to controls and no oviposition occurred in apples treated with 100% Nulure. Apple maggot females arrived in similar numbers on apples treated with 10% Nulure and untreated apples, but only 5% of those arriving on Nulure-treated apples showed ovipositor boring with no egg deposition while 60% of females arriving on untreated apples showed ovipositor boring activity and laid an average of 2.5 eggs per apple. In another experiment, individual AM females displayed similar behavioral responses to 10% Nulure-treated apples; none of the 56 females tested on treated apples displayed ovipositor boring activity, but 59% of the females (N=56) tested on untreated apples displayed ovipositor boring within 5 min of their arrival. Ninetyeight percent of AM females stayed and fed on fruit surfaces for 5 min on Nulure-treated apples without ovipositor boring compared to only 2% on untreated apples. Of the females that arrived on untreated apples, 39% flew away within 5 min without ovipositor boring compared to only 2% of those that arrived on Nulure-treated apples. Results of these two behavioral experiments suggest that upon arrival on a protein bait-treated apple, an apparent change of behavior occurs in AM females and instead of attempting to oviposit, they attempt to feed on fruit surfaces resulting in reduced oviposition activity. These results indicate that the feeding and oviposition-related activities of AM females are probably mutually exclusive and that the feeding behavior preempts oviposition activities on host fruits treated with higher rates of protein hydrolysate baits.     

Are the apple maggot,Rhagoletis pomonella,and blueberry maggot,R. mendax,distinct species? Implications for sympatric speciation

Rhagoletis pomonella (Walsh) and R. mendax (Curran) (Diptera: Tephritidae) are major economic pests of apple and blueberry fruits, respectively, in eastern North America. The taxonomic status of these flies as distinct species has been in dispute because of their close morphological similarity, broadly overlapping geographic distributions and inter-fertility in laboratory crosses. Starch gel electrophoresis of soluble proteins was performed to establish the extent of genetic differentiation and levels of gene flow between blueberry infesting populations of R. mendax and apple and hawthorn infesting populations of R. pomonella. R. mendax and R. pomonella were found to be genetically distinct sibling species as eleven out of total of twenty-nine allozymes surveyed possessed species specific alleles. Data from three sympatric apple and blueberry fly populations in Michigan indicated that these flies do not hybridize in nature and gave no evidence for nuclear gene introgression. Differences in host plant recognition were implicated as important pre-mating barriers to gene flow between R. pomonella and R. mendax; a result supporting a sympatric mode of divergence for these flies.

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Résumé R. pomonella Walsh and R. mendax Curran sont respectivement deux mouches très nuisibles aux pommes et aux myrtilles du N E des USA. La position taxonomique de ces mouches comme espèces distinctes a été longtemps mise en doute par suite de leur grande ressemblance morphologique, de l'important chevauchement de leurs répartitions et de leur interfécondité au laboratoire. L'électophorèse sur gel d'amidon de protéines solubles a été utilisé pour établir l'importance de la différenciation génétique et du flux génique entre R. mendax contaminant des myrtilles et R. pomonella contaminant des pommiers et des aubépines. R. mendax et R. pomonella se sont révélées des espèces jumelles car, à l'exception de 11 alolozymes sur 29, chaque espèce possédait des allèles spécifiques. Les données concernant 3 populations sympatriques de mouches des myrtilles et des pommes du Michigan ont montré que des mouches ne s'hybrident pas dans la nature et n'ont fourni aucune indication sur une introgression de gènes nucléaires. Des différences concernant la découverte de hôtes sont impliquées comme obstacles prézygotiques importants au flux génique entre R. pomonella et R. mendax; ce résultat conforte l'hypothèse d'une divergence sympatrique de ces mouches.

     

Production and electrophoretic verification of F1 hybrids between the sibling speciesRhagoletis pomonella andR. cornivora

To investigate the potential for natural interspecific hybridization in theRhagoletis pomonella group of sibling fruit fly species, an attempt was made to hybridizeR. pomonella, the apple maggot fly, withR. cornivora, a non-pest feeding on dogwoods. Laboratory hybridization was difficult, with only five F₁ hybrids produced. Electrophoretic analysis of the hybrids confirmed their hybrid origin. Allele frequency data for three diagnostic loci in up to 5,596 individuals of the parental species, gathered from published and unpublished studies, indicate that the two sibling species hybridize in nature very infrequently, if at all.     

Influence of Host Plant Structure and Microclimate on the Abundance and Behavior of a Tephritid Fly

Microclimate and host plant architecture significantly influence the abundance and behavior of insects. However, most research in this field has focused at the invertebrate assemblage level, with few studies at the single-species level. Using wild Solanum mauritianum plants, we evaluated the influence of plant structure (number of leaves and branches and height of plant) and microclimate (temperature, relative humidity, and light intensity) on the abundance and behavior of a single insect species, the monophagous tephritid fly Bactrocera cacuminata (Hering). Abundance and oviposition behavior were signficantly influenced by the host structure (density of foliage) and associated microclimate. Resting behavior of both sexes was influenced positively by foliage density, while temperature positively influenced the numbers of resting females. The number of ovipositing females was positively influenced by temperature and negatively by relative humidity. Feeding behavior was rare on the host plant, as was mating. The relatively low explanatory power of the measured variables suggests that, in addition to host plant architecture and associated microclimate, other cues (e.g., olfactory or visual) could affect visitation and use of the larval host plant by adult fruit flies. For 12 plants observed at dusk (the time of fly mating), mating pairs were observed on only one tree. Principal component analyses of the plant and microclimate factors associated with these plants revealed that the plant on which mating was observed had specific characteristics (intermediate light intensity, greater height, and greater quantity of fruit) that may have influenced its selection as a mating site.     

Olfactory response of three parasitoid species (Hymenoptera: Braconidae) to volatiles of guavas infested or not with fruit fly larvae (Diptera: Tephritidae)

The olfactory responses of the native parasitoids Doryctobracon areolatus (Szépligeti) and Asobara anastrephae (Muesebeck) and of the exotic parasitoid Diachasmimorpha longicaudata (Ashmead) to guava (Psidium guajava L.) infested or not with fruit fly larvae were evaluated. D. areolatus and D. longicaudata females responded to the odors of uninfested rotting guavas, although D. areolatus was also attracted to fruits at the initial maturation (turning) stage. The females of these species recognized the volatiles of guavas containing Ceratitis capitata (Wied.) larvae. However, in bioassays involving fruits with larvae of different instars, D. longicaudata females were not able to separate between fruits containing C. capitata larvae at the initial instars and larvae at the third instar. In the evaluations of volatiles released by guavas containing C. capitata and Anastrepha fraterculus (Wied.) larvae, the D. longicaudata females were oriented toward the volatiles of fruits containing both host species, but differed significantly from volatiles of guavas containing C. capitata larvae. The D. areolatus females also showed responses to both species, although with a preference for volatiles of fruits containing A. fraterculus larvae. The A. anastrephae females were oriented toward the odors of fruits infested with both fruit fly species. In the shade house, D. longicaudata females were oriented to volatiles of rotting fruits containing larvae or not, but could not significantly differentiate between hosts. D. areolatus females were not attracted toward fruits on the ground in the shade house, regardless of host, suggesting that this parasitoid does not forage on fallen fruits.     

Effects of fruit fly host, fruit species, and host egg to female parasitoid ratio on the laboratory rearing of Biosteres arisanus

The host suitability of the oriental fruit fly, Bactrocera dorsalis (Hendel), for development of Biosteres arisanus (Sonan), a braconid parasitoid, was compared with three other fruit fly species, namely, Mediterranean fruit fly, Ceratitis capitata Weidemann, melon fly, Bactrocera cucurbitae Coquilett, and Malaysian fruit fly, Bactrocera latifrons (Hendel). In addition, effects of five different fruit species, namely, Carica papaya L. (solo papaya), Musa sapientum (L.) O. Ktze. (apple banana), Mangifera indica (L.) (Haden mango), Terminalia catappa (L.) (false kamani), and Citrus aurantiifolia (Christman) Swingle (common lime), on the parasitization rate of B. dorsalis and sex ratio of parasitoid progenies were evaluated. Effects of host egg to female B. arisanus ratios on parasitoid progeny yields were likewise determined. The host suitability of fruit flies for development of B. arisanus was ranked as: B. dorsalis>C. capitata=B. latifrons=B. cucurbitae. Based on percent parasitization of B. dorsalis, preference of B. arisanus females for host eggs varied with fruit species, however, preferential oviposition displayed by female parasitoids did not influence sex ratios of subsequent parasitoid progenies. Increases in host egg to female parasitoid ratios of 5:1, 10:1, 20:1, 25:1, and 30:1 corresponded with increases in parasitoid progeny yield reaching a plateau at 20:1.