Associational resistance mediated by natural enemies

Abstract.  1. Associational resistance theory suggests that the association of herbivore-susceptible plant species with herbivore-resistant plant species can reduce herbivore density on the susceptible plant species. Several casual mechanisms are possible but none has so far invoked natural enemies. Associational resistance mediated by natural enemies was tested for by examining densities of a gall fly, Asphondylia borrichiae (Diptera: Cecidomyiidae), and levels of parasitism on two closely related seaside plants, Borrichia frutescens and Iva frutescens , when alone and when co-occurring.
2. Both Borrichia and Iva grow alone or together on small offshore islands in Florida. Each host plant species has its own associated race of fly, but both races of fly are attacked by the same four species of parasitoids. Borrichia normally has a higher density of galls than Iva , and galls are larger on Borrichia than on Iva .
3. Gall size, gall abundance, parasitism levels, and parasitoid community composition were quantified on both Borrichia and Iva on islands where each species grew alone or together. Some islands were then manipulated by adding Borrichia to islands supporting only Iva , and by adding Iva to islands supporting only Borrichia . Subsequent gall densities and gall parasitism levels on the original native species were then examined.
4. On both natural and experimentally manipulated islands, gall densities on Iva were significantly lowered by the presence of Borrichia . This is because bigger parasitoid species that were common on Borrichia galls, which are bigger, spilled over and attacked the smaller Iva galls. Thus, parasitism rates on Iva were higher on islands where Borrichia co-occurred than on islands where Borrichia were absent. Most parasitoids from Iva were too small to successfully attack the large Borrichia galls and so gall density on Borrichia was unaffected by the presence of Iva .     

Effects of large‐scale host plant addition and removal on parasitoid‐mediated associational resistance in the gall midge Asphondylia borrichiae

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Indirect competitive effects of stemborers on a gall community

Interspecific competition between phytophagous insects using the same host plant occurs frequently and can strongly affect population densities of competing species. Competition between gallmakers and stemborers could be especially intense because both types of herbivore are unable to avoid competition by relocation during their immature stages. For apical meristem gallmakers the main result of competition is likely to be the interruption of resources to the gall by the stemborers' devouring of stem contents. The proximate effect of such competition could be to reduce gall size, thereby increasing the number of chambers per gall unit volume, and reducing the size and potential reproductive output of the gallformer. In addition, smaller galls may be more susceptible to attack from size‐limited parasitoids, resulting in a second indirect effect of competition. Using a community of galling and stemboring insects on the saltmarsh shrub Iva frutescens L. (Asteraceae), we measured for indirect effects of competition. We examined the primary indirect effect of competition on gall midge crowding and the secondary effects on parasitism rates and parasitoid guild composition. Results indicated that galls co‐occurring with stemborers were smaller, crowding of gall inhabitants was 22% greater, and the composition of the parasitoid guild was altered relative to galls on unbored stems. The overall parasitism rate was not different between galls on bored vs. unbored stems. These results show that competition resulting from the presence of stemborers has the potential to affect the gall midge Asphondylia borrichiae Rossi & Strong (Diptera: Cecidomyiidae) and secondarily to affect its guild of hymenopteran parasitoids.     

Does gall diameter affect the parasitism rate of Asphondylia borrichiae (Diptera: Cecidomyiidae)?

Abstract. 1. Fertilized field plots of Borrichia frutescens (L.) de Candolle produced plants with a higher apical-leaf nitrogen content than control plots.
2. Gall frequency of the cecidomyiid Asphondylia borrichiae Rossi & Strong on stems of B.frutescens , increased significantly on fertilized plots after approximately 3 months.
3. Fertilizer treatment did not result in changes in stem density but did tend to increase proportion of stems flowering and overall plant size. Galls are not normally found on flowering stems.
4. Galls grew at a faster rate and to a larger final size on fertilized plots.
5. On three sampling dates, per cent parasitism of galls was the same on fertilized and control plots; therefore, this study does not support the galldiameter hypothesis.     

Description of the soybean pod gall midge,Asphondylia yushimai sp. n. (Diptera: Cecidomyiidae), a major pest of soybean and findings of host alternation

The soybean pod gall midge is an important pest of soybean in Japan and is known to occur also in Indonesia and China. This gall midge is described from Japan as Asphondylia yushimai sp. n. and is clearly distinguished from its congeners by the arrangement of the lower frontal horns of the pupa and the sequence of the mtDNA COI region. It is concluded that Prunus zippeliana Miquel is a winter host of the soybean pod gall midge since haplotypes of the soybean pod gall midge coincide with those of the Prunus fruit gall midge that produces fruit galls on P. zippeliana. In addition, phenological and distributional information on the two gall midges and on their host plants supports the identification of the winter host. In Japan, the soybean pod gall midge overwinters as a first instar in the fruit galls on P. zippeliana and emerges as an adult from the galls in May. In summer and autumn, the soybean pod gall midge has two or more generations in the pods of soybean, Glycine max (L.) Merrill or wild fabaceous and caesalpiniaceous plants. Thus host alternation by A. yushimai is confirmed. This is the second finding of host alternation by a species of Asphondylia, the first instance being that of Asphondylia gennadii (Marchal) in Cyprus.     

Non-random distribution among a guild of parasitoids: implications for community structure and host survival

Abstract 1. Immature stages of the gall midge, Asphondylia borrichiae, are attacked by four species of parasitoids, which vary in size and relative abundance within patches of the gall midge’s primary host plant, sea oxeye daisy (Borrichia frutescens). 2. In the current study, a bagging experiment found that the smallest wasp, Galeopsomyia haemon, was most abundant in galls exposed to natural enemies early in the experiment, when gall diameter is smallest, while the wasp with the longest ovipositor, Torymus umbilicatus, dominated the parasitoid community in galls that were not exposed until the 5th and 6th weeks when gall diameter is maximal. 3. Moreover, the mean number of parasitoids captured using large artificial galls were 70% and 150% higher compared with medium and small galls respectively, while stem height of artificial galls significantly affected parasitoid distribution. Galls that were level with the top of the sea oxeye canopy captured 60% more parasitoids compared with those below the canopy and 50% more than galls higher than the plant canopy. 4. These non‐random patterns were driven primarily by the differential distribution of the largest parasitoid, T. umbilicatus, which was found significantly more often than expected on large galls and the smallest parasitoid of the guild, G. haemon, which tended to be more common on stems level with the top of the plant canopy. 5. Large Asphondylia galls, especially those located near the top of the Borrichia canopy, were more likely to be discovered by searching parasitoids. Results using artificial galls were consistent with rates of parasitism of Asphondylia galls in native patches of sea oxeye daisy. Gall diameter was 19% greater and the rate of parasitism was reduced by almost 50% on short stems; as a result, gall abundance was 24% higher on short stems compared with ones located near the top of the plant canopy. 6. These results suggest that parasitoid community composition within galls is regulated by both interspecific differences in ovipositor length and preferences for specific gall size and/or stem length classes.     

Coastal insect herbivore communities are affected more by local environmental conditions than by plant genotype

Abstract.

• 1 We compared the effects of plant genotype and local environment on population densities of a community of coastal insect herbivores in west-central Florida. Reciprocal transplants of four genotypes of three species of coastal plants, Borrichia frutescens, Iva frutescens and Limbricata, were made in July 1992 between a series of off-shore islands.
• 2 For each plant species, phytophagous insects with a wide range of feeding modes including gall-makers, stem borers, leaf miners and sap suckers were affected more by local environment than by plant genotype. Whereas host genotype had a significant effect on the population densities of gall-makers on B.frutescens in the spring of 1993, no significant effect on the denrities of any other insect species was found and the effect on the gall-makers on Borrichia disappeared in the summer, 1 year after the transplants had been made. In our study, local environment had by far the greatest effect on insect population densities among islands. This is an unusual result because in other studies over 80% of the insect species examined have been affected by plant genotype (Karban, 1992). This result is consistent with that reported by Stiling (1994), who censused populations of two phytophagous insects on reciprocal transplantr ot Borrichia in north Florida.
• 3 Local environment also had an effect on insect population densities within islands. This result contrasts with similar studies performed in north Florida (Stiling, 1994), where population densities did not differ within areas, and underlies how some biotic processes may change with in the same community even over relatively small changes in species range.

     

Gall size determines the structure of the Rabdophaga strobiloides host–parasitoid community

Abstract.  1. The relationship between gall size and mortality of the willow pinecone gall midge Rabdophaga strobiloides (Diptera: Cecidomyiidae) was examined by determining the fate of all galls in a 30-ha area in central Alberta, Canada over 4 years. It was found that gall size has a large effect on the type and intensity of mortality experienced by the gall midge, and consequently this factor has the potential to influence the dynamics of the host–parasitoid interaction through the creation of phenotypic refuges.
2. Total midge mortality ranged from 51% to 78% over the course of the study and was dominated by parasitism by Torymus cecidomyiae (Hymenoptera: Torymidae) and Gastrancistrus sp. (Hymenoptera: Pteromalidae) as well as predation by birds. Gall size had a strong, non-linear effect on the attack rates of each of these natural enemies.
3. Birds attacked the smallest size classes. Torymus cecidomyiae preferentially attacked medium diameter galls and thus avoided predation by birds in smaller galls. Gastrancistrus sp. preferentially attacked the largest galls and consequently suffered lower rates of predation by both T. cecidomyiae and birds.
4. This study emphasises the importance of understanding the interactions among mortality factors in order to describe adequately the susceptibility of R. strobiloides to parasitism and predation, and ultimately its population dynamics.     

Rapid gall midge adaptation to a resistant willow genotype

Abstract 1 We conducted two experiments to investigate why a basket willow Salix viminalis L. genotype, known to be highly resistant to the leaf-roller gall midge Dasineura marginemtorquens (Bremi), should support very high gall densities in a field plantation at Tälle, south Sweden.
2 The first experiment was a field test of the hypothesis of fine-scale host adaptation in the gall midge/willow system. Support for the hypothesis would be established if midges originating from resistant willows and those originating from nearby susceptible willows differed in their abilities to initiate galls and complete development on resistant plants.
3 The objective of the second experiment was to explore whether there was a genetic basis to the trait for virulence in the midge population and to investigate any potential trade-offs this trait may entail.
4 Our results indicate that there was a fine-scaled microgeographic genetic structure to the midge population at Tälle. Midges originating from resistant plants had a heritable trait that enabled them to establish galls on resistant plants.
5 Midges able to initiate galls on the resistant genotype had longer developmental time on the susceptible genotype. This suggests that there is a physiological cost associated with being adapted to the resistant willow genotype.
6 We suggest that driving forces behind the observed host adaptation are selection imposed on the midge population by very strong willow resistance and restricted gene flow in the midge populations due to the special life history features of D. marginemtorquens .     

New gall midge taxa (Diptera: Cecidomyiidae) from Australian Chenopodiaceae

Abstract  Five new species and a new genus of gall midge are described from flower galls on native chenopod plants in Eyre Peninsula, South Australia. Asphondylia vesicaria sp. n. induces galls on Atriplex vesicaria ; A. mcneilli sp. n. on Sclerolaena diacantha ; and A. tonsura sp. n. on Enchylaena tomentosa . Infested flowers develop into galls and produce no seeds. DNA analysis of part of the cytochrome-c oxidase subunit I mitochondrial gene supported the morphological and biological differences between each of the new species and the previously described A. floriformis (Veenstra-Quah & Kolesik) and A. sarcocorniae (Veenstra-Quah & Kolesik) that induce galls on leaves and branches, respectively, of Sarcocornia quinqueflora (Chenopodiaceae) in Australian salt marshes. A new genus, Dactylasioptera gen. n. and two new species of Lasiopterini, D. adentata sp. n. and D. dentata sp. n. are described – both were reared from galls of A. mcneilli and A. tonsura .     

Sex ratio in relation to season and host plant quality in a monogenous stem-galling midge (Diptera: Cecidomyiidae)

Abstract.  1. The sex determination mechanism in gall midges is little understood, although it is known that the females of several species primarily or exclusively produce unisexual broods throughout their lifetime.
2. The gall midge Izeniola obesula Dorchin is a multivoltine species, inducing multi-chambered stem galls on the salt-marsh plant Suaeda monoica . Each gall contains 5–70 individuals, all being the progeny of a single female. Sampling of more than 450 galls, from which adult midges were reared, suggested that I .  obesula exhibits strict monogeny, resulting in galls that contain either all female or all male progeny.
3. Characterisation of the growth pattern of young S .  monoica shoots revealed that shoots in apical positions grew more rapidly than shoots in more basal positions. Galls that were induced on such shoots were larger and yielded more adult midges.
4. No difference in the site of gall induction was found between male and female galls, with galls of either sex being mainly induced on more rapidly growing shoots. It is concluded that I .  obesula females cannot control the sex of their progeny, and that both female-producers and male-producers strive to maximise their reproductive success by choosing the faster-growing shoots for gall induction.
5. Female galls were larger and more abundant than male galls at almost all times. The sex ratio among galls fluctuated throughout the year, ranging from 4:1 in spring to 1:1 in winter. The skewed sex ratio among galls possibly results from greater mortality rates among male galls than among female galls, due to either primary or secondary factors. Alternatively, it is possible that the number or fitness of male-producers in the population is reduced relative to female-producers.     

Tritrophic interactions and trade-offs in herbivore fecundity on hybridising host plants

Abstract. 1. Interspecific plant hybridisation can have important evolutionary consequences for hybridising plants and for the organisms that they interact with on multiple trophic levels. In this study the effects of plant hybridisation on the abundance of herbivores and on the levels of herbivore parasitism were investigated. 2. Borrichia frutescens, B. arborescens, and their hybrid (B. × cubana) were censused for Asphondylia borrichiae galls and Pissonotus quadripustulatus plant hoppers in the Florida Keys. Levels of egg parasitism were determined by dissecting parental and hybrid stems and galls for herbivore and parasite eggs and larvae. Stem toughness and gall size are plant‐mediated modes of protection from parasitism and these were also measured. For gall midges, fly size was measured as an estimate of fecundity. 3. Field censuses indicated that herbivore abundances varied on hybrid hosts relative to parent plant species and that the different herbivore species exhibited different patterns of abundance. Asphondylia borrichiae gall numbers followed the additive pattern of abundance while P. quadripustulatus numbers most closely resembled the dominance pattern. 4. Parasitism of P. quadripustulatus eggs was high on B. frutescens and the hybrids, and low on B. arborescens, which also had significantly tougher stems. Asphondylia borrichiae suffered the highest levels of parasitism on B. frutescens, the host plant which produced the smallest galls. On B. arborescens, which produced the largest galls, levels of A. borrichiae parasitism were lowest. Both parasitism and gall size were intermediate on the hybrid plants. Galls on B. arborescens and hybrid plants produced significantly smaller flies then those from B. frutescens suggesting that, when selecting hosts from among parent species and hybrids, gall flies may face a trade‐off between escape from natural enemies and maximising fecundity.     

Feeding by Hessian fly [Mayetiola destructor (Say)] larvae does not induce plant indirect defences

Abstract.  1. Recent research has addressed the function of herbivore-induced plant volatiles in attracting natural enemies of feeding herbivores. While many types of insect herbivory appear to elicit volatile responses, those triggered by gall insects have received little attention. Previous work indicates that at least one gall insect species induces changes in host-plant volatiles, but no other studies appear to have addressed whether gall insects trigger plant indirect defences.
2. The volatile responses of wheat to feeding by larvae of the Hessian fly Mayetiola destructor (Say) (Diptera: Cecidomyiidae) were studied to further explore indirect responses of plants to feeding by gall insects. This specialist gall midge species did not elicit a detectable volatile response from wheat plants, whereas a generalist caterpillar triggered volatile release. Moreover, Hessian fly feeding altered volatile responses to subsequent caterpillar herbivory.
3. These results suggest that Hessian fly larvae exert a degree of control over the defensive responses of their host plants and offer insight into plant-gall insect interactions. Also, the failure of Hessian fly larvae to elicit an indirect defensive response from their host plants may help explain why natural enemies, which often rely on induced volatile cues, fail to inflict significant mortality on M. destructor populations in the field.     

Trade-off in oviposition strategy: choosing poor quality host plants reduces mortality from natural enemies for a salt marsh planthopper

Abstract.  1. Both host plant nutrition and mortality from natural enemies have been predicted to significantly impact host plant selection and oviposition behaviour of phytophagous insects. It is unclear, however, if oviposition decisions maximise fitness.
2. This study examined whether the salt marsh planthopper Pissonotus quadripustulatus prefers higher quality host plants for oviposition, and if oviposition decisions are made so as to minimise mortality at the egg stage.
3. A controlled laboratory experiment and 4 years of field data were used to assess the rates of planthopper oviposition on higher quality 'green' and lower quality 'woody' stems of the host plant Borrichia frutescens . The numbers and percentages of healthy eggs and eggs that were killed by parasitoids or the host plant were recorded.
4. In all years, including the laboratory experiment, Pissonotus planthoppers laid more eggs on lower quality woody stems than on higher quality green stems. While host plant related egg mortality was higher in woody stems, the percentage of eggs parasitised was much greater in green stems. This resulted in a lower total mortality of eggs on woody stems.
5. The results of this study demonstrate that, although Pissonotus prefers lower quality host plants for oviposition, this actually increases fitness. These data seem to support the enemy free space hypothesis, and suggest that for phytophagous insects that experience the majority of mortality in the egg stage, oviposition choices may be made such that mortality is minimised.     

Dispersal and foraging behaviour of Platygaster californica: hosts can't run, but they can hide

Abstract.  1. Host–parasitoid models often identify foraging behaviour and dispersal distance as important for system persistence.
2. Laboratory observations and field trials were used to characterise foraging behaviour and dispersal capability of Platygaster californica Ashmead (Platygasteridae), a parasitoid of the gall midge Rhopalomyia californica Felt (Cecidomyiidae).
3. Although foraging parasitoids meticulously searched plants in laboratory observations, none of the laboratory trials resulted in 100% parasitism, and the proportion of parasitism declined as midge egg density increased.
4. The field trials showed that the distribution of parasitism over distance from a central release point was hump-shaped, as predicted by a simple diffusion model. Mean parasitoid dispersal distance was 4.5 m, considerably farther than the 1.7 m mean midge dispersal found in previous work.
5. Although the parasitoid appears to search thoroughly for midge eggs and to disperse farther than the midge, the results of this study show how this host–parasitoid system may persist due to spatially variable incomplete parasitism.     

Genetic and ecological differences between <Emphasis Type="Italic">Asphondylia yushimai</Emphasis> and the ivy gall midge, <Emphasis Type="Italic">Asphondylia</Emphasis> sp. (Diptera: Cecidomyiidae), with a new distribution record of the former from Hokkaido and South Korea

The soybean pod gall midge, Asphondylia yushimai, is known to utilize Laurocerasus zippeliana (Rosaceae) and Osmanthus heterophyllus (Oleaceae) as autumn–spring hosts. In addition, ivy, Hedera rhombea (Araliaceae), was thought to be a candidate for an additional autumn–spring host. However, our genetic analysis indicated that no haplotypes of the ivy fruit gall midge, Asphondylia sp., were identical to any of the haplotypes of A. yushimai. Furthermore, the life-history traits of the ivy fruit gall midge, such as voltinism, host-plant range, lower development threshold temperature (LDT), and developmental speed, were clearly different from those of A. yushimai. Thus, the results from genetic analysis and life-history traits revealed that the ivy fruit gall midge was not identical to A. yushimai and that H. rhombea is not an additional autumn–spring host plant for A. yushimai. We also discovered through morphological observation and genetic analysis that A. yushimai is distributed in Hokkaido and South Korea, and that the ivy fruit gall midge exhibits host plant alternation, utilizing both the fruit of Phytolacca americana (Phytolaccaceae) and the flower buds of Paederia foetida (Rubiaceae) as spring–autumn hosts.     

Ecological divergence among morphologically and genetically related Asphondylia species (Diptera: Cecidomyiidae), with new life history data for three congeners including the Alpinia fruit gall midge

Ecological data is crucial for determining the degree of reproductive isolation among closely related species, and in identifying the factors that have produced this divergence. We studied life history traits for three Asphondylia (Diptera: Cecidomyiidae) species that induce fruit galls either on Alpinia, Ligustrum or Aucuba, and we compared the traits with those published for three other closely related Japanese Asphondylia species. We found that the six species were significantly differentiated in important life history traits, such as host range, voltinism, lower developmental threshold temperature, thermal constant and diapausing season. The data indicate that divergence in the assessed life history traits evolves before morphological divergence, and such ecological divergence could strengthen isolating barriers among the taxa. We present scenarios on how host range expansion, host plant shift and host organ shift for galling initiate the early stages of speciation. We also highlight the importance of ecological data in identifying cryptic species. Specifically, we confirm that Alpinia intermedia (Zingiberaceae) is not an autumn–spring host of the soybean pod gall midge Asphondylia yushimai based on many differences in the life history traits between the Alpinia fruit gall midge Asphondylia sp. and A. yushimai.     

The interactions of plant clone and abiotic factors on a gall-making midge

Within and around Tampa Bay, Florida, monoclonal populations of the sea daisy, Borrichia frutescens, can be found on small, isolated islands growing within the intertidal zone. Stem tips of Borrichia are attacked by the gall-making cecidomyiid, Asphondyliaborrichiae. We used reciprocal transplants of Borrichia clones between islands to assess the importance of plant genotype and local environmental conditions (shade and host-plant nitrogen) on gall abundance. In another experiment, we controlled for host genotype effects by inducing differences in local environmental conditions through the addition of NH₄NO₃ fertilizer and/or shade to field plots at the only monoclonal site with a large enough population of Borrichia to facilitate the experiment. We also examined the effect of these variables on attack levels of Asphondylia by parasitoids. In the reciprocal transplant, while some Borrichia clones always supported more galls than others, regardless of environmental conditions, all four clones developed more galls when they were placed in the shade, compared to those in the sun, at all four sites. In addition, some islands always supported more galls than others and we found a significant clone × site interaction. In the single-clone experiment, Borrichia in fertilized- and shaded-only plots developed more Asphondylia galls than those from nonmanipulated control plots, and plants that received both shading and fertilizer developed the most galls. Although shade and fertilization produced an additive increase in plant nitrogen content, their effects resulted in a synergistic decrease in C:N ratio. Neither shading nor host plant nitrogen content had a significant effect on levels of parasitism between experimental and control plants. Our results suggest that genetic differences in Borrichia's susceptibility to Asphondylia attack are important in shaping the distribution of galls, but environmental factors such as soil nitrogen and degree of shading are at least as important as genetic differences between host plants. Received: 12 June 1997 / Accepted: 6 April 1998     

Evolutionary conservatism of geographic variation in host preference in Callosobruchus maculatus

Abstract.  1. A study of host preference of four pairs of populations of the cowpea weevil Callosobruchus maculates was carried out. The pairs had different geographical origins.
2. One population of each pair had been maintained for about 110 generations on cowpea Vigna unguiculata , the other population had been maintained on mung bean V. radiata . Half of the tested females from each population were raised on cowpea and exposed to this host prior to the assay; the other half was raised on mung bean. This design permitted assessment of the relative contributions of geographical origin, recent host use in the laboratory, and individual experience, to variation in host preference.
3. Host preference was assayed by letting the females oviposit on an equal-weight mixture of cowpea and mung seeds; two experiments were performed six generations apart.
4. Both experiments revealed a strong effect of geographical origin: populations originating from Nigeria laid a much greater proportion (68–86%) of their eggs on cowpea than those originating from Uganda and Yemen (30–42%); those from Cameroon were intermediate (56–60%). These preferences were not affected consistently by about 110 generations of laboratory evolution on one or the other host, or by experience of individual females.
5. These results indicate considerable geographical variation in host preference, and suggest that host preference is behaviourally inflexible and evolutionarily conserved.