Density-dependent egg mortality in early stages of gall induction by the fruit gall midge <Emphasis Type="Italic">Asphondylia aucubae</Emphasis> Yukawa and Ohsaki

Density-dependent mortality has been considered a symptom of intraspecific competition. We examined the occurrence of such mortality in the early stages of gall induction by the gall midge Asphondylia aucubae Yukawa et Ohsaki (Diptera: Cecidomyiidae). Female midges deposit eggs into young fruit of the dioecious shrub Aucuba japonica Thunberg to induce gall formation. Each host fruit received 0–67 eggs (mean 18.5 eggs) from multiple females, whereas established galls each contained one to ten larvae. Midges suffered intense mortality (65–90%) at the egg stage. Egg mortality occurred even in fruit in which no larvae had hatched, suggesting that this mortality cannot be wholly attributed to larval interference. Egg mortality was affected by fruit size, i.e., resource capacity. Midges distributed more eggs in larger fruit. Egg mortality increased as the per-fruit density increased relative to fruit size. In contrast, the mortality of hatchlings was density-independent. Our results suggest that A. aucubae intensely compete for gall-induction substrates, which are spatiotemporally rare resources.     

Nutrition as a facilitator of host‐race formation: the shift of a stem‐boring beetle to a gall host

1. The importance of host‐race formation to herbivorous insect diversity depends on the likelihood that successful populations can be established on a new plant host. A previously unexplored ecological aid to success on a novel host is better nutritional quality. The role of nutrition was examined in the shift of the stem‐boring beetle Mordellistena convicta to fly‐induced galls on goldenrod and the establishment there of a genetically distinct gall host race. 2. First, larvae of the host race inhabiting stems of Solidago gigantea were transplanted into stems and galls of greenhouse‐grown S. gigantea plants. At the end of larval development, the mean mass of larvae transplanted to galls was significantly greater than the mass of larvae transplanted to stems, indicating a likely nutritional benefit during the shift. This advantage was slightly but significantly diminished when the gall‐inducing fly feeding at the centre of the gall died early in the season. Additionally, there was a suggestion of a trade‐off in the increased mortality of smaller beetle larvae transplanted into galls. 3. In a companion experiment, S. gigantea gall‐race beetle larvae were likewise transplanted to S. gigantea stems and galls. Besides the expected greater mass in galls, the larvae also exhibited adaptations to the gall nutritional environment: larger inherent size, altered tunnelling behaviour, and no diminution of mass pursuant to gall‐inducer mortality. 4. In a third line of inquiry, chemical analyses of field‐collected S. gigantea plants revealed higher levels of mineral elements important to insect nutrition in galls as compared with stems.     

Density-dependent regulation of Aedes cantans (Diptera: Culicidae) in natural and artificial populations

Abstract.

• 1 Mortality and size variation (siphon length) in Aedes cantans larvae were examined in natural populations in northern England in 1989 and 1990.
• 2 Under crowded conditions, density-dependent competition led to reduction in the size of both larvae and adults and increased larval mortality.
• 3 Larvae were also maintained in cages in the field at different densities. Results paralleled those for the natural populations in the ponds; larvae maintained at high densities showed increased mortality and reduced size.
• 4 Possible density-dependent factors leading to mortality and size reduction include cannibalism and contact inhibition leading to food shortages.
• 5 The main density-independent factor contributing to larval mortality was habitat desiccation.

     

Population dynamics of the yew gall midge Taxomyia taxi and its chalcid parasitoids: a 24-year study

Abstract.

• 1 The dynamics of three populations of Taxomyia taxi (Inchbald) (Diptera: Cecidomyiidae) and its chalcid parasitoids have been studied over a 24-year period. Most individuals have a 2-year life cycle but some develop in 1 year. Details of within-generation mortalities in T.taxi are used for life table analyses.
• 2 Mortality in the period between emergence of adult T.taxi and larval infestation of buds appears to be density-dependent and is the largest component of overall mortality. In 2-year life cycles, this mortality and that caused by Torymus nigritarsus (Walker) contribute equally to variance in overall mortality. In 1-year cycles, mortality caused by Mesopolobus diffinis (Walker) is density-dependent and accounts for most within-gall losses.
• 3 T. nigritarsus, which attacks only 2-year galls, is absent from all populations for a number of years in the middle of the study period. Its searching efficiency declines as its density and that of its host increase.
• 4 Densities of M.diffinis are strongly correlated with those of available hosts. Percentage parasitism of 2-year galls is less than that of 1-year galls, suggesting the occurrence of enemy-free space.
• 5 Although there are some correlations in densities and mortality between trees, the dynamics of populations on each are frequently different.
• 6 An earlier analysis of 10 years' data failed to demonstrate density-dependent effects. The extension to a 24-year run has revealed such effects, but also demonstrates long-term fluctuations in population densities, with corresponding changes in the balance of advantage between 1- and 2-year life cycles of T.taxi.

     

Factors affecting refuge from parasitoid attack in a cynipid wasp, Aphelonyx glanduliferae

We examined seasonal patterns of gall morphology, growth, and survivorship of the agamic generation of a cynipid wasp, Aphelonyx glanduliferae, and discussed its mortality factors, especially from the point of view of refuge from parasitoid attack. Although the initiation period varied greatly among individual galls, the larvae of A. glanduliferae grew rapidly and reached their maximum size within 3 weeks before pupating in late September to early October. This growth period corresponded to the period when the gall walls became thinner. Parasitoid attack, which was the principal factor in the mortality of A. glanduliferae in the tree crown, was concentrated around the pupation period of the cynipid. Gall walls were significantly thinner in galls attacked by parasitoids than in those still containing a living cynipid. Therefore, the period available to parasitoids seems to be limited by both gall wall thickness and cynipid size. Thus, the growth pattern of A. glanduliferae larvae can have significance in that it narrows the window of vulnerability to parasitoids to a particular period. Although delaying gall initiation will also shorten the exposure period to parasitoid attacks, it was likely to increase the risk of death from gall abortion caused by seasonal degradation in the quality of host plant tissues. Although many cynipids were killed by disease in the galls that fell to the ground, the falling of mature galls to the ground may be another way to a parasitoid-free space. It is thus suggested that a trade-off among life history traits against multiple factors operates in the refuge of A. glanduliferae from parasitoid attack. Received: May 15, 2001 / Accepted: February 1, 2002     

Oxycephalomyia, gen. nov., and life history strategy of O. styraci comb. nov. (Diptera: Cecidomyiidae) on Styrax japonicus (Styracaceae)

A new genus Oxycephalomyia is described to contain the gall midge that was previously known as Asteralobia styraci (Shinji). Oxycephalomyia styraci, comb. nov., produces leaf vein galls on Styrax japonicus (Styracaceae). The adult of O. styraci is redescribed, and its full‐grown larva and pupa are described for the first time. The annual life cycle of the gall midge in northern Kyushu was clarified; the first instars overwinter in the galls on the host plant. However, the galls of O. styraci mature much later in the season than those of other gall midges with a similar life history pattern, and the durations of second and third larval instars are remarkably short. Such a life history pattern is considered to have an adaptive significance in avoiding larval parasitism, particularly by early attackers. The number of host axillary buds as oviposition sites decreased in bearing years and increased in off years, but there was no sign of oviposition site shortage even in bearing years, probably due to the low population density of the gall midge. An unidentified lepidopteran that feeds on galled and ungalled host buds and a Torymus sp. that attacks pupae of O. styraci were recognized as mortality factors of the gall midge.     

Life history strategy and adult and larval behavior of Macrodiplosis selenis (Diptera: Cecidomyiidae), a species that induces leaf‐margin fold galls on deciduous Quercus (Fagaceae)

Life historical, behavioral and ecological traits of Macrodiplosis selenis, which induces leaf‐margin fold galls on Quercus serrata, Q. mongolica and Q. dentata (Fagaceae) in Japan and South Korea, were studied. Daily activity and larval development indicate that M. selenis is a diurnal and univoltine gall midge. In April, females lay their eggs both on upper and under surfaces of fresh leaves. The duration of the egg stage varies from 5 to 9 days, depending on daily temperatures. Hatched larvae crawl to the upper surface of the leaf margin, where they start to induce galls. Larvae become full‐grown in October, drop to the ground in November and overwinter in cocoons on the ground, while larvae of congeners mature in May and drop to the ground in June. A relatively long period of the second larval stadium from July to October on the host trees seems to be effective for M. selenis in avoiding summer mortalities caused by predation and aridity on the ground and by ectoparasitoids that attack mature larvae or pupae on the host leaves. The spatial distribution pattern of M. selenis leaf galls is contagious and the mean gall density per leaf is significantly correlated with the mean crowding. This study adds new insights of life history strategy and adult and larval behavioral pattern to the ecological knowledge of gall midges, and these kinds of information are essential for further studies of M. selenis population dynamics and interactions with other Quercus‐associated herbivores.     

Larval distributions of the ectoparasitoid wasp Eurytoma robusta relative to the host tephritid gall fly Urophora cardui

Parasitism may explain the patchy distributions of host populations. The present paper is a study of larval distributions of the parasitoid Eurytoma robusta in galls of the tephritid gall fly Urophora cardui. It focuses on E. robusta's choice of U. cardui gall and whether this changes relative to the rate of parasitism. Oviposition patterns were inferred by direct counts of larvae in galls and genetically, for both species, using indirect relatedness estimates between gall‐members. Furthermore, rates of parasitism in four populations were monitored for 4 years. The modal distribution of E. robusta larvae per gall was one and independent of the level of parasitism. The mean number of E. robusta per gall did not differ from Poisson distributions at different parasitism rates. We were not able to demonstrate a parasitoid preference for gall size. In contrast, parasitoids may have a negative effect on gall growth. Relatedness estimates showed that E. robusta gall members were often unrelated, whereas U. cardui were siblings. Thus, larval distributions of E. robusta suggest that oviposition behaviour is generally constrained and density independent. In four populations monitored over 4 years, parasitism was initially high (up to 70%), but suddenly declined with no apparent effect on fly (gall) abundance.     

A plant pathogen causes extensive mortality in an invasive insect herbivore

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Effects of gall volume on survival and fecundity of gall-making aphidsAdelges japonicus (Homoptera: Adelgidae)

The number of larvae reaching maturity within the gall of Adelges japonicus was positively related to gall volume, and the relationship between the number of mature larvae and gall volume did not change with different densities of colonized larvae. The population changes in the bud galls of A. japonicus were surveyed by collecting the galls, which did not suffer predation or parasitism within the galls, from young artificial plantations of Picea jezoensis over two years. In the year when the density of colonized larvae was high, they suffered a 42% mortality within the galls, whereas mortality was nearly zero in the low density year. The numbers of larvae per gall were positively correlated with gall volume. The regression lines of the number of colonized larvae on gall volume did not differ significantly in the regression coefficients between the two years, whereas the intercept was significantly higher in the year when the density of colonized larvae was higher. However, different within-gall mortality nullified this difference, and the regression lines of the number of mature larvae on gall volume had no significant difference both in the regression coefficients and the intercepts. This suggests that the number of mature larvae per gall was limited by available resources within the gall which were positively related to gall volume. In 25% of the galls in which mature larvae inhabited, the space within the galls were completely filled by the larvae, indicating that space was one of the limiting resources. Gall volume also affected the number of adults that emerged from the gall and the potential number of their progeny.     

Vascular implications of Dasineura sp. galls' establishment on Peumus boldus stems

• Some chewing larvae are capable of inducing galls in the host vascular cylinder, e.g. Dasineura sp. (Cecidomyiidae) on Peumus boldus stems. Due to the medicinal and economic importance of P. boldus, the anatomical and functional implications of establishment of Dasineura sp. on P. boldus stems were investigated. We asked if establishment of Dasineura sp. in P. boldus stems induces abnormalities at the cellular and organizational level of the vascular system that increase during gall development in favour of the hydric status of the gall.
• Anatomical alterations induced in the stems during gall development were determined. Cytohistometric analyses in mature galls were compared to non-galled stems, and water potential and leaf area of non-galled stems were compared with galled stems.
• Dasineura sp. establishes in the vascular cambium, leading to delignification and rupture of xylem cells, inhibiting formation of phloem and perivascular sclerenchyma. Gall diameter increases together with larval feeding activity, producing a large larval chamber and numerous layers of nutritive tissue, vascular parenchyma, and sclerenchyma. These anatomical alterations do not affect the leaf area of galled stems but favour increased water flow towards these stems.
• The anatomical alterations induced by Dasineura sp. in P. boldus stems guarantee water and nutrient supply to the gall and larva. After the inducer exits stems, some host branches no longer have vascular connections with the plant body.

     

Studies on the distribution pattern of the pine needle gall midge,Thecodiplosis japonensis Uchida etInouye (Diptera: Cecidomyiidae) in a pine forest

Numerical changes and distribution patterns of the pine needle gall midge, Thecodiplosis japonensisUchida etInouye , were studied during the period from 1978 to 1979 in a young plantation of Pinus thunbergii in Shiga Prefecture, Japan. The survivorship curve of this species was characterized by a low mortality of larvae in galls and two high mortalities before the formation of galls and during the overwintering period in soil. The within and between-trees distributions of eggs and larvae in galls were examined by using the regression method. The egg distribution per shoot was aggregative both within and between host plants. The within-tree variations in numbers of eggs per shoot were related to the differences in the abundance of available needles for oviposition per shoot among the canopy layers. The between-tree variations reflected the heterogeneous emergence of adult females in the study plot. The degree of aggregation increased from egg to gall stage in both within- and between-tree distributions and the increase was explained by the different mortality of larvae within trees and the inversely density-dependent mortality between trees. The distribution patterns in the soil habitat stages were examined by the patchness index (). This species showed aggregative distributions in soil stages. There was a correlation in spatial patterns of adult emergence between the successive generations. The distribution properties of this species were discussed in connection with the population dynamics and the availability of host plants in the study plot.     

The problem of optimal clutch size in a tritrophic system: the oviposition strategy of the thistle gallfly Urophora cardui (Diptera,Tephritidae)

The problem of optimal clutch sizes is a central theme in life history theory. Optimal allocation of eggs is especially complicated for insects in tritrophic systems. In this study we analyze some of the processes determining clutch sizes of the thistle gallfly Urophora cardui, a monophagous tephritid fly associated with Cirsium arvense. U. cardui forms multilocular shoot galls, which vary broadly in their size and number of their gall cells. We investigate various fitness consequences of gall size. An analysis of the number of cells per gall (which is correlated with gall diameter and gall weight) showed that in U. cardui there is mutual facilitation rather than larval competition. Increasing numbers of larvae per gall led to a decreasing mortality and increasing larval weight. Larval weight in turn was positively correlated with the probability of survival to adulthood and with adult weight and fecundity. Thus, all fitness parameters measured favoured large galls. Clutch sizes in oviposition experiments were distinctly larger than the number of gall cells of field populations and in cage experiments, suggesting high mortality of eggs and/or early larval instars. There was a significant relationship between the internal structure (i.e., the size of the growing point) of the bud and clutch size, suggesting that U. cardui females are able to measure bud quality and adapt clutch sizes accordingly. Clutch size was positively correlated with the female's age at first oviposition and negatively with the number of previous ovipositions and previously laid eggs. Since the potential egg capacity per female is higher than the average number of larvae it is likely to produce during its short adult lifespan, U. cardui females tend to be time-limited rather than egglimited, which might favour large clutches once an appropriate oviposition site has been located. As the development of the gall and hence the fate of a clutch depends on a number of unpredictable factors, exclusive concentration of eggs in a few large clusters would involve risks which could be avoided by increasing the number of clutches. Therefore we interpret the high variation of clutch sizes in U. cardui as a mixed strategy of bet hedging and gambling.     

Density-related mortality of the flour beetle,Tribolium confusum duval

Life tables were constructed to assess the relative importance of some factors causing mortality of Tribolium confusum and to gauge their response to increasing population density. Observations focussed on three population densities (100, 400 and 800 individual/8 g medium) from the egg to the adult stage. The medium was not renewed in order to maximize predatory interactions. Generation mortality at densities 100, 400 and 800 was 42%, 50% and 74% respectively, i. e. density-dependent. Mortality in the first 10 days was also density-dependent reaching a maximum of 27% at density 800: predation by small larvae on eggs seemed the principal causative factor. The overall pattern of larval mortality was density-independent. Data on the mortality of pupae and callows were ultimately consistent with an inversely density-dependent pattern. Apparently, only mortality occurring within the first 10 days was capable of population regulation.     

The life history of a gall‐inducing mite: summer phenology,predation and influence of gall morphology in a sugar maple canopy

• 1 Eriophyoid mites are among the most ubiquitous gall‐inducing arthropods, and are adapted species‐specifically to a broad diversity of plants, although their life histories remain poorly studied outside agricultural systems.
• 2 We examined the seasonal phenology of a leaf‐galling eriophyid mite, the maple spindle gall mite Vasates aceriscrumena (MSGM), in naturally occurring stands of sugar maple Acer saccharum in south‐central Ontario in 2007 and 2008.
• 3 Galls were first induced in spring (mid‐May) and were devoid of mites by late August. In the study region, MSGM appears to have at least two generations, with overwintering, deutogyne females that initiate galls in spring (mid‐May) after leaf flush, giving rise to a generation of protogyne (primary) females and a few morphologically similar males (<1 for every 10 females) and, subsequently, to a new generation of deutogyne females in mid‐July to early August. In July, some galls can be highly crowded, with 50–200 individuals per gall.
• 4 In addition, a tarsonemid mite, Tarsonemus acerbilis, was found in approximately 40% of MSGM galls examined. As much as 95.4% of galls in 2007 and 97.4% in 2008 that contained tarsonemid larvae did not contain MSGM eggs (by contrast, only 2.3% of tarsonemid‐free galls contained no MSGM eggs), suggesting that these juveniles feed, at least opportunistically, on MSGM eggs.
• 5 Gall ostiole morphology appeared to influence both MSGM and Tarsonemus densities within galls, with ‘open’ ostioles (versus ‘closed’) being much more susceptible to invasion by the tarsonemid. The latter is likely to be an important regulator of MSGM populations. We hypothesize that the two ostiole types are the result of selection pressures on the gall inducer, favouring closed gall entrances for increased protection, and possibly also on the host tree, favouring open galls to increase predator access.

     

Oviposition choice and larval survival of Dasineura marginemtorquens (Diptera: Cecidomyiidae) on resistant and susceptible Salix viminalis

Abstract.

• 1 Substantial intraspecific variation exists in Salix viminalis resistance to the gall midge Dasineura marginemtorquens. Earlier work has found this variation to have a large genetic component. Willow clones are stable in their resistances between midge generations and different nutrient levels in both field and laboratory culture.
• 2 This study reports the results of laboratory experiments on female oviposition choice and larval survival on potted plants from clones that are very different in resistance as determined in field studies.
• 3 In choice experiments using pairs of plants, the average female midge did not prefer susceptible willow clones over resistant ones for oviposition. In about one third of the replicates, midges actually laid more eggs on the resistant clone. Further work is necessary to examine the nature of variation among midges in discrimination of these plant types.
• 4 Resistance is manifested as great differences in larval survival. Six days after oviposition survival was 92% on susceptible plants but only 6% on resistant ones. Galls developed on all of the susceptible plants, while in 73% of the resistant plants galls were not even initiated.
• 5 The plant traits causing resistance are enigmatic. Larval behaviour suggests that resistant plants interfere with feeding behaviour. On resistant plants, most larvae wander for more than 24 h without initiating any galls before dying. On susceptible plants many first instar larvae begin feeding and initiate galls within this period.

     

Competition during early ontogeny: Effects of native and invasive planktivores on the growth,survival, and habitat use of bluegill

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Population regulation of a mosquitoArmigeres theobaldi with description of the animal fauna in zingiberaceous inflorescences

1. Inflorescences of some Curcuma and Zingiber (Zingiberaceae) in tropical Asia provide an unique aquatic habitat being discrete, small, and made of numerous smaller compartments (the bracts).
2. The aquatic community in inflorescences of Curcuma in northern Thailand was composed of immature Diptera, of which the biting midge Dasyhelea and the mosquito Armigeres theobaldi were the commonest. No important competitors, predators or pathogenic parasites for the mosquito were confirmed.
3. Inter-inflorescence distribution of the mosquito was contagious. Within each inflorescence, the fourth-instar larvae or pupae usually occupied bracts singly.
4. The k-value analysis detected density-dependent mortality due to contest competition in the mosquito larvae.
5. Variations in the larval and pupal mosquito size were density-independent and remarkably small as compared with size variations known for other mosquitoes.
6. These population attributes (large density-dependent mortality with density-independent, minimally variable individual size) appear unique among mosquitoes, arising from conspecific killing, efficient foraging (inter-bract movement by crawl and single occupation of bracts), and availability of host plant tissues as supplementary food.
7. A simple population model suggested that a small proportion of adult females lay eggs.

     

The change of the distribution pattern of the large weevil,Hyposipalus gigas Fabricius (Coleoptera,Rhynchophoridae) within a single generation. A preliminary note

The distribution pattern of adults of the large weevil, Hyposipalus gigas on logs of the Japanese black pine and that of larvae in pine stumps were examined by using the m^*−m method (Iwao , 1968). The distribution of adults on logs was contagious, probably due to their response to local heterogeneity of the environment. The larval distribution was also contagious, but there was no significant correlation between the number of larval bores and the stump size. The mortality of young larvae seemed not to be related with the number of larvae per stump, but it was related with the density per unit area of bark surface. There was some sign of inversely density-dependent or ‘all-or-none’ type mortaltiy in the late larval stage.     

VARIABLE SELECTION ON EUROSTA'S GALL SIZE. II. A PATH ANALYSIS OF THE ECOLOGICAL FACTORS BEHIND SELECTION

We examined phenotypic selection exerted by natural enemies on the gall-making fly Eurosta solidaginis in an extensive field study of 16 populations, spanning four generations. Gall-makers that induce small galls are vulnerable to the attack of Eurytoma gigantea. This imposes upward directional selection on gall size. Insectivorous birds, predominantly the downy woodpecker, are more likely to attack larvae that induce large galls than small ones, and this imposes downward directional selection. We used path analysis to explore the relative contributions of these natural enemies to the net directional selection on gall size. The path models further examined several ecological factors that influence selection intensity through their effects on parasitoid and bird attack rates. Net directional selection varied more strongly with E. gigantea attack than bird attack. Competitive interactions among birds and the three parasitoid species, including E. gigantea, were evidenced by low winter bird attack rates in fields where a high proportion of galls contained the overwintering parasitoids. Eurytoma gigantea attack was heavier in fields where mean gall size was small and bird attack heavier in fields where mean gall size was large. Neither birds nor E. gigantea showed simple density-dependent attack. Data suggested a form of frequency-dependent attack by birds but not by E. gigantea.