Influence of feeding guild on insect response to host plant fertilization

Abstract.

• 1 The densities of insect herbivores in fertilized and unfertilized field plots of goldenrods, Solidago altissima (Compositae), were monitored over a period of 4 years.
• 2 A total of seventeen insect taxa occurred on the plots over the course of the study, including sap feeders, leaf chewers, leaf miners, leaf gallers and stem gallers with multiple representatives in each of these feeding guilds.
• 3 Nine of the seventeen taxa significantly increased in density on fertilized plots in at least one year of the study, two taxa showed marginally significant increases on fertilized plots, two significantly decreased in density on fertilized plots in at least one year, and the remaining taxa were unaffected by the fertilizer treatment.
• 4 The effects of fertilization on the insects were not strongly related to feeding guild; the group of insects that increased on fertilized plots was functionally diverse, and for the most part members of the same guild did not respond to the fertilizer treatment in consistent ways.
• 5 Differences between fertilized and unfertilized plots were greatest in the fourth year. The insects that showed delayed responses to fertilizer treatment may have been affected by changes in microclimate that developed slowly over the course of the study, suggesting that long-term studies may be necessary to detect effects of host plant stress on insect herbivores.

     

Limitations of FST hemispheres in lotic benthos research

• 1 FST hemispheres have been proposed as a method for assessing flow characteristics near the river bottom. We suspected that the hemispheres were too big for this goai and that the ground plate would significantly affect near-bottom microhydraulics.
• 2 The results we present have confirmed our assumption about the limitations of this method: the correlation between FST results and current velocity (measured by an anemometer, φ= 1.2cm) was best at 40% of depth (‘mean current velocity’, coefficient of determination r²= 0,58) and decreased to r²= 0.24 at 0.6 cm above the bottom; the correlation with (calculated) shear stress was only r²= 0.23. A correlation between FST results and macroinvertebrate abundance was found for only four of eight investigated taxa and was similar to the correlation between abundance and ‘mean current velocity’.
• 3 We conclude that for fieldwork the FST hemispheres have about the same limitations as has a conventional (i.e. propeller-type) anemometer. With the hemispheres we could not obtain better data than with other methods.

     

The use of long‐term monitoring data for studies of planktonic diversity: a cautionary tale from two Swiss lakes

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Insect diversity over 36 years at a protected Sierra Nevada (California) site: towards an evaluation of the insect apocalypse hypothesis

1. Recent authors have suggested that declines of insect abundance or diversity, documented first for particular insect taxa of high interest (e.g., butterflies, bees), may apply to insect diversity more generally. This has led to an urgent call for analysis of additional longitudinal datasets to examine trends in general insect diversity.
2. Here we present a dataset gathered from 1982 to 2018 by advanced undergraduate students and graduate students enrolled in a taxonomy course that involved collecting as many insect families as possible over a 5-week period at a high-elevation protected forested site in the Sierra Nevada, California, USA.
3. The data do not support any consistent gain or loss of family-level richness between 1982 and 2018 (no linear trend); a non-linear model suggested a possible small decrease in family-level richness collected between 1986 and 1990, followed by a gradual increase from 1990 to 2018. Neither weather variables nor collector experience or skill appeared to explain among-year variation in collected insect diversity.
4. We urge caution in attempting to draw conclusions from single-site, longitudinal datasets like this one; a definitive answer to the hypothesis of a broad, global decline of insect diversity will require the joint analysis of many datasets like the one we share here.

     

Macroinvertebrate community succession in Wolf Point Creek, Glacier Bay National Park, Alaska

• 1 Macroinvertebrate community development in Wolf Point Creek in Glacier Bay National Park, Alaska formed by ice recession was investigated from 1991 to 1994 as part of a long‐term study of colonization now exceeding 20 years. Chironomidae, the first taxon to colonize the stream, still dominated the community comprising 75–95% by number, but species succession was apparent.
• 2 Species richness in August increased from five species in 1978 to 11 in 1991 and 16 in 1994.
• 3 Diamesa species, abundant in 1978 at densities exceeding 2 750 m^‐2, were not collected in 1994, while Pagastia partica dominated the community with densities exceeding 10 000 m^‐2.
• 4 Sixteen taxa, never previously collected, colonized the stream between 1991 and 1994 including representatives of Coleoptera, Muscidae, Trichoptera, and the first noninsect taxon, Oligochaeta. Colonization by new taxa was associated with an increase in summer water temperature and the development of riparian vegetation.
• 5 Inter‐specific competition is suggested as a possible factor in species succession and is incorporated into a taxa richness model of community development in postglacial streams incorporating stable and unstable channels.

     

Endophyte fungus confers resistance to ryegrass: Argentine stem weevil larval studies

ABSTRACT.

• 1 Conferred insect resistance arises from a mutualistic relationship between claviciptaceous endophytic fungi and various grasses. A number of secondary compounds are responsible for the insect resistance. This paper describes studies of the relationship between the endophytic fungus Acremonium lolii Latch, Christensen & Samuels and the Argentine stem weevil Listronotus bonariensis (Kuschel) (Curculionidae); in particular, the effects on larval feeding behaviour, growth and survival.
• 2 Larvae are stem borers. Tunnel length and distance bored into the basal sheath zone of ryegrass was reduced when the plants were infected by A. lolii.
• 3 Lolitrem B, a potent mammalian neurotoxin, levels were highest in A. lolii infected grasses and ranged from 2.61 to 7.08 μg/g dry wt, the higher value being recorded in summer.
• 4 Lolitrem B extracted from A. lolii-infected perennial ryegrass reduced larval growth rates and survival.
• 5 Lolitrem B acted via the insect stomach rather than by absorption through the integument.
• 6 The potential role of endophytic fungi in plants is discussed briefly in relation to herbivory.

     

Testing for seasonality in alpine streams: How does altitude affect freshwater insect life cycles?

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Herbivorous insect species in the tree canopy of a relict South African forest

Abstract.

• 1 The herbivorous insects on twelve species of evergreen broadleafed trees were repeatedly sampled over a period of 11 months in a small relict forest on the east coast of South Africa. This extraordinarily speciose forest patch has an unusually high proportion of endemic tree species, some of which are extremely rare.
• 2 The insect herbivore fauna (number of species) seems to be markedly depauperate compared to that reported on native, broadleafed trees from other parts of the world. Some possible reasons for this are discussed.
• 3 The total number of herbivorous insect species on each tree species was strongly correlated with the local relative abundance of the host plant species.
• 4 There was no relationship between the total number of insect herbivore species on each tree species and the relative taxonomic isolation of the trees. The proportion of seemingly unique (= specialist) herbivorous insect species (i.e. those that occurred on one tree species only) was greatest on taxonomically isolated trees.
• 5 A fundamental deficiency in the interpretation of the data in this study, and of many other similar studies that report on the number of insect species on plants, is discussed, namely the lack of clarity on the closeness of the association between individual insect herbivore species and their respective host plants.

     

Spatial and seasonal variability in the trophic role of aquatic insects: An assessment of functional feeding group applicability

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The frequency of detection of density dependence in insect orders

Abstract.

• 1 A priori, there are no obvious reasons why patterns should exist in the frequency of density dependence across insect orders. However, orders may reflect related factors which influence population regulation (e.g. life-history patterns and ecology) and are difficult to quantify. The frequency of occurrence of density dependence is compared in 171 time series (of ten or more generations) from Lepidoptera, Hemiptera, Diptera, Odonata, Hymenoptera and Coleoptera. A posteriori attempts are made to identify the cause of observed patterns.
• 2 Buhner's (1975) test found non-delayed density dependence more frequently in Odonata than Lepidoptera and Hymenoptera, which in turn showed non-delayed density dependence more frequently than Diptera, Hemiptera and Coleoptera. Similarly, detection was greater for Odonata than other orders using Dennis & Taper's (1993) test for density dependence and Crowley's (1992) test for attraction. Varley & Gradwell's (1960) test found density dependence less frequently in Hemiptera than other orders. These differences were independent of time series length, temporal trends and numbers of generations per year.
• 3 The reasons for observed patterns in detection of density dependence (and attraction) in insect orders are not clear; however, plausible explanations are differences in: (i) intrinsic growth rate, which is correlated with body size (although evidence to support this hypothesis is weak); (ii) the sampling method used; or (iii) whether individuals come from a single population or many populations.
• 4 Using Turchin's (1990) test, delayed (lag 2) density dependence was detected most frequently in Hymenoptera, which often show delayed diapause or are parasitoids.

     

Do pools impede drift dispersal by stream insects?

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Littoral microcrustacea in Lake 302S in the Experimental Lakes Area of Canada: acidification and recovery

• 1 The littoral microcrustacean community (Cladocera and Copepoda) was examined from 1989 to 1991 in a lake experimentally acidified to pH 4.5, and from 1992 to 1997 during the early stages of pH recovery.
• 2 Cladoceran abundance declined significantly from 1989 to 1991 (pH 4.5), but species richness did not change. Acantholeberis curvirostris, Simocephalus serrulatus, Latona spp. (Latona setifera, L. parviremis), and all species of chydorid Cladocera declined markedly in abundance while at pH 4.5. The abundance of cyclopoid copepods was low and Ceriodaphnia and calanoid copepods were absent.
• 3 Recovery of the community was subsequently monitored as pH was incrementally changed to a target pH 5.1 in 1992 and 1993, and to 5.8 in 1994–97. Species richness remained unchanged. Chydorid cladocerans remained at low abundance in 1992, and only Chydorus cf. brevilabris increased substantially from 1993 to 1996. Non‐chydorid Cladocera increased in abundance in 1992, declined again in 1993, then gradually increased (mainly due to Ophryoxus gracilis) in 1994–96. All species declined in 1997 as minnows recolonized the lake. The calanoid copepod Leptodiaptomus minutus was present in low numbers in 1997.
• 4 The microcrustacean community in the littoral zone of Lake 302S has not yet shown consistent signs of recovery from acidification.

     

Alternative seed defence mechanisms in a palo verde (Fabaceae) hybrid zone: effects on bruchid beetle abundance

Abstract.

• 1 We asked three questions about the patterns of relative abundance of insect herbivores across host plant taxa at a palo verde hybrid zone. (1) What is the morphological structure of the hybrid zone and does this suggest a certain pattern of introgression? (2) How are putative parental seed defence mechanisms expressed in hybrid plants? (3) Do ovipositing females prefer host plant taxa on which their offspring have best survivorship?
• 2 Morphologically, hybrids were either intermediate or tended to resemble one parental species. Previous studies have suggested that unidirectional introgression results in loss of parental defence mechanisms against herbivores. Hybrid plants in general lacked seed coat resistance and early pod abscission which are known to act as plant defence mechanisms against bruchid beetles in the parental palo verde trees.
• 3 All other sources of bruchid mortality that we examined did not vary across parental and hybrid taxa, with the possible exception of egg parasitism which occurred at a lower frequency on one parental palo verde species.
• 4 Thus, survivorship of bruchid offspring should be greater on hybrid palo verdes.
• 5 Patterns of egg densities suggested that females may prefer hybrid hosts in some years but not others. An oviposition choice experiment conducted in the field, however, showed bruchids have no preference for hybrids over one of the parental species.
• 6 These results suggest that some insect herbivores may have higher densities on hybrid host plants because they are less resistant.

     

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.

     

Dynamic distribution modelling of the swamp tigertail dragonfly Synthemis eustalacta (Odonata: Anisoptera: Synthemistidae) over a 20-year bushfire regime

1. Intensity and severity of bushfires in Australia have increased over the past few decades due to climate change, threatening habitat loss for numerous species. Although the impact of bushfires on vertebrates is well-documented, the corresponding effects on insect taxa are rarely examined, although they are responsible for key ecosystem functions and services. Understanding the effects of bushfire seasons on insect distributions could elucidate long-term impacts and patterns of ecosystem recovery.
2. Here, the authors investigated the effects of recent bushfires, land-cover change, and climatic variables on the distribution of a common and endemic dragonfly, the swamp tigertail (Synthemis eustalacta) (Burmeister, 1839), which inhabits forests that have recently undergone severe burning. The authors used a temporally dynamic species distribution modelling approach that incorporated 20 years of community-science data on dragonfly occurrence and predictors based on fire, land cover, and climate to make yearly predictions of suitability. The authors also compared this to an approach that combines multiple temporally static models that use annual data.
3. The authors found that for both approaches, fire-specific variables had negligible importance for the models, while the percentage of tree and non-vegetative cover were most important. The authors also found that the dynamic model outperformed the static ones, based on cross-validation omission rate. Model predictions indicated temporal variation in area and spatial arrangement of suitable habitat, but no patterns of habitat expansion, contraction, or shifting.
4. These results highlight not only the efficacy of dynamic modelling to capture spatiotemporal variables such as vegetation cover for an endemic insect species, but also provide a novel approach to mapping species distributions with sparse locality records.

     

Insect community response to switchgrass intercropping and stand age of loblolly pine (Pinus taeda) plantations

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Interactions between phytophagous insects and their Opuntia hosts

Abstract.

• 1 The cactophagous insect community on opuntias is analysed to show the number of insect species in different taxa. An extension of this analysis gives the average species complement on large and small opuntias.
• 2 A highly significant positive correlation is found between the total number of phytophagous insect species on individual Opuntia species and a measure of the overall ‘architecture’ of their host plants.
• 3 The specificity of the phytophagous insects on opuntias is briefly considered and the community as a whole analysed by guilds. The co-evolution of the Opuntia-feeding insects and their hosts has culminated in a community of specialist insects to the exclusion of nearly all generalist phytophages.
• 4 The life history strategies of the Opuntia-feeding insects are reviewed. Common to all developmental stages are morphological and behavioural adaptations that reduce the risk of attack by natural enemies. This is clearly the consequence of living on structurally simple host plants where there is little place to hide.
• 5 The possible influence of insect herbivores on Opuntia evolution is discussed.
• 6 An understanding of the interactions between the phytophagous insect community and opuntias has clear implications for the biological control of alien Opuntia weeds.

     

Interaction effects between local flower richness and distance to natural woodland on pest and beneficial insects in apple orchards

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Hedgerows surrounding organic apple orchards in north‐west Spain: potential to conserve beneficial insects

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