Plant stress and larval performance of a dipterous gall former

According to the plant vigour hypothesis, galling insects should respond positively and perform better on vigorous plants or plant parts, the opposite of the predictions of the plant stress hypothesis. I carried out field experiments to analyse the effects of sustained abiotic stress on the interactions between the common reed (Phragmites australis) and a gall-forming fly (Lipara lucens). The reed shoot diameter (a measure of plant vigour) is strongly affected by environmental conditions, where dry and/or nutrient-poor habitats produce thinner (stressed) shoots. L. lucens gall density is negatively correlated with shoot diameter. In a survival experiment with a wide range of shoot diameters, larval mortality was also highly correlated with shoot quality. Gall formation was higher on thinner, stressed shoots. An analysis of the gall tissues revealed that galls induced by L. lucens contain a high amount of a nutrient-rich feeding tissue. The impact of L. lucens is higher on thinner shoots. The results of this study showed that L. lucens performs better on stressed hosts, which contradicts the plant vigour hypothesis for galling insects. The low nutrient availability in the stressed shoots can be compensated by the production of galls with a nutrient-rich feeding tissue.     

Leaf buds, a factor in host selection by Battus philenor butterflies

ABSTRACT.

• 1 Field and laboratory experiments identified a character intrinsic to Aristolochia reticulata Nutt. host plants, the terminal leaf bud, that is involved in host-selection behaviour by female pipevine swallowtail butterflies (Battus philenor L.) searching for oviposition sites.
• 2 In the field, the frequency with which females landed on non-host buds declined seasonally as the proportion of host foliage that consisted of buds decreased. Female butterflies did not land on non-host species in proportion to their abundance; rather, females landed on those non-host species whose buds resembled those of A.reticulata.
• 3 A.reticulata plants whose terminal leaf bud was concealed by plastic tape were less susceptible to oviposition in the field than were control plants.
• 4 Female butterflies released in a large, outdoor enclosure were conditioned to search for leaf buds only when exposed to a host species bearing a prominent terminal leaf bud.
• 5 The significance of conditioning of leaf-bud searching behaviour is discussed with respect to discrimination between hosts and non-hosts, between host species, and among plants within a host species.

     

Interaction of pre-attack and induced monoterpene concentrations in host conifer defense against bark beetle-fungal complexes

Two pine species (Pinus resinosa, P. banksiana) responded to inoculation with fungi carried by bark beetles by rapidly increasing monoterpene concentrations at the entry site. Changes in total monoterpenes were more pronounced than changes in proportionate compositions. The extent and rate of host response was affected by fungal species, the viability of the inoculum, and host tree species. In general, host responses were highest to fungi that are phytopathogenic and consistently associated with the major bark beetles in the study region. Simple mechanical wounding cannot account for the observed allelochemical changes, as aseptic inoculations elicited only minor reactions. Similarly, inoculation with autoclaved inviable fungi generally elicited intermediate responses, suggesting that both structural and metabolic fungal properties are important. Responses by jack pine, P. banksiana, were generally more rapid and variable than those of red pine, P. resinosa. Dose-toxicity experiments with synthetic compounds demonstrated that monoterpene concentrations present in vivo only a few days after simulated attack are lethal to most beetles. Constitutive (pre-attack) monoterpene levels can also exert some toxicity. Because bark beetles engage in pheromone-mediated mass attacks that can deplete host defenses, constitutive monoterpene levels, while a necessary early phase of successful plant defense, appear insufficient by themselves. Such interactions between constitutive and induced defense chemistry may be important considerations when evaluating general theories of plant defense.     

Chemical barriers to adaptation by a specialist herbivore

Summary Depressaria pastinacella, the parsnip webworm (Lepidoptera: Oecophoridae), feeds throughout eastern North America on Pastinaca sativa (wild parsnip) and few other species. The assumption that specialist herbivores such as the parsnip webworm are adapted to hostplant chemistry, and are therefore unaffected by chemical variation in hostplants, was tested. Flower buds from plants grown first in the greenhouse and then in the field were fed to ultimate instar webworms. Plant phenotype had a significant effect on virtually all webworm food utilization parameters. While nutritional factors (i.e., nitrogen content) were correlated with approximate digestibility, two constituents of the flowers — bergapten and xanthotoxin, both linear furanocoumarins — independently accounted for a significant amount of variation in food utilization indicies. The physiological effects of these furanocoumarins were confirmed in artificial diet experiments. Despite the fact that the two most important furanocoumarins in parsnip flowers relative to webworm feeding and growth are isomers, differing only in the positioning of a methoxy substituent, they have different physiological effects; while xanthotoxin in general has no effect on growth, bergapten decreases growth and digestibility of the diet. These results underscore the need in studies of plant-animal interactions to examine individual chemical components rather than classes of compounds.     

Phytochemical correlates of herbivory in a community of native and naturalized cruciferae

Oviposition and larval feeding behaviors of the crucifer specialist Pieris napi macdunnoughii correlate with leaf glucosinolate profils of plant species in a natural community. Profiles are species-specific in this group of eight Cruciferae, but particular glucosinolates are shared by subsets of the community. Pieris accepts two lethal naturalized weeds whose glucosinolate profiles resemble that of an indigenous foodplant. The results suggest that specific glucosinolates constitute insect behavioral cues which are only loosely linked evolutionarily to foodplant suitability, and further suggest that allelochemically similar community associates influence the coevolution of individual plant species with insect herbivores.     

Opposing effects of elevated CO2 and N deposition on Lymantria monacha larvae feeding on spruce trees

The effects of elevated atmospheric CO₂ and increased wet N deposition on leaf quality and insect herbivory were evaluated in nine model ecosystems composed of 7-year-old spruce trees (Picea abies) and three understorey species established on natural forest soil. Each model ecosystem was grown in a simulated montane climate, and was exposed to one of three CO₂ concentrations (280, 420, and 560 μl l⁻¹), and to one of three levels of N deposition (0, 30, and 90 kg ha⁻¹ year⁻¹) for 3 years. In the 3rd year of the experiment second to third instars of the nun moth (Lymantria monacha) were allowed to feed directly on current-year needles of top canopy branches of each tree for 12 days. Specific leaf area (SLA), water content, and N concentration decreased in needles exposed to elevated CO₂, whereas the concentrations of starch, condensed tannins, and total phenolics increased. Increased N deposition had no significant effect on SLA, and water content, but the concentrations of starch, condensed tannins, and total phenolics decreased, and sugar and N concentrations increased. Despite higher relative consumption rates (RCRs) larvae consumed 33% less N per unit larval biomass and per day at the two high CO₂ treatments, compared to those feeding on 280 μl l⁻¹-needles, but they maintained similar N accumulation rates due to increased N utilization efficiencies (NUE). However, over the 12-day experimental period larvae gained less N overall and reached a 35% lower biomass in the two high-CO₂ treatments compared to those at 280 μl l⁻¹. The effects of increased N deposition on needle quality and insect performance were generally opposite to those of CO₂ enrichment, but were lower in magnitude. We conclude that altered needle quality in response to elevated CO₂ will impair the growth and development of L. monacha larvae. Increasing N deposition may mitigate these effects, which could lead to altered insect herbivore distributions depending on regional patterns of N deposition. Received: 8 June 1998 / Accepted: 27 October 1998     

CO2 and light effects on deciduous trees: growth, foliar chemistry, and insect performance

This study examined the effects of CO₂ and light availability on sapling growth and foliar chemistry, and consequences for insect performance. Quaking aspen (Populus tremuloides Michx.), paper birch (Betula papyrifera Marsh.), and sugar maple (Acer saccharum Marsh.) were grown in controlled environment greenhouses under ambient or elevated CO₂ (38.7 and 69.6 Pa), and low or high light availability (375 and 855 μmol m⁻² s⁻¹). Because CO₂ and light are both required for carbon assimilation, the levels of these two resources are expected to have strong interactive effects on tree growth and secondary metabolism. Results from this study support that prediction, indicating that the relative effect of rising atmospheric CO₂ concentrations on the growth and secondary metabolism of deciduous trees may be dependent on light environment. Trees in ambient CO₂-low light environments had substantial levels of phytochemicals despite low growth rates; the concept of basal secondary metabolism is proposed to explain allocation to secondary metabolites under growth-limiting conditions. Differences between CO₂ and light effects on the responses of growth and secondary metabolite levels suggest that relative allocation is not dependent solely on the amount of carbon assimilated. The relative growth rates and indices of feeding efficiency for gypsy moth (Lymantria dispar L.) larvae fed foliage from the experimental treatments showed no significant interactive effects of light and CO₂, although some main effects and many host species interactions were significant. Gypsy moth performance was negatively correlated with CO₂- and light-induced increases in the phenolic glycoside content of aspen foliage. Insects were not strongly affected, however, by treatment differences in the nutritional and secondary chemical components of birch and maple. Received: 15 July 1998 / Accepted: 23 December 1998     

Ingestion and excretion of two transgenic Bt corn varieties by slugs

The release of transgenic Bacillus thuringiensis (Bt) corn expressing various Cry endotoxins has raised concern that these endotoxins are disseminated in the food web and may adversely affect non-target beneficial organisms, such as predators and organisms of the decomposer food web. We therefore investigated in a laboratory study, whether the Cry1Ab and Cry3Bb1 protein from Bt corn could potentially be transferred to such organisms by measuring the Cry protein content in the two common agricultural slug pests Arion lusitanicus and Deroceras reticulatum and their feces. We measured Cry1Ab and Cry3Bb1 protein concentration in leaves, intestines, and feces of corn leaf-fed slugs using ELISA and determined how much of the ingested protein is excreted by the slugs. Cry3Bb1 concentration in leaves of DKC5143Bt corn was significantly higher than Cry1Ab concentration in leaves of N4640Bt corn. While slugs were feeding on corn leaves, the Cry3Bb1 and Cry1Ab proteins were found in intestines and feces of both slug species. Bt protein concentrations in intestines of Cry3Bb1 corn-fed slugs were in both slug species higher than in Cry1Ab corn fed slugs, whereas no differences between Cry3Bb1 and Cry1Ab protein in feces were found. After slugs had ceased feeding on Bt corn, Cry1Ab was detectable in fresh slug feces for a significantly longer time and often in higher amounts than the Cry3Bb1. Our results indicate that both Cry proteins are likely to be transferred to higher trophic levels and to the decomposer food web. Since different Bt proteins seem to vary in their degradation, they have different transfer probabilities. This should be considered in risk assessments for non-target arthropods.     

河北省和山东省棉蚜天敌蜘蛛名录

经15年来的调查,已明确河北省和山东省的棉蚜天敌蜘蛛种类,计11科、59种,即:(1)隐石蛛科1种,(2)卷叶蛛科2种,(3)园蛛科5种,(4)肖蛸科6种,(5)球腹蛛科5种,(6)皿蛛科7种.(7)狼蛛科7种,(8)管巢蛛科8种,(9)蟹蛛科13种,(10)逍遥蛛科2种,(11)跳蛛科3种。同时明确其天敌河北省为11科、51种,山东省为11科、50种。     

Effects of genetic structure of Lupinus arboreus and previous herbivory on Platyprepia virginalis caterpillars

Two leaf-feeding caterpillars, western tussock moth (Orgyia vetusta) and ranchman's tiger moth (Platyprepia virginalis) are abundant on Lupinus arboreus along the California coast. Previous experiments and observations suggested that feeding caused by either of these two folivores could reduce the performance and possibly the abundance and distribution of the other species. Previous common garden experiments also indicated that genetically determined characteristics of the host plants were important for O. vetusta. Here we examined the effects of familial origin of the host plant, and previous damage caused by O. vetusta on the abundance of P. virginalis. Plants with parents from one of three locations had higher numbers of P. virginalis than plants with parents from the other two locations. However, this effect of plant origin depended on the statistical analysis and was not as strong as the effect of prior damage by O. vetusta on numbers of P. virginalis. Counter to our expectation, bushes that supported higher levels of damage by O. vetusta in the previous summer had more P. virginalis caterpillars. This strong effect could result by both moth species selecting bushes with the same traits or as the result of herbivory by O. vetusta enhancing the susceptibility of bushes for P. virginalis. Received: 28 October 1998 / Accepted: 14 March 1999     

Mechanisms of interspecific competition that result in successful control of Pacific mites following inoculations of Willamette mites on grapevines

We have successfully reduced populations of economically damaging Pacific mites by introducing another economically unimportant herbivorous mite to grapevines. Although this technique has proven successful in greenhouse and commercial applications, we had a poor understanding of why it worked. Experiments were conducted in a Zinfandel vineyard to assess the relative importance of different forms of competition between these two species and the importance of the timing of the introduction. The early season release of Willamette mites that occurred as shoots were first elongating reduced populations of Pacific mites, indicating that some form of competition occurred in this experiment. However, introductions of Willamette mites 4 weeks later did not reduce populations of Pacific mites. An early season release is essential for control. Control shoots did not receive additions of Willamette mites and had high population densities of Pacific mites. In contrast to this, Pacific mite populations were reduced on treatments with Willamette mites, regardless of whether the two species overlapped spatially or were separated by a barrier. This indicated that physical interference was not responsible, that induced resistance was involved, and that the effect was systemic throughout the shoot.     

Pests and Diseases of Betelvine Piper Betle and Their Natural Enemies in India

Betelvine (Piper betle) is an important cash crop in India. The cultivation of this crop is seriously threatened by several pest insects, for example the homopterans Aleurocanthus rugosa, Dialeuroides pallida, Aphis gossypii and Tricentrus gibbosulus and the thysanopterans Membrothrips indicus, Zaniothrips ricini, Mymarothrips garuda and Aroidothrips longistylus and by the plant pathogenic fungi Curvularia lunata, Phytophthora parasitica, Colletotrichum piperis and Fusarium moniliforme. The pest and pathogen species are attacked by natural enemies belonging to the Coleoptera, Diptera, Heteroptera, Hymenoptera, Aranea and Acari. While occurrence of non-acarine enemies is confined to certain seasons, the acarine enemies are associated with the insect pests and fungal pathogens throughout the year. The predatory mites Walzia indiana and Proctolaelaps pygmaeus and the mycophagous mites Unguizetes clavatus and Acarus sp. seem to have potential to control the insect pests and fungal pathogens, respectively.     

Population dynamics models in plant-insect herbivore-pesticide interactions

We consider several population dynamics models in investigating data from controlled experiments with aphids in broccoli patches surrounded by different margin types (bare or weedy ground) and three levels of insecticide spray (no, light, or heavy spray). We carry out parameter estimation computations along with statistical analysis to compare autonomous versus non-autonomous model dynamics. We conclude with a brief discussion of some not-so-subtle pitfalls that can arise when using quantitative measures of model fit-to-data to make biological inferences as well as offer a positive example of how one might combine a priori biological hypothesis and intuition with rather sophisticated (from a field biology viewpoint) mathematical methodologies to suggest synergisms.     

Oral secretions from herbivorous lepidopteran larvae exhibit ion channel-forming activities

Feeding insects introduce oral secretions (OS) into the wounded tissue of the attacked plant. Various OS-derived molecules must be involved in subsequent processes including the induction of plant defence reactions. Using the planar lipid bilayer membrane technique, isolated OS were analyzed with respect to their membrane activities. Transmembrane ion fluxes were generated by OS of eight different lepidopteran larvae, all of which form comparable ion channels in artificial membranes. Currents were characterized by long lasting open times and conductivities from 250pS up to 1100pS. Channels formed by Spodoptera exigua secretions showed a preference for cations over anions. OS also induced a transient increase of the cytosolic calcium concentration in soybean cells, determined by the aequorin technique. Known compounds of the OS, fatty-acid-glutamine conjugates, also interfered with the membrane but were unable to form stable channels. Since ion fluxes and depolarization are early responses upon insect feeding, OS-derived components may be involved in the elicitation process by direct interaction with the plant membranes.     

Enemy-free space: a new name for an ancient ecological concept

While the importance of enemy-free space in structuring animal communities well merits the attention recently given to it, the concept is far more ancient than has been suggested. Far from Charles Darwin being the first to argue this, as recently claimed, it has long been known that major aspects of species' lifestyles are moulded by the impact of natural enemies. Examples from the literature spanning a period of more than two millenia, some of them far more specific than Darwin's comment, are cited.