Plants have evolved a plethora of different chemical defenses covering nearly all classes of (secondary) metabolites that represent a major barrier to herbivory: Some are constitutive; others are induced after attack. Many compounds act directly on the herbivore, whereas others act indirectly via the attraction of organisms from other trophic levels that, in turn, protect the plant. An enormous diversity of plant (bio)chemicals are toxic, repellent, or antinutritive for herbivores of all types. Examples include cyanogenic glycosides, glucosinolates, alkaloids, and terpenoids; others are macromolecules and comprise latex or proteinase inhibitors. Their modes of action include membrane disruption, inhibition of nutrient and ion transport, inhibition of signal transduction processes, inhibition of metabolism, or disruption of the hormonal control of physiological processes. Recognizing the herbivore challenge and precise timing of plant activities as well as the adaptive modulation of the plants’ metabolism is important so that metabolites and energy may be efficiently allocated to defensive activities.

Plant Defense Against Herbivores: Chemical Aspects

We assessed the state of knowledge regarding the effects of large-scale pollution with neonicotinoid insecticides and fipronil on non-target invertebrate species of terrestrial, freshwater and marine environments. A large section of the assessment is dedicated to the state of knowledge on sublethal effects on honeybees (Apis mellifera) because this important pollinator is the most studied non-target invertebrate species. Lepidoptera (butterflies and moths), Lumbricidae (earthworms), Apoidae sensu lato (bumblebees, solitary bees) and the section “other invertebrates” review available studies on the other terrestrial species. The sections on freshwater and marine species are rather short as little is known so far about the impact of neonicotinoid insecticides and fipronil on the diverse invertebrate fauna of these widely exposed habitats. For terrestrial and aquatic invertebrate species, the known effects of neonicotinoid pesticides and fipronil are described ranging from organismal toxicology and behavioural effects to population-level effects. For earthworms, freshwater and marine species, the relation of findings to regulatory risk assessment is described. Neonicotinoid insecticides exhibit very high toxicity to a wide range of invertebrates, particularly insects, and field-realistic exposure is likely to result in both lethal and a broad range of important sublethal impacts. There is a major knowledge gap regarding impacts on the grand majority of invertebrates, many of which perform essential roles enabling healthy ecosystem functioning. The data on the few non-target species on which field tests have been performed are limited by major flaws in the outdated test protocols. Despite large knowledge gaps and uncertainties, enough knowledge exists to conclude that existing levels of pollution with neonicotinoids and fipronil resulting from presently authorized uses frequently exceed the lowest observed adverse effect concentrations and are thus likely to have large-scale and wide ranging negative biological and ecological impacts on a wide range of non-target invertebrates in terrestrial, aquatic, marine and benthic habitats.

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Effects of neonicotinoids and fipronil on non-target invertebrates

This paper reviews the literature concerning digestion and nutrient content of pollen. Four topics are addressed in detail: 1) The mechanism of pollen digestion by animals; 2) The efficiency of mechanical and digestive removal of pollen content by various animals; 3) Range and taxonomic distribution of pollen nutrients, and 4) Adaptive hypotheses proposed to associate pollen chemistry with pollinator reward. Studies on the mechanism(s) of pollen digestion remain inconclusive, but suggest that differences in digestibility among pollen types may reflect differences in pollen wall porosity, thickness, and composition. Although hummingbirds reportedly digest pollen very poorly, most animals studied, including those that do not regularly consume pollen, can digest 50–100% of ingested grains. Overlooked and recent research of pollen protein content shows that pollen grains may contain over 60% protein, double the amount cited in some studies of pollen-feeding animals. Adaptive hypotheses that associate pollen starch and pollen caloric content with pollinator reward remain unsubstantiated when critically viewed through the lens of phylogeny.

Pollen nutritional content and digestibility for animals

A number of aposematic butterfly and diurnal moth species sequester unpalatable or toxic substances from their host plants rather than manufacturing their own defensive substances. Despite a great diversity in their life histories, there are some general features in the selective utilization of plant secondary metabolites to achieve effective protection from predators. This review illustrates the biochemical, physiological, and ecological characteristics of phytochemical-based defense systems that can shed light on the evolution of the widely developed sequestering lifestyles among the Lepidoptera.

Sequestration of defensive substances from plants by Lepidoptera.

Although plant-herbivore and plant-pollinator interactions have traditionally been studied separately, many traits are simultaneously under selection by both herbivores and pollinators. For example, secondary compounds commonly associated with herbivore defense have been found in the nectar of many plant species, and many plants produce nectar that is toxic or repellent to some floral visitors. Although secondary compounds in nectar and toxic nectar are geographically and phylogenetically widespread, their ecological significance is poorly understood. Several hypotheses have been proposed for the possible functions of toxic nectar, including encouraging specialist pollinators, deterring nectar robbers, preventing microbial degradation of nectar, and altering pollinator behavior. All of these hypotheses rest on the assumption that the benefits of toxic nectar must outweigh possible costs; however, to date no study has demonstrated that toxic nectar provides fitness benefits for any plant. Therefore, in addition to these adaptive hypotheses, we should also consider the hypothesis that toxic nectar provides no benefits or is tolerably detrimental to plants, and occurs due to previous selection pressures or pleiotropic constraints. For example, secondary compounds may be transported into nectar as a consequence of their presence in phloem, rather than due to direct selection for toxic nectar. Experimental approaches are necessary to understand the role of toxic nectar in plant-animal interactions.

http://people.umass.edu/lsadler/adlersite/adler/Oikos00.pdf

The ecological significance of toxic nectar

The influence of 63 dietary allelochemicals (alkaloids, terpenes, glycosides,etc.) on the feeding behaviour of bees (Apis mellifera) was tested in terms of deterrency and attraction. For 39 compounds a deterrent (mostly alkaloids, coumarins and saponins) and for 3 compounds an attractive response (mostly terpenes) was obtained in choice tests, which allowed the calculation of respective ED50-values. Under no-choice conditions, 17 out of 29 allelochemicals caused mortality at concentrations between 0.003 and 0.6%. Especially toxic were alkaloids, saponins, cardiac glycosides and cyanogenic glycosides. These data show that bees which are confronted with plant allelochemicals in nectar and pollen, are not especially adapted (i.e. insensitive) to the plants' defence chemistry. GLC and GLS-MS data are given on the alkaloid composition of nectar and pollen ofBrugmansia aurea, Atropa belladonna andLupinus polyphyllus.

Attraction, deterrence or intoxication of bees (Apis mellifera) by plant allelochemicals

other signals, for instance mechanical ones, under natural conditions. It is important to point out that two compounds, MP and MN, appear to act at the same time as a pheromone for the honeybee and as a kairomone for the parasitic mite Varroa jacobsoni. These compounds, emitted by honeybee larvae, first attract the mites, and then the honeybee workers for the capping of the cells. The analysis of the composition of the pheromone blend and of the kairomone blend, in relation to the age of the larvae, is in progress. This work was supported by grants from the National Institute of Agricultural Research (INRA), the National Center of Scientific Research (CNRS), the French Ministry of Environment (SRETIE), the ECC (DG.VI), and Intermiel. We thank Drs. R. Brossut, C. Descoins, C. Noirot, J. Pain, G. Ourisson, and F. Ruttner for critical reading of the manuscript, and J. E. Walker for her assistance in the English translation.

Carrier-mediated uptake of digoxin by larvae of the cardenolide sequestering moth, Syntomeida epilais

Abstract Oncopeltus fasciatus (Hemiptera) is morphologically and physiologically adapted to sequester cardiac glycosides for its chemical defence against predators. As a prerequisite for cardenolide utilization this bug has to take up and store the dietary cardenolides. Using isolated midguts it could be shown experimentally that the resorption of [3H]digoxin is time-, and temperature-dependent. The hydrogen ion concentration for uptake is optimal between pH 5.5 and 7 and an activation energy of 45 to 49 kJ/mol can be derived from an Arrhenius plot. Uptake can be inhibited competitively by the polar cardiac glycoside convallatoxin. Sulfhydryl group reagents (N-ethylmaleimide), membrane destabilizing compounds and respiratory chain inhibitors reduce digoxin resorption, as do inhibitors of sugar transport (e.g., phloridzin). The experimental data thus provide evidence for a cardenolide carrier in midgut cells of O. fasciatus.

Andreas Detzel

Papers

Attraction, deterrence or intoxication of bees (Apis mellifera) by plant allelochemicals

Carrier-mediated uptake of digoxin by larvae of the cardenolide sequestering moth, Syntomeida epilais

Evidence for a Cardenolide Carrier in Oncopeltus fasciatus (Dallas) (Insecta: Hemiptera)