Biotic invaders are species that establish a new range in which they proliferate, spread, and persist to the detriment of the environment. They are the most important ecological outcomes from the unprecedented alterations in the distribution of the earth's biota brought about largely through human transport and commerce. In a world without borders, few if any areas remain sheltered from these im- migrations. The fate of immigrants is decidedly mixed. Few survive the hazards of chronic and stochastic forces, and only a small fraction become naturalized. In turn, some naturalized species do become invasive. There are several potential reasons why some immigrant species prosper: some escape from the constraints of their native predators or parasites; others are aided by human-caused disturbance that disrupts native communities. Ironically, many biotic invasions are apparently facilitated by cultivation and husbandry, unintentional actions that foster immigrant populations until they are self-perpetuating and uncontrollable. Whatever the cause, biotic invaders can in many cases inflict enormous environmental damage: (1) Animal invaders can cause extinctions of vulnerable native species through predation, grazing, competition, and habitat alteration. (2) Plant invaders can completely alter the fire regime, nutrient cycling, hydrology, and energy budgets in a native ecosystem and can greatly diminish the abundance or survival of native species. (3) In agriculture, the principal pests of temperate crops are nonindigenous, and the combined expenses of pest control and crop losses constitute an onerous "tax" on food, fiber, and forage production. (4) The global cost of virulent plant and animal diseases caused by parasites transported to new ranges and presented with susceptible new hosts is currently incalculable. Identifying future invaders and taking effective steps to prevent their dispersal and establishment con- stitutes an enormous challenge to both conservation and international commerce. Detection and management when exclusion fails have proved daunting for varied reasons: (1) Efforts to identify general attributes of future invaders have often been inconclusive. (2) Predicting susceptible locales for future invasions seems even more problematic, given the enormous differences in the rates of arrival among potential invaders. (3) Eradication of an established invader is rare, and control efforts vary enormously in their efficacy. Successful control, however, depends more on commitment and continuing diligence than on the efficacy of specific tools themselves. (4) Control of biotic invasions is most effective when it employs a long-term, ecosystem- wide strategy rather than a tactical approach focused on battling individual invaders. (5) Prevention of invasions is much less costly than post-entry control. Revamping national and international quarantine laws by adopting a "guilty until proven innocent" approach would be a productive first step. Failure to address the issue of biotic invasions could effectively result in severe global consequences, including wholesale loss of agricultural, forestry, and fishery resources in some regions, disruption of the ecological processes that supply natural services on which human enterprise depends, and the creation of homogeneous, impoverished ecosystems composed of cosmopolitan species. Given their current scale, biotic invasions have taken their place alongside human-driven atmospheric and oceanic alterations as major agents of global change. Left unchecked, they will influence these other forces in profound but still unpredictable ways.

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Biotic invasions: causes, epidemiology, global consequences, and control

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.

This article discusses the insect CYP genes and P450 enzymes. Cytochrome P450, or CYP genes, constitutes one of the largest family of genes, with representatives in virtually all living organisms, from bacteria to protists, plants, fungi, and animals. The multiple functions of insect P450 enzymes, their complex biochemistry, and their toxicological and physiological importance are presented. Physiological functions are not restricted to one branch of the P450 evolutionary tree, and the ramifications that seem typical of “environmental response genes” are found in both microsomal and mitochondrial P450 encoding genes. The CYP3 and CYP4 clans seem the most diverse, and the smaller CYP2 and mitochondrial CYP clans seem to have proportionally more genes found as orthologs across species. There are many physiological functions for P450s that just are not known or suspected. CYP4C7 was shown to be selectively expressed in the corpora allata and to metabolize JH and its precursors to new metabolites.

Insect CYP Genes and P450 Enzymes

https://www.cell.com/trends/plant-science/pdf/S1360-1385(04)00063-9.pdf

Interactions of insect pheromones and plant semiochemicals

4.1 – Insect Cytochrome P450

Larvae of Creatonotos transiens (Lepidoptera, Arctiidae) and Zonocerus variegatus (Orthoptera, Pyr-gomorphidae) ingest 14C-labeled senecionine and its N-oxide with the same efficiency but sequester the two tracers exclusively as N-oxide. Larvae of the non-sequestering Spodoptera littoralis eliminate efficiently the ingested alkaloids. During feeding on the two alkaloidal forms transient levels of senecionine (but not of the N-oxide) are built up in the haemolymph of S. littoralis larvae. Based on these results, senecionine [18O]N-oxide was fed to C. transiens larvae and Z. variegatus adults. The senecionine N-oxide recovered from the haemolymph of the two insects shows an almost complete loss of 18O label, indicating reduction of the orally fed N-oxide in the guts, uptake of the tertiary alkaloid and its re-N-oxidation in the haemolymph. The enzyme responsible for N-oxidation is a soluble mixed function monooxygenase. It was isolated from the haemolymph of the sequestering arctiid Tyria jacobaeue and purified to electrophoretic homogeneity. The enzyme is a flavoprotein with a native M, of 200000 and a subunit M, of 51 000. It shows a pH optimum at 7.0, has its maximal activity at a temperature of 40–45°C and an isoelectric point at pH 4.9. The reaction is strictly NADPH-dependent (Km, 1.3 μM). From 20 pyrrolizidine alkaloids so far tested as substrates, the enyzme N-oxidizes only alkaloids with structural elements which are essential for hepatotoxic and genotoxic pyrrolizidine alkaloids (i.e. 1,2-double bond, esterification of the allylic hydroxyl group, presence of a second free or esterified hydroxyl group at carbon 7). A great variety of related alkaloids and xenobiotics were tested as substrate, none was accepted. The K, values of senecionine, monocrotaline and heliotrine, representing the three main types of pyrrolizidine alkaloids, are 1.3 μM, 12.5 μM and 290 μM, respectively. The novel enzyme was named senecionine N-oxygenase (SNO). The enzyme was partially purified from two other arctiids. The three SNOs show the same general substrate specificity but differ in their affinities towards the main structural types of pyrrolizidine alkaloids. The enzymes from the two generalists (Creatonotos transiens and Arctia caja) display a broader substrate affinity than the enzyme from the specialist (Tyria jacobaeae). The two molecular forms of pyrrolizidine alkaloids, the lipophilic protoxic tertiary amine and its hydrophilic nontoxic N-oxide are discussed in respect to their bioactivation and detoxification in mammals and their role as defensive chemicals in specialized insects. Pyrrolizidine-alkaloid-sequestering insects store the alkaloids as nontoxic N-oxides which are reduced in the guts of any potential insectivore. The lipophilic tertiary alkaloid is absorbed passively and then bioactivated by cytochrome P 450 oxidase.

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The two Faces of Pyrrolizidine Alkaloids: the Role of the Tertiary Amine and its N‐Oxide in Chemical Defense of Insects with Acquired Plant Alkaloids

Pyrrolizidine alkaloids in Chromolaena odorata. Chemical and chemoecological aspects

http://www.fzi.uni-freiburg.de/pdf/p-Creatonotine.pdf

Transformation of plant pyrrolizidine alkaloids into novel insect alkaloids by Arctiid moths (Lepidoptera)

Larvae of the arctiid moth Tyria jacobaeae reared on Senecio jacobaea or S. vulgaris take up and store pyrrolizidine alkaloids (PAs) from their host plants. Individual PAs are taken up without preference. The PA patterns found in the insect bodies correspond to the PA composition of their host plants. Like plants the insects store PAs as N-oxides, and larvae as well as pupae are specifically able to N -oxidize any tertiary PA. Callimorphine (O9-(2-methyl-2-acetoxybutanoyl)-retronecine), an insect PA well known from several arctiids, was found in pupae and imagines of Tyria which as larvae had been fed on S. jacobaea. It is accompanied by small amounts of its isomer O7-(2-methyl-2-acetoxybutanoyl)-retronecine named isocallimor-phine. The callimorphines may well account for 45% of total PAs found in the insect. Only small amounts of callimorphine were detected in pupae of Tyria which as larvae had been fed on S. vulgaris. [14C]Callimorphine N -oxide was isolated and identified from Tyria pupae which as larvae received [14C]retronecine. It is suggested that Tyria is able to esterify retronecine, derived from hydrolysis of ingested plant PAs with a necic acid produced by the insect. During metamorphosis the formation of callimorphine is restricted to the early stage of pupation.

Sequestration, N-Oxidation and Transformation of Plant Pyrrolizidine Alkaloids by the Arctiid Moth Tyria jacobaeae L

Zonocerus variegatus L. (Pyrgomorphidae) is a polyphagous, aposematic, West African grasshopper, the dry season populations of which are nowadays a pest in agriculture and forestry. The insects seem to gain protection from predation by storing pyrrolizidine alkaloids (PAs) from plant sources: (1) Bernays et al. (1977) reported storage of monocrotaline as the main PA in specimens reared on Crotalaria retusa (Fabaceae); (2) Boppre et al. (1984) found a close relative, Z. elegans, to be attracted to sources of dry PA-containing plants as well as to pure PAs, suggesting a pharmacophagous relationship of Zonocerus to PAs (cf. Boppre, 1986). In this paper, we report on sequestration of PAs by Zonocerus from the Siam weed, Chromolaena odorata King & Robinson (Asteraceae) and discuss a striking influence that secondary compounds of an introduced non-host plant can have on native insect populations.

Andreas Biller

Papers

The two Faces of Pyrrolizidine Alkaloids: the Role of the Tertiary Amine and its N‐Oxide in Chemical Defense of Insects with Acquired Plant Alkaloids

Pyrrolizidine alkaloids in Chromolaena odorata. Chemical and chemoecological aspects

Transformation of plant pyrrolizidine alkaloids into novel insect alkaloids by Arctiid moths (Lepidoptera)

Sequestration, N-Oxidation and Transformation of Plant Pyrrolizidine Alkaloids by the Arctiid Moth Tyria jacobaeae L

The non-nutrional relationship of Zonocerus (Orthoptera) to Chromolaena (Asteraceae)