Gabriel Guillet

Multiple modes of insecticidal action of three classes of polyacetylene derivatives from Rudbeckia hirta

Polyacetylene derivatives (PADs) occurring in Rudbeckia hirta (Asteraceae) were isolated or chemically synthesized and examined for their insecticidal properties against mosquito larvae under different light regimes: dark (D), visible (VIS) and visible+near-ultra-violet (VIS+UV). A straight chain polyine, 1-tridecene-3,5,7,9,11-pentayne, was highly toxic under all light regimes tested although a thiarubrine, 3-(1-propynyl)-6-(3,5-hexadien-1-ynyl)-1,2-dithiacyclohexa-3,5-diene, and a thiophene, 2-(1-propynyl)-5-(3,5-hexadien-1-ynyl)-thiophene, showed a toxicity that significantly varied between the light regimes. The thiarubrine was more toxic against mosquito larvae under D or VIS+UV conditions while the thiophene had a more pronounced toxicity only in presence of VIS+UV irradiation. The distinctive insecticidal properties in darkness of the thiarubrine compare to the thiophene were also confirmed in trials with larvae of a herbivorous insect, Manduca sexta. Such a variability in the light-modulated toxicity to insects for different biosynthetically related PADs emphasizes a diversity in the insecticidal mechanisms of action.

How do insect herbivores cope with the extreme oxidative stress of phototoxic host plants

Plants of the Asteraceae and Hypericaceae possess secondary compounds that induce photooxidation in insect herbivores that consume them. One of the well-established modes of action of these substances is peroxidation of membrane lipids. Some herbivores counteract these defences by avoidance of light and tissues rich in phototoxins or the ability to detoxify these secondary substances. The cytochrome P-450 polysubstrate monooxygenase systems involved, the metabolic products, and a new putative toxin pump have been described. Dietary antioxidants (β-carotene, vitamin E, ascorbate) are additional defences against phototoxicity. They reduce mortality in herbivores exposed to phototoxins and some specialist herbivores have high constitutive levels. Adapted specialist insects also have higher constitutive levels of superoxide dismutase (SOD) and respond to phototoxins in their diet by the induction of catalase (CAT), glutathione reductase (GR), and increased levels of reduced glutathione (GSH). Artificial inhibition of the enzymes SOD and CAT had little effect on phototoxicity but inhibition of GSH synthesis in herbivores enhanced photooxidative effects of administered phototoxins on lipid peroxidation. While insects have many mechanisms to overcome plant photooxidants, the Asteraceae appear to have adopted a strategy of counterattack. We suggest and provide preliminary evidence that a second group of secondary substances, the sesquiterpene lactones, occurring in the Asteraceae can attack key antioxidant defences to synergise phototoxins.

Behavioral and biochemical adaptations of generalist and specialist herbivorous insects feeding on Hypericum perforatum (Guttiferae)

Abstract The behavior and biochemical adaptations of 3 generalist insect species (Tettigonia viridissima L., Ruspolia nitidula Scopoli, and Conocephalus discolor Thunberg) and 3 specialists (Galeruca tanaceti L., Chrysolina geminata Payrtull, and Cloantha perspicillaris Boisduval) to the hypericin-containing leaves of Hypericum perforatum were investigated in southwestern France. The generalists preferentially fed on the part of the leaf lacking the phototoxic, hypericin-laden dark glands. The specialists showed no discriminatory feeding pattern but exhibited a negative phototaxis that is presumed to be an efficient strategy to circumvent the light-induced toxicity of hypericin. The constitutive and hypericin-inducible activities of glutathione reductase and glutathione S-transferase, 2 antioxidant enzymes which are considered to be biochemical adaptations used by phytophagous insects to attenuate the oxidative stress caused by photosensitization, were determined in the fatbody and midgut of T. viridissima, C. discolor, G. tanaceti, and G. geminata. The specialist insects had lower constitutive activities of glutathione S-transferase and glutathione reductase than the generalists, although the application of hypericin induced the activity of glutathione S-transferase in specialist insects only. Insects with different lifestyles therefore are capable of circumventing the phototoxic effects of hypericin by appropriate behavioral and biochemical strategies.

Volatile monoterpenes in porophyllum gracile and p. ruderale (asteraceae): identification, localization and insecticidal synergism with α-terthienyl

Abstract Volatiles occurring in Porophyllum gracile and P. ruderale (Asteraceae) were identified in order to assess their role in integrated chemical defences against insects. Headspace and steam distillation techniques yielded a total of 12 mono- and sesquiterpenes and fatty acid derivatives. The glandular secretory cavities present on leaves of P. ruderale were large enough to permit direct sampling and were found to be particularly rich in volatile monoterpenes (73.6% of the integrated FID trace). Using reduction of relative growth rate of third instar Ostrinia nubilalis (Lepidoptera: Pyralidae) larvae as an index of insecticidal activity, the volatiles released from the secretory cavities located on the leaves of P. ruderale had no significant effect alone, but they synergized the effects of α-terthienyl, a toxic light-activated secondary compound also present in P. gracile and P. ruderale. This synergistic interaction was shown to be related to an enhanced accumulation of the α-terthienyl in O. nubilalis larvae when they were exposed to the volatiles emitted from the foliar secretory cavities of P. ruderale.

Synergistic Insecticidal Mode of Action between Sesquiterpene Lactones and a Phototoxin, α‐Terthienyl

The synergistic insecticidal action of characteristic defensive substances produced by the plant family Asteraceae was investigated under controlled laboratory conditions. Sesquiterpene lactones isolated from Asteraceae that may form, through a Michael addition process, conjugates with glutathione were administered in a meridic diet to a herbivorous insect, Manduca sexta. By administering sesquiterpenes, variable in vivo reduced glutathione levels were observed in the insect larvae. When the Asteraceae‐derived photooxidant α‐terthienyl was co‐administered, lipid peroxidation and larval mortality were significantly enhanced in the treated groups of insects with lowered in vivo glutathione levels.

Symposium-in-Print Synergistic Insecticidal Mode of Action between Sesquiterpene Lactones and a Phototoxin, α-Terthienyl

Abstract The synergistic insecticidal action of characteristic defensive substances produced by the plant family Asteraceae was investigated under controlled laboratory conditions. Sesquiterpene lactones isolated from Asteraceae that may form, through a Michael addition process, conjugates with glutathione were administered in a meridic diet to a herbivorous insect, Manduca sexta. By administering sesquiterpenes, variable in vivo reduced glutathione levels were observed in the insect larvae. When the Asteraceae-derived photooxidant α-terthienyl was co-administered, lipid peroxidation and larval mortality were significantly enhanced in the treated groups of insects with lowered in vivo glutathione levels.

Behavioral adaptations of two phytophagous insects feeding on two species of phototoxic Asteraceae

Two phototoxic plants of the Asteraceae family were studied in relation to species of phytophagous insects for which they are hosts:Argyrotaenia velutinana Wlk. feeding onChrysanthemum leucanthemum L. andChlorochlamys chloroleucaria (Guenée) colonizingRudbeckia hirta L. The toxicity of these two plants is related to the presence of acetylenes and thiophenes that induce a light-mediated production of deleterious singlet oxygen and other free radicals (phototoxicity). Results showed that females ofA. velutinana laid their eggs preferentially in the shade and the larvae adopted hiding behaviors, such as bending of ligulate corollas and silk spinning to build opaque shelters. By avoiding direct exposure to the sun, both behaviors may reduce phototoxicity associated with ingested plant materials. Furthermore, larvae ofC. chloroleucaria demonstrated a preference in the field for pollen, which constitutes a nonphototoxic tissue of their host plant. Experimental alterations of these specific behaviors induced important biological consequences for larvae of both insects such as mortality or reduction of larval growth rate. These results reinforce the idea that behavior may constitute an efficient adaptation to avoid phototoxicity.

Advances in Insect Chemical Ecology: Phytochemical diversity of insect defenses in tropical and temperate plant families

One of the most intriguing features of the chemical ecology of plant–insect interactions is the remarkable number of different phytochemical defenses found in plants. A single plant may contain five or six biosynthetic groups of secondary metabolites andwithin each group these defensesmay includemany structurally related analogs and derivatives. Across the different species of higher plants, there is a bewildering array of different substances and modes of actions of substances. During the course of our research on phytochemical defenses in specific plant families, we have become interested in the raison d’etre for this diversity, as have many other researchers (Romeo et al., 1996). How much do we know about different types of defenses in plants? How did they arise? Are some of these secondary metabolites “redundant,” with no function? How do they interact with one another? In this chapter, we will address some of these issues with observations on the defenses of several plant families against insects, using results from our own research and the published literature. In general, we do not have a comprehensive picture of the different types of defenses in plants. Over a decade ago, Soejarto and Farnsworth (1989) estimated that of the 250 000 species of flowering plants, only 5000 species had been thoroughly investigated according to the Natural Product Alert (NAPRALERT) database, leaving 98% of species with potential for phytochemical discovery. Taking a more focussed view of the potential for discovering drugs of phytochemical origin in tropical forests, Mendelsohn and Balick (1995) estimated that current prescription drugs represent only 12% of what might be there to discover. Although steady progress in identifying the phytochemical defenses of plants is being made (thousands of compounds per year), the percentage of higher plants studied is

Phototoxic polyacetylenes from Viguiera annua and adaptations of a chrysomelid beetle, Zygogramma continua, feeding on this plant

Abstract Two major polyacetylenes, trans -1,3,5,11-tridecatetraen-7,9-diyne and 1, cis -3, trans -5, trans -11-tridecatetraen-7,9-diyne, possessing light-activated toxicity (phototoxicity) were isolated from the leaves of Viguiera annua (M. E. Jones) Blake (Asteraceae). Despite the insecticidal properties of these polyacetylenic derivatives, a field survey revealed that larvae of Zygogramma continua Le Conte (Coleoptera: Chrysomelidae) were able to feed and develop normally on the highly phototoxic leaves of V. annua . Results suggested that the light-avoidance behaviour, and the inducibility of some antioxidant enzymes, in Z. continua larvae were important features explaining the tolerance of this insect to the phototoxic polyacetylenes occurring in V. annua .

PRODUCTION OF GLANDS IN LEAVES OF POROPHYLLUM SPP. (ASTERACEAE) : ECOLOGICAL AND GENETIC DETERMINANTS, AND IMPLICATIONS FOR INSECT HERBIVORES

1 The mean number of translucent glands containing potent insecticidal allelochemicals in leaves of Porophyllum gracile was observed to vary up to nearly twofold under field conditions in the area of Tucson, Arizona (USA). 2 The production of glands appeared to be a beneficial trait to reduce herbivory in the field since individuals of P. gracile bearing 2.5 glands per leaf were submitted to an approximately fivefold higher herbivory pressure than those having 4.8 glands per leaf. 3 To investigate if the production of glands in leaves of P. gracile is under genetic control, seeds were collected from individuals for which this trait varied under field conditions. There was no correlation for the mean number of glands per leaf between individuals of P. gracile sampled in the field and their progeny seedlings grown under controlled conditions. This suggested that the production of glands in leaves of P. gracile is not strongly hereditary. 4 The resource allocated to the formation of foliar glands, which was determined as the ratio of total volume of glands per unit area of leaf, in seedlings of P. ruderale and P. gracile was enhanced by up to one order of magnitude by both nitrogen fertilization, 15 vs. 0 mm of NOin the watering solution, and high light regime, 380 vs. 50 ,mole photon m-2 s-5. These results suggest that the production of glands in leaves of Porophyllum spp. depends greatly on resource availability. 5 The natural plasticity in the production of glands in leaves of P. ruderale was in turn employed in controlled laboratory experiments to confirm the potential benefit of the glands to repress insect herbivory as observed in the field. It was demonstrated that adults of the red-legged grasshopper are repelled by the volatiles emitted from the glands and that they feed less on leaves bearing more glands.