Eric Thibout

LEEK ODOR ANALYSIS BY GAS CHROMATOGRAPHY AND IDENTIFICATION OF THE MOST ACTIVE SUBSTANCE FOR THE LEEK MOTH, Acrolepiopsis assectella

When crushed, the leek,Allium porrum emits propyl propanethiosulfinate. The unstable thiosulfinate decomposes during GC analysis in numerous compounds, except on very short columns. The propyl propanethiosulfmate is the most attractive substance for the leek moth,Acrolepiopsis assectella. This thiosulfinate is not active onPlutella xylostella orPlodia interpunctella and seems to be repulsive onEphestia kuehniella.

Identification of leek-moth and diamondback-moth frass volatiles that stimulate parasitoid,Diadromus pulchellus

Acrolepiopsis assectella andPlutella xylostella frass volatiles, trapped on Tenax GC, were examined by capillary gas chromatography. In both moths, the same three disulfides, dimethyl, dipropyl, and methyl propyl, were the most abundant substances, but in different proportions. The synthetic disulfides elicited the same behavioral response by the parasitoid,Diadromus pulchellus as frass. The plant origin of these substances is discussed.

How a specialist and a non-specialist insect cope with dimethyl disulfide produced by Allium porrum

Damaged Allium plants produce and release sulfur allelochemicals, presumably to prevent insect herbivory. Defensive sulfur compounds, particularly dimethyl disulfide (DMDS), are highly toxic for non‐adapted species. The toxicity of DMDS in these insects is due to disruption of the cytochrome oxidase system of their mitochondria. The purpose of this study was to compare susceptibility to DMDS in a specialist and a non‐specialist insect of Allium plants, i.e., Acrolepiopsis assectella Zeller (Lepidoptera: Acrolepiidae) and Callosobruchus maculatus Fabricius (Coleoptera: Bruchidae) a specialist insect of Leguminosae. Results showed that A. assectella larvae are less susceptible to DMDS than C. maculatus adults. This bruchid becomes more tolerant after a first exposure to 0.2 µl l−1 of DMDS, with second‐exposure toxicity depending on the time lapse between exposures. Higher second‐exposure tolerance could be due to selection and detoxification. To answer this question, the activity of glutathione S‐transferase (GST), a key enzyme in the detoxification system, was analyzed after DMDS exposure of C. maculatus adults and larvae and of A. assectella larvae. Exposure to DMDS increased GST activity in C. maculatus adults and larvae. This finding implies that induced GST is involved in C. maculatus tolerance to DMDS. Exposure to DMDS had no effect on GST activity in A. assectella. Adaptations underlying A. assectella tolerance to sulfur compounds are discussed.

STIMULATION OF REPRODUCTIVE ACTIVITY OF FEMALES OF ACROLEPIOPSIS ASSECTELLA (LEPIDOPTERA: HYPONOMEUTOIDEA) BY THE PRESENCE OF EUPYRENE SPERMATOZOA IN THE SPERMATHECA

This work completes previous experiments and sums up the various influences of mating upon the sexual and reproductive activity of leek‐moth females.

Susceptibility of Callosobruchus maculatus (Coleoptera: Bruchidae) and its Parasitoid Dinarmus basalis (Hymenoptera: Pteromalidae) to Sulphur-Containing Compounds: Consequences on Biological Control

Abstract In West Africa Callosobruchus maculatus F. causes major losses during storage of cowpea (Vigna unguiculata Walper). The larvae of C. maculatus are parasitized inside the seeds by Dinarmus basalis (Rondani). African farmers introduce aromatic plants into storage systems at the beginning of the storage period that release toxic volatile compounds into the headspace of the stores. The susceptibility of C. maculatus and D. basalis to two plant sulphur-containing compounds, methylisothiocyanate (MITC) and dimethyl disulfide (DMDS), was analyzed under laboratory conditions. The adults of C. maculatus and D. basalis had the same susceptibility to MITC but the parasitoid was more susceptible to DMDS than its host. The larvae were less affected by the treatments than the adults; the seed coat acts as a barrier for volatile penetration. Susceptibility to DMDS was high when the C. maculatus larvae were extracted from the seeds. DMDS was more toxic than MITC and differences in penetration inside the seeds could explain these results. The higher susceptibility of D. basalis to the treatments could have consequences on biological control in storage systems. The presence or absence of leaves of the shrub Boscia senegalensis Lamarck (Capparaceae), which release MITC into the jar atmosphere could have an impact on numbers of C. maculatus and D. basalis. When D. basalis adults were put into the storage systems in the absence of B. senegalensis leaves, successive generations of the parasitoids maintained the C. maculatus population at a low density. In the presence of B. senegalensis leaves, the D. basalis population was more affected by the treatment than its host C. maculatus and was incapable of preventing the increase in bruchid population. This traditional method appears to increase seed weight losses by limiting the efficiency of the biological control.

Increased Sulfur Precursors and Volatiles Production by the Leek Allium porrum in Response to Specialist Insect Attack

To defend themselves against herbivory, plants use a variety of direct and indirect strategies involving induced increases in secondary substances. Species of the Allium genus (Alliaceae), such as the leek Allium porrum (L.), produce nonprotein sulfur amino acids derived from cysteine, i.e., alk(en)yl-cysteine sulfoxides that are precursors of volatile thiosulfinates and disulfides. These defend most species including the specialist leek moth, Acrolepiopsis assectella. We determined by measuring the increase in the sulfur precursor propyl-cysteine sulfoxide (PCSO) if production of this precursor is induced in response to moth attack and mechanical wounding. The concentration of PCSO was determined by HPLC in 2- or 6-mo-old leeks after attacks of various intensity either by the specialist leek moth or by a generalist moth, Agrotis ipsilon. Injury-induced release of sulfur volatiles was measured by GC/MS after the attacks. Results showed an increase in the production of sulfur compounds in both the precursor and volatile form, occurring only in association with intensive attacks by leek moths. The increase in sulfur precursors also led to an increase in the release of sulfur volatiles. This induced response may provide an effective defense strategy against the plant’s main natural enemy, both directly and indirectly by attracting entomophagous insects.

Transfer, Perception, and Activity of Male Pheromone of Acrolepiopsis assectella with Special Reference to Conspecific Male Sexual Inhibition

In the leek moth, Acrolepiopsis assectella, the male, stimulated by a calling female, produces a sexual pheromone that is active on the female. The male-produced pheromone blend contains eight alkanes previously isolated from the male hair-pencils. We used EAG techniques to study the effect of the pheromone on females reared on leek or on artificial diet and on males reared on leek. The optimal stimulation duration appeared to be 1.6 sec exposure to the pheromone or components. The concentrations tested on the antenna were of the order of 1012 molecules/cm3. The hair-pencil extract tested was of an estimated concentration of around 109 molecules/cm3. The antennal responses are expressed relative to responses to a standard, amyl acetate, but also as an absolute value. Generally, females reared on artificial diet and males reared on leek responded better than females reared on leek. Of the alkanes tested, those present in hair-pencils gave higher responses, with hexadecane always giving the strongest response. A possible inhibiting activity of male leek moth pheromone on the sexual behavior of conspecific males was investigated. The behavior of sexually stimulated males was observed in the presence of other males, hair-pencil extracts, and different compounds either pure or in a mixture. The experiments established that in this species, male pheromone inhibits wing fluttering duration of conspecific males. This inhibition was obtained not only with fluttering males as a source of pheromone but also with all the alkanes tested. The inhibition was due to hair-pencil chemicals, particularly if these were perceived by olfaction plus contact. Wing fluttering increased the inhibitory activity of male-derived alkanes.

Solid-phase microextraction–gas chromatography–direct deposition infrared spectrometry as a convenient method for the determination of volatile compounds from living organisms

Abstract Gas chromatography–direct deposition infrared spectrometry (GC–DD-IR) permits coupling of GC to IR at a level of sensitivity of routine GC–MS coupling but the presence of ice resulting from living organisms limits the usefulness of the system. Headspace solid-phase microextraction (SPME) coupled to GC–DD-IR leads to a rigorous absence of water and can be applied to unknown volatiles trapped in situ in combination with SPME–GC–MS. For instance we succeeded in identification of the asparagus fly male pheromone and 1 min of an individual emission is sufficient to obtain a good IR spectrum in real time.

Origin of kairomones in the leek moth (Acrolepiopsis assectella, Lep.) frass. Possible pathway from methylthio to propylthio compounds.

Feeding leek moths on an artificial diet has shown that dimethyl and dipropyl disulfides and methyl-propyl disulfide found in frass arise from sulfur compounds specific toAllium. The addition of either propyl or methyl disulfide or their precursors to the diet leads to appearance of the three disulfides in the frass. This implies the transformation of theS-propyl moiety toS-methyl and vice versa by an as yet unknown mechanism.

The cocoon and humidity in the development of Acrolepiopsis assectella (Lep.) pupae: consequences in adults.

Abstract In a saturated atmosphere, the cocpon of the leek moth can absorb up to two‐thirds of its weight in water. It is shown that this phenomenon enables the pupae to lose less water in the course of their development, particularly during pupal moulting. Although quantitatively minor, this protection is qualitatively important: it has no effect on mortality or the duration of pupal development, but does affect vitellogenesis and fertility in females. It thus clearly has an adaptive role towards the environment.