Brigitte Frérot

GENETIC ISOLATION BETWEEN TWO SYMPATRIC HOST‐PLANT RACES OF THE EUROPEAN CORN BORER, OSTRINIA NUBILALIS HUBNER I. SEX PHEROMONE, MOTH EMERGENCE TIMING, AND PARASITISM

Abstract Adaptation to different environments may be a powerful source of genetic differentiation between populations. The biological traits selected in each environment can pleiotropically induce assortative mating between individuals of these genetically differentiated populations. This situation may facilitate sympatric speciation. Successful host shifts in phytophagous insects provide some of the best evidence for the ecological speciation that occurs, or has occurred, in sympatry. The European corn borer, Ostrinia nubilalis (Lepidoptera: Crambidae), colonized maize after its introduction into Europe by humans about 500 years ago. In northern France, two sympatric host races feed on maize (Zea mays) and mugwort (Artemisia vulgaris), respectively. We investigated the factors involved in the genetic isolation of these two races at a field site near Paris, France. We identified two biological differences that might make a significant contribution to the genetic divergence between sympatric populations feeding on the two host plants. First, assortative mating may be due to differences in the moth emergence pattern between the two races: mugwort-race moths emerged on average 10 days earlier than maize-race moths. In addition, the males emerged earlier than females in both races. Hence, the likelihood of mating between maize-race males and mugwort-race females was higher than that of mating between mugwort-race males and maize-race females. Second, the females feeding on mugwort and maize produced sex pheromones with different E/Z isomeric ratios of Δ-11-tetradecenyl acetate. This difference in mate recognition systems reinforces the potential for assortative mating in the two races. During the experiment, overwintering mortality was much lower on maize than on mugwort. This difference was due to a braconid parasitoid wasp, Macrocentrus cingulum, that killed more than 50% of the larvae overwintering on mugwort but did not infest larvae diapausing on maize. Hence, by colonizing maize, European corn borer populations probably escaped from numerous predators, competitors, and parasitoids, such as M. cingulum. This decrease in host-associated selection may have favored the colonization of this new host. Finally, throughout this experiment we observed selection at two allozyme loci (or at linked loci): Tpi and Mpi. The Tpi locus is tightly linked with the genes involved in the response of the male to the sex pheromone and in developmental timing. The location of these traits on the Z chromosome may play a role in shortening the time required for the evolution of premating barriers.

Host-plant specialization in pheromone strains of the European corn borer Ostrinia nubilalis in France.

European corn borer (ECB) feeding on maize (Zea mais), mugwort (Artemisia vulgaris), and hop (Humulus lupulus) are genetically different in France and referred to as host-plant races. Here, we investigated sex pheromone composition as a possible trait linked to the host plant. ECB host races were sampled from 13 different sites in France. GC-MS analysis of female pheromone showed that 175 out of 176 maize females belonged to the Z type with one hybrid. In contrast, mugwort and hop females belonged almost exclusively to the E type. No Z females were found on these plants and only 2 females out of 169 were hybrids. In the three sites of sympatry, the hybrid proportion was far from Hardy–Weinberg expectations. Wind tunnel experiments showed that 76–79% of maize males from three populations were attracted by Z females, whereas neither mugwort nor hop males were. Mugwort males from Toussus-le-Noble were attracted by E females originating from an American maize strain. These data showed that maize, mugwort, and hop host races of O. nubilalis differ not only in their host plant but also in the sex pheromone they use. Because mugwort and hop are putative ancestral host plants, these results are discussed from the point of view of evolutionary scenarios for the emergence of Z and E strains.

Genetic structure of European and Mediterranean maize borer populations on several wild and cultivated host plants

Target pests may become resistant to Bacillus thuringiensis (Bt) toxins produced by trangenic maize (Zea mays L.). Untreated refuge areas are set aside to conserve high frequencies of susceptibility alleles: a delay in resistance evolution is expected if susceptible individuals from refuges mate randomly with resistant individuals from Bt fields. In principle, refuges can be toxin‐free maize or any other plant, provided it hosts sufficiently large pest populations mating randomly with populations from Bt‐maize fields. Our aim was to examine the suitability of several cultivated or weedy plants [pepper (Capsicum frutescens L.), sorghum (Sorghum spec.), sunflower (Helianthus annuus L.), cocklebur (Xanthium spec.), cantaloupe (Cucumis melo L.), and hop (Humulus lupulus L.)] as refuges for Ostrinia nubilalis (Hübner) (Lepidoptera: Crambidae) and Sesamia nonagrioides Lefebvre (Lepidoptera: Noctuidae), two major maize pests in southern Europe. Larvae of both species were collected on these plants. Their genetic population structure was examined at several allozyme loci. We found little or no evidence for an influence of geographic distance, but detected a significant host‐plant effect on the genetic differentiation for both species. Ostrinia nubilalis populations from sunflower, pepper, cocklebur, and sorghum appear to belong to the same genetic entity as populations collected on maize, but to differ from populations on hop. Accordingly, females from pepper and cocklebur produced exclusively the ‘Z’ type sexual pheromone, which, in France, characterizes populations developing on maize. Qualitatively, these plants (except hop) could thus serve as refuges for O. nubilalis; however, they may be of little use quantitatively as they were found much less infested than maize. Sesamia nonagrioides populations on maize and sorghum reached comparable densities, but a slight genetic differentiation was detected between both. The degree of assortative mating between populations feeding on both hosts must therefore be assessed before sorghum can be considered as a suitable refuge for this species.

Optimization of coffee berry borer, Hypothenemus hampei Ferrari (Col., Scolytidae), mass trapping with an attractant mixture

Mass trapping is a technique currently being considered to control the coffee berry borer (CBB), Hypothenemus hampei Ferrari (Col., Scolytidae), as part of the integrated management of its populations. However, the research undertaken in this field has variable and sometimes contradictory results. The purpose of this study carried out with a single type of trap (experimental model ‘1B’) was to evaluate the technique and define parameters for its efficient use. The first result obtained was detection of the excellent attractant properties of the ethanol–methanol mixture, and its efficiency related to the proportion of the two alcohols. Coffee borers were not attracted to traps with caffeine, green coffee powder, freshly pulped and hulled coffee berries and ground CBB. It was shown that the ethanol–methanol mixture (1 : 1) release rate varied without affecting the capture rate, and that the red colour of the trap substantially increased CBB attraction. In terms of trap set‐up, it was found that capture rates were three times better with a trap height at 1.2 m, than that for a position near ground level. In addition, the best density of traps for achieving efficient mass trapping was 22 units per hectare. These results show the importance of developing an attractant using pure compounds, and of improving the trap and the trapping technique in line with CBB behaviour.

Evidence for allelochemical attraction of the coffee berry borer,Hypothenemus hampei, by coffee berries.

Petri dish choice tests conducted on the coffee berry borer (CBB),Hypothenemus hampei, showed that females were able to discriminate between coffee berries at different ripening stages. A Y-shaped glass olfactometer was used to demonstrate that coffee berries emitted volatile chemicals that elicited upwind movement by female CBB. Olfactometer tests with three different solvent extracts of berries showed that at least some of the attractive chemical(s) released by the coffee berries could be extracted with acetone.

Importance of plant physical cues in host acceptance for oviposition by Busseola fusca

Plant and surrogate stems exhibiting specific combinations of physical cues were used to determine which plant‐related stimuli influence the oviposition of Busseola fusca (Fuller) (Lepidoptera: Noctuidae). The number of eggs and egg batches laid per female increased with an increase in diameter of both natural and artificial stems. Direct observations of the oviposition behaviour (walking, antennating, and sweeping with the ovipositor) indicated that the female moths preferred oviposition supports with a large diameter and non‐pubescent or smooth surfaces over pubescent or rough ones. Pubescence and rough surfaces significantly affected the behavioural steps leading to oviposition by interfering with the ovipositor sweep process necessary to find a suitable oviposition site. Furthermore, more eggs and egg batches were laid on soft than rigid supports. The rigidity of the support affected the proper insertion of the ovipositor for egg deposition. Our results underline the importance of physical stimuli in B. fuscas choice of an oviposition site, which may facilitate the identification of potential host plants or preferred oviposition sites on a plant for this species.

Trapping of the coffee berry borer Hypothenemus hampei Ferr. (Col., Scolytidae) within a mesh-enclosed environment: interaction of olfactory and visual stimuli

Coffee berry borer (CBB) colonizing females were released within a mesh‐tent and recaptured in traps to study the interaction of olfactory and visual stimuli during the flight phase. A 1:1 methanol–ethanol mixture was used as an attractant in traps constructed from multiple funnels of red or white. The first aim of the present experiments was to verify that the results obtained in a laboratory olfactometer, in which insects were constrained to walk and in which visual and olfactory stimuli were independently manipulated, could be extended to an outdoor arena enclosed by a net tent. The second aim was to test different emission odour rate, and finally develop a prototype trap which could be used for subsequent studies of CBB population dynamics in the field. Out of 3 000 females released, over 45% were recaptured, among which 95% were collected from traps with methanol–ethanol mixture. The red traps were more attractive than the white traps. The lower the rate of emission of odour, the greater the number of females recaptured. These results show the importance of vision and olfaction for the CBBs flight approach to the trap.

Progression in field infestation is linked with trapping of coffee berry borer, Hypothenemus hampei (Col., Scolytidae)

Phenology of the coffee plant and infestation by coffee berry borer Hypothenemus hampei Ferrari were studied in relation to trapping of adult females in kairomone‐baited traps in a coffee plantation in New Caledonia. In a 0.4 ha coffee field, a group of 27 trees located along a transect beginning at an early infestation point was selected. The number of green, red and dry coffee berries, along with the number of larvae, adult males and females per berry was determined monthly from October 1993 to July 1994. Twelve, red multifunnel traps, each baited with a solution of methanol:ethanol (1:1 ratio, a mean solution release rate of 1 g/day) were placed within the coffee field, along the transect, within the selected trees, grouped in four zones named 1–4. Two additional traps were located outside the plantation.

Effect of physiological status on olfactory and visual responses of female Hypothenemus hampei during host plant colonization

The response of Hypothenemus hampei (Ferr.) (Coleoptera, Scolytidae) (coffee berry borer, CBB) females to olfactory stimuli was demonstrated to be related with the physiological status; especially with age, whether they were mated or not, and whether they have already oviposited. A special behaviour, the abandonment of the native berry, was also found to play a major role. In contrast, response to visual stimuli (false coffee berries) was found to be independent of the physiological status of CBB females. Variations in behavioural responses to visual and olfactory stimuli are discussed in relation to colonization strategy.

Identification of hairpencil secretion from maleMamestra brassicae (L.) (Lepidoptera: Noctuidae) and electroantennogram studies.

Extracts of maleMamestra brassicae (L.) hairpencils were analyzed by capillary gas chromatography (GC) and by GC-mass spectrometry (GC-MS). The extracts were found to consist of six components. Benzaldehyde, 2-methylpropanoic acid, 2-methylbutanoic acid, and phenol were present in the extracts as well as the previously identified benzyl alcohol and phenyl ethanol. The two major components were 2-phenylethanol and 2-methylbutanoic acid. They represented, respectively, 74% and 12.5% of the total blend in 3-day-old male extracts. Electroantennograms were recorded on male and female antennae in response to stimulation by hairpencil compounds. Male and female antennae responded to each chemical but the female responses were significantly higher than those of the males.