Hainan Gu

Response of Cydia pomonella to selection on mobility: laboratory evaluation and field verification

1. The codling moth Cydia pomonella (L.), largely regarded as a sedentary species, shows great variation in flight capacity among individuals in the laboratory, as has been reported for tethered flight. The occurrence of individuals with the ability to fly over long distances is considered ecologically significant for the colonisation of new habitats in response to deteriorating environmental conditions. The work reported here was designed to investigate the importance of the genetic component in regulating dispersal in C. pomonella.

Trade-off between mobility and fitness in Cydia pomonella L. (Lepidoptera: Tortricidae)

Abstract.  1. The codling moth Cydia pomonella, one of the most important pest insects of apple fruit orchards worldwide, is regarded as a ‘sedentary’ insect, but the natural populations consist of both mobile and sedentary genotypes that display different dispersal capacity in the field. The current study investigated whether there is a fitness consequence of being mobile in this species by comparing two strains with contrasting levels of mobility obtained through bi‐directional selection.

Nutritional value of floral nectar sources for flight in the parasitoid wasp, Cotesia glomerata

Abstract.  This study investigates the feeding of the parasitoid Cotesia glomerata on the floral nectar of Anethum graveolens and Origanum vulgare and evaluates the nutritional value of these food sources for increasing flight capacity. A significantly higher proportion of females are observed feeding on A. graveolens immediately after being released onto the floral nectar plants compared with those of the group released onto O. vulgare, although female wasps from both groups spent a similar period of time feeding on both nectar sources and there is no significant difference in their body weight increase. Laboratory tests on computer‐linked flight mills demonstrate that adult feeding significantly influences flight capacity, as defined by the number of flights, the longest single flight and the total distance flown during a given period of time, in female wasps but not in males, albeit the extent of influence varying with the type of food sources. After ad libitum feeding on the floral nectar of A. graveolens, female wasps perform flight activity as well as those feeding on honey, which is superior to those feeding on sucrose, in all the measured parameters. Feeding on the floral nectar of O. vulgare has a similar effect to feeding on the extrafloral nectar of Vicia faba and significantly increases the total distance flown but not the number of flights or the longest single flight. Feeding on sucrose does not increase flight capacity in the parasitoids beyond those having access to water only. The findings of this study suggest that judicious selection of floral nectar plants and habitat manipulation might play a key role in the biological control of pest insects.

Superparasitism in Cotesia glomerata: response of hosts and consequences for parasitoids

Abstract.  1. Superparasitism occurs in Cotesia glomerata (Hymenoptera: Braconidae), a gregarious endoparasitoid of Pieris spp. (Lepidoptera: Pieridae). The response of P. brassicae larvae to superparasitism and the consequences for the parasitoid were examined in order to elucidate the ecological significance of this behaviour.

Selection on olfactory response to semiochemicals from a plant–host complex in a parasitic wasp

Parasitic wasps (parasitoids) use volatiles from the plants infested by phytophagous insects to locate host herbivores, but their behavioural response to such semiochemicals is highly variable. Bi-directional selection on Cotesia glomerata (Hymenoptera: Braconidae) was conducted to investigate the importance of genetic variation in the olfactory response of parasitoids. Female wasps were assessed for flight orientation and landing success in response to the hexane extract from a plant–host complex in a wind tunnel. After the first generation of selection, two strains significantly differentiated in both flight orientation and landing success, and their divergence continued with further selection. The two selected strains genetically differentiated in olfactory perception rather than upwind flight ability. The realized heritability was estimated as 0.248 for flight orientation and 0.216 for landing success. The selection experiment further demonstrated that a prior exposure to the semiochemicals significantly enhanced the subsequent response of female wasps, independent of genetic differences. These results suggest that both genetic component and environmental conditioning have played an important role in the evolution of host selection and utilization by the parasitoid in a tritrophic system.

Single-locus sex determination in the parasitoid wasp Cotesia glomerata (Hymenoptera: Braconidae).

The parasitoid Cotesia glomerata usually produces female-biased sex ratios in the field, which are presumably caused by inbreeding and local mate competition (LMC); yet, sibling mating increases the production of males, leading to the male-biased sex ratio of broods in the laboratory. Previous studies have suggested that the sex allocation strategy of C. glomerata is based on both partial LMC in males and inbreeding avoidance in females. The current study investigated the presence of single-locus complementary sex determination (sl-CSD) as a sex-determining mechanism in this species through inbreeding experiment, cytological examination and microsatellite analysis. Cytological examination detected diploid males in nine of 17 single pairs of sibling mating, thus in agreement with the proportion of matched matings predicted by the sl-CSD model. Sex ratio shifts in these matched sibling matings were consistent with the sl-CSD model with less viable diploid males. The haploid males have a single set of maternal chromosomes (n=10), whereas diploid males possess a double set of chromosomes (2n=20). Microsatellite analyses confirmed that diploid males produced from the matched matings inherited segregating genetic materials from both parents. Thus, this study provides the first solid evidence for the presence of sl-CSD as a sex-determining mechanism in the braconid genus Cotesia.

Genetic variation in behavioral response to herbivore-infested plants in the parasitic wasp, Cotesia glomerata (L.) (Hymenoptera: Braconidae).

Female Cotesia glomerata (L.) relies on stimuli from herbivore-infested plants to select suitable hosts, but behavioral response to such stimuli is highly variable among individuals. This study investigates a genetic component of phenotypic variability in both short-range host-search and long-range host-location behaviors in the tritrophic system consisting of cabbage plants (Brassica oleracea L.), cabbage butterfly (Pieris brassicae L.) and the parasitoid, by comparing full-sib families established from a laboratory population and isofemale strains from a field population. Short-range host-search behaviors were examined within a Petri-dish test arena, and long-range host-location behaviors assessed in a wind tunnel. Significant differences among full-sib families were shown in the duration of walking on a plant-herbivore complex (i.e., a leaf section with two host caterpillars, their silk and feces) and searching off the complex, and the total time elapsed for wasps to locate a host larva after release into the test arena. Flight responses to and landing choices between the intact and the herbivore-infested plants were also significantly different among these families. Effects of families on both short-range host-search and long-range host-location behaviors were consistent, without significant influences of host larvae from which wasps emerged. The analysis of isofemale strains reveals that strains account for significant variation in the oriented flight response to herbivore-infested plants, and the “isofemale heritability” for this behavioral character is estimated as 0.447. The results suggest that genetic variation exists at different behavioral levels of the host-selection process in this parasitoid.

Spring colonisation of orchards by Anthonomus pomorum from adjacent forest borders

The early‐season dispersal of the overwintered apple blossom weevil, Anthonomus pomorum (L.) (Coleoptera: Curculionidae), is a crucial stage in the colonisation of dwarf apple orchards adjacent to forests. We have conducted release‐recapture studies with 1700 to 4000 marked weevils at two orchard sites in Switzerland over 2 years to characterise the spatial and temporal pattern of the dispersal process. The dispersal and colonisation of orchards in spring by overwintered weevils is dependent upon the prevailing temperature.

Tolerance as a mechanism of resistance to Thrips palmi in common beans

Tolerance as a mechanism of resistance to the melon thrips, Thrips palmi Karny (Thysanoptera: Thripidae), in common beans, Phaseolus vulgaris L., was evaluated under field and greenhouse conditions. Seven resistant (Brunca, BH‐5, BH‐60, BH‐130, BH‐144, EMP 486, and FEB 115) and five susceptible (PVA 773, EMP 514, BAT 477, APN 18, and RAZ 136) bean genotypes were assessed according to adult and larval populations, visual damage and reproductive adaptation scores, and yield components in field trials. From these genotypes, four resistant (Brunca, BH‐130, EMP 486, and FEB 115) and two susceptible (APN 18 and RAZ 136) genotypes were selected for quantification of proportional plant weight and height increase changes due to thrips infestation in greenhouse tests. Under medium to high thrips infestation in the field, most resistant genotypes tended to have higher reproductive adaptation and lower yield losses, though they did not always suffer less damage, as compared to susceptible genotypes. In the greenhouse, resistant genotypes showed less reduction in plant dry weight and height increase than did some susceptible ones under the same infestation pressure. Results from both field trials and greenhouse tests suggest the possible expression of tolerance as a mechanism of resistance to T. palmi in the resistant genotype EMP 486, and confirm the existence of antixenosis in FEB 115, whereas tolerance might be combined with other resistance mechanisms in Brunca.

Diel patterns of locomotor activity in Cydia pomonella: age and sex related differences and effects of insect hormone mimics

Abstract. The diel pattern of locomotor activity in the codling moth Cydia pomonella L. (Lepidoptera: Tortricidae) was investigated in the laboratory, using a computer‐based infra‐red actograph. The level of locomotor activity varied with sex and age. On the day of emergence, females were more active than males, but on days 2, 4, 6 and 12 males were approximately twice as active as females. Males reached their highest activity on day 4 and females on day 12. Both sexes were most active at dusk. Their activity patterns were fitted to a set of polynomial regression models. Treatment with 400 p.p.m. of Juvenile Hormone mimic, fenoxycarb, stimulated locomotor activity and provoked a marked activity peak at dawn in both virgin and mated females. A similar treatment with the ecdysone agonist tebufenozide showed a neutral effect on the locomotor activity of females. Possible implications of these findings are discussed in relation to the physiology of the moth and to its dispersal behaviour.