ABSTRACT
Solitary parasitoids of colony-forming hosts may produce quasi-gregarious broods, which favours sibmating on the natal patch and local mate competition (LMC). We examined seasonal variations in brood size and sex ratio in three species of solitary parasitoids of aphids associated with trophobiotic ants. Adialytus arvicola, a parasitoid of Sipha agropyrella on grasses, had the smallest broods (mean=4.2, maximum 19), while Lysiphlebus hirticornis, a parasitoid of Metopeurum fuscoviride on tansy, had the largest broods (mean=32.0, maximum 265). In Pauesia pini, a parasitoid of Cinara piceicola on Norway spruce, broods comprised an average of 5.8 (maximum 41) individuals; brood size increased during early summer when hosts became more available but remained the same later in the season. In all three species the sex ratio at eclosion was female-biased, with broods containing approximately two daughters for each son in both A. arvicola and L. hirticornis; the degree of female-bias was least in P. pini. The sex ratio did not vary with brood size. In A. arvicola, the variance of the number of sons declined with an increase in brood size, consistent with "precise" sex allocation. In contrast, in L. hirticornis, the overall sex-ratio variance was greater than its binomial expectation, while it did not differ from binomial in P. pini. A large proportion of broods contained only sons or only daughters, especially in A. arvicola. An excess of male-only broods is expected if constrained females (which can produce only sons) contribute offspring prior to mating. The number of male-only broods, however, did not differ from the number expected if all females are mated and allocate offspring sexes binomially, except in P. pini. In the latter species, broods with two daughters (as opposed to two sons) exceeded binomial expectations. We propose that P. pini is largely outbreeding, while the strongly female-biased sex ratio in A. arvicola and L. hirticornis is consistent with partial sibmating and LMC. Ant-parasitoid interactions could account for a different population mating structure in the three parasitoid species. Both A. arvicola and L. hirticornis mimic the epicuticular hydrocarbon pattern of their aphid hosts. Eclosing wasps are ignored by honeydew-collecting ants and hence can mate and forage on the natal patch. In contrast, P. pini generally depart the mummy area to avoid attacks by trophobiotic ants and mate off patch.
ABSTRACT
The intensity of the mutualistic relationship between aphids and ants depends mainly on the composition and amount of honeydew. We used the model system Tanacetum vulgare-Metopeurum fuscoviride to study age-related differences in honeydew production and composition and its effect on the mutualism between M. fuscoviride and the ant Lasius niger. First and second instar larvae of M. fuscoviride produced only half of the amount of honeydew as older larvae or adults. There were, however, no differences between age classes in the total honeydew sugar concentration, which averaged approx. 80 &mgr;g sugar/&mgr;l honeydew. Honeydew sugar composition also did not differ between age classes, and melezitose was the dominant sugar (59% in all classes). The amino acid concentration, by contrast, increased significantly with aphid age, reaching 22.6 nmol per &mgr;l honeydew in adult M. fuscoviride. This increase was mainly caused by asparagine and glutamine, while there were no differences in the concentrations of the five other regularly detected amino acids and cystine, respectively. The intensity of ant-attendance was significantly lower in colonies of first and second instar larvae than in colonies of older age classes. Ant-attendance correlated with the amount of honeydew produced, and not with the total amino acid concentration.
ABSTRACT
Broad bean (Vicia faba), an annual plant bearing extrafloral nectaries (EFN) at the base of the upper leaves, is regularly infested by two aphid species, Aphis fabae and Acyrthosiphon pisum. EFN and A. fabae are commonly attended by the ant, Lasius niger, while Ac. pisum usually remains uninfested. Sugar concentration and sugar composition of extrafloral nectar did not change significantly after aphid infestation. The sugar concentration was significantly higher in EFN (c. 271 µg µl(-1)) than in the honeydew of A. fabae (37.5 µg µl(-1)). The presence of small A. fabae colonies had no significant effect on ant attendance of EFN, which remained at the same level as that on plants without A. fabae. Obviously, there was no significant competitive effect between the two sugar sources. We suggest that the high sugar concentration in the extrafloral nectar may outweigh the higher quality (due to the presence of melezitose) and quantity of aphid honeydew. Ants and the presence of EFN influenced aphid colony growth. While A. fabae colonies generally grew better in the presence of ants, Ac. pisum colonies declined on plants with EFN or A. fabae colonies. We conclude that EFN may provide some degree of protection for V. faba against those sucking herbivores that are not able to attract ants.
ABSTRACT
The honeydew composition and production of four aphid species feeding on Tanacetum vulgare, and mutualistic relationships with the ant Lasius niger were studied. In honeydew of Metopeurum fuscoviride and Brachycaudus cardui, xylose, glucose, fructose, sucrose, maltose, melezitose, and raffinose were detected. The proportion of trisaccharides (melezitose, raffinose) ranged between 20% and 35%. No trisaccharides were found in honeydew of Aphis fabae, and honeydew of Macrosiphoniella tanacetaria consisted of only xylose, glucose and sucrose. M. fuscoviride produced by far the largest amounts of honeydew per time unit (880 µg/aphid per hour), followed by B. cardui (223 µg/aphid per hour), A. fabae (133 µg/aphid per hour) and M. tanacetaria (46 µg/aphid per hour). The qualitative and quantitative honeydew production of the aphid species corresponded well with the observed attendance by L. niger. L. niger workers preferred trisaccharides over disaccharides and monosaccharides when these sugars were offered in choice tests. The results are consistent with the ants' preference for M. fuscoviride, which produced the largest amount of honeydew including a considerable proportion of the trisaccharides melezitose and raffinose. The preference of L. niger for B. cardui over A. fabae, both producing similar amounts of honeydew, may be explained by the presence of trisaccharides and the higher total sugar concentration in B. cardui honeydew. M. tanacetaria, which produced only low quantities of honeydew with no trisaccharides was not attended at all by L. niger.
ABSTRACT
Males of the aphid hyperparasitoid Dendrocerus carpenteri (Curtis) were attracted by a sex pheromone released by conspecific females. The intensity of this cue, and thus female attractiveness, depended both on the female's mating status and her age. Only virgin females younger than 2 h were consistently recognized as mates by foraging males. Male age did not influence foraging and mating success. Empty mummies, from which females had emerged within the previous 10 min were attractive to males and examined intensively. Rain reduced the searching success of males, although the host plant Vicia faba provided sheltered places. Wind did not reduce mating success but prevented both sexes from leaving the host plant. Since the time of female attractiveness seems to be very limited, wind may have an enormous effect on the mating success of D. carpenteri in the field and thus on the population dynamics of this species.
ABSTRACT
The parasitoid wasp genus Lysiphlebus (Hymenoptera: Braconidae: Aphidiinae) contains a taxonomically poorly resolved group of both sexual (arrhenotokous) species and asexual (thelytokous) clones. Maximum-parsimony and maximum-likelihood analyses of mitochondrial DNA sequence data from specimens collected across Western Europe showed that asexuality, which does not appear to be caused by the bacterium Wolbachia, is concentrated in two geographically widespread lineages, the older of which diverged from the closest extant sexual taxa approximately 0.5 million years ago. However, the DNA sequences of a nuclear intron (elongation factor-1α) showed no congruence with this pattern, and a much higher frequency of heterozygotes with very high allelic diversity was observed among the asexual females compared to that among females from the sexual species. This pattern is consistent with maternally inherited asexuality coupled with a history of rare sex with members of several closely related sexual populations or species. Our observations reinforce recent arguments that rare sex may be more important for the persistence of otherwise asexual lineages than hitherto appreciated.
ABSTRACT
We tested the protective function of larval wax covers in the two ladybird beetle species, Scymnus nigrinus and S. interruptus, against cannibalism, predation and ant aggression, and its importance for the distribution of both species in the field. Cannibalism was generally very low and not influenced by the presence or absence of the wax cover, or by larval size. Fourth-instar larvae of three ladybird species, Adalia bipuncata, Exochomus quadripustulatus and Harmonia quadripunctata, consumed Scymnus larvae-which are much smaller-regularly, independent of the presence or absence of waxes. By contrast, first-instar larvae of the three species had generally little success when attacking Scymnus spp. larvae. Wax-covered S. interruptus larvae survived significantly more attacks by the predacious carabid beetle Platynus dorsalis than larvae without wax cover. Wax-covered S. interruptus larvae and S. nigrinus larvae survived attacks by workers of the ant species, Lasius niger and Formica polyctena, respectively, significantly more often than larvae without wax covers. We show that, in the field, Scymnus larvae have higher densites in ant-attended resources than in unattended ones and conclude that both Scymnus species benefit from the ability to feed in ant-attended aphid colonies by a reduced predation risk.
ABSTRACT
Searching patterns and oviposition success of the aphid parasitoid Aphidius rosae were analysed in different abiotic conditions (simulated wind, simulated drizzling rain, simulated shower, high temperature, low humidity, "standard conditions") in the laboratory. Residence times, time allocation and oviposition success did not differ significantly between females foraging on rose shoots during periods with high temperature (>33°C) and low humidity (35%-40% relative humidity) and females searching under standard laboratory conditions (20°C, 65%-70% relative humidity, no wind). A wind speed of 2 m/s reduced the oviposition numbers significantly, and females spent much more time resting than searching for hosts. Females did not leave a shoot during wind, but departed quickly after wind had ceased. Simulated drizzling rain or simulated showers had the strongest effects on A. rosae foraging. Both types of rain prevented all foraging activities including departure from the shoot, and females laid no eggs during rain. After rain had stopped, females were mainly engaged with cleaning themselves and laid relatively few eggs compared to standard conditions. The impact of unfavourable abiotic environmental factors like wind or rain may help to explain why many parasitoids lay only a small proportion of their available eggs when foraging in the field.
ABSTRACT
The searching patterns of the aphid parasitoid Aphidius rosae were analysed at three different spatial scales: leaves, shoots and bushes. Parasitoid females searched aphid-infested leaves and shoots intensively and remained on average more than twice as long on infested than on uninfested shoots. Patch times and oviposition numbers per shoot were highly variable both between females and for different shoot visits within females. However, at the shoot and bush level low oviposition numbers were generally found. The time spent on different behavioural patterns (searching, resting, feeding, host handling) changed significantly during subsequent shoot visits of individual females but oviposition success was not influenced by this change. Parasitoids searched individual leaves and shoots mainly by walking, while moving between shoots occurred exclusively by flight. The travel time between shoots (i.e. flight time) accounted for less than 1% of the residence time in a bush. At the bush level foraging was characterized by a high ability to localize infested shoots and consequently little time was wasted in searching on uninfested shoots. The pattern of resource exploitation of individual females was consistent with the distribution of A. rosae larvae in field samples taken from individual rose bushes.
