Wolfgang Völkl

Aphids or their parasitoids : who actually benefits from ant-attendance?

The influence of ant-attendance on patterns of parasitism was studied for two parasitoid species attacking the black bean aphid Aphis fabae (Scop.), Trioxys angelicae (Haliday) and Lysiphlebus cardui (Marshall) on the host-plants Evonymus europaeus (L.) and Cirsium arvense (L.). Foraging Trioxys angelicae females were heavily attacked by honeydew-collecting ants. However, ants did not treat Lysiphlebus cardui females in an aggressive way. Ant attendance provided a considerable protection for A.fabae against T.angelicae, but did not prevent a high parasitization success of L.cardui

The importance of adverse weather conditions for behaviour and population ecology of an aphid parasitoid.

1. Weather conditions are known to influence insect behaviour and population dynamics. We investigated how wind and rain affect reproductive success and parasitization patterns of the aphid parasitoid Aphidius rosae in rose bushes. 2. A model was developed to simulate the life history of females foraging in the field under different environmental conditions. The model was parameterized using field and laboratory data on foraging behaviour of A. rosae. Field data on the daily occurrence of wind and rain were used for the simulation of different weather conditions. 3. When 1 day in the life of a female was modelled, both wind and rain reduced the number of aphid colonies and rose bushes visited. During rain, the parasitoid ceased all foraging activities, resulting in a negative correlation between the number of eggs being laid in visited aphid colonies, and the duration of rain during a day. During wind, the parasitoid continued to forage at a reduced rate, which led to increasing rates of parasitism in encountered aphid colonies during long wind intervals. 4. In a field study, we linked patterns of parasitism in aphid colonies to the weather conditions at the time when the eggs were laid by the parasitoids. The results confirmed the predictions of the model and showed that wind and rain change the distribution of parasitism among host colonies. 5. When the lifetime reproductive success of foraging females was simulated, both wind and rain affected parasitoid reproductive success. Even if weather conditions were benign, the reproductive success of females was greatly reduced compared to the maximum achievable oviposition numbers in the absence of any bad weather periods. Weather conditions had a less pronounced effect on parasitoid life history if travel mortalities between patches were taken into consideration. 6. Both bad weather conditions and travel mortality led to a strong skew in the frequency distribution of female reproductive success. A small proportion of females was able to realize oviposition numbers close to the maximum lifetime fecundity, as measured in the laboratory.

Age-specific patterns in honeydew production and honeydew composition in the aphid Metopeurum fuscoviride : implications for ant-attendance

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.

MOLECULAR MARKERS INDICATE RARE SEX IN A PREDOMINANTLY ASEXUAL PARASITOID WASP

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.

Honeydew amino acids in relation to sugars and their role in the establishment of ant‐attendance hierarchy in eight species of aphids feeding on tansy (Tanacetum vulgare)

Abstract.  The ratio of the concentration of honeydew total amino acids to total sugars in the honeydew of eight species of aphids, all feeding on tansy, Tanacetum vulgare (L.), was determined and correlated with honeydew production and ant‐attendance. The honeydew of the five ant‐attended aphid species [Metopeurum fuscoviride (Stroyan), Trama troglodytes (v. Hayd), Aphis vandergooti (Börner), Brachycardus cardui (L.), Aphis fabae (Scopoli)] was rich in total amino acids, ranging from 12.9 to 20.8 nmol µL−1 compared with the unattended aphid Macrosiphoniella tanacetaria (Kalt.) with only 3 nmol µL−1. Asparagine, glutamine, glutamic acid and serine (all nonessential amino acids) were the predominant amino acids in the honeydew of all species. The total concentration of amino acids in the phloem sap of tansy was much higher (78.7 nmol µL−1) then in the honeydew samples, and the predominant amino acids were glutamate (34.3%) and threonine (17.7%). A somewhat unexpected result was the finding that those aphid species with the highest total amino acid concentration in the honeydew always had the highest concentration of sugars. The lowest amino acid–sugar combined value was 104–28.8 nmol µL−1 in the non ant‐attended species M. tanacetaria, and the highest value was an average of 270–89.9 nmol µL−1 for the three most intensely attended aphid species M. fuscoviride, A. vandergooti and T. troglodytes. There is no evidence that any single amino acid or group of amino acids in the honeydew acted as an attractant for ant‐attendance in these eight aphid species. The richness of the honeydew (rate of secretion × total concentration of sugars), along with the presence of the attractant sugar melezitose, comprised the critical factors determining the extent of ant‐attendance of the aphids feeding on T. vulgare. The high total amino acid concentration in sugar‐rich honeydews can be explained by the high flow‐through of nutrients in aphids that are particularly well attended by ants.

Interactions between ants attendingAphis fabae ssp.cirsiiacanthoidis on thistles and foraging parasitoid wasps

We describe the behavioral interactions between honeydew-collecting workers of the ants Lasius nigerand Myrmica ruginodisand females of three species of aphidiid wasps (Lysiphlebus cardui, Lysiphlebus testaceipes, Trioxys angelicae)foraging for their aphid host, Aphis fabaessp. cirsiiacanthoidis,on thistles. Using field and laboratory experiments, we show that the ant-parasitoid interactions are species specific. Workers of both ant species generally attacked and killed females of T. angelicae,but they ignored those of L. cardui.This pattern was not altered when we anesthetized the wasps slightly with carbon dioxide to reduce their mobility. Prior contacts between L. carduiand either conspecific L. nigerfrom a different nest or workers of a different ant species (M. ruginodis)did not influence L. nigersnonaggressive behavior. The number of aphids parasitized by L. testaceipeswas significantly reduced in aphid colonies attended by L. niger,although this parasitoid was rarely attacked by ants. In encounters between these species of ants and wasps, ant aggression is consistent with differences in wasp behavior. We suggest that, in addition, chemical cues located in the cuticula may enable L. carduito avoid detection by honeydew-collecting ants.

Foraging strategies in solitary parasitoids: the trade-off between female and offspring mortality risks

SummaryIt is often assumed that oviposition rate is the currency that parasitoids should maximize in order to maximize reproductive success. Female parasitoids foraging in a patchy environment face a variety of mortality risks that influence the survival of both themselves and their offspring. Maximizing oviposition rate ignores these risks. A model is developed to analyse the influence of female and offspring mortality risks on optimal patch residence time in time-limited solitary parasitoids. The optimal compromize between minimizing a females own mortality risks and the mortality risks of her offspring in characterized. The optimal patch residence time is shown to be dependent on the relative magnitude of these mortality risks, as well as the rate with which reproductive success accumulates while on a patch. If travel time between patches is not fixed but a random variable, the optimal patch residence time decreases. However, variability in travel time increases expectations of total reproductive success. The model is illustrated with a case study in two aphid parasitoids.

Behavioral and morphological adaptations of the coccinellid,Platynaspis luteorubra for exploiting ant-attended resources (Coleoptera: Coccinellidae)

Larvae of the coccinellidPlatynaspis luteorubra were found significantly more often in ant-attended aphid colonies than in unattended colonies. The larva ofP. luteorubra are protected against detection by the antLasius niger by their unusual shape, by inconspicuous movements, and presumably by chemical camouflage. Pupae were attacked byL. niger but protected by their dense hair cover. Adults were also attacked and responded either by fleeing or by pressing their body tightly against the plant surface. Fourth-instar larvae foraging in ant-attended aphid colonies had a higher success rate in capturing large prey items and benefitted from this by higher adult weights. Larval parasitism was higher in individuals feeding in unattended than in ant-attended colonies.

Ant‐attendance as a critical factor in the biological control of the banana aphid Pentalonia nigronervosa Coq. (Hom. Aphididae) in Oceania

Abstract: Colonies of the banana aphid Pentalonia nigronervosa Coq. were heavily attended by ants in banana plantations on Tongatapu Island, Kingdom of Tonga. Ant‐attendance reduced the density of indigenous predators of P. nigronervosa considerably. Most important was the reduction by ants of the parasitism of Aphis gossypii by the aphelinid wasp Aphelinus gossypii. The introduced aphid parasitoid Aphidius colemani was also heavily attacked by various ant species, and we conclude that ant‐attendance may limit the economic impact of this successfully introduced parasitoid species.

Pea aphid clonal resistance to the endophagous parasitoid Aphidius ervi

The physiological mechanism of resistance to the endophagous braconid Aphidius ervi Haliday (Hymenoptera, Braconidae) by a pink clone (PC) of Acyrthosiphon pisum (Harris) (Homoptera, Aphididae) has been investigated. Comparative data on parasitoid development and associated host biochemical changes in the resistant PC aphids and in a susceptible green clone (GC) of A. pisum are reported. When the PC aphids were attacked as early 4th instars, the developing parasitoid larvae showed a strongly reduced increase in size, compared to those synchronously developing in GC aphids, and were unable to produce a regular mummy. In contrast, parasitism of 2nd instar PC aphids, allowed completion of parasitoid development, but adults had a prolonged developmental time, due to a longer duration of parasitoids final (3rd) instar. In all cases, teratocytes, cells deriving from the A. ervi serosal membrane, and the proteins abundantly synthesised by them, were never found in the haemolymph of parasitised PC aphids. Host castration, as demonstrated by total protein incorporation into reproductive tissues, was total in the majority of early (2nd instar) parasitised host aphids, while it was limited when later instars (4th) of PC aphids were parasitised. This is partly due to the absence of the cytolytic activity of teratocytes on host embryos, which, through their persistence, may compete for nutritional resources with the developing parasitoid larvae. In parasitised PC aphids, this competitive effect is further aggravated for the parasitoid by the absence of the regulated amino acid titre increase in the host haemolymph, which is regularly observed in GC aphids. Failure of teratocyte development in the PC clone of the pea aphid is, then, the major functional constraint accounting for the reduction/inhibition of A. ervi larval growth. The reported results allow to assess in vivo the role of teratocytes in the host physiological redirection and nutritional exploitation by the parasitoid, and to integrate and validate the proposed physiological model of host-parasitoid interactions in the system A. pisum-A.ervi.