Main scientific findings
Below, there is a list of findings that the author of this homepage finds exciting. In addition, Wolfgang Völkl’s publications are full of unique observations of natural history, including those on individual species published in German.
Ants and parasitoid life-history
A main finding is the role of ants in affecting life-histories of aphid parasitoid, with species of the genus Lysiphlebus being able to use chemical mimicry to avoid ant attack. He extended his work to other parasitoids including those attacking root-feeding (i.e belowground-living) aphids.
Völkl, W. (1992). Aphids or their parasitoids: who actually benefits from ant-attendance? J. Anim. Ecol., 61:273-281.
Völkl, W. (1994). The effect of ant-attendance on the foraging behaviour of the aphid parasitoid Lysiphlebus cardui. Oikos, 70:149-155.
Völkl, W. & Mackauer, M. (1993). Interactions between ants and parasitoid wasps foraging for Aphis fabae spp. cirsiiacanthoidis on thistles. J. Insect Behav., 6:301-312.
Liepert, C., & Dettner, K. (1993). Recognition of aphid parasitoids by honeydew-collecting ants - the role of cuticular lipids in a chemical mimicry system. Journal of Chemical Ecology, 19:2143-2153.
Völkl, W., Liepert, C., Birnbach, R., Hübner, G. & Dettner, K. (1996). Chemical and tactile communication between the root aphid parasitoid Paralipsis enervis and trophobiotic ants: consequences for parasitoid survival. Experientia 52:731-738.
Ants and hyperparasitoids
Wolfgang Völkl extended his work to ask in more detail how hyperparasitoids, i.e. wasps that attack the primary (aphid) parasitoid while still living inside an aphid, manage to escape ant aggression. Again, he found it to be a mixture of chemical, morphological and behavioural adaptations.
Völkl, W., Hübner, G. & Dettner, K. (1994). Interactions between Alloxysta brevis (Hymenoptera, Cynipoidea, Alloxystidae) and honeydew collecting ants: How an aphid hyperparasitoid overcomes ant aggression by chemical defense. J. Chem. Ecol., 20:2901-2915.
Hübner, G. & Völkl, W. (1996). Behavioral strategies of aphid hyperparasitoids to escape aggression by honeydew-collecting ants. J. Insect Behav., 9:143-157.
Ants and aphid predators
In addition to his work on aphid parasitic wasps, Wolfgang also explored the interactions of aphid predators with their prey and with ants, to unravel the different behavioural strategies of these organisms.
Völkl, W. (1995). Behavioural and morphological adaptations of the coccinellid Platynaspis luteorubra for exploiting ant-attended resources. J. Insect Behav., 8:653-670.
Völkl, W. & Vohland, K. (1996). Wax covers in larvae of two Scymnus species: do they enhance coccinellid larval survival? Oecologia, 107:498-503.
Linking individual behaviour and population dynamics in parasitic wasps
Wolfgang summarized his many experiments on a variety of species in a number of articles where he and his co-authors argued that the behavioural strategies, that are themselves the result of the interactions with hosts, ants, and other organisms, are the main factor explaining the patterns seen at the population and community level. This concerns in particular the rates of parasitism which are important for biocontrol.
Mackauer, M. & Völkl, W. (1993). Regulation of aphid population densities by aphidiid wasps: Does parasitoid foraging behaviour or hyperparasitism limit impact? Oecologia, 94:339-350.
Völkl, W. (1997). Interactions between ants and aphid parasitoids: patterns and consequences for resource utilization. Ecol. Stud., 130:225-240.
The role of abiotic factors for insect behaviour
Every ecologist knows that weather is important for the publication dynamics of almost all species. Nevertheless, the effect of weather has largely been studied with a focus on population ecology, i.e. whether temperature, rainfall etc. can regulate populations (probably not). Wolfgang noticed that small insects such as parasitoids are endangered by rain drops or wind and start to explore the behavioural response of the insects to simulated rainfall and wind. Similary, he wondered what parasitoids did at night and tested this in the laboratory.
Fink, U. & Völkl, W. (1995). The effect of abiotic factors on foraging and oviposition success of the aphid parasitoid, Aphidius rosae. Oecologia, 103:371-378.
Weisser, W.W., Völkl, W. & Hassell, M.P. (1997). The importance of adverse weather conditions for behaviour and population ecology of an aphid parasitoid. J. Anim. Ecol., 66:386-400.
Schwörer, U., Völkl, W. & Hoffmann, K.H. (1999). Foraging for mates in the parasitic wasp, Dendrocerus carpenteri: Impact of unfavourable weather conditions and parasitoid age. Oecologia, 119:73-80.
Völkl, W. & Kranz, P. (1995). Nocturnal activity and resource utilization in the aphid hyperparasitoid, Dendrocerus carpenteri. Ecol. Entomol., 20:293-297.
Schwörer, U. & Völkl, W. (2001). The foraging behavior of Aphidius ervi (Haliday) (Hymenoptera: Braconidae: Aphidiinae) on different spatial scales: resource utilization and suboptimal weather conditions. Biol. Control, 21:111-119.
Weinbrenner, M. & Völkl, W. (2002). The oviposition behaviour of the aphid parasitoid, Aphidius ervi: Are wet aphids recognized as host? Entomol. Exp. Appl., 103:51-59.
The role of aphid honeydew in ant-aphid interactions
When back in Bayreuth after his stay in Bonn, Wolfgang used Tansy (Tanacetum vulgare L.) as a model system which hosts at least five aphid species, some of which are ant-tended. He noted in field observations that there were great differences between these aphid species in the rate and the intensity in which were tended by ants. Wolfgang explored possible reasons for these differences, exploring aphid behaviour, aphid morphology, and the quantity and composition of the aphid honeydew. While it was known that aphid honeydew differs in composition from the composition of the plant phloem sap, it was not known that the honeydew of different aphid species feeding on the same plant species can differ dramatically in the sugar content. It turned out that ants preferred aphids that produced most honeydew, but in particular those where the honeydew included trisaccherides, e.g. melezitose and raffinose. These sugars are not contained in the phloem sap of the plants, i.e. they serve to attract ants.
Völkl, W., Woodring, J., Fischer, M., Lorenz, M.W. &. Hoffmann, K.H. (1999). Ant-aphid mutualisms: The impact of honeydew production and honeydew sugar composition on ant preferences. Oecologia, 118:483-491.
Fischer, M.K., Hoffmann, K.H. & Völkl, W. (2001). Competition for mutualists in an ant-homopteran interaction mediated by hierarchies of ant-attendance. Oikos, 92:531-541.
Engel, V., Fischer, M.K., Wäckers, F.L. & Völkl, W. (2001). Interactions between extrafloral nectaries, aphids and ants: are there competition effects between plant and homopteran sugar sources? Oecologia, 129:577-584.
Fischer, M.K., Völkl, W., Schopf, R. & Hoffmann, K.H. (2002). Age-specific patterns in honeydew production and honeydew composition in the aphid Metopeurum fuscoviride: implications for ant-attendance. J. Insect Physiol., 48:319-326.
Comparative behavioural ecology of aphid parasitoids and hyperparasitoids
Together with colleagues Wolfgang studied the behaviour and the interaction with their host of a wide range of aphid parasitoids and also hyperparasitoids. He summarized this knowledge in a number of reviews, and contributed ecological knowledge and specimen to studies on the phylogeny of the species. The reviews contain many insights that Wolfgang gained in his field and experimental work, rather than being mere summaries of the literature.
Sullivan, D. J. & Völkl, W. (1999). Hyperparasitism: Multitrophic ecology and behavior. Annu. Rev. Ent., 44:291-315.
Völkl, W. & Mackauer, M. (2000). The oviposition behaviour of aphidiine wasps (Hymenoptera: Braconidae, Aphidiinae): morphological adaptations and evolutionary traits. Can. Ent., 132:197-212.
Kambhampati, S., Völkl, W. & Mackauer, M. (2000). Phylogenetic relationships among genera of Aphidiinae (Hymenoptera: Braconidae) based on DNA sequence of mitochondrial 16S rRNA gene. Syst. Entomol., 25:437-445.
Völkl, W., Mackauer, M., Pell, J.K. & Brodeur, J. (2007). Predators, parasitoids and pathogens of aphids. In: van Emden, H. & Harrington, R. (eds): Aphids as crop pests. CABI, Wallington, 87-233.