Regulation of aphid populations by aphidiid wasps: does parasitoid foraging behaviour or hyperparasitism limit impact?

Abstract

Aphidiid parasitoids (Hymenoptera: Aphidiidae) of aphids generally exploit only a small percentage of the available host resources in the field. This limited impact on aphid populations has often been explained as a consequence of hyperparasitism. We propose that a wasp's reproductive strategy, as opposed to hyperparasitism, is the dominant factor in aphidiid population dynamics. A wasp's foraging efficiency and oviposition decisions are influenced by several variables, including searching behaviour between and within patches, host choice (as modified by the aphids' defensive behaviours), and plant structural complexity. Two broadly different patterns of host exploitation have evolved in aphidiid wasps in relation to ant-aphid mutualism. Firstly, in species that are exposed to predation and hyperparasitism, a female may leave a patch before all suitable hosts are parasitized. Because predators and hyperparasitoids tend to aggregate at high aphid or aphidiid densities, or in response to aphid honeydew, this strategy enables females to reduce offspring mortality by "spreading the risk" over several host patches. Secondly, in species that have evolved mechanisms to avoid aggression by mutualistic ants, females are able to exploit a hyperparasitoid-free resource space. Such species may concentrate their eggs in only a few aphid colonies, which are thus heavily exploited. Although hyperparasitism of species in the first group tends to reach high levels, its overall impact on aphid-aphidiid population dynamics is probably limited by the low average fecundity of most hyperparasitoids. We discuss the foraging patterns of aphidiid wasps in relation to aphid population regulation in general, and to classical biological control in particular. We argue that a parasitoid's potential to regulate the host population is largely determined by its foraging strategy. In an exotic parasitoid, a behavioural syndrome that has evolved and presumably is adaptive in a more diverse (native) environment may, in a more uniform (managed) environment, result in suboptimal patch-leaving and oviposition decisions, and possibly increased resource usage.

Spatial population dynamics of a specialist aphid parasitoid, Lysiphlebus hirticornis Mackauer (Hymenoptera: Braconidae: Aphidiinae): evidence for philopatry and restricted dispersal

“…M. fuscoviride colonies are normally attended by ants, especially Lasius niger L. (Hymenoptera: Formicidae) (Mackauer and Vö lkl, 1993). From the aphid, ants obtain an important source of nutrients in the form of honeydew; in return, they act as guards, warding off predators and parasitoids (Vö lkl, 1997).…”

Landscape complexity is not a major trigger of species richness and food web structure of European cereal aphid parasitoids

“…For example, Monmany and Aide (2009) found higher parasitism rates of herbivorous insects in some Argentinian forests as a result of higher numbers of generalist primary parasitoids. These differences between and within guild, as well as landscape structure and agricultural intensification might influence food chain structure and therefore total parasitism rates and biocontrol effectiveness (Mackauer and Völkl 1993).…”

Landscape scale study of diversity and interaction in cereal aphids - parasitoids system (Homoptera; Hymenoptera)

“…Pregledom rezultata sličnih studija (Tomanović & Brajković, 2001;Sigsgaard, 2002;Roschewitz et al, 2005;Vollhardt et al, 2008;Tomanović et al, 2008;Caballero-López et al, 2011, 2012Gagic et al, 2011Gagic et al, , 2012Lohaus et al, 2012;Kos et al, 2012) može se zaključiti da sastav i struktura gilde parazitoida biljnih vašiju značajno varira ne samo između regiona, već i između godina u okviru istog regiona. Imajući u vidu različito ponašanje porazitoida i njihovu preferenciju ka biljnim vašima (Powell & Zhi-Li, 1983;Mackauer & Völkl, 1993;Bueno et al, 2003;Lo Pinto et al, 2004;Langley et al, 2006;Rehman & Powell, 2010) kao i biotičke odnose, pre svega kompeticiju, unutar gilde parazitoida (McBrien & Mackauer, 1990;Brodeur & Rosenheim, 2000), prisustvo ili odsustvo određenih vrsta parazitoida, kao i njihova relativna zastupljenost mogu imati veliki uticaj na uspešnost biološke kontrole biljnih vaši. Treba istaći da je parazitiranost bila najviše prisutna kod vrste S. avenae, vrlo malo kod vrste M. dirhodum, a potpuno odsutna kod preostale dve vrste.…”