Non‐random distribution among a guild of parasitoids: implications for community structure and host survival

Abstract

1. Immature stages of the gall midge, Asphondylia borrichiae , are attacked by four species of parasitoids, which vary in size and relative abundance within patches of the gall midge's primary host plant, sea oxeye daisy ( Borrichia frutescens ).2. In the current study, a bagging experiment found that the smallest wasp, Galeopsomyia haemon , was most abundant in galls exposed to natural enemies early in the experiment, when gall diameter is smallest, while the wasp with the longest ovipositor, Torymus umbilicatus , dominated the parasitoid community in galls that were not exposed until the 5th and 6th weeks when gall diameter is maximal.3. Moreover, the mean number of parasitoids captured using large artificial galls were 70% and 150% higher compared with medium and small galls respectively, while stem height of artificial galls significantly affected parasitoid distribution. Galls that were level with the top of the sea oxeye canopy captured 60% more parasitoids compared with those below the canopy and 50% more than galls higher than the plant canopy.4. These non-random patterns were driven primarily by the differential distribution of the largest parasitoid, T. umbilicatus , which was found significantly more often than expected on large galls and the smallest parasitoid of the guild, G. haemon , which tended to be more common on stems level with the top of the plant canopy.5. Large Asphondylia galls, especially those located near the top of the Borrichia canopy, were more likely to be discovered by searching parasitoids. Results using artificial galls were consistent with rates of parasitism of Asphondylia galls in native patches of sea oxeye daisy. Gall diameter was 19% greater and the rate of parasitism was reduced by almost 50% on short stems; as a result, gall abundance was 24% higher on short stems compared with ones located near the top of the plant canopy.6. These results suggest that parasitoid community composition within galls is regulated by both interspecific differences in ovipositor length and preferences for specific gall size and/or stem length classes.

The Gall MidgeAsphondylia borrichiae(Diptera: Cecidomyiidae): An Indigenous Example of Host-Associated Genetic Divergence in Sympatry

“…Prolonged isolation and differences in host quality may result in host-plant associated variation in the size and search image of the midge's parasitoids. Previous bagging studies using galls in situ suggest that T. umbilicatus becomes the dominant member of the parasitoid guild when gall diameter exceeds 7-9 mm (most likely because smaller members of the parasitoid guild are unable to reach developing insects with their shorter ovipositors) (Weis & Abrahamson 1985;Stiling & Rossi 1994;Rossi et al 2006). Lastly, it should be mentioned that I. imbricata, which primarily inhabits beach dunes (rather than marshes like B. frutescens and I. frutescens) was probably the last plant colonized by the midge (and its parasitoids).…”

“…Parasitism is a primary mortality factor for A. borrichiae and aggregate parasitism levels can reach 100% at some sites causing local extinction of the midges and it has been hypothesized that decreased size and fecundity of the midge on the 2 Iva spp. is partially balanced by the acquisition of enemyfree-space on these derived species (Stiling et al 1992(Stiling et al , 1994Rossi & Stiling 1995;Moon & Stiling 2002;Rossi et al 2006).…”

Manner of Apical Meristem Destruction Affects Growth, Reproduction, and Survival of Sea Oxeye Daisy

“…Adult A. borrichiae are small (<3 mm long), fragile, weak-flying gall midges that live for only 2-3 days, during which their primary activity is breeding [2,[44][45][46]. Female A. borrichiae collect the conidia of a fungal symbiont and deposit the conidia with her eggs inside the apical meristematic tissue of the host plant during oviposition, which induces a gall to form [42,45,47,48]. Larvae feed on the fungal symbiont, which draws nutrients from plant tissues, and they pupate within the gall [47,48].…”

“…Female A. borrichiae collect the conidia of a fungal symbiont and deposit the conidia with her eggs inside the apical meristematic tissue of the host plant during oviposition, which induces a gall to form [42,45,47,48]. Larvae feed on the fungal symbiont, which draws nutrients from plant tissues, and they pupate within the gall [47,48]. With occasional exceptions, female midges appear to avoid ovipositing in flower heads [40,42], and the presence of a gall typically prevents the apical meristems from flowering [33,42,45].…”

“…With occasional exceptions, female midges appear to avoid ovipositing in flower heads [40,42], and the presence of a gall typically prevents the apical meristems from flowering [33,42,45]. Galls persist on the host plant during spring and summer for about seven to ten weeks and senesce after midge eclosion, typically resulting in the death of the galled apical meristem [33,45] [43,47] have found galling on B. frutescens to be very high (approximately 30-50% of apical meristems) and a strong positive correlation between stem number and gall count (Anthony M. Rossi, unpublished data).…”