Spatial patterns of Trybliographa rapae parasitism of Delia radicum larvae in oilseed rape and cauliflower

Abstract

Trybliographa rapae (Westwood) is an important parasitoid of Delia radicum (L.). Parasitism of D. radicum larvae by T. rapae in relation to host density on canola (oilseed rape) and cauliflower roots was examined at 10 field sites in Germany and Switzerland. For roots with host larvae, the proportion of roots with one or more parasitized hosts increased with increasing host density. However, for these infested roots, the parasitism of individual larvae was not consistently related to host density. When considering only roots on which there were parasitized larvae and the opportunity for multiple attacks, the proportion of larvae that were parasitized decreased with increasing host density in the field locations, and in a cage study under controlled conditions. A model of patch-finding and number of attacks by female parasitoids suggests that patch-finding is density-dependent, but that low attack rate and interference effects limit numbers of attacks to three or less per visit to a host patch; the reduced number of attacks per visit leads to the inverse relationship of larval parasitism with host density in the host patches visited. The interplay of the densitydependent and inversely density-dependent processes appears to be responsible for the inconsistency of density dependence of overall larval parasitism in this and previous studies. In the laboratory, adult female T. rapae parasitized hosts at £4 cm deep in soil, but not at 6 cm deep. From the depth distribution of larval feeding sites in the field, we infer that between 4% and 20% of Delia larvae may be in a physical refuge from T. rapae parasitism, which may have a stabilizing influence on the host-parasitoid interaction.

Habitat selection of a parasitoid mediated by volatiles informing on host and intraguild predator densities

“…Under natural conditions, parasitoids with this type of functional response are aggregated in patches of high host density (Hassell and May 1973 ; Jones and Hassell 1988 ). Indeed, high host densities were most attractive to T. rapae females in the present study, which is in agreement with previous findings (Neveu et al 2002 ; Hemachandra et al 2007 ). Similarly, entomopathogenic fungi often display clumped distributions in the soil environment (Meyling and Eilenberg 2007 ).…”

“…HIPVs and other host-derived cues have been shown to guide female parasitoids to locate and evaluate host patches before oviposition (Vet and Dicke 1992 ; Wäschke et al 2013 ). It is also well established that host density plays a role when parasitoid females select suitable habitats for oviposition (Waage 1983 ; Connor and Cargain 1994 ; Hemachandra et al 2007 ; Girling et al 2011 ). The mechanism involved may be an increase in emitted HIPVs with higher host densities (Girling et al 2011 ).…”

“…We used larvae of the cabbage root fly, Delia radicum L., a herbivore of roots of cruciferous plants in temperate climates (Finch 1989 ), feeding on white cabbage plants, as a model host habitat. Host habitat location by T. rapae females is mainly guided by olfactory cues with HIPVs released by the plant in combination with host-related cues (e.g., saliva, feces) informing the parasitoid of host presence (Brown and Anderson 1999 ; Neveu et al 2002 ; Nilsson et al 2012 ) and host density (Hemachandra et al 2007 ). As pathogens, we used the generalist entomopathogenic fungi Metarhizium brunneum Petch and Beauveria bassiana (Balsamo) Vuillemin (Ascomycota: Hypocreales), which are ubiquitous in the soil environment (Meyling and Eilenberg 2007 ) and occur naturally at higher densities in plant rhizospheres than in the surrounding soil (Bruck 2010 ).…”

Afrotropical Cynipoidea (Hymenoptera)

“…Attack Anthomyiidae and occasionally other calyptrate flies in dung, fungi, debris, etc (Belizin 1963, Block et al 1987, Griffiths 1993, Harukawa and Kikamashiro 1939, Hemachandra et al 2007, Hennig 1976, Hummel et al 2010, Jones and Hassell 2008, Kerrich and Quinlan 1960, Kieffer 1913a, Makarenko 1968, Neveu et al 1996, 2000, Nilsson et al 2011, 2012, Nordlander 1981, Quinlan 1978, Tamer 1994, Wilkes and Wishart 1953, Wishart and Monteith 1954, and plenty of others, plus lots of additional label data). The host of the African species is not known.…”

Effects of conservation strip and crop type on natural enemies ofDelia radicum

“…High parasitism rates of D. radicum have been reported (Bromand ; Hemachandra et al. ; Meyling et al. ) and are suggested to help stabilize fly populations at farm scale from one generation to the next (Finch and Collier ; Finch ).…”