ARS | Publication request: Host density drives spatial variation in parasitism of the alfalfa weevil, Hypera postica, across dryland and irrigated alfalfa cropping systems

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 30, 2012
Publication Date: February 1, 2013
Repository URL: http://handle.nal.usda.gov/10113/56103
Citation: Rand, T.A. 2013. Host density drives spatial variation in parasitism of the alfalfa weevil, Hypera postica, across dryland and irrigated alfalfa cropping systems. Environmental Entomology. 42(1):116-122.

Interpretive Summary: Biological control against the alfalfa weevil, a destructive pest of alfalfa, has resulted in the establishment of nine parasitoid wasps, which attack and kill various stages of the alfalfa weevil, in the US. Despite a massive effort by the USDA to redistribute a number of species throughout the US, there remains little follow-up information on whether these species are present, or doing any good, in many regions. In this study we surveyed parasitoid populations associated with alfalfa weevil larvae across 30+ sites spanning the eastern Montana and western North Dakota border over two years. Multiple sites were sampled in two habitat types differing in physical characteristics, flood-irrigated and dryland (rain-fed) alfalfa fields. Irrigated fields are higher yielding but also more intensively disturbed habitats due to increased harvest frequency and repeated flooding. Given evidence that both habitat disturbance and pest numbers, which often increase with increased plant growth, can influence parasitoids, we expected that the percentage of weevils killed by parasitoids and/or the relative importance of different species would differ across these two system types. Of four larval parasitoids species previously released/recovered in the region, two were present in our study, Bathyplectes curculionis and Oomyzus incertus with 37.2% and 3.5% of weevil larvae killed by each respectively. Levels of parasitism differed between habitat types, but the effect was entirely driven by differences in weevil numbers which were higher in irrigated than dryland fields. Parasitoid responses to weevil numbers varied between years and species. B. curculionis killed a higher percentage of weevil larvae as their numbers increased across sites in 2009 but the opposite pattern was found in 2010 when weevil numbers were higher overall. In contrast, the percentage of larvae killed by O. incertus increased with pest numbers across sites in 2010. Our results suggest that these two parasitoid species may be differentially effective at different weevil densities.

Technical Abstract: Biological control against the alfalfa weevil, Hypera postica (Gyllenhal), a destructive pest of alfalfa, has resulted in the establishment of nine hymenpoteran parasitoid species in the USA. Despite widespread redistribution of a number of these species, there remains little post-release data on their establishment and potential effectiveness in many regions. In this study we surveyed parasitoid populations associated with alfalfa weevil larvae across 30+ sites in eastern Montana and western North Dakota over two years. Replicate sties were sampled in two habitat types which differ in their physical characteristics, flood-irrigated and dryland alfalfa fields. Irrigated systems are more productive but also more intensively disturbed habitats due to increased harvest frequency and repeated flooding. Given evidence that both habitat disturbance and herbivore density, which often increases with productivity, can influence parasitoid dynamics, we expected that parasitism levels and/or the relative importance of different species would differ across these two system types. Of four larval parasitoids species previously released or recovered in the region, two were present in this study, Bathyplectes curculionis (Thomson) and Oomyzus incertus (Ratzenberg), with average levels of parasitism of 37.2% and 3.5% respectively. Levels of parasitism differed between habitat types, but the effect was entirely driven by concomitant differences in host densities which were higher in irrigated than dryland fields. Parasitoid responses to host density varied both across years and species. B. curculionis exhibited positive density dependent parasitism rates across sites in 2009 but the opposite pattern in 2010 when host densities were higher regionally. In contrast O. incertus exhibited positive density dependence in 2010. Our results suggest that these two species may be differentially effective at different host densities which may thus represent an important axis along which parasitoids exhibit spatial complementarity in function.