RESULTS Landscape Composition and Parasitoid Community Structure

Question 1. Does surrounding landscape composition affect the species richness, life history strategies (generalists versus specialists), and overall parasitism rates of this guild of parasitoids in wild and agricultural sunflower habitats?

In the summers of 2004 and 2005, I surveyed agricultural and self seeding sunflower fields throughout the Great Central Valley, CA (22 agricultural sites, 6 “wild” sites). Each week during flower bloom, I collected 100 flowers from each site and dissected these in the lab to rear each of the sunflower moth larvae present in individual diet cups. I recorded larval head capsule size, parasitoid emergence, and identification. This dataset provides temporal information on the herbivore density, parasitism, and parasitoid guild parameters across survey sites and the summer season. For the purposes of this study, I have focused on the parasitoids attacking the 2nd _{– 4}th _instar larvae of H. electellum. Based on past parasitism survey efforts (Chen and Welter, 2005, Nerney, unpublished data), these instars are the most commonly parasitized life stages of this host.

To address temporal patterns in parasitoid guild composition observed in the parasitism surveys described above experimentally in 2005, I placed sentinel larvae in flowers at five self seeding sunflower sites to examine differences in these parasitoid guild factors along temporal as well as spatial gradients. Three times over the course of active host larval period (July, August and September), I placed laboratory reared sunflower moth larvae (2nd _{– 3}rd instar) in flowers of standardized age, which I introduced into the field in potted plants raised in the greenhouse. These larvae were then exposed to parasitism for 8 degree days and brought back to the lab (field recovery rates 76.9%) to be reared in individual diet cups. This allows for a comparison of parasitoid parameters across sites with a controlled number of degree days at each of the sites within the California Central Valley. This sentinel experiment is currently being expanded (summer of 2006) to a total of 10 sites across a gradient of agricultural and wild sunflower habitats to examine the effect of proximity to several different habitat types, including host plants of the alternate hosts of the most common generalist parasitoids of sunflower moth.

Specialist Parasitoid Genetic Population Structure

Question 2. Does the specialist parasitoid Dolichogenidea homoeosoma of sunflower moth exhibit genetic population structure at the habitat or geographic scales in the Central Valley?

Using preliminary collections of D. homoeosoma, we identified a total of 9 microsatellite markers, or polymorphic tandem repeat sequences in the genome (Douhovnikoff et al., 2005), which we are currently using to analyze the genetic population structure of this specialist parasitoid of sunflower moth at the habitat (wild versus agricultural sites) and geographic scale (Counties within California, Great Central Valley wide and in comparison with specimens reared from sunflower moth in Kansas, North Dakota and Texas). By genotyping specimens reared from sunflower moth at all of the sites surveyed for sunflower moth parasitism in 2004, 2005, and 2006 as well as specimens obtained from collaborators in other states, I am able to compare the relative genetic relatedness of these georeferenced individual samples using a spatial statistical model for landscape genetics (Guillot et al., 2005). Temporal Factors of Parasitoid Guild Composition

Question 3. Does the temporal resource limitation (shorter time frame of host availability) in agricultural sunflower fields exclude some of the parasitoids of H. electellum?

To determine the feasibility of timing sunflower plantings to maintain or increase generalist parasitoid populations for biological control purposes in other crops, I surveyed parasitoid community parameters in 22 agricultural and 8 “wild” sunflower sites during the months of June, July, August, September, and October of 2004 and 2005. I recorded the timing of peak flowering at each patch as well as the parameters of overall parasitism, parasitoid species richness, and ratio of specialists to generalists and then plotted the occurrence of each of the six most common parasitoid species along a time axis.

RESULTS Landscape Composition and Parasitoid Community Structure

Overall parasitism and parasitoid species richness do not appear to be influenced by surrounding crop type, while the ratio of generalist to specialist parasitoids may be influenced by proximity to some orchard crops. I am currently investigating this observed pattern to determine the inter-year consistency of this result.

Number of parastioids of each species at each site 6 4 1 1 0 6 3 11 8 3 5 3 1 4 17 6 2 2 1 9 7 4 0 3 1 0 0 4 2 0 2 0 0 1 0 5 2 0 1 0 1 2 0 2 4 6 8 10 12 14 16 18 Dolicogenidea homoeosoma Macrocentrus sp.

Bracon sp. Ichneumonid 1 Ichneumonid 2 Tachinid 1 Tachinid 2

species in d ivid u als r eared f ro m H . e lect ellu m co llect ed in s u rve y YB1 n=78 YB2 n=128 YB3 n=221 SL1 n=87 SL2 n=30 CS n=90

Fig. 1. Number of parasitoids of each species at each of 6 wild sites in the Great Central Valley, CA.

Specialist Parasitoid Genetic Population Structure

The specialist parasitoid D. homoeosomae of sunflower moth does not exhibit genetic population structure at the habitat (wild versus agricultural) level nor at the level of counties within California. Genotype structure is exhibited at the broader geographic level (California compared to Central States). I am currently analyzing additional samples to define the specific structure and scale of this organization.

Temporal Factors of Parasitoid Guild Composition

Generalist parasitoids attacking sunflower moth, H. electellum, are more common than the main specialist parasitoid earlier in the sunflower blooming season. This pattern is reversed towards the end of the blooming season. This pattern is observed throughout the Great Central Valley and most strongly in landscape contexts that are predominantly orchard crops. I am currently investigating the parasitoid community structure (ratio of

generalists to specialists) in an expanded set of sites near orchard crops that provide alternate hosts for the generalist parasitoids to understand the role of alternate host species.

DISCUSSION

Workers in the field of parasitoid biology are working towards an understanding of the rules that can conceptually unify parasitoid community ecology, insect population biology, and the applied practice of biological control (Hawkins, 1994). A combination of laboratory, field, landscape, and population level perspectives will be necessary in this effort. These preliminary findings demonstrate a potentially useful combination of field parasitism surveys, sentinel experiments, and molecular tools to further our understanding of the factors that determine parasitoid community structure and parasitoid-host population dynamics.

ACKNOWLEDGEMENTS

I thank my collaborators Stephen C. Welter, George Roderick, Vladimir Douhovnikoff, and Yolanda Chen. Research has been funded by The Robert van den Bosch Scholarship in Biological Control, The Land Institute, and the Division of Organisms and Environment at the University of California, Berkeley.

REFERENCES

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Charlet, L.D., Kopp, D.D., Oseto, C.Y. 1987. Sunflowers: Their history and associated insect community in the Northern Great Plains. Bull. Entomol. Soc. Am. 33, 69:75.

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Roderick, G.K., Navajas, M. 2003. Genes in new environments: genetics and evolution in biological control. Nature Reviews; Genetics 4, 2003.

Sheehan, W. 1994. Parasitoid community structure. In: Parasitoid community ecology. In: Hawkins B.A., Sheehan, W. (eds.), Oxford University Press. Cambridge, U.K., pp. 472-491.

Thies, C., Steffan-Dewenter, I., Tscharntke, T. 2003. Effects of landscape context on herbivory and parasitism at different spatial scales. Oikos 101, 18-25.

SUSCEPTIBILITY OF THE OLIVE FRUIT FLY, BACTROCERA OLEAE (DIPTERA: