Influences of genetic population structure, landscape composition and herbivore phenology on parasitism of sunflower moth, Homoeosoma electellum, in California
This research examines influences upon the parasitoid guild of sunflower moth, Homoeosoma electellum Hulst (Lepidoptera: Pyralidae), a North American native specialist herbivore and pest of crop sunflower, Helianthus annuus var. macrocarpus (L.). I approach the system from the genetic population, landscape, and habitat scales to determine the relevance of different variables towards future efforts in conservation biological control of sunflower moth.
At the population level, I used a set of 9 microsatellites, or polymorphic neutral genetic markers, to examine the population genetic structure and level of gene flow within the most important specialist parasitoid of sunflower moth, Dolichogenidea homoeosomae (Hymenoptera: Braconidae). I found that gene flow rates in this species are high, and that there are no signals of population substructure at the level of wild versus agricultural settings or local- regional geography. At the continental scale the population structure is detectable but gene flow still occurs. This points to a high degree of genetic mixing within this species in North America and indicates that genetic specialization towards wild or agricultural sunflower habitats is very unlikely. At the landscape scale I performed a field survey of sunflower moth larvae at 60 agricultural sunflower sites throughout the Central Valley of California during three summer growing seasons in 2003-2005. I then compared total parasitism rate, parasitoid species richness and relative proportions of generalist parasitoids to the proportion of habitat in the 1km radius circular area around each field in annual crops, orchards, riparian habitat and wild sunflower habitat. To follow this, I set up a sentinel larvae experiment to test the effects of surrounding habitat type upon the parasitism parameters. In the survey, I found a significant positive relationship between parasitism rate and the two other parasitoid guild parameters: parasitoid species richness and proportion of parasitism due to generalist parasitoids. The proportion of the area surrounding the field in orchard habitat was positively correlated with parasitism, parasitoid species richness and relative impact by generalists compared to the specialist parasitoid. While not statistically significant, parasitoid guild parameters in the sentinel larvae experiment generally followed the trends found in the survey. At the flower and habitat scale I examined herbivore phenology in self seeding sunflower, Helianthus annuus var. annuus (L.) in 5 different patches during three peak bloom summer seasons to determine the relative abundance and within flower-head co-ocurrence rates of the major sunflower herbivores and their parasitoids. In addition to sunflower moth, H. electellum, I found several other common flower consuming herbivores in the wild sunflower habitat in California. One in particular, Plagiomimicus spumosum (Lepidoptera: Noctuidae), exhibited an earlier peak in and higher overall abundance relative to H. electellum and was present in high densities at all of the sites surveyed. This herbivore was rarely found in flowers with conspecifics or other florivore species and was statistically negatively correlated with other species at the flower-head level. This indicates that this species may be exerting competitive pressure within the flower consuming guild in wild settings. Observed larval parasitism of P. spumosum averaged 12.2%, compared to 33.3% parasitism of H. electellum. Two parasitoid species were found to attack both H. electelluma and P. spumosum: Bracon nuperus and Erynnia tortricis. Given that the co-occurrence of the two most abundant herbivores in flowers is significantly lower than that expected by chance and they share at least two parasitoid species, further experimental investigation of the interaction between these herbivores and the consequences for community composition is warranted.
In summary, this research demonstrates that while the current population structure of the specialist parasitoid of sunflower moth does not show genetic substructure that could lead towards improved control of sunflower moth in agricultural fields, the role of generalist parasitoids and in particular those associated with nearby orchard habitat could be very important in biological control. In addition, complex interactions amongst herbivores in the wild sunflower habitats may contribute to the differences observed in parasitism rate between the wild and agricultural sunflower settings. These results show that using the tools and theories of population genetics, landscape and community ecology, we can contribute valuable information towards efforts in conservation biological control and sustainable agricultural practices.