2012 Annual Report
Field plots. In 2011, 16 plots of hard red spring wheat were established, each surrounded by a 15 ft border of alfalfa. The alfalfa was not mowed throughout the duration of the experiment. Treatments consisted of a factorial design incorporating high and low aphids (R. padi) densities and high and low sugar treatments. Plants were infested with laboratory-reared aphids, but there was no evidence of successful introduction: aphids had disappeared within 24 h of placement in the field. In each plot, a grid of 35 evenly spaced sample location points was established within each plot. Sugar treatments were established by applying a 15% sucrose solution every week in high treatment plots and every 3 wk in low treatment plots. Sucrose treatments were applied 1 d prior to sampling along the grid. Insect communities. Sampling was conducted for 7 wk, and was initiated on June 9. Insects within plots were sampled using four methods: sweepnetting, quadrats, cores and whole plant counts. Sweepnetting was conducted three times by sweeping a 30 ft transect parallel and central to the area of sugar application. Sweeps were conducted 10, 40 and 70 ft from the sugar patch. Arthropods on the soil surface were sampled every two weeks using 0.25 m2 metal quadrats (twice per plot). All predators were removed from the soil surface within the quadrats using mouth aspirators. Soil cores (four per plot) were taken along gridpoints three times over the season. Arthropods within the soil cores were extracted from Berlese funnels over 7 d. Every week, wheat plants within a 1 ft2 area around four gridpoints were examined for 5 min, and all insects (pests and predators) were hand-collected. Predators (between 5-10 per sample method per date) were frozen at -20 C for gut content analysis (both sugar and predation). The remaining predators were identified to species level. Honeydew availability. Three honey dew traps were placed in each plot immediately after sucrose was applied to alfalfa each week. Tin weigh boats were stapled to wooden stakes and placed in the ground next to wheat plants in order to collect honeydew. Traps were collected when insect sampling was complete the next day. Traps were collected and placed at -20°C until anthrone tests are conducted to assess sugar concentrations. Yield. Wheat was harvested on August 18. Test weight and percent moisture were assessed for each of six samples collected per plot in order to calculate yield.
Preliminary Results. Whole plant counts indicated that aphid densities were consistently low throughout all plots; thus there was no distinction between treatments of high and low aphid density. Predators observed foraging on wheat plants included various ant species, nabids, pentatomids, spiders, green lacewings, and phalangiids. Ants were the most abundant predator identified in cores with Crematogaster, Ponera and Lasius most common throughout plots. Additional predators collected in soil cores included carabid larvae, as well as other Coleoptera larvae, diplurans, staphylinids, cucujids, and spiders. Collembola and thrips were common pests collected in cores and sweepnet samples, respectively. All specimens collected in 2011 have been identified, and gut content analyses are underway.
2012 progress. 16 field plots were again established in 2012 with the same high and low sucrose application rates, and sampling efforts focused on quadrat and whole plant samples. The intensive sampling included only eight of the plots in 2012, representing high and low sugar resources. Predation on aphids was studied more intensively in 2012, including a cage study to exclude predators, two 36-hr continuous observation periods on restrained aphids under field conditions, and collection and preservation of predators near outplanted aphid colonies. We hope to resolve whether predation is a major factor in restricting the colonization of wheat fields by R. padi.