Investigators: Mahmut Dogramaci, Z B Mayo, Robert J. Wright, Dept. of Entomology, University of Nebraska-Lincoln, and John C. Reese, Dept. of Entomology, Kansas State University, Manhattan.

Contact: Robert Wright (rwright2@unl.edu)

Project description: Interactions of the parasitoid Lysiphlebus testaceipes (Cresson) and the greenbug, Schizaphis graminum (Rondani), on greenbug-resistant 'Cargill 607E' (antibiosis), 'Cargill 797' (primarily tolerance), and -susceptible 'Golden Harvest 510B' sorghum, Sorghum bicolor (L.) Moench, were tested using three levels of biotype I greenbug infestation. The parasitoid infestation rate was 0.5 female and 1.0 male L. testaceipes per plant. For all three greenbug infestation levels, the parasitoid brought the greenbug under control (i.e., prevented the greenbugs from killing the plants) on both resistant hybrids, but it did not prevent heavy leaf damage at the higher greenbug infestation rates. At the low greenbug infestation rate (50 greenbugs per resistant plant when parasitoids were introduced), greenbugs damaged 5 and 18% of the total leaf area on 'Cargill 797' and 'Cargill 607E', respectively, before greenbugs were eliminated. Leaf damage was higher for the intermediate infestation study (120 greenbugs per plant), 21% and 30% leaf area were damaged on the resistant sorghum hybrids 'Cargill 797' and 'Cargill 607E', respectively. At the high greenbug infestation rate (300 greenbugs per plant), heavy damage occurred: 61% on 'Cargill 607E' and 75% on 'Cargill 797'. The parasitoids did not control greenbugs on the susceptible sorghum hybrid 'Golden Harvest 510B'. L. testaceipes provided comparable control on both greenbug-resistant hybrids. This study supports previous studies indicating that L. testaceipes is effective in controlling greenbugs on sorghum with antibiosis resistance to greenbugs. Furthermore, new information is provided indicating that L. testaceipes is also effective in controlling greenbugs on a greenbug-tolerant hybrid.

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2) Project title: Ladybeetle development in alfalfa

Investigators: Alexander P. Cunningham, Stephen D. Danielson, Dept. of Entomology, and James Brandle, School of Natural Resource Sciences, University of Nebraska-Lincoln, Lincoln, NE, 68583-0816

Contact: Stephen Danielson (sdanielson1@unl.edu )

Project description: Ladybeetle and alfalfa development rates were monitored in alfalfa fields in Southeast Nebraska in 2004 and 2005. Remote-sensing equipment was used to collect year-round temperature measurements in each field studied. Degree-day accumulation was recorded and compared to development stages of alfalfa and ladybeetles. Sweep net sampling was used to determine predominant ladybeetle life stages throughout the growing season. This experiment will be useful in determining if a correlation exists between alfalfa and natural enemies of alfalfa pests in Nebraska. Another aim is to determine whether harvest timing may be manipulated to optimize the biological control of alfalfa pests by ladybeetles.

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3) Project title: Combined effects of herbicides and Sphenoptera jugoslavica on diffuse knapweed (Centaurea diffusa) population dynamics

Investigators: Robert Wilson, Panhandle Research & Extension Center, University of Nebraska, 4502 Avenue I, Scottsbluff, NE 69361, K. George Beck, and Philip Westra, Department of Bioagricultural Sciences and Pest Management, Colorado State University, Fort Collins, CO 80523

Contact: Robert Wilson (rwilson1@unl.edu)

Project description: Sphenoptera jugoslavica (Coleoptera:Buprestidae) negatively influences diffuse knapweed populations, but the influence is inconsistent in space and time. In spring 1998, a 3-yr experiment was established to determine whether low rates of picloram or clopyralid would increase the percentage of plants infested with S. jugoslavica, thereby potentially enhancing its capacity to control diffuse knapweed. The experiment was conducted at three sites in Colorado, where S. jugoslavica was released in 1994. Picloram and clopyralid were applied separately to plots at 35, 70, or 140 g ha-1 during June or September, and a non–herbicide-treated control was added. Density and cover measurements were collected in permanent quadrats three times during each growing season to determine whether herbicides influenced diffuse knapweed growth and population dynamics. Diffuse knapweed plants were harvested outside the permanent quadrats to determine the percentage of plants bearing S. jugoslavica larvae. During the spring after herbicide application, all picloram rates and the 35-g ha-1 rate of clopyralid applied the previous June increased the percentage of plants infested by S. jugoslavica approximately 25% compared with the nonsprayed control. None of the herbicide treatments increased the percentage of plants infested by S. jugoslavica 2 yr after application, indicating that the herbicides' positive effects on the percentage of plants infested by S. jugoslavica lasted only 1 yr. Results indicate that combining S. jugoslavica with low rates of picloram or clopyralid applied in June can improve diffuse knapweed control compared with using S. jugoslavica alone.

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4) Project title: Impact of Ceutorhynchus litura feeding on root carbohydrate levels in Canada thistle (Cirsium arvense)

Investigators: Gary L. Hein, and Robert G. Wilson. Panhandle Research & Extension Center, University of Nebraska, 4502 Avenue I, Scottsbluff, NE 69361.

Contact: Gary Hein (ghein1@unl.edu)

Project description: Canada thistle is a serious perennial weed found throughout the northern regions of the United States and Canada. The weevil, Ceutorhynchus litura (F.), was first released in Canada in 1965 as a potential biological control agent for Canada thistle; however, its impact as a control agent has been sporadic. The objective of this study was to characterize C. litura impacts on the carbohydrate profile in Canada thistle roots through the growing season and to evaluate the potential for this biological control agent in causing stress to Canada thistle. Field plots, infested with C. litura, were established and extensively sampled for C. litura infestations. By sampling C. litura–damaged and undamaged Canada thistle shoots–roots through the season, we were able to establish the profile of free sugars and fructans in the roots and compare these levels to the presence and extent of insect damage. Levels of all free sugars and fructans were consistently found to be depressed in roots from C. litura–damaged shoots early in the summer during and shortly after the larval feeding period. Ceutorhynchus litura feeding in Canada thistle shoots appears to disrupt the movement of photoassimilates from leaves to roots. Late-season levels of free sugars and fructans indicate that roots do recover from these depressed levels, and in several instances, significant overcompensation occurred in the damaged roots. Measurement of free sugars and fructans to identify sublethal impacts of control tactics may allow the strategic combining of complementary tactics to maximize the impact of stresses on Canada thistle.

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Mahmut Dogramaci, Z B Mayo, Robert J. Wright & John C. Reese. 2005. Tritrophic interaction of parasitoid Lysiphlebus testaceipes (Hymenoptera: Aphidiidae), greenbug, Schizaphis graminum (Homoptera: Aphididae), and greenbug-resistant sorghum hybrids. J. Economic Entomology 98(1):202–209.

Gary L. Hein & Robert G. Wilson. 2004. Impact of Ceutorhynchus litura feeding on root carbohydrate levels in Canada thistle (Cirsium arvense). Weed Science 52(4): 628–633.

Robert Wilson, K. George Beck & Philip Westra. 2004. Combined effects of herbicides and Sphenoptera jugoslavica on diffuse knapweed (Centaurea diffusa) population dynamics. Weed Science: 52(3): 418–423

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