AN INTEGRATIVE APPROACH TO COMBAT THE WHEAT STEM SAWFLY, Cephus cinctus (HYMENOPTERA: CEPHIDAE)

Abstract

The wheat stem sawfly (WSS) is a native grass-feeding insect and a major threat to wheat production across the Great Plains of North America. For over a century, WSS has caused significant damage to growers – not just by feeding within the stem and reducing yield, but particularly by causing wheat lodging. Economic losses are estimated at $350 million annually, with over $40 million in Colorado alone. Managing WSS has proven challenging and requires integrated tactics from multiple fronts. The goal of my PhD was to address WSS management through a combination of complementary, interdisciplinary approaches.To sustainably manage a pest that threatens food security, entomologists must first understand its biology, life cycle, and management history. In Chapter 1, I conducted a comprehensive review of WSS, detailing its biological traits and tracing the evolution of control strategies. Although WSS has long been a key pest, no predictive phenology model existed to guide management decisions. Phenology models are essential in pest management, helping forecast insect emergence and inform control timing. Therefore, in Chapter 2, I developed the first degree-day model (DDM) for WSS using 13 years of field data from Colorado. Wheat fields in northeastern Colorado were sampled annually from 2011 to 2023. Using local temperature data, we estimated the heat units (growing degree days) required for key life events. A generalized additive model (GAM) predicted adult emergence at 148 GDD, peak flight at 224 GDD, and flight end at 354 GDD, using a base temperature of 10°C and upper threshold of 30°C. We also identified weather variables associated with population shifts. These findings provide a powerful tool to improve WSS monitoring and management. I also explored resistance in wheat cultivars. While semi-solid stems have been the focus of past breeding efforts, growers reported potential resistance in a hollow-stem, herbicide-resistant cultivar – Clearfield Plus (CL Plus). I evaluated three cultivars: one resistant, one susceptible, and CL Plus. Across lab and field trials over three years, I measured oviposition, infestation, and cutting rates. Results showed that CL Plus cultivars exhibited resistance traits. To investigate further, I analyzed volatile organic compound (VOC) profiles using thermal desorption-gas chromatography/mass spectrometry (TD-GC/MS). One CL Plus cultivar had a VOC profile similar to the resistant line, suggesting possible chemical cues underlying resistance. This work opens avenues to explore the role of plant volatiles in WSS oviposition behavior and plant defense. Chapter 3 addressed the effect of crop rotation. As rotations have become more diverse in northeastern Colorado, I assessed whether they influenced WSS infestation and larval survival. Post-harvest wheat stems were collected over two years from fields with different rotation histories, ranging from wheat–fallow to wheat–corn–millet–fallow. Surprisingly, crop rotation alone did not appear to reduce WSS pressure. However, fields with resistant cultivars showed lower infestation, reinforcing the role of host plant resistance as the most effective strategy—regardless of crop sequence. Chapter 4 focused on biological control, specifically the role of braconid parasitoids. These wasps have historically played a key role in WSS suppression in the northern Great Plains, but their impact in Colorado has been minimal. I conducted cage experiments comparing parasitism rates among Bracon spp. populations from Colorado, Nebraska, and North Dakota. All populations showed similar parasitism potential, suggesting no mismatch between Colorado Bracon spp. and local WSS. To explore the low parasitism levels further, I performed a population genetics study. I extracted DNA from both WSS and their parasitoids to assess whether population structure or host-associated differentiation (HAD) could explain the pattern. Analyses of genetic diversity, phylogenetic trees, and haplotype networks revealed that one key parasitoid, Bracon cephi, may be undergoing population subdivision, possibly due to the rapid expansion of its host. In contrast, WSS populations in Colorado continue to expand, showing high haplotype diversity. I also investigated Bracon lissogaster populations from both wild grasses and wheat. Although higher genetic diversity was observed in wild grasses, there was no strong evidence of HAD. However, sample size needs to be increased substantially for a solid HAD analysis. These chapters reflect a multi-faceted effort to understand and manage WSS through phenology modeling, cultivar resistance, crop rotation, and biological control. This work lays the groundwork for more precise, integrated, and regionally tailored pest management strategies.