‘Beneficial Bug Baler’: A Novel Technique for Mass Relocation of Bracon in Support of Cephus cinctus Biocontrol in Wheat, Triticum aestivum, 2024

The performance of baled wheat, Triticum aestivum L., straw was evaluated as a method to facilitate the movement, release, and introduction or augmentation of Bracon cephi populations for the biological control of Cephis cinctus in wheat in Nebraska and Colorado. Following identification of Bracon establishment in hard red winter wheat (var. ‘Ruth’) at the UNL High Plains Agriculture Laboratory (HPAL), Sidney, NE (1320 m elevation) grain was harvested using a combine equipped with a stripper header (25 Jul 2023) and the standing residue was cut by swather (draper head without conditioning rollers or auger to minimize compression of wheat straw) on 13 Sep 2023. Swathed wheat straw windrows were subsequently collected by bale, or loose, to observe differences in Bracon mortality from transported wheat straw. Thirteen round bales of straw, weighing approximately 135 kg bale⁻¹, were collected (5 Oct 2023) using a Case International 8420 hardcore baler adjusted to a loose setting with an objective of reducing straw compaction during the baling process and associated mortality of Bracon larvae within the straw. On the same day, 3 straw bales were loaded onto a flatbed trailer using a 3-pronged bale spear for transport to a Nebraska farm (DS Farm) (approximately 13 km east from HPAL, 1240 m elevation), while the remaining 11 bales were stacked (2 high) on the field edge where they remained until they were transported by flatbed trailer to Colorado (USDA-APHIS permit 526-23-285-20929). On 6 Nov 2023, two 1250 L modified chemical storage tanks were filled with approximately 50 kg of loose straw tote⁻¹ that had been hand raked on the same day from straw that remained in the same field where the bales had been collected previously. The remaining bales and loose straw totes were then transported to the USDA-ARS Central Great Plains Field Station, Akron, CO (CGPFS, 1380 m elevation). Both DS Farm and CGPFS were selected as Bracon release locations due to their high population densities of C. cinctus (DS and PH, personal obs.). Two bales and 1 modified chemical tote of loose straw were unloaded onto grassland abutting an emerged field of winter wheat (var. ‘Brawl CS Plus’) field on DS Farm. The grassland habitat consisted of the following approximate diversity and population densities of plant species: Bouteloua curtipendula (50%), Pascopyrum smithii (15%), Schizachyrium scoparium (20%), B. gracilis (10%), Sorghasrum nutans (5%), Andropogan gerardii (<5%), Panicum virgatum (<5%), and Dalea purpurea (<1%). One bale was immediately (11/06/2023) spread into a windrow perpendicular to the planted wheat field, while the remaining straw bale and loose straw were spread, respectively, in a similar orientation on 22 Apr 2024 to assess overwintering conditions on Bracon emergence. At CGPFS, the straw bales were spread at two locations at two separate times. One location (CGPFS-N17) consisted of a 1.2-ha area subdivided into 4.57 m by 35.1 m plots of either wheat (var. ‘Amplify’) or Thynopyrum intermedium. The other location (CGPFS-N18) was a wheat (var. ‘Amplify’) plot (36.6 m by 374.9 m) bordered by Agropyron cristatum. One wheat straw bale was spread at each of the two locations on 6 Nov 2023, while the second bale and loose straw were spread on 22 Apr 2024. To verify the survival of Bracon adults emerging from the baled straw, soil emergence traps (Bug dorm model number BT 2016; 60 cm × 60 cm  × 60 cm) were set up prior to the onset of insect emergence in the spring (8 May 2024). At each location, 2 traps were set up over straw in each unfurled bale (1 at the leading/outer edge of the unfurled strip and 1 in the center) as well as in the strips of loose straw. Trap collection vials were filled with 150 ml of a 50% propylene glycol solution and emptied approximately every 2 wk (6 collections total) between 20 May 2024 until insects had stopped emerging on 2 Aug 2024. The straw in each trap was weighed to standardize insect numbers since straw thickness varied across bales. To determine whether emergent Bracon parasitoids established and attacked C. cinctus in the wheat adjacent to bale spreading plots, we collected wheat stems just prior to harvest at each location. All samples were collected from CGPFS on 2 Jul 2024, and DS Farm on 3 Jul 2024. Approximately 25 wheat stems were collected every ~10–20 paces at 8 points along each of 2 transects: 1 paralleling the wheat field edge (~1 m from the field edge, where bales were added) and the second 35 and 50 m in from the wheat field edge at the CGPFS-N18 and DS Farm, respectively, At CGPFS-N18 and CGPFS-N17, approximately 100 wheat stems were collected in each of the 4 wheat plots. Post-harvest samples were also collected at each location to determine post-harvest parasitism of C. cinctus from Bracon; including parasitism of C. cinctus pre-pupae within their hibernaculae (within wheat stubs). Post-harvest samples were collected at CGPFS on 1 Aug 2024, and the DS Farm on 26 Jul 2024. We followed the same general sampling plan as described for pre-harvest sampling, only with a 0.25-m linear row of wheat residue (including stubble cut by C. cinctus) collected at each of the sampling points along each transect. All stem and stubble material was stored at 2°C until dissected to determine the presence of C. cinctus and Bracon following standard protocols.

As multiple observations were taken from each bale, the average number of emergent parasitoids per kg of straw was used in analyses to help account for pseudo-replication. A Wilcoxon rank-sum test was used to compare means between parasitoid emergence from treatment groups. This test was selected due to the skewed distribution of data and unequal sample sizes between treatment groups. Both CGPFS and DS locations were analyzed together (i.e., there was insufficient replication to analyze potential location effects on emergence).

A chi-square test of independence on Bracon pupal count data was used to compare parasitism rates between pre- and post-harvest samples. To accurately calculate parasitism, every dissected stem within a sample was determined to have evidence of C. cinctus (1) or no C. cinctus present (0) and parasitism (1) or no evidence of parasitism (0). Data were then filtered such that parasitism was calculated based on the proportion of C. cinctus-infested stems only (i.e., data coded with a 1). Separate chi-square tests were used for CGPFS and DS Farm data to account for potential differences in parasitism due to environmental and management effects on parasitism rates.

This experiment indicates that swathing, baling, transporting, and then spreading wheat straw (sourced from a post-harvest field) is a viable methodology for the introduction of Bracon into new habitats, for the purpose of the biological control of C. cinctus in wheat production. However, bale density (i.e., the amount of straw compressed within a bale) has a significant (n = 5 for baled straw, n = 3 for loose straw, W = 0, p = 0.03571) effect on the emergence of Bracon (Table 1) with approximately 42–52 more emergent Bracon per kg from loose, unbaled straw. A vast majority of the Bracon in our samples were Bracon cephi with only 1 B. lissogaster identified. Bracon mortality within bales appeared to be due to crushing, presumably due to the action of baling. However, additional preliminary work has indicated that very low-density bale settings may improve survival. For instance, soft-core balers exist that reduce compaction in the interior of the bale (we used a hardcore baler). Importantly, even with lower survival of Bracon, we estimate that 1, 135 kg bale (the weight of the bales in this study) supported the emergence of ~4725 adult Bracon bale⁻¹ (assuming no mortality) at our estimated emergence rates. Further, our round bales were relatively small (<1 m diameter) compared with common round bands. Thus, an opportunity exists to refine techniques for Bracon collection and release using larger, looser baling methods. Cephus cinctus parasitism from Bracon was apparently higher during the pre-harvest compared to post-harvest sampling (Table 2) at the release sites. While the Bracon pupal sample counts were significantly lower in post-harvest samples from CGPFS (X² = 6.6006, df = 1, p = 0.01019) and DS Farm (X² = 15.185, df = 1, p = 9.748e−05), some number of Bracon pupae were likely culled by wheat harvest machinery. Thus, our post-harvest Bracon pupal counts likely underrepresent Bracon activity during the growing season. Importantly, we consistently observed higher parastiism at wheat field edges relative to the field interior (Table 3). The distance of C. cinctus parasitism within the field suggests that the wheat landscape was broadly affected by the Bracon release. While additional data would be required to confirm that neither of our Bracon introduction sites had significant, extant populations of Bracon already in wheat fields, location managers have indicated considerable densities of C. cinctus at these locations for about 10 years (DS and PK, personal obs.). Further, no Bracon has been collected from CGPFS at least in the previous 2–3 y despite intensive sampling (PN and AO, unpublished data).¹

Footnotes