1. August 2013
Developing Methods to Conserve and Enhance Native Bracon Parasitoids as Wheat Stem Sawfly Biocontrol Agents.
B. Tegner Jacobson, Ryan Bixenmann, and David Weaver
Institute on Ecosystems and the Land Resources and Environmental Sciences Department, Montana State University
2013 Institute on Ecosystems Summer Internship Program
INTRODUCTION
RESULTS
DISCUSSION Cont.
Wheat stem sawflies (Cephus cinctus) are a wheat pest in Montana causing more than $350 million in damage to North American wheat crops annually. The sawflies lay their eggs inside the wheat stems which then hatch, eat the inside of the stem, and cut it in such a way that the majority of the stem falls over making the grain difficult to recover during harvest. Conventional methods of pest control are unable to make an impact on these sawflies because they remain inside the stems of the wheat during the majority of their life. Previous work has demonstrated that the native parasitoids, Bracon cephi and B. lissogaster, can significantly reduce sawfly damage.
This experiment investigates land management practices that will potentially increase the abundance and lifespan of native parasitoids that feed on the sawfly larvae. In addition, we demonstrate the effectiveness of floral diversity on sawfly infestation and mortality and on pollinator diversity along wheat field margins. We have explored the effect that the floral and insect diversity has on the habitats bordering wheat fields as well as the effect that nectar availability has on the lifespan of the parasitoids. Practices based on this information could be used by farmers in order to foster increased parasitoid populations that will help to decrease the sawfly infestation of their fields.
This is important in the case of both B. lissogaster and B. cephi because both wasps lay their eggs inside of the sawfly larvae to parasitize them. This increase in fecundity leads to the expectation that managed habitats where nectar is available will increase the parasitism of the pests (Heimpel & Jervis, 2005).
The floral abundance in the sampled sites changed over the summer as different flowering species went through their respective life stages and passed through the flowering stage over the course of the summer (Figure 5). What was observed however was that the abundance of flowers did not necessarily increase the abundance of floral visiting insects at those sites, but instead there was a negative trend observed between the floral and insect abundance (Figure 8). When diversity was observed, a different trend was seen, with increased floral diversity there was seen an increase of insect diversity (Figure 7). This is important because other studies have observed correlations between floral diversity and pest control (Kremen & Chaplin-Kramer, 2007) (Heimpel & Jervis, 2005). Data from our research project also support these findings, as is seen in the decreased levels of infestation as floral diversity increased (Figure 4).
Figure 5: The change in flower abundance from June to July. *
CONCLUSIONS
From the information discussed above it would be recommended for wheat farmers to plant a diverse set of native wildflowers at the edges of their fields in order to build up the most diverse community of flowering plants. This diversity and nectar availability would help build up small parasitoid communities (Heimpel & Jervis, 2005) and increase the lifespans of larger parasitoid communities. The diverse floral community would ensure that as different flowering plant species went through their life cycles there would be nectar available to the parasitoid population. The higher diversity of flowers would most likely attract a more diverse floral visitor and pollinator community which would help ensure its continuation as a field-margin habitat for the braconid wasps (Noordijk, Delille, Schaffers & Sýkora, 2009).
n=7 A B
Figure 1: A) a Bracon lissogaster parasitoid specimen; B) a Cephus cinctus sawfly specimen.
Figure 2: The most common insects arranged by morphospecies.
Figure 6: The average lifespans of braconid wasps after treatment with nectar.
METHODS
First a total of 10 transect sites were established along the edges of wheat fields outside of Bozeman. The flowering and non-flowering plant species in the transects were then observed and recorded on a weekly basis. Pan traps were also set along these transects to collect samples of the insect species found in the individual transects.
The collected insect species were then assigned a morphospecies in the lab and the diversity of the insects was calculated using the Simpson diversity index. This index was then compared to the number of flowering plants present in that transect.
Samples of the smooth brome grass in these Transects were taken and dissected to determine the average sawfly infestation along the transect.
In the lab the braconid wasps were taken immediately after emergence and randomly split into two different groups. The first group was given a synthesized nectar, and the second was given water. The lifespans of these two groups were then recorded and analyzed to see if there was any effect between the two treatments.
REFERENCES
1.Heimpel, G. E., & Jervis, M. A. (2005). Does floral nectar improve biological control by parasitoids. In F. Wäckers, P. van Rijn & J. Bruin (Eds.), Plant-Provided Food and Herbivore-Carnivore Interactions Cambridge University Press.
2. Kremen, C., & Chaplin-Kramer, R. (2007). Insects as providers of ecosystem services: Crop pollination and pest control. In Stewart, A.J.A., New, T.R., Lewis, O.T. (Eds.), Insect Conservation Biology (pp ). The Royal Entomological Society.
3. Noordijk, J., Delille, K., Schaffers, A. P., & Sýkora, K. (2009). Optimizing grassland management for flower-visiting insects in roadside verges. Biological Conservation, 142,
n=16
n=16
n=4
Figure 3:The average infestation observed was highest in sampling sites near the fallow wheat fields.
Figure 7: The positive correlation between insect and floral diversity.
ACKNOWLEDGMENTS
I would like to thank Ryan Bixenmann, David Weaver, and Martha Sellers for their contribution to this project and their support, without which this project would not be possible.
I would also like to thank the Linfield Splitting lab for their support and efforts in splitting all the smooth brome samples while looking for infestation. I would also like to thank the Institute on Ecosystems, the MSU LRES Department and the Montana Wheat and Barley Committee for their support and assistance in this research.
This material is based on work supported by the National Science Foundation under Grant EPS Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
DISCUSSION
The infestation of the sawflies was found to be highest in the sampling patches nearest to the fallow fields and lowest in the sampling sites nearest to the fields containing crops (Figure 3). These results are most likely due to the emergence of the sawfly adults out of the stems from the previous year in the fallow fields and moving out towards the grasses and wheat that were not yet infested.
The braconid wasps in the laboratory experiment had an average lifespan of ~14 days when nectar was available to them as compared to an average of ~6 days when nectar was unavailable to them (Figure 6). The increased lifespan of these braconid wasps is significant because “under most field conditions, increased lifespan will lead to higher fecundity because more time is available for host location and egg maturation” (Heimpel & Jervis, 2005).
Figure 4: The decreasing trends of sawfly infestation and floral diversity.
Figure 8: The negative trend between insect and floral diversity.