1. 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.
INTRODUCTION
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).
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).
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
August 2013
2013 Institute on Ecosystems Summer Internship Program
METHODS
ACKNOWLEDGMENTS
DISCUSSION Cont.
CONCLUSIONS
RESULTS
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.
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-1101342. 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.
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. 349-382). 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, 2097-2103.Figure 3:The average infestation observed was highest
in sampling sites near the fallow wheat fields.
0
10
20
30
40
50
60
70
19-Jun 26-Jun 3-Jul 10-Jul 17-Jul
NumberOfFloweringStems
Date
Temporal Change in Flower
Abundance
1E 1W 2E 2W 3E 3W 4E 4W 5E 5W
Figure 1: A) a Bracon lissogaster parasitoid specimen;
B) a Cephus cinctus sawfly specimen.
A B
0%
2%
4%
6%
8%
10%
12%
14%
16%
18%
20%
Crop Fallow Field
PercentInfestation
Field Type Adjacent to Sampling Site
Infestation by Field Type
Figure 5: The change in flower abundance from
June to July.
Figure 6: The average lifespans of braconid
wasps after treatment with nectar.
Figure 7: The positive correlation between insect
and floral diversity.
Figure 8: The negative trend between insect and floral
diversity.
Figure 4: The decreasing trends of sawfly infestation
and floral diversity.
0 1 2 3 4 5 6 7
Diptera 12
Diptera 13
Hymenoptera 22
Hymenoptera 29
Hymenoptera 42
Coleoptera 5
Diptera 16
Diptera 22
Diptera 4
Hymenoptera 10
Hymenoptera 18
Hymenoptera 20
Hymenoptera 24
Hymenoptera 32
Coleoptera 4
Diptera 9
Hymenoptera 39
Hymenoptera 48
Hymenoptera 50
Unwinged Hymenoptera 1
Diptera 3
Diptera 31
Hymenoptera 11
Hymenoptera 27
Hymenoptera 30
Diptera 17
Diptera 2
Hymenoptera 2
Hymenoptera 3
Lepidoptera 3
Hymenoptera 12
Hymenoptera 17
Hymenoptera 6
Hymenoptera 14
Hymenoptera 15
Hymenoptera 16
Hymenoptera 9
Hymenoptera 19
Diptera 7
Hymenoptera 38
Diptera 20
Coleoptera 3
Hemiptera 9
Diptera 6
Hemiptera 1
Hymenoptera 8
Diptera 1
Diptera 25
Ln(Insect Count)
TopMorphospecies
Species Distribution by Morphospecies
Figure 2: The most common insects arranged by
morphospecies.
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).
n=4
0
2
4
6
8
10
12
14
16
18
20
No Nectar Available Nectar Available
DaysLived
Braconid Treatment Group
Average Lifespan of Braconid Wasps
n=7 n=7
R² = 0.3234
0
2
4
6
8
10
12
14
16
18
0 0.2 0.4 0.6 0.8 1 1.2 1.4
InsectDiversity(1/SimpsonDiversityIndex)
Floral Diversity (1/Simpson Diversity Index)
Insect Diversity Versus Floral Diversity
R² = 0.0464
0
10
20
30
40
50
60
70
80
0 10 20 30 40 50 60 70
InsectAbundance(NumberofInsects)
Floral Abundance (Number of Flowering Plants)
Insect Versus Floral Abundance
*
n=16n=16
R² = 0.0219
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
AverageSawfliesPerStem
Floral Diversity (1/Simpson Diversity Index)
Average Sawflies Per Stem Versus Floral
Diversity