Quantifying dispersal of bark beetles and their predators
This study aims to quantify the dispersal range of three species of Coleoptera associated with red pine
trees (Pinus resinosa). Experiments described below are part of a long-term research project gathering
more knowledge on interaction between below and above ground herbivory in pine forests and its effect
on forest gap formation (Raffa et al., LTREB, proposal project to the NSF-2002). We will use markrecapture experiments to study dispersion of Ips pini Say (Scolytidae), a common bark-beetle which
attacks stressed or dying pine trees, and its most abundant predator, the checkered beetle Thanasimus
dubius Fabricius (Cleridae) in red pine stands of Wisconsin. Recent experimental evidence has clearly
shown the importance of T. dubius on bark beetles dynamics (Reeve, 1997; Turchin et al., 1999; Aukema
and Raffa, 2002). This experiment will also consider the dispersal of another bark-beetle quite abundant
in Wisconsin and which colonizes the root collar, the red turpentine beetle Dendroctonus valens
(Scolytidae).
While scolytids are usually reported to disperse less than two kilometers (Zumr, 1992; Turchin and
Thoeny, 1993; Barak et al., 2000; Dodds and Ross 2002), T. dubius has been reported to disperse farther
than the range of its prey in its southern distribution. One third of individual dispersal is indeed estimated
to occur farther than 2 km, some of them are even recaptured at a distance of 8 km (Cronin et al., 2000).
These data give us a precious estimate of what should be the limits (radius) of our grid of traps in order to
select the appropriate spatial scale.
The main objective of the project will be to characterize dispersal range of each beetle, by using quantities
like median dispersal distance (distance at which 50% of insects disperse) and other dispersal quantiles
(33.3, 66.7 and 95% for instance). While Ryall and Fahrig (2005) have addressed the effect of habitat
fragmentation on I. pini – T.dubius ratios in the field, this is yet the first attempt to measure on the same
geographical area dispersal abilities of both root and stem colonizers and predators in the red pine system.
Field sites
Three potential sites which are primarily forested with red pine have been located in southern Wisconsin.
One of them, Mirror Lake (ML) is free from decline but the two others, Spring Green (SG) and Kettle
Murrain (KM) are including a spot of decline inside their range. They are all located at 1 hour driving
distance from Madison and the following figure 1 shows approximately their respective location.
Fig. 1: ML= Mirror Lake, SG= Spring Green, KM= Kettle Murrain; WS= West Salem, BRF= Black River Falls
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These three sites will be plenty enough during year 2006. They could be reused during the 2007 season
with a different order/site of release or we could also use two others potential sites located farther on West
Salem (WS) and Black River Falls (BRF).
Trapping system
At each site, multiple-funnel traps (Lindgren, 1983) will be arranged in a cross-shaped pattern (4 traps at
each distance). Traps will be set at distances of 50, 100, 200, 300, 400, 500, 750, 1000, 1250, 1500 m
from the release point (figure 2). Traps will be suspended between two trees with a string.
Figure 1: trapping grid
Legend: o = trap
Each member of its guild seems to be quite abundant in this area (Raffa, personal communication) and
insects are relatively easy to catch by baiting traps with attractive mixture of semiochemicals (Table 1
below). Inside the collecting cup, a small piece of No-Pest Strip (Biostrip. Reno., active ingredient: 2-2dichlorovinyl dimethyl phosphate) will be added to prevent beetles escape (Cronin et al., 2000).
Species
Ipsdienol
+
lanierone
Δ-3 carene
frontalin
+
α-pinene
I. pini
+++
0
0
D. valens
“0”
+++
“0”
T. dubius
+
0
+++
Table 1: preferences of I. pini, D. valens and T. dubius for several semiochemicals.
Our study is on dispersal and not on pheromone preference and we want to minimize any side effect
interfering in beetle dispersal. We will thus avoid baiting the same trap with a mix of chemicals
supposedly attractive for two species and we will have only one species released at a time in each site.
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The traps will be baited with bubble caps of racemic ipsdienol (50%+/50%- ; release rate: 0.5-1.0 mg/day,
RD-0589) and lanierone to catch I. pini. Δ-3 carene will be used to catch D. valens. For T. dubius, we will
use a 0.5 mL vial of frontalin (release rate: 0.5-1.0 mg/day, RD-0244), and a 120 mL bottle of α-pinene
released using cotton wick. Indeed, T. dubius still responds to frontalin in the Great Lakes region (Reeve,
personal communication).
As attraction to a pocket (spot of decline) could produce an attractive bias on beetle movement, we will
set the grid of SG and KM sites in such way that the release point will not be too closed from the pocket
(200 m at least, see appendix A, figure 1 modified). The arm diving inside the pocket will be compared to
those free of decline, and thus, the relative distorting effect of a spot on insect dispersal could be assessed.
Habitat loss is able to change the ratio of predator abundance to prey in this system (Ryall and Fahrig,
2005) and in others (Kareiva, 1987, Zabel and Tscharntke, 1998). Thus, it wouldn’t be surprising that the
relative fragmentation of habitat observed on our study area (and which is quite unavoidable) could also
create differences of dispersal between sites. This is part of the variability inherent of any field work and
the goal of this study is not primarily to evaluate the loss of habitat. Yet, we could attribute to each site an
index of habitat loss by calculating the ratio between the surface area of forest and the total surface area
of the site. If showing a significant effect, it would be incorporated in our model. As some traps are going
to be set in clearances, this index will at least control this factor of variability on dispersal. Aerial pictures
and GIS could provide useful tools to produce easily such index.
Mark – recapture system
I. pini will be released from logs either naturally infested in non-study sites or artificially in the laboratory
in the case of early releases (May). Rearing facilities are available at University of Wisconsin-Madison
and are equipped with emergence metallic cages. Before beetle emergence, infested logs will be dusted
with fluorescent powder and set at the center of the grid (release point, figure 1). I. pini will be marked
with this powder when they emerge from logs. Used on another bark beetle, D. frontalis, this technique
seems to mark at least half of emerging individuals (Turchin and Thoeny, 1993). We will need to release
at least several thousand of marked I. pini, mainly because the rate of recapture is very low whereas
several hundreds (400-500) should be enough for T. dubius (Cronin et al., 2000). D. valens and T. dubius
caught in one site by using the trapping grid will be released at the center of the grid of an another site
randomly selected. Before release, beetles of both species (D. valens and T. dubius) will be brought back
to the laboratory and stored at 10˚C for less than 7 days or until a sufficient number will be collected. D.
valens seems to be especially abundant at KM, whereas T. dubius are reported to be more easily caught
around ML. To increase our capture efficiency of T. dubius, we will add 10 more traps at the “Lenz
Patrulis” site (5 km distance from ML). D. valens and T. dubius are bigger, so they will be marked by
painting a small spot of paint on the pronotum. We could also try to mark them with fluorescent powder.
By this way, the same techniques will be used for all insects. A 10% subsample of marked individuals
will be kept to be later compared to those having dispersed through the grid. It will give some information
about the relative size of dispersers. D. valens and T. dubius will be released from a tree and kept initially
on a cage attached to the bole to calm them before release.
Traps will be checked every week at least during a month, and then until no more marked insects are
recaptured since two weeks. According to previous mark recapture experiments, most of recaptures
should occur during the first four weeks (Cronin et al., 2000). Marked insect recaptured will be counted
and cumulated for each trapping distance, and further analyzed by the diffusion model. We will also
record the sex of each recaptured D. valens and T. dubius and its elytra length (mm) and the mean length
and sex ratio of recaptured insects will be compared to those of the 10% subsample subtracted before
release.
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Statistical analysis
A diffusion model has been developed in previous studies on dispersal of bark beetles and predators
(Turchin and Thoeny, 1993; Cronin et al., 2000). It assumes simple diffusion from a release point and the
loss of individuals from mortality or dispersal termination.
The number of marked beetles C(r) recaptured at a particular distance r is expressed by the following
equation: C(r) = Ar-1/2 exp (-r/B). A is a scaling parameter and B= D/δ with D being the diffusion
coefficient and δ the loss rate of individuals. The parameters A and B will be estimated by regressing C(r)
on r. If needed, we will transform the data of capture (log transformation) before any regression to respect
conditions of homoscedasticity. Providing we know the estimate of B, we can then calculate the dispersal
quantiles: percentage of beetles dispersing at a particular distance (for instance 33.3, 66.7 and 95%)
A two way ANOVA using species and sites as main factors will compare values of B and of the median
dispersal distance.
Timetable
Except unfortunate events, the following timetable should lead us to perform 2 releases of each species
during the 2006 season.
Infest logs
in the lab
Early
March
Set grids
of traps
Mid
March
Capture/release
Time
1st : Early - Mid May
ML
T. dubius
Sites
SG
D. valens
KM
I. pini (*)
2nd : Mid June (**)
I. pini
T. dubius
D. valens
Table 2: timetable of mark recapture tasks
Legend: ML = Mirror Lake, SG = Spring Green, KM = Kettle Murrain
Recaptures of beetles released during the second release should be over through mid or late July. Traps
could be then removed.
(*) depending on artificial infestation success
(**) if insect emergence is delayed or amount of individuals limited, only one attempt could take place
Note: the proposed timing release in table 2 is an example of randomization between site and order.
Needs from UW
• Funnel traps: 40 (traps) * 3 (sites) + 10 (extra) = 130
• At least 100 baits of each of the followings:
- 0.5 mL vial of frontalin (release rate: 0.5-1.0 mg/day, RD-0244)
- bubble caps of racemic ipsdienol (50%+/50%- ; release rate: 0.5-1.0 mg/day, RD-0589)
- bubble caps of lanierone
• Ten 1 Liter bottles of α-pinene (40 traps * 2 releases* 120 mL = 9.6 L)
• 1 L bottles of Δ-3 carene
REFERENCES
Aukema, B.H., and Raffa, K.F. 2002. Relative effects of exophytic predation, endophytic predation and
intraspecific competition on a subcortical herbivore: consequences to the reproduction of Ips pini and
Thanasimus dubius. Oecologia, 133: 483-491.
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Barak, A.V., McGrevy, D. and Tokaya, G. 2000. Dispersal and recapture of marked, overwintering
Tomicus piniperda (Coleoptera: Scolytidae) from Scotch pine bolts. Great Lakes Entomol., 33: 69-80.
Cronin, J.T., Reeve, J.D., Wilkens R., and Turchin P. 2000. The pattern and range of movement of a
checkered-beetle predator relative to its bark-beetle prey. Oikos, 90: 127-138.
Dodds, K.J. and Ross, D.W. 2002. Sampling range and range of attraction of Dendroctonus pseudotsugae
pheromone-baited traps. Can. Entomol., 134: 343-355.
Kareiva, P. 1987. Habitat fragmentation and the stability of predator-prey interactions. Nature, 326: 388390.
Lindgren, B.S. 1983. A multiple funnel trap for scolytid beetles (Coleoptera). Can. Entomol., 115: 299302.
Raffa, K. F., Aukema, B., Clayton, M. K., Reeve, J. D., and Zhu, J. 2002. Long Term Research in
Environmental Biology, proposal to the National Science Foundation: Interaction of below and above
ground herbivory in forest gap formation: Long term analysis of underlying mechanisms and spatiotemporal patterns.
Reeve, J.D. 1997. Predation and bark beetle dynamics. Oecologia, 112: 48-54.
Ryall, K.L. and Fahrig, L. 2005. Habitat loss decreases predator-prey ratios in a pine-bark bettle system.
Oikos, 110: 265-270.
Turchin, P. and Thoeny, W.T. 1993. Quantifying dispersal of southern pine beetles with mark-recapture
experiments and a diffusion model. Ecological applications, 3: 187-198.
Turchin, P., Taylor, A.D. and Reeve, J.D. 1999. Dynamical role of predators in population cycles of a
forest insect: An experimental test. Science, 285: 1068-1071.
Zabel, J. and Tscharntke, T. 1998. Does fragmentation of Urtica habitats affect phytophagous and
predatory insects differentially? Oecologia, 116: 419-425.
Zumr, B. 1992. Dispersal of the spruce bark beetle Ips typographus (L.) (Col., Scolytidae) in spruce
woods. Journal of Applied Entomology, 114: 348-352.
Appendix A: Fig 1 modified: trapping grid with a pocket (spot of decline) in one arm; Legend: o = trap
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