Are parasitized Plodia better competitors An empirical test using Maximum Likelihood CAMERON, T'C',

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Are parasitized Plodia better competitors? An
empirical test using Maximum Likelihood CAMERON,
T.C., METCALFE, D., WEARING, H.J., ROHANI, P.
SAIT, S.M.UNIVERSITY OF LEEDS, UNIVERSITY OF
GEORGIA, UNIVERSITY OF CAMBRIDGE
Tom Cameron Centre for Biodiversity
Conservation University of Leeds bgytcc_at_leeds.ac.u
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Introduction Natural populations of insects are
subject to attack from a range of natural
enemies. Many of these natural enemies, either
true parasites or parasitoids, do not ever or
immediately kill their host. The lag that occurs
between attack and host death results in mixed
populations of healthy and "infected" hosts.
Empirical data has suggested that parasitized
hosts may be either weaker or stronger
competitors. Little is known about how the
competitive nature of infected hosts affects the
survival and dynamics of the surviving healthy
host populations. Objectives Believing the
competitive nature of infected animals has
important consequences for the dynamics of a host
population and therefore management. We
undertook to go beyond current methods where
survival of parasitized or healthy hosts are
plotted across natural densities or healthy and
parasitized hosts compete at un-natural
densities. Our objective was to derive real
competition coefficients for our system of
Indian meal moths (Plodia interpunctella) and
their koinobiont endoparsitoid, Venturia
canescens so that the results could be compared
with current theoretical models of competition
and provide a framework for future research.
Experiment The experiment followed the response
surface design (Inouye, 2002). Varying ratios of
healthy or individually paraistized 3rd
instar larvae were allowed to compete at 4
different densities (only 3 presented here). FIG
1 Experimental enclosures were
monitored daily and adult moths, or adult wasps
were removed and frozen for later analysis. Data
on survival, time to emergence and adult dry
weight were collected and were fit to standard
competition equations. We solved the issue of
encapsulation by using two morphs of Plodia
interpunctella. We parasitized a golden morph and
used wild type (Picture 1) as our healthy larvae.
This permitted accurate encapsulation rate data
and appropriate measures were taken in our
analysis. Analysis The Model NLh(t1)NLh(t)1
K(NLh(t)(a12NLp(t)))-ß NLp(t1)NLp(t)1K(NL
p(t)(a21NLh(t)))-ß NLh NLp are the number
of healthy parasitised hosts at different times
(t) respectively, K is an index of the carrying
capacity and a ß are inter and intraspecific
competition coefficients. The model is based on
Comins Hassell (1976) and only the survival
data is used in this poster. The other data when
complete will be presented elsewhere. We used
Nonlinear regression (least sum of squares, Max
likelihood of data with normal errors) to derive
the values of the interspecific competition
coefficients. With ß set equal to 1 then the
returned values of a are relative to
intraspecific competition. This also allows for
fewer parameters in the model that need to be
addressed. All analysis was conducted in s-plus
(Insightful, Version 6.1 for windows student
edition) and plots in sigmaplot 8.0 (spss for
windows).
Results Conclusions
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The densities were chosen based on known minimum
food requirements for healthy larvae so that the
chosen densities would range from well below to
well above carrying capacity.
Survival of both healthy and parasitised larvae
is reduced with increasing total density of
competing insects Increased density of
parasitised larvae relative to healthy larvae
results in increased survival for both moths and
parasitoids
Picture 1 Wild type Golden Plodia