Oviposition pattern of phytophagous insects: on the importance of host population heterogeneity |
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Authors: | Emmanuel Desouhant Domitien Debouzie Frédéric Menu |
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Institution: | (1) UMR CNRS 5558 “Biométrie, Génétique et Biologie des Populations”, Université Claude Bernard, 43, bd du 11 novembre 1918, 69622 Villeurbanne Cedex, FRANCE Fax: (33) 04 78 89 27 19 e-mail: desouhan@biomserv.univ-lyon1.fr, FR |
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Abstract: | Frequency distributions of insect immatures per host are often fitted to contagious distributions, such as the negative binomial,
to deduce oviposition pattern. However, different mechanisms can be involved for each theoretical distribution and additional
biological information is needed to correctly interpret the fits. We chose the chestnut weevil Curculio elephas, a pest of the European chestnut Castanea sativa, as a model to illustrate the difficulties of inferring oviposition pattern from fits to theoretical distributions and from
the variance/mean ratio. From field studies over 13–16 years, we show that 20 out of the 31 yearly distributions available
fit a negative binomial and 25 a zero-inflated Poisson (ZIP). No distribution fits a Poisson distribution. The ZIP distribution
assumes heterogeneity within the fruit population. There are two categories of host: the first comprises chestnuts unsuitable
for weevil oviposition or in excess relative to the number of weevil females, and the second comprises suitable fruits in
which oviposition behavior is random. Our results confirm this host heterogeneity. According to the ZIP distribution, the
first category of hosts includes on average 74% of the chestnuts. A negative binomial distribution may be generated by either
true or false contagion. We show that neither interference between weevil females, nor spatial variation in the infestation
rate exist. Consequently, the observed distributions of immatures are not the result of false contagion. Nevertheless, we
cannot totally exlude true contagion of immatures. In this paper we discuss the difficulty of testing true contagion in natural
conditions. These results show that we cannot systematically conclude in favour of contagion when fitting a distribution such
as the negative binomial or when a variance/mean ratio is higher than unity.
Received: 22 September 1997 / Accepted: 15 December 1997 |
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Keywords: | Oviposition pattern Negative binomial distribution Zero inflated Poisson distribution Host heterogeneity Chestnut weevil |
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