OVIPOSITION PATTERN OF PHYTOPHAGOUS INSECTS - ON THE IMPORTANCE OF HOST POPULATION HETEROGENEITY

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 i s 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 patter n from fits to theoretical distributions and from the variance/mean ra tio. From field studies over 13-16 years, we show that 20 out of the 3 1 yearly distributions available Rt a negative binomial and 25 a zero- inflated Poisson (ZIP). No distribution fits a Poisson distribution. T he ZIP distribution assumes heterogeneity within the fruit population. There are two categories of host: the first comprises chestnuts unsui table for weevil oviposition or in excess relative to the number of we evil females, and the second comprises suitable fruits in which ovipos ition behavior is random. Our results confirm this host heterogeneity. According to the ZIP distribution, the first category of hosts includ es on average 74% of the chestnuts. A negative binomial distribution m ay be generated by either true or false contagion. We show that neithe r interference between weevil females, nor spatial variation in the in festation rate exist. Consequently, the observed distributions of imma tures are not the result of false contagion. Nevertheless, we cannot t otally exlude true contagion of immatures. In this paper we discuss th e difficulty of testing true contagion in natural conditions. These re sults show that we cannot systematically conclude in favour of contagi on when fitting a distribution such as the negative binomial or when a variance/mean ratio is higher than unity.