The Distribution of Eggs Per Host in a Herbivorous Insect − Intersection Of
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Journal of Animal Blackwell Publishing Ltd − Ecology 2006 The distribution of eggs per host in a herbivorous insect 75, 387–398 intersection of oviposition, dispersal and population dynamics HEINRICH ZU DOHNA Yale University, School of Forestry and Environmental Studies, 370 Prospect Street, New Haven, CT 06511, USA Summary 1. The dynamics of parasitic organisms depend critically upon the frequency distribu- tion of parasite individuals per host. However, the processes giving rise to this frequency distribution have rarely been modelled and tested for organisms with complex host selection behaviour. 2. In this study Microrhopala vittata, a chrysomelid beetle, was used to investigate how oviposition behaviour, movement and density of host plants interact in shaping the fre- quency distribution of egg clusters per host in the field. 3. Enclosures were stocked with two different host species and different beetle densities and various stochastic process models were fitted to egg cluster count data obtained from these enclosures. The different models were derived considering different scenar- ios, in particular whether or not plant density limits oviposition rate, whether or not ovi- positing females actively seek out the most attractive plant within their perception radius and whether a female’s oviposition rate is determined by plant intrinsic factors, the plant’s egg cluster load or the surrounding beetle density. 4. The model parameters fitted to cage data were used to describe the frequency dis- tribution of egg cluster counts obtained in a release experiment in the field. A total of 220 beetle pairs were released at five locations in a field where this beetle was not observed previously. Each release point was at a border between the two host species. 5. One model predicted for the preferred host species the egg cluster count frequencies in the field from parameters estimated in the cages. This model assumed that egg clusters present on a plant increased subsequent oviposition on this plant. All other models could not describe the distribution of egg cluster counts for either of the two host species. 6. The results suggest that females seek out attractive hosts actively and the attractive- ness of a plant increases with its egg cluster load. This behaviour creates a frequency distribution of egg clusters per host that depends only on beetle density but not on plant density. This conclusion has important implications for modelling insect–plant interactions. Key-words: Chrysomelidae, dispersal, egg count distribution, oviposition behaviour, stochastic processes. Journal of Animal Ecology (2006) 75, 387–398 doi: 10.1111/j.1365-2656.2006.01059.x asite individuals per host (e.g. Anderson & May 1978; Introduction May & Anderson 1978; Hassell, Southwood & Reader The dynamics of organisms with parasitic lifestyle 1987; Hassell et al. 1991). Knowledge of the frequency depend critically on the frequency distribution of par- distribution of local densities is often necessary for predicting the change in global densities of locally © 2006 The Author. Correspondence and present address: Heinrich zu Dohna, interacting species for a simple reason (Chesson 1996, Journal compilation Rockefeller University, Laboratory of Populations, 1230 York 1998): if the change of a species’ density is determined © 2006 British Avenue, New York, NY 10021, USA. Tel: (212) 327 8886; by local interactions, it is a function of local densities. Ecological Society Fax: (212) 327 8888; E-mail: [email protected] In this case the overall change in density is calculated 388 by applying the function for the change in density to Adults of M. vittata emerge in late April after hiber- H. zu Dohna local densities and averaging the resulting values over nation and start to mate and oviposit immediately all local densities. The common practice of applying (Cappuccino 1991). The oviposition period lasts until the function for the change in density to the global mid-June. Males and females feed and mate multiple (i.e. the mean) density is equivalent to exchanging times during the oviposition period and females lay the order between applying a function and taking the multiple clutches (personal observations). Eggs are laid mean. However, the order of these two operations can- in clusters of two to four eggs on the upper leaves and not be exchanged for a non-linear function. The proper are covered with excrement (Messina & Root 1980). procedure, namely averaging the values of the function Only egg clusters were counted in this study. Unpublished for change in density over the local densities, requires data showed no connection between the number of egg information about the distribution of densities at the clusters per leaf and the number of eggs per egg clusters. local scale of interaction. The number of egg clusters per leaf is therefore approx- This situation applies not only to parasites but also imately proportional to the number of eggs per leaf. to many herbivorous insects which undergo an immo- The larvae are leaf miners that burrow directly into bile larval stage, during which larvae interact on the the oviposition leaf after hatching and stay on the same scale of a host plant. To model the dynamics of these plant until pupation. An increased number of eggs per species, one has to understand the processes shaping leaf increases the mine establishment success and there- the distribution of egg counts per host plant. This dis- fore the survival to pupation of each individual larva tribution is a result of potentially complex interactions (Damman 1994). Pupation occurs in the study region of movement and oviposition behaviour of females as between mid-July and August (Clark 1983). The adults well as the distribution of hosts. Because of their com- usually stay on the same plant before they hibernate in plexity, these processes have usually been investigated the ground. Although M. vittata is, in principle, cap- in isolation for herbivorous insects. able of flying, I have not observed any attempt to fly by There is much research on oviposition choice by these beetles during 3 years of close observation. females in insects (for review see Mayhew 1997), its implications for community dynamics (e.g. Hassell 1978; Hassell et al. 1991; Hanski & Singer 2001) as well as for the evolution of host specialization (Futuyma The aim of this study was to understand how oviposi- 1983; Jaenike 1990; Mayhew 1997). Distributional pat- tion and movement behaviour of the beetles interact terns of insects resulting from the interaction of insect with host density in shaping the distribution of egg movement and host plant distribution have been stud- cluster counts. A common garden experiment (density ied extensively (e.g. Root & Kareiva 1984; Kareiva experiment) was performed to determine how the egg 1985). However, little is known about how movement, cluster distribution depends on the density of con- oviposition behaviour and host density interact in specifics. A release experiment yielded data about the shaping the distribution of egg counts per host for her- distribution of egg clusters generated in the field when bivorous insects. all females start at one point. These experiments will be This study investigated whether cage observations of described in detail in the next section. oviposition can predict the frequency distribution of As a framework for the interpretation of the experi- egg clusters per plant in the field for Microrhopala mental results, I will outline a general mathematical vittata Fabricius, a chrysomelid beetle, and two of its formulation of the oviposition process during which goldenrod host species, Solidago altissima L. and S. adults disperse, oviposit and die. Using this general rugosa Miller. Results from cage experiments were used framework I will derive more specific stochastic to build a mathematical description of the oviposition models, which were fitted to the data from the density process. The model was tested against patterns in the experiment and used to predict patterns from the field. The goal of this approach was to develop empir- release experiment. ically grounded models for dynamics of populations Let n(x, t) denote the density of ovipositing females with non-linear localized interactions. and m(x, t) the egg cluster density at the spatial location = x (x1, x2) and time t. The density of ovipositing females (henceforth referred to as beetle density) Methods changes at any given location over time due to move- ment, mortality and the termination of oviposition. The model presented here does not describe the M. vittata is a chrysomelid beetle which, in Connecticut, change in beetle density but rather for a given trajec- © 2006 The Author. USA, occurs on three goldenrod species, S. altissima, tory of beetle density, n(x, t), the change in egg cluster Journal compilation S. juncea Aiton and S. rugosa. S. rugosa, although the density m(x, t), which is given by the ordinary differ- © 2006 British most common of these three species in the study region, ential equation: Ecological Society, Journal of Animal is used only occasionally as a host. S. juncea is the least common host in the region and was not investigated in ∂m Ecology, 75, =⋅ nmnp φ(, , ) (1) 387–398 this study. ∂t 389 where φ(m, n, p) denotes the rate of oviposition per In the alternative scenario of ‘active plant selection’ the Frequency female as function of egg cluster density m, beetle den- probability of a plant being chosen depends on how distribution of egg sity n, and plant density p. Time dependency of ovi- attractive the plant is to an ovipositing female relative cluster counts position is captured in n(x, t), the density of ovipositing to the neighbouring plants. To simplify this situation a females. At any point in time this approach considers female was assumed to only select the most attractive plant only the current beetle density and cannot incorporate within its perception radius. The attractiveness of a plant any effect an individual’s history of host encounter could depend on plant intrinsic characteristics or its egg could have on its oviposition rate.